**Section 2**

**Polymers and Biopolymers** 

192 Infrared Spectroscopy – Materials Science, Engineering and Technology

Freitas, JCC.; Bonagamba, TJ. & Emmerich, FG. (1999). 13C High-resolution solid-state NMR

Hammond, TE.; Cory, DG.; Ritchey, M. & Morita, H. High resolution solid state 13C n.m.r.

Ibarra, JV. & Juan, R. (1985). Structural changes in humic acids during the coalification

Ibarra, JV.; Muñoz, E. & Moliner, R. (1996). FTIR study of the evolution of coal structure

Japanese Industrial Standards Committee. JIS M 8814. (2003). *Coal and coke*. Determination of

Kalkreuth, W. & Chornet, E. (1982). Peat hydrogenolysis using H2/CO mixtures:

Lau, FS.; Roberts, MJ.; Rue, DM.; Punwani, DV; Wen, WW. & Johnson, PB. (1987). Peat beneficiation by wet carbonization. *Int J Coal Geol,* Vol.8, pp. 111–121. Mursito, AT.; Hirajima, T. & Sasaki, K. (2010). Upgrading and dewatering of raw tropical

Mursito, AT.; Hirajima, T.; Sasaki, K. & Kumagai S. (2010). The effect of hydrothermal

Noto, S. (1991). *Peat engineering handbook*. Civil Engineering Research Institute of Hokkaido

Orem, WH.; Neuzil, SG.; Lerch, EL. & Cecil, CB. (1996). Experimental early–stage

Painter, PC.; Snyder, RW.; Starsinic, M.; Coleman, MM.; Kuehn, DW. & Davis, A. (1981).

Van Krevelen, DW. (1950). Graphical–statistical method for the study of structure and

Xuguang, S. (2005). The investigation of chemical structure of coal macerals via transmitted– light FT–IR microspectroscopy. *Spectrochim Acta*, Vol.62, pp. 557–564. Yoshida, T. & Maekawa, Y. (1987). Characterization of coal structure by CP/MAS carbon-13

Yoshida, T.; Sasaki, M.; Ikeda, K.; Mochizuki, M.; Nogami, Y. & Inokuchi, K. (2002)

Prediction of coal liquefaction reactivity by solid state 13C NMR spectral data. *Fuel*,

gross calorific value by the bomb calorimetric method, and calculation of net

Micropetrological and chemical studies of original material and reaction residues.

dewatering of Pontianak tropical peat on organics in wastewater and gaseous

coalification of a peat sample and a peatified wood sample from Indonesia. *Org* 

Concerning the application of FT–IR to the study of coal: a critical assessment of band assignments and the application of spectral analysis programs. *Appl Spectrosc*,

during the coalification process. *Org Geochem,* Vol.24, pp. 725–735.

study of peat carbonization. *Energ Fuel*, Vol.13, pp. 53–59.

calorific value. Japanese Standards Association. Tokyo

peat by hydrothermal treatment. *Fuel,* Vol.89, pp. 635–41.

of Canadian peats. Fuel 1985;64:1687–1695.

process. *Fuel*, Vol.64, pp. 650–656.

*Fuel Process Technol*, Vol.6, pp. 93–122.

products. *Fuel,* Vol.89, pp. 3934–3942.

Development Bureau

Vol.35, pp. 475–485.

Vol.81, pp. 1533–1539.

*Geochem*, Vol.24, pp. 111–125.

Spedding, PJ. (1988). Peat. *Fuel*, Vol.67, pp. 883–900.

reaction processes of coal. *Fuel*, Vol.29, pp. 269–284. World Energy Council (WEC). (2001). Survey of Energy Resources. *Peat*.

NMR spectrometry. *Fuel Process Technol*, Vol.15, pp. 385–395.

**9** 

*Mexico* 

**FTIR – An Essential Characterization** 

Guillermo Andrade Espinosa and José L. Dávila Rodríguez

Infrared spectroscopy is an essential and crucial characterization technique to elucidate the structure of matter at the molecular scale. The chemical composition and the bonding arrangement of constituents in a homopolymer, copolymer, polymer composite and polymeric materials in general can be obtained using Infrared (IR) spectroscopy (Bhargava et al. 2003)

The FTIR spectrometers obtain the IR spectrum by Fourier transformation of the signal from an interferometer with a moving mirror to produce an optical transform of the infrared signal. Numerical Fourier analysis gives the relation of intensity and frequency, that is, the IR spectrum. The FTIR technique can be used to analyze gases, liquids, and solids with

The spectrometer may operate in transmission or reflection, but also in attenuated total reflection (ATR) mode, which have been widely used during the last two decades. The transmission mode is very suitable for quantitative analysis, since the main parameters to apply on the Beer-Lambert law are known or easily estimated. The reflection mode is used when polymer is not well dissolved at room temperature, and a film or pellets of the sample is characterized. Such is the case of polyolefins and some engineering polymers like PET.

The ATR objective has a polished face of diamond, germanium or zinc selenide (ZnSe) that is pressed into contact with the sample (Sawyer et al. 2008). Infrared reflection is attenuated by absorption within a surface layer a few micrometers deep. In this case, a good contact is required, but that is easy for most polymers, and the advantage is that no sample preparation is required. Therefore, powders, films, gels and even polymer solutions can be

The absorption versus frequency characteristics of light transmitted through a specimen irradiated with a beam of infrared radiation provides a fingerprint of molecular structure. Infrared radiation is absorbed when a dipole vibrates naturally at the same frequency in the absorber. The pattern of vibrations is unique for a given molecule, and the intensity of absorption is related to the quantity of absorber. In the IR region, each group has several

**1. Introduction** 

characterized.

minimal preparation. (Lee, 1977).

**Technique for Polymeric Materials** 

José R. Rangel Méndez, Nancy V. Pérez Aguilar,

*Instituto Potosino de Investigación Científica y Tecnólogica,* 

*A.C. (IPICYT), Divisiόn de Ciencias Ambientales* 

Vladimir A. Escobar Barrios,

## **FTIR – An Essential Characterization Technique for Polymeric Materials**

Vladimir A. Escobar Barrios, José R. Rangel Méndez, Nancy V. Pérez Aguilar, Guillermo Andrade Espinosa and José L. Dávila Rodríguez *Instituto Potosino de Investigación Científica y Tecnólogica, A.C. (IPICYT), Divisiόn de Ciencias Ambientales Mexico* 

#### **1. Introduction**

Infrared spectroscopy is an essential and crucial characterization technique to elucidate the structure of matter at the molecular scale. The chemical composition and the bonding arrangement of constituents in a homopolymer, copolymer, polymer composite and polymeric materials in general can be obtained using Infrared (IR) spectroscopy (Bhargava et al. 2003)

The FTIR spectrometers obtain the IR spectrum by Fourier transformation of the signal from an interferometer with a moving mirror to produce an optical transform of the infrared signal. Numerical Fourier analysis gives the relation of intensity and frequency, that is, the IR spectrum. The FTIR technique can be used to analyze gases, liquids, and solids with minimal preparation. (Lee, 1977).

The spectrometer may operate in transmission or reflection, but also in attenuated total reflection (ATR) mode, which have been widely used during the last two decades. The transmission mode is very suitable for quantitative analysis, since the main parameters to apply on the Beer-Lambert law are known or easily estimated. The reflection mode is used when polymer is not well dissolved at room temperature, and a film or pellets of the sample is characterized. Such is the case of polyolefins and some engineering polymers like PET.

The ATR objective has a polished face of diamond, germanium or zinc selenide (ZnSe) that is pressed into contact with the sample (Sawyer et al. 2008). Infrared reflection is attenuated by absorption within a surface layer a few micrometers deep. In this case, a good contact is required, but that is easy for most polymers, and the advantage is that no sample preparation is required. Therefore, powders, films, gels and even polymer solutions can be characterized.

The absorption versus frequency characteristics of light transmitted through a specimen irradiated with a beam of infrared radiation provides a fingerprint of molecular structure. Infrared radiation is absorbed when a dipole vibrates naturally at the same frequency in the absorber. The pattern of vibrations is unique for a given molecule, and the intensity of absorption is related to the quantity of absorber. In the IR region, each group has several

FTIR – An Essential Characterization Technique for Polymeric Materials 197

nanotubes are chemically treated in order to improve its interaction with other entities like polymeric matrix, for example. In this sense, oxidized nitrogen-doped nanotubes were analyzed by FTIR, corroborating that functional groups are present in these carbon

The xerogels are materials that have precise microstructure and they are synthesized by sol– gel synthesis. Two steps are implied in the sol-gel process, hydrolysis and condensation, which are very sensitive to reaction conditions, being faster or slower and giving xerogels with different characteristics, in terms of structure, surface area, and porosity (Fidalgo and

In this sense, the trace of each reaction with time, hydrolysis and condensation, is important in order to establish the end and beginning of each of these reactions. FTIR is a very useful characterization technique to follow reactions during sol-gel synthesis, since there are

In the case of hydrolysis, silanol is produced from reaction between silane and water molecules. The silanol exhibits a strong absorbance around 945 cm−1, which can be followed

The condensation reaction, where the silanol groups react between them, creating siloxane groups, gives an intense absorbance band at 1080 cm-1. Therefore, it is also possible to see

The main functional group that is produced during hydrolysis of silanes is silanol, which has a characteristic absorbance band at 945 cm−1. The hydrolysis reaction, usually, is carried out in presence of some alcohol, depending of the silane. For example, if the hydrolysis is carried out with tetra methoxi silane, then the chosen alcohol is methanol, or with tetra ethoxy silane, the preferred alcohol is ethanol. This is because the by-product of hydrolysis reaction is an alcohol. Therefore, the intense signals in the infrared bands located at 945 and 880 cm−1, corresponding to silanol and ethanol (for example) can be followed during

In the Figure 1 it is shown the spectra for hydrolysis of tetra ethoxy silane (TEOS) in presence of ethanol at different reaction time. The absorbance band at 945 cm−1 can be assigned to the vibration of the Si-O from the silanol residual groups [Si-(OH)4], whereas the band at 880 cm−1 can be assigned to OH groups from ethanol. The time at which no more increment of the absorbance band of silanol and OH groups occurs was established as the

From Figure 1, it can be seen that the absorbance bands corresponding to silanol and OH, from ethanol, increased up to 30 min. After this time, there was no increase of these bands (spectra not shown), which can indicate that the hydrolysis reaction has been completed.

Determination of the time needed to carry out the hydrolysis reaction is critical since the knowledge of the complete conversion to silanol groups is required to promote a more

specific functional groups that participate in the sol-gel process.

variation of such band with time (Andrade et al., 2010).

time at which the hydrolysis reaction was completed.

nanotubes.

**2. Xerogels** 

Ilharco, 2005).

during the reaction.

**2.1 Kinetics by FTIR** 

hydrolysis reaction.

efficient condensation reaction.

and different patterns of vibration such as: stretching, bending, rocking, etc. The absorbance of these bands is proportional to their content based on the Lambert–Beer law. Because of the complexities of the structures (for example, polybutadiene unit has 1,4-cis, 1,4-trans and 1,2-vinyl microstructure while polyisoprene unit has other additional 3,4-microstructure), the general methods for quantitative analysis require external standards, which are usually characterized by primary technique like Nuclear Magnetic Resonance (Zhang et al, 2007). Nevertheless, it is possible to characterize polymers, quantitatively, using FTIR as it will be discussed later.

Thus, infrared spectroscopy permits the determination of components or groups of atoms that absorb in the infrared at specific frequencies, permitting identification of the molecular structure (Bower, 1989; Koenig, 2001). These techniques are not limited to chemical analysis. In addition, the tacticity, crystallinity, and molecular strain can also be measured. Copolymer compositions can be determined as block copolymers absorb additively, and alternating copolymers deviate from this additivity due to interaction of neighboring groups.

The aim of this chapter is to present some examples in the context of different areas, in order to remark the importance of FTIR to determine different aspects as identification of functional groups, to elucidate mechanisms of reaction or even to determine the time of reaction.

The track of a reaction in real time when xerogels are synthesized, interaction of different materials like chitin with polyurethane during synthesis of biocomposite, identification of functional groups attached to nanotubes during their modification and establishment of microstructure of polybutadiene during its hydrogenation are the main examples of the use of FTIR, even being semi-quantitative, quantitative or qualitative analysis, that will be presented and analyzed.

The synthesis of xerogels involves basically two steps, hydrolysis and condensation. The follow of such steps is important to establish the time needed to change process conditions and optimize the xerogel. Even when FTIR analysis is a qualitative technique, this analysis could be used as a semi-quantitative indirect measure of the variation of functional groups, during hydrolysis or condensation reactions.

Biocomposites are very fascinating materials since they offer characteristics of two or more different materials, in order to have very specific features that would be practically impossible to obtain by every single material of biocomposite. Chitin is an abundant biopolymer obtained from shrimp, insects and some vegetal species. This material is capable to remove some contaminants like fluoride from water. Nevertheless, in order to improve the mechanical characteristics of chitin, in order to be applied in water treatment in real conditions, it must be supported. Polyurethane is a very versatile polymer due to its chemical structure. During its synthesis, interactions between functional groups take place in order to create the urethane group. The synthesis of biocomposite must bear in mind that interaction between compounds is essential to create a mechanical and chemical resistant material. FTIR with ATR analysis was carried out to characterize a biocomposite based on chitin and polyurethane, demonstrating that interaction between them occurs.

The characterization of carbon nanotubes usually is carried out using Raman spectroscopy. Nonetheless, the use of FTIR allows the determination of functional groups when carbon nanotubes are chemically treated in order to improve its interaction with other entities like polymeric matrix, for example. In this sense, oxidized nitrogen-doped nanotubes were analyzed by FTIR, corroborating that functional groups are present in these carbon nanotubes.

## **2. Xerogels**

196 Infrared Spectroscopy – Materials Science, Engineering and Technology

and different patterns of vibration such as: stretching, bending, rocking, etc. The absorbance of these bands is proportional to their content based on the Lambert–Beer law. Because of the complexities of the structures (for example, polybutadiene unit has 1,4-cis, 1,4-trans and 1,2-vinyl microstructure while polyisoprene unit has other additional 3,4-microstructure), the general methods for quantitative analysis require external standards, which are usually characterized by primary technique like Nuclear Magnetic Resonance (Zhang et al, 2007). Nevertheless, it is possible to characterize polymers, quantitatively, using FTIR as it will be

Thus, infrared spectroscopy permits the determination of components or groups of atoms that absorb in the infrared at specific frequencies, permitting identification of the molecular structure (Bower, 1989; Koenig, 2001). These techniques are not limited to chemical analysis. In addition, the tacticity, crystallinity, and molecular strain can also be measured. Copolymer compositions can be determined as block copolymers absorb additively, and alternating copolymers deviate from this additivity due to interaction of neighboring

The aim of this chapter is to present some examples in the context of different areas, in order to remark the importance of FTIR to determine different aspects as identification of functional groups, to elucidate mechanisms of reaction or even to determine the time of

The track of a reaction in real time when xerogels are synthesized, interaction of different materials like chitin with polyurethane during synthesis of biocomposite, identification of functional groups attached to nanotubes during their modification and establishment of microstructure of polybutadiene during its hydrogenation are the main examples of the use of FTIR, even being semi-quantitative, quantitative or qualitative analysis, that will be

The synthesis of xerogels involves basically two steps, hydrolysis and condensation. The follow of such steps is important to establish the time needed to change process conditions and optimize the xerogel. Even when FTIR analysis is a qualitative technique, this analysis could be used as a semi-quantitative indirect measure of the variation of functional groups,

Biocomposites are very fascinating materials since they offer characteristics of two or more different materials, in order to have very specific features that would be practically impossible to obtain by every single material of biocomposite. Chitin is an abundant biopolymer obtained from shrimp, insects and some vegetal species. This material is capable to remove some contaminants like fluoride from water. Nevertheless, in order to improve the mechanical characteristics of chitin, in order to be applied in water treatment in real conditions, it must be supported. Polyurethane is a very versatile polymer due to its chemical structure. During its synthesis, interactions between functional groups take place in order to create the urethane group. The synthesis of biocomposite must bear in mind that interaction between compounds is essential to create a mechanical and chemical resistant material. FTIR with ATR analysis was carried out to characterize a biocomposite based on

The characterization of carbon nanotubes usually is carried out using Raman spectroscopy. Nonetheless, the use of FTIR allows the determination of functional groups when carbon

chitin and polyurethane, demonstrating that interaction between them occurs.

discussed later.

groups.

reaction.

presented and analyzed.

during hydrolysis or condensation reactions.

The xerogels are materials that have precise microstructure and they are synthesized by sol– gel synthesis. Two steps are implied in the sol-gel process, hydrolysis and condensation, which are very sensitive to reaction conditions, being faster or slower and giving xerogels with different characteristics, in terms of structure, surface area, and porosity (Fidalgo and Ilharco, 2005).

In this sense, the trace of each reaction with time, hydrolysis and condensation, is important in order to establish the end and beginning of each of these reactions. FTIR is a very useful characterization technique to follow reactions during sol-gel synthesis, since there are specific functional groups that participate in the sol-gel process.

In the case of hydrolysis, silanol is produced from reaction between silane and water molecules. The silanol exhibits a strong absorbance around 945 cm−1, which can be followed during the reaction.

The condensation reaction, where the silanol groups react between them, creating siloxane groups, gives an intense absorbance band at 1080 cm-1. Therefore, it is also possible to see variation of such band with time (Andrade et al., 2010).

## **2.1 Kinetics by FTIR**

The main functional group that is produced during hydrolysis of silanes is silanol, which has a characteristic absorbance band at 945 cm−1. The hydrolysis reaction, usually, is carried out in presence of some alcohol, depending of the silane. For example, if the hydrolysis is carried out with tetra methoxi silane, then the chosen alcohol is methanol, or with tetra ethoxy silane, the preferred alcohol is ethanol. This is because the by-product of hydrolysis reaction is an alcohol. Therefore, the intense signals in the infrared bands located at 945 and 880 cm−1, corresponding to silanol and ethanol (for example) can be followed during hydrolysis reaction.

In the Figure 1 it is shown the spectra for hydrolysis of tetra ethoxy silane (TEOS) in presence of ethanol at different reaction time. The absorbance band at 945 cm−1 can be assigned to the vibration of the Si-O from the silanol residual groups [Si-(OH)4], whereas the band at 880 cm−1 can be assigned to OH groups from ethanol. The time at which no more increment of the absorbance band of silanol and OH groups occurs was established as the time at which the hydrolysis reaction was completed.

From Figure 1, it can be seen that the absorbance bands corresponding to silanol and OH, from ethanol, increased up to 30 min. After this time, there was no increase of these bands (spectra not shown), which can indicate that the hydrolysis reaction has been completed.

Determination of the time needed to carry out the hydrolysis reaction is critical since the knowledge of the complete conversion to silanol groups is required to promote a more efficient condensation reaction.

FTIR – An Essential Characterization Technique for Polymeric Materials 199

One of the most studied biosorbent is chitin, which is an abundant biopolymer found in crustaceans, insects and fungus. This biopolymer is commercially purified by alkaline deproteinization, acid demineralization and decoloration by organic solvents of crustaceans wastes (Pastor, 2004). An additional stronger alkaline treatment of chitin produces deacetylated chitin. If the acetylation degree (DA) decreases at 39% or less, the biopolymer is named chitosan. Hence, the DA of chitin is variable and depends on the process conditions (alkali concentration, contact time, temperature, etc.), which produces DA values from 100 to 0%. Because of this, chitin is known as the biopolymer which has a DA from 100 to 40%; likewise, when the chitinous biopolymer has DA lower than 40%, the biopolymer is named chitosan. Chitosan is, therefore, a biopolymer with structure very similar to that of chitin (see Figure 2); however, chitosan solubility is much greater,

Fig. 2. Chemical structures of a) chitin and b) chitosan (Elnashar, 2010).

Chitin is capable to remove many contaminants from water. A wide variety of studies have demonstrated the ability of chitin to uptake substances as metals, anions and organics.

especially in acid mediums.

Fig. 1. Spectra of hydrolysis of TEOS at different times.

In the case of the condensation reaction, the silanols are reacting, between them, to produce siloxane groups. The siloxane group has a characteristic absorbance band, corresponding to the vibration mode (Si-O-Si) at 1080 cm-1, that can be followed with time. In fact, from Figure 1, it is noted that after 15 minutes of reaction, there is an important increment of the band at 1080 cm-1 compared to the spectrum at 5 minutes of reaction. It is important to have in mind that hydrolysis and condensation can take place at same time, but condensation is catalyzed with alkaline chemicals, as amines. The fact that the band at 1080 cm-1 had not an increment, from 15 to 30 minutes, could be attributed to the absence of alkaline catalyst in the media reaction. It was shown (Andrade et al., 2010) that the absorbance was increased when an alkaline catalyst was added to the media reaction, and the band corresponding to silanol (945 cm-1) was reduced with time.

#### **3. Biocomposites**

Biocomposites i.e. composite materials comprising one or more phase(s) derived from a biological origin (Fowler et al., 2006), are very interesting materials since they offer characteristics of two or more different materials, in order to have very specific features that would be practically impossible to obtain by every single material of biocomposite. Many types of biocomposites have been proposed, depending of the application objective of each material. Water treatment researchers have recently proposed the use of biocomposites as adsorbent materials, in order to remove many different contaminants from water. Synthesis of biocomposites to use them as adsorbents has been necessary, for example, because biosorbents (adsorbents from a biological origin) usually have poor physical/chemical resistance. The roll of the matrix in biocomposite is to reinforce the biosorbent, by establishing physical and/or chemical links between both phases.

Si-O

OH

In the case of the condensation reaction, the silanols are reacting, between them, to produce siloxane groups. The siloxane group has a characteristic absorbance band, corresponding to the vibration mode (Si-O-Si) at 1080 cm-1, that can be followed with time. In fact, from Figure 1, it is noted that after 15 minutes of reaction, there is an important increment of the band at 1080 cm-1 compared to the spectrum at 5 minutes of reaction. It is important to have in mind that hydrolysis and condensation can take place at same time, but condensation is catalyzed with alkaline chemicals, as amines. The fact that the band at 1080 cm-1 had not an increment, from 15 to 30 minutes, could be attributed to the absence of alkaline catalyst in the media reaction. It was shown (Andrade et al., 2010) that the absorbance was increased when an alkaline catalyst was added to the media reaction, and the band corresponding to

Biocomposites i.e. composite materials comprising one or more phase(s) derived from a biological origin (Fowler et al., 2006), are very interesting materials since they offer characteristics of two or more different materials, in order to have very specific features that would be practically impossible to obtain by every single material of biocomposite. Many types of biocomposites have been proposed, depending of the application objective of each material. Water treatment researchers have recently proposed the use of biocomposites as adsorbent materials, in order to remove many different contaminants from water. Synthesis of biocomposites to use them as adsorbents has been necessary, for example, because biosorbents (adsorbents from a biological origin) usually have poor physical/chemical resistance. The roll of the matrix in biocomposite is to reinforce the biosorbent, by

establishing physical and/or chemical links between both phases.

Fig. 1. Spectra of hydrolysis of TEOS at different times.

silanol (945 cm-1) was reduced with time.

**3. Biocomposites** 

One of the most studied biosorbent is chitin, which is an abundant biopolymer found in crustaceans, insects and fungus. This biopolymer is commercially purified by alkaline deproteinization, acid demineralization and decoloration by organic solvents of crustaceans wastes (Pastor, 2004). An additional stronger alkaline treatment of chitin produces deacetylated chitin. If the acetylation degree (DA) decreases at 39% or less, the biopolymer is named chitosan. Hence, the DA of chitin is variable and depends on the process conditions (alkali concentration, contact time, temperature, etc.), which produces DA values from 100 to 0%. Because of this, chitin is known as the biopolymer which has a DA from 100 to 40%; likewise, when the chitinous biopolymer has DA lower than 40%, the biopolymer is named chitosan. Chitosan is, therefore, a biopolymer with structure very similar to that of chitin (see Figure 2); however, chitosan solubility is much greater, especially in acid mediums.

Fig. 2. Chemical structures of a) chitin and b) chitosan (Elnashar, 2010).

Chitin is capable to remove many contaminants from water. A wide variety of studies have demonstrated the ability of chitin to uptake substances as metals, anions and organics.

FTIR – An Essential Characterization Technique for Polymeric Materials 201

One research has recently reported characterization of the polyurethane-chitin interaction in a novel fluoride biosorbent biocomposite, using attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) (Davila-Rodriguez et al., 2009). Such a biocomposite was produced by mixing chitin flakes with polyurethane (60:40 w/w) during the polymerization reaction, which was carried out under intense stirring. The biocomposite consisted, therefore, of chitin flakes covered by a thin film of polyurethane of approximately 1 m thickness, according to different observations performed by scanning electron microscopy. The novel composite material showed greater chemical resistance compared to pure chitin, measured as a decrease of around ten times in the loss of mass when the material was submerged in an acid aqueous medium at pH 5 (from 19.6 to 1.5%). These results demonstrated the strong interaction between chitin and polyurethane on their contact surface. Nevertheless, FTIR-ATR methodology was used in order to get a better understanding of the biopolymer-matrix links formed during the biocomposite synthesis. For the FTIR-ATR analysis, a sample of the chitin-polyurethane biocomposite was used as produced. The FTIR-ATR instrument used was a Nicolet 6700 (Thermo Scientific) operating in the wavenumber range of 650 to 4000 cm-1, with ZnSe crystal, 32 scans and 4 cm-1

Analysis of FTIR-ATR spectra showed that the biocomposite spectrum was very similar to that of polyurethane. In addition, it is important to point out that main bands of chitin spectrum were at the same wavenumber intervals that those of polyurethane (see Figure 4).

Fig. 4. FTIR-ATR spectra of chitin, polyurethane and chitin-polyurethane biocomposite

(modified from Davila-Rodriguez et al., 2009).

**3.1 Methods** 

resolution.

**3.2 Results and discussion** 

Nevertheless, in order to improve the mechanical characteristics of chitin to be applied for water treatment in real conditions, it must be supported. The use of polymers as supporting matrix of biosorbents has given encouraging results. In this sense, polyurethane has been one of the most used polymers.

Polyurethane includes a group of polymers derived from the isocyanates, organic compounds which have the isocyanate group (-N=C=O) in their structures and, owing to this, are very reactive. Polyurethane shows advantages as easy handling and the possibility to obtain malleable and resistant biocomposites. The Figure 3 shows a general polyurethane prepolymer structure and how isocyanate group reaction gives rise to different type of chemical linkages.

Fig. 3. General chemical structure of polyurethane prepolymer (modified from Hepburn, 1982).

Chemical reaction between the isocyanate and a primary amine group produces the urea group, whereas the reaction between isocyanate and hydroxyl group forms the urethane group. Both urea and urethane are very stable chemical groups, which make the polyurethane a polymer very useful in applications that must resist extreme conditions of temperature, friction and UV radiation (Hepburn, 1982). Due to the high reactivity of the isocyanate with the primary amine and hydroxyl groups, it is valid to assume that the polyurethane establish strong unions with biosorbents to form biocomposites. Hence, such unions make polyurethane a good support matrix for biosorbents. In fact, more and more studies have been recently published about the use of polyurethane as support of biosorbents as seaweed (Alhakawati & Banks, 2004; Zhang & Banks, 2006), moss, sunflower waste and maize plant (Zhang & Banks, 2006), bacteria (Vullo et al., 2008; Mao et al., 2010), fungi (Sudha & Abraham, 2003; Li et al., 2008) and chitin (Davila-Rodriguez et al., 2009). Most of these studies have focused on the biocomposite biosorption capacity; however, the interaction polyurethane-biosorbent has been practically not studied. Infrared spectroscopy is a good methodology to characterize such interaction.

### **3.1 Methods**

200 Infrared Spectroscopy – Materials Science, Engineering and Technology

Nevertheless, in order to improve the mechanical characteristics of chitin to be applied for water treatment in real conditions, it must be supported. The use of polymers as supporting matrix of biosorbents has given encouraging results. In this sense, polyurethane has been

Polyurethane includes a group of polymers derived from the isocyanates, organic compounds which have the isocyanate group (-N=C=O) in their structures and, owing to this, are very reactive. Polyurethane shows advantages as easy handling and the possibility to obtain malleable and resistant biocomposites. The Figure 3 shows a general polyurethane prepolymer structure and how isocyanate group reaction gives rise to different type of

Fig. 3. General chemical structure of polyurethane prepolymer (modified from Hepburn,

is a good methodology to characterize such interaction.

Chemical reaction between the isocyanate and a primary amine group produces the urea group, whereas the reaction between isocyanate and hydroxyl group forms the urethane group. Both urea and urethane are very stable chemical groups, which make the polyurethane a polymer very useful in applications that must resist extreme conditions of temperature, friction and UV radiation (Hepburn, 1982). Due to the high reactivity of the isocyanate with the primary amine and hydroxyl groups, it is valid to assume that the polyurethane establish strong unions with biosorbents to form biocomposites. Hence, such unions make polyurethane a good support matrix for biosorbents. In fact, more and more studies have been recently published about the use of polyurethane as support of biosorbents as seaweed (Alhakawati & Banks, 2004; Zhang & Banks, 2006), moss, sunflower waste and maize plant (Zhang & Banks, 2006), bacteria (Vullo et al., 2008; Mao et al., 2010), fungi (Sudha & Abraham, 2003; Li et al., 2008) and chitin (Davila-Rodriguez et al., 2009). Most of these studies have focused on the biocomposite biosorption capacity; however, the interaction polyurethane-biosorbent has been practically not studied. Infrared spectroscopy

one of the most used polymers.

chemical linkages.

1982).

One research has recently reported characterization of the polyurethane-chitin interaction in a novel fluoride biosorbent biocomposite, using attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) (Davila-Rodriguez et al., 2009). Such a biocomposite was produced by mixing chitin flakes with polyurethane (60:40 w/w) during the polymerization reaction, which was carried out under intense stirring. The biocomposite consisted, therefore, of chitin flakes covered by a thin film of polyurethane of approximately 1 m thickness, according to different observations performed by scanning electron microscopy. The novel composite material showed greater chemical resistance compared to pure chitin, measured as a decrease of around ten times in the loss of mass when the material was submerged in an acid aqueous medium at pH 5 (from 19.6 to 1.5%). These results demonstrated the strong interaction between chitin and polyurethane on their contact surface. Nevertheless, FTIR-ATR methodology was used in order to get a better understanding of the biopolymer-matrix links formed during the biocomposite synthesis. For the FTIR-ATR analysis, a sample of the chitin-polyurethane biocomposite was used as produced. The FTIR-ATR instrument used was a Nicolet 6700 (Thermo Scientific) operating in the wavenumber range of 650 to 4000 cm-1, with ZnSe crystal, 32 scans and 4 cm-1 resolution.

#### **3.2 Results and discussion**

Analysis of FTIR-ATR spectra showed that the biocomposite spectrum was very similar to that of polyurethane. In addition, it is important to point out that main bands of chitin spectrum were at the same wavenumber intervals that those of polyurethane (see Figure 4).

Fig. 4. FTIR-ATR spectra of chitin, polyurethane and chitin-polyurethane biocomposite (modified from Davila-Rodriguez et al., 2009).

FTIR – An Essential Characterization Technique for Polymeric Materials 203

could occur: the first is a three-coordinated N atom within the sp2- hybridized network, with the presence of additional electrons. The second type is the pyridine type (two-coordinated N) which can be incorporated in the nanotube lattice; an additional carbon atom is removed from the framework. N-MWCNTs have stacked-cone morphology (bamboo-type) and the degree of tubular perfection decreases as a result of the N incorporation into the hexagonal carbon lattice (Terrones et al., 2002). Moreover, properties of CNTs can also be modified by

H2O2). Under acidic oxidation bonds of carbon nanotubes could follow several reactions, for example the formation of quinones which further evolve to different oxygen acidic groups, such as phenolic, lactonic and carboxylic groups. The attached oxygenated groups change the surface chemistry of carbon nanotubes, in particular their wetting behavior and also can be used to attach different chemical groups or to improve their chemical interaction with other substances (Hirsch, 2002; Niyogi et al., 2002). Studies reported previously about oxidation of CNTs in liquid phase suggest that defects at the tips and sidewalls are the most reactive sites to attach oxygen functional groups to the nanotubes structure. These bonded atoms pull the reactive C atom out of the base plane, reducing the curvature strain of nanotubes and creating holes and pores. Long periods of oxidation lead to the shortening and destruction of the nanotubes (Ago et al., 1999; Ovejero et al., 2006; Zhang et al., 2003). Raman spectroscopy is widely used to evidence the enhancement in defects density along the walls of the carbon nanotubes when oxidation is applied. The main bands of carbon materials are the graphite band (G-band) and the disorder-induced phonon mode (D-band). The relative intensity between the D-band and G-band (ID/IG ratio) indicate a continuously covalent bonding of oxygenated groups along the walls of nanotubes, disrupting the aromatic system of π-electrons (Liu et al., 2004). Complementary information can be obtained with FTIR spectroscopy; it allows determination of functional groups attached to surface of CNTs. This tool was used in some studies carried out by our research group with N-MWCNT, MWCNT and SWCNT, which were oxidized with nitric acid solution (70%) at 80 ± 3 °C for 5 h, as detailed by Perez-Aguilar et al., (2010). Oxidized carbon nanotubes were identified as ox-N-MWCNT, ox-MWCNT and ox-SWCNT, and their morphology was observed by electronic microscopy (Figure 5). As the oxidation of carbon nanotubes was carried out in liquid phase, oxygen containing functional groups were attached along their entire length. Reactions between carbon materials and nitric acid involve strong electrophilic species in solution, which form nitrogen oxides that eventually are reduced to N2 through oxidation of carbon. Introduction of a small content of nitrogen has been reported, mainly

due to the introduction of nitro (-NO2) groups (Chen and Wu, 2004; Zawadski, 1980).

Fig. 5. Micrographs of carbon nanotubes observed by (a) SEM and (b) HRTEM for oxidized N-MWCNTs; HRTEM for (c) oxidized MWCNTs and (d) oxidized SWCNTs. Oxidation with

nitric acid at 80 ± 3 ⁰C by 5 h

+, H2SO4, K2Cr2O7,

oxidation with thermal treatment or with acidic solutions (HNO3, KMnO4

Such an aspect contributed to produce the similarity between biocomposite and polyurethane spectra. This fact was predictable because chitin and polyurethane have similar chemical groups on their structures, including amine, amide and carbonyl. Nevertheless, some of the bands observed in the biocomposite spectrum were different in intensity compared to those of polyurethane and chitin. Bands located at 660, 1590 and 3300 cm-1, corresponding to amine/amide groups, and the band located at 3350 cm-1, corresponding to hydroxyl group, experimented appreciable changes in intensity. This phenomenon could be due to the chitin-polyurethane chemical interaction. As previously mentioned, primary amine and hydroxyl groups (both present in chitin structure) are able to react chemically with the polymer functional groups i.e. isocyanate, to form urea and urethane groups, respectively.

On the other hand, it is important to say that chitin-polyurethane interaction has an important physical component, since attraction forces as Van der Waals interactions and hydrogen bonds are present at the interface chitin-polyurethane. This aspect could have caused that chitin-polyurethane chemical interaction was not strong enough to be more clearly observed by means of the FTIR-ATR analysis. In addition, chitin was added to the biocomposite in solid form, which limits the achievability of amine and hydroxyl groups of chitin surface due to a steric impediment. Other important aspect to point out is that polyurethane film thickness over the interface chitin-polyurethane could be irregular, which could difficult to reach the chitin-polyurethane interface by infrared light.

FTIR-ATR characterization of interface chitin-polyurethane could be improved by controlling the uniformity of polyurethane film thickness. This would be achieved by polymerizing the polyurethane in the necessary quantity to cover a high-sized chitin flake (e.g. 1 cm diameter), trying to produce a very thin film. As reported in literature, infrared light penetration depth can range from some hundred nanometers to several micrometers (Kane et al., 2009). Therefore, 1 μm thickness for polyurethane film is sufficient to infrared light reaches the chitin-polyurethane interface; this is part of future work in this research.

#### **4. Carbon nanotubes: Synthesis, properties and modification**

Carbon nanotubes (CNT) are key elements in nanotechnology. The structure of carbon nanotubes is depicted as a rolled segment of a graphene sheet, formed by linking each carbon atom to three equivalent neighbors by covalent bonds resulting in a hexagonal network. Carbon nanotubes can be structured by only one graphene sheet (single-wall carbon nanotubes, SWCNT) or by several coaxial graphene sheets (multiwall carbon nanotubes, MWCNT) (Dresselhaus et al., 2001). Synthesis of carbon nanotubes can be carried out by catalytic chemical vapor deposition (CVD) from carbon-containing gaseous compounds which decompose catalytically on transition-metal particles at temperatures lower than 1000 °C. CVD processes are becoming the major way for synthesizing carbon nanotubes in a controlled way (Loiseau et al., 2006). It is possible to modify and control the physicochemical properties of carbon nanotubes by doping processes, introducing either non-carbon atoms or molecules at small concentrations in the plane of the graphene lattice.

Doping carbon nanotubes with nitrogen atoms at lower concentrations than 6.5 wt% induce crystalline disorder in the graphene sheets, as well as an excess of electron donors on the nitrogen-rich areas may result in a more reactive structure compared to pure carbon nanotubes. For nitrogen-doped carbon nanotubes (N-MWCNT) two types of C–N bonds

Such an aspect contributed to produce the similarity between biocomposite and polyurethane spectra. This fact was predictable because chitin and polyurethane have similar chemical groups on their structures, including amine, amide and carbonyl. Nevertheless, some of the bands observed in the biocomposite spectrum were different in intensity compared to those of polyurethane and chitin. Bands located at 660, 1590 and 3300 cm-1, corresponding to amine/amide groups, and the band located at 3350 cm-1, corresponding to hydroxyl group, experimented appreciable changes in intensity. This phenomenon could be due to the chitin-polyurethane chemical interaction. As previously mentioned, primary amine and hydroxyl groups (both present in chitin structure) are able to react chemically with the polymer functional groups i.e. isocyanate, to form urea and

On the other hand, it is important to say that chitin-polyurethane interaction has an important physical component, since attraction forces as Van der Waals interactions and hydrogen bonds are present at the interface chitin-polyurethane. This aspect could have caused that chitin-polyurethane chemical interaction was not strong enough to be more clearly observed by means of the FTIR-ATR analysis. In addition, chitin was added to the biocomposite in solid form, which limits the achievability of amine and hydroxyl groups of chitin surface due to a steric impediment. Other important aspect to point out is that polyurethane film thickness over the interface chitin-polyurethane could be irregular, which

FTIR-ATR characterization of interface chitin-polyurethane could be improved by controlling the uniformity of polyurethane film thickness. This would be achieved by polymerizing the polyurethane in the necessary quantity to cover a high-sized chitin flake (e.g. 1 cm diameter), trying to produce a very thin film. As reported in literature, infrared light penetration depth can range from some hundred nanometers to several micrometers (Kane et al., 2009). Therefore, 1 μm thickness for polyurethane film is sufficient to infrared light reaches the chitin-polyurethane interface; this is part of future work in this research.

Carbon nanotubes (CNT) are key elements in nanotechnology. The structure of carbon nanotubes is depicted as a rolled segment of a graphene sheet, formed by linking each carbon atom to three equivalent neighbors by covalent bonds resulting in a hexagonal network. Carbon nanotubes can be structured by only one graphene sheet (single-wall carbon nanotubes, SWCNT) or by several coaxial graphene sheets (multiwall carbon nanotubes, MWCNT) (Dresselhaus et al., 2001). Synthesis of carbon nanotubes can be carried out by catalytic chemical vapor deposition (CVD) from carbon-containing gaseous compounds which decompose catalytically on transition-metal particles at temperatures lower than 1000 °C. CVD processes are becoming the major way for synthesizing carbon nanotubes in a controlled way (Loiseau et al., 2006). It is possible to modify and control the physicochemical properties of carbon nanotubes by doping processes, introducing either non-carbon atoms or molecules at small concentrations in the plane of the graphene lattice. Doping carbon nanotubes with nitrogen atoms at lower concentrations than 6.5 wt% induce crystalline disorder in the graphene sheets, as well as an excess of electron donors on the nitrogen-rich areas may result in a more reactive structure compared to pure carbon nanotubes. For nitrogen-doped carbon nanotubes (N-MWCNT) two types of C–N bonds

could difficult to reach the chitin-polyurethane interface by infrared light.

**4. Carbon nanotubes: Synthesis, properties and modification** 

urethane groups, respectively.

could occur: the first is a three-coordinated N atom within the sp2- hybridized network, with the presence of additional electrons. The second type is the pyridine type (two-coordinated N) which can be incorporated in the nanotube lattice; an additional carbon atom is removed from the framework. N-MWCNTs have stacked-cone morphology (bamboo-type) and the degree of tubular perfection decreases as a result of the N incorporation into the hexagonal carbon lattice (Terrones et al., 2002). Moreover, properties of CNTs can also be modified by oxidation with thermal treatment or with acidic solutions (HNO3, KMnO4 +, H2SO4, K2Cr2O7, H2O2). Under acidic oxidation bonds of carbon nanotubes could follow several reactions, for example the formation of quinones which further evolve to different oxygen acidic groups, such as phenolic, lactonic and carboxylic groups. The attached oxygenated groups change the surface chemistry of carbon nanotubes, in particular their wetting behavior and also can be used to attach different chemical groups or to improve their chemical interaction with other substances (Hirsch, 2002; Niyogi et al., 2002). Studies reported previously about oxidation of CNTs in liquid phase suggest that defects at the tips and sidewalls are the most reactive sites to attach oxygen functional groups to the nanotubes structure. These bonded atoms pull the reactive C atom out of the base plane, reducing the curvature strain of nanotubes and creating holes and pores. Long periods of oxidation lead to the shortening and destruction of the nanotubes (Ago et al., 1999; Ovejero et al., 2006; Zhang et al., 2003).

Raman spectroscopy is widely used to evidence the enhancement in defects density along the walls of the carbon nanotubes when oxidation is applied. The main bands of carbon materials are the graphite band (G-band) and the disorder-induced phonon mode (D-band). The relative intensity between the D-band and G-band (ID/IG ratio) indicate a continuously covalent bonding of oxygenated groups along the walls of nanotubes, disrupting the aromatic system of π-electrons (Liu et al., 2004). Complementary information can be obtained with FTIR spectroscopy; it allows determination of functional groups attached to surface of CNTs. This tool was used in some studies carried out by our research group with N-MWCNT, MWCNT and SWCNT, which were oxidized with nitric acid solution (70%) at 80 ± 3 °C for 5 h, as detailed by Perez-Aguilar et al., (2010). Oxidized carbon nanotubes were identified as ox-N-MWCNT, ox-MWCNT and ox-SWCNT, and their morphology was observed by electronic microscopy (Figure 5). As the oxidation of carbon nanotubes was carried out in liquid phase, oxygen containing functional groups were attached along their entire length. Reactions between carbon materials and nitric acid involve strong electrophilic species in solution, which form nitrogen oxides that eventually are reduced to N2 through oxidation of carbon. Introduction of a small content of nitrogen has been reported, mainly due to the introduction of nitro (-NO2) groups (Chen and Wu, 2004; Zawadski, 1980).

Fig. 5. Micrographs of carbon nanotubes observed by (a) SEM and (b) HRTEM for oxidized N-MWCNTs; HRTEM for (c) oxidized MWCNTs and (d) oxidized SWCNTs. Oxidation with nitric acid at 80 ± 3 ⁰C by 5 h

FTIR – An Essential Characterization Technique for Polymeric Materials 205

Fig. 6. FTIR spectra of (a) oxidized nitrogen-doped carbon nanotubes, ox-N-MWCNT; (b) oxidized multiwall carbon nanotubes, ox-MWCNT; (c) oxidized single-wall carbon

into their structures promotes degradation by heat or light, making them materials with low outdoor resistance. The hydrogenation of these materials is an effective method to improve their performance for outdoor or high temperatures applications (De Sarkar et al., 1997). In this context, the determination of microstructure (relative quantities of isomers), before and after the hydrogenation, is important since chemical and mechanical resistances are related

In the case of butadiene-containing polymers, the microstructure of the polybutadiene segment (i.e. the relative amount of 1,2 vinyl, 1,4-trans and 1,4-cis bonds) plays an important role in determining their thermal and mechanical properties, as well as their interaction with other materials in the production of composites, such as high-impact polystyrene, modified asphalt, pressure sensitive adhesives, etc. It is also known that anionic polymerization gives copolymers with well-controlled monomer distribution and microstructure, a narrow molecular weight distribution, and the possibility to have end group functionality (Escobar et al., 2000). Hydrogenation of styrene-diene copolymers has received special interest (Bhattacharjee et al., 1993) since the double bond present in the diene segment of the elastomer is susceptible to thermal and oxidative degradation . Hydrogenation of styrenediene block copolymers allows the production of thermo-oxidative resistant thermoplastics, and selective saturation of diene units (over the rigid units of styrene) permits to control mechanical and thermal properties. Thus, FTIR is essential to determine quantitatively the microstructure, using a liquid cell. Microstructure can be evaluated with time of hydrogenation by FTIR, allowing the establishment of the mechanism of hydrogenation for

nanotubes, ox-SWCNT, obtained by attenuated total reflectance (ATR)

to it.

styrene-butadiene copolymers.

After the oxidation Van der Waals forces that kept CNTs aggregated were overcome by repulsion forces, as result of reducing the π-conjugation of nanotubes and enhancing the surface dipoles by oxygen-containing functional groups. This effect was monitored by determining the elemental composition of pristine nanotubes and partially oxidized CNTs.

An important increment in oxygen content was registered as the carbon content was reduced, caused by the destruction of graphene lattice by electrophilic reaction. The hydrogen content also increased in oxidized nanotubes, maybe as a result of the introduction of carboxylic groups. However, for N-MWCNTs the nitrogen content remained almost constant, probably these atoms might change from pyridinic-type or quaternary-type to an aliphatic form. Equilibrium could be established between the consumption of nitrogen atoms of the lattice and the attachment of nitrogen atoms in some sites of the lattice as nitro groups, as occurs when oxidizing some organic molecules or activated carbons with nitric acid (Zawadski, 1980; Chen and Wu, 2004).

### **4.1 FTIR characterization of oxidized carbon nanotubes**

Functional groups attached to oxidized carbon nanotubes were identified by Fouriertransformed infrared spectroscopy by attenuated total reflectance, ATR-FTIR in a Nicolet 6700 FT-IR spectrophotometer at 1068 scans, in the frequency interval of 4000 cm-1 to 650 cm-1 with resolution of 8 cm-1.

The main functional group attached to oxidized carbon nanotubes was carboxylic. Comparison between spectra of pristine nanotubes and oxidized N-MWCNTs showed that the bands at 1444 cm-1, 1373 cm-1 and 1251 cm-1, attributed to vibration of MWCNTs and C-N bonding in N-MWCNTs, were overcome by several vibrations in carbonyl and carboxylic functionalities in the range of 1720 to 1250 cm-1 (Choi et al., 2004; Misra et al., 2007). A strong band appeared from 3600 cm-1 to 3300 cm-1 by the stretching of the bonding -OH of carboxylic group in oxidized nanotubes (Zawadski, 1980; Chen and Wu, 2004).

The nitro group (–NO2) is isoelectronic with the carboxylate ion group and their spectra are very similar, but a weak feature related to nitro group was observed at 1535 cm-1 for oxidized nanotubes (Zawadski, 1980). By other side, spectra obtained for three types of oxidized CNTs (Figure 6) showed broad features from 3400 cm-1 to 3000 cm-1 by the stretching of the bonding -OH in carboxylic and hydroxyl groups. Signals near 1710 cm-1 and 1685 cm-1 were of carbonyl vibrations in carboxyl bonding, a weak band about 1640 cm-1 of quinones, and the broad band from 1200 to 1000 cm-1 was attributed to single-bonded oxygen atoms such as phenols and lactones.

Two weak signals related to the nitro group were observed at 1538 and 1340 cm-1 for ox-N-MWCNTs. Similar spectra have been reported for oxidized multiwall carbon nanotubes; characteristic peaks were assigned to carboxylic, carbonyl, and hydroxyl groups (Wang et al., 2007). These results have probe that spectra carefully acquired by FTIR-ATR are a useful tool for identification of chemical groups attached to surface of carbon nanotubes chemically modified.

#### **5. Microstructure of styrene-butadiene copolymers and its hydrogenation**

Elastomers like polybutadiene or thermoplastic elastomers like styrene-butadiene-styrene copolymers are widely used in diverse industries. However, the presence of double bonds

After the oxidation Van der Waals forces that kept CNTs aggregated were overcome by repulsion forces, as result of reducing the π-conjugation of nanotubes and enhancing the surface dipoles by oxygen-containing functional groups. This effect was monitored by determining the elemental composition of pristine nanotubes and partially oxidized CNTs. An important increment in oxygen content was registered as the carbon content was reduced, caused by the destruction of graphene lattice by electrophilic reaction. The hydrogen content also increased in oxidized nanotubes, maybe as a result of the introduction of carboxylic groups. However, for N-MWCNTs the nitrogen content remained almost constant, probably these atoms might change from pyridinic-type or quaternary-type to an aliphatic form. Equilibrium could be established between the consumption of nitrogen atoms of the lattice and the attachment of nitrogen atoms in some sites of the lattice as nitro groups, as occurs when oxidizing some organic molecules or activated carbons with nitric

Functional groups attached to oxidized carbon nanotubes were identified by Fouriertransformed infrared spectroscopy by attenuated total reflectance, ATR-FTIR in a Nicolet 6700 FT-IR spectrophotometer at 1068 scans, in the frequency interval of 4000 cm-1 to 650 cm-1

The main functional group attached to oxidized carbon nanotubes was carboxylic. Comparison between spectra of pristine nanotubes and oxidized N-MWCNTs showed that the bands at 1444 cm-1, 1373 cm-1 and 1251 cm-1, attributed to vibration of MWCNTs and C-N bonding in N-MWCNTs, were overcome by several vibrations in carbonyl and carboxylic functionalities in the range of 1720 to 1250 cm-1 (Choi et al., 2004; Misra et al., 2007). A strong band appeared from 3600 cm-1 to 3300 cm-1 by the stretching of the bonding -OH of

The nitro group (–NO2) is isoelectronic with the carboxylate ion group and their spectra are very similar, but a weak feature related to nitro group was observed at 1535 cm-1 for oxidized nanotubes (Zawadski, 1980). By other side, spectra obtained for three types of oxidized CNTs (Figure 6) showed broad features from 3400 cm-1 to 3000 cm-1 by the stretching of the bonding -OH in carboxylic and hydroxyl groups. Signals near 1710 cm-1 and 1685 cm-1 were of carbonyl vibrations in carboxyl bonding, a weak band about 1640 cm-1 of quinones, and the broad band from 1200 to 1000 cm-1 was attributed to single-bonded

Two weak signals related to the nitro group were observed at 1538 and 1340 cm-1 for ox-N-MWCNTs. Similar spectra have been reported for oxidized multiwall carbon nanotubes; characteristic peaks were assigned to carboxylic, carbonyl, and hydroxyl groups (Wang et al., 2007). These results have probe that spectra carefully acquired by FTIR-ATR are a useful tool for identification of chemical groups attached to surface of carbon nanotubes chemically

Elastomers like polybutadiene or thermoplastic elastomers like styrene-butadiene-styrene copolymers are widely used in diverse industries. However, the presence of double bonds

carboxylic group in oxidized nanotubes (Zawadski, 1980; Chen and Wu, 2004).

**5. Microstructure of styrene-butadiene copolymers and its hydrogenation** 

acid (Zawadski, 1980; Chen and Wu, 2004).

oxygen atoms such as phenols and lactones.

with resolution of 8 cm-1.

modified.

**4.1 FTIR characterization of oxidized carbon nanotubes** 

Fig. 6. FTIR spectra of (a) oxidized nitrogen-doped carbon nanotubes, ox-N-MWCNT; (b) oxidized multiwall carbon nanotubes, ox-MWCNT; (c) oxidized single-wall carbon nanotubes, ox-SWCNT, obtained by attenuated total reflectance (ATR)

into their structures promotes degradation by heat or light, making them materials with low outdoor resistance. The hydrogenation of these materials is an effective method to improve their performance for outdoor or high temperatures applications (De Sarkar et al., 1997). In this context, the determination of microstructure (relative quantities of isomers), before and after the hydrogenation, is important since chemical and mechanical resistances are related to it.

In the case of butadiene-containing polymers, the microstructure of the polybutadiene segment (i.e. the relative amount of 1,2 vinyl, 1,4-trans and 1,4-cis bonds) plays an important role in determining their thermal and mechanical properties, as well as their interaction with other materials in the production of composites, such as high-impact polystyrene, modified asphalt, pressure sensitive adhesives, etc. It is also known that anionic polymerization gives copolymers with well-controlled monomer distribution and microstructure, a narrow molecular weight distribution, and the possibility to have end group functionality (Escobar et al., 2000). Hydrogenation of styrene-diene copolymers has received special interest (Bhattacharjee et al., 1993) since the double bond present in the diene segment of the elastomer is susceptible to thermal and oxidative degradation . Hydrogenation of styrenediene block copolymers allows the production of thermo-oxidative resistant thermoplastics, and selective saturation of diene units (over the rigid units of styrene) permits to control mechanical and thermal properties. Thus, FTIR is essential to determine quantitatively the microstructure, using a liquid cell. Microstructure can be evaluated with time of hydrogenation by FTIR, allowing the establishment of the mechanism of hydrogenation for styrene-butadiene copolymers.

FTIR – An Essential Characterization Technique for Polymeric Materials 207

The structures corresponding to the polybutadiene isomers, which can be evaluated as it is

It is important to mention that the studied styrene-butadiene copolymers and the polybutadiene homopolymer were synthesized by anionic polymerization. Therefore, just

These isomers are present at different relative quantities, depending on the polymerization conditions. One key factor to modify the microstructure is the presence of chemical named microstructure modifier, which has an interaction with the counterion of the initiator,

Considering the Beer-Lambert law, and knowing the extinction coefficients for each isomer at specifics wave numbers: 698, 728, 910 and 966 cm-1, for the case of styrene-butadiene copolymers, it is possible to evaluate microstructure. The Beer-Lambert law can be expressed as:

 A*<sup>i</sup>* = K*ij* C*j* L (3) Thus, the equation system can be expressed as a matrix, which can be solved since L and K*ij* are constants. The reported (Huang, 1995) values for the extinction coefficient are shown in

Entity A698 A728 A910 A966 1,4-cis 0.385 0.551 0.037 0.058 1,4-trans 0.005 0.007 0.055 2.542 1,2-vinyl 0.153 0.05 3.193 0.098 styrene 2.703 0.038 0.064 0.05

Table 1. Extinction coefficients for styrene and isomers of styrene-butadiene copolymers

three isomers are possible to obtain, the 1,4-cis; 1,4-trans and the 1,2-vinyl.

Extinction coefficient (K)

modifying the bond length and promotes the ion pair formation.

detailed in the next section, are shown in Figure 7.

Fig. 7. Isomer structure for the polybutadiene

**5.2.1 Microstructure and composition** 

Table 1.

Most of styrene-butadiene copolymers hydrogenation studies have focused on global saturation of polybutadiene double bonds (i.e. 1,4-trans, 1,4-cis and 1,2-vinyl) and on obtaining high saturation percentages (>90%), however, only a few of them have made a distinction on the saturation of the different types of double bonds present in polybutadiene (Escobar et al., 2000).

The evaluation of microstructure and composition (relative quantity of polystyrene in the copolymer) in styrene-butadiene copolymers by FTIR has been conventionally carried out considering just two of three isomers, and the third isomer percentage has been obtained by difference. The previous characterization by RMN is essential in order to contrasts the results. However, if it is considered that evaluation of microstructure and composition, of such butadiene-based copolymers, by FTIR takes in account the Beer-Lambert law, expressed as:

$$\mathbf{A} = \mathbf{K} \,\, \mathbf{C} \,\, \mathbf{L} \tag{1}$$

where:

A represents the absorbance at certain wave number,

K is the extinction coefficient at certain wave number,

C is the solution concentration,

L is the path length for the laser light

Therefore, it is possible to express the Beer-Lambert law (Huang, 1995) for each component as follows:

$$\mathbf{A} \mathbf{\bar{\cdot}} \mathbf{\bar{\cdot}} \mathbf{\bar{\cdot}} \mathbf{\bar{\cdot}} \mathbf{\bar{\cdot}} \mathbf{L} \tag{2}$$

where:

A*i* represents the absorbance at wave number *i*,

K*ij* represents the extinction coefficient for *j* unit at wave number *i*,

C*j* is the solution concentration of *j* unit,

L is the path length for the laser light

In the case of liquid cell the path length, L, is the spacer thickness between the KBr windows.

#### **5.1 Experimental**

The samples of commercial products were weighted (0.25 g) and dissolved in carbon bisulfide (10 ml). A liquid cell with KBr window was used with a spacer thickness of 0.2 mm. The FTIR equipment was a Magna 560 from Nicolet, with 32 scans, in the mid-red: 4000 a 400 cm-1. These samples also were characterized by H1 NMR.

#### **5.2 Results and discussion**

The results for microstructure and composition determination of polybutadiene portion in styrene-butadiene copolymers are presented in the following section. Such characterization was carried out using the indicated spectrometer and contrasted with the results obtained by H1 NMR.

Most of styrene-butadiene copolymers hydrogenation studies have focused on global saturation of polybutadiene double bonds (i.e. 1,4-trans, 1,4-cis and 1,2-vinyl) and on obtaining high saturation percentages (>90%), however, only a few of them have made a distinction on the saturation of the different types of double bonds present in polybutadiene

The evaluation of microstructure and composition (relative quantity of polystyrene in the copolymer) in styrene-butadiene copolymers by FTIR has been conventionally carried out considering just two of three isomers, and the third isomer percentage has been obtained by difference. The previous characterization by RMN is essential in order to contrasts the results. However, if it is considered that evaluation of microstructure and composition, of such butadiene-based copolymers, by FTIR takes in account the Beer-Lambert law,

A = K C L (1)

Therefore, it is possible to express the Beer-Lambert law (Huang, 1995) for each component

A*i* = K*ij* C*j* L (2)

In the case of liquid cell the path length, L, is the spacer thickness between the KBr

The samples of commercial products were weighted (0.25 g) and dissolved in carbon bisulfide (10 ml). A liquid cell with KBr window was used with a spacer thickness of 0.2 mm. The FTIR equipment was a Magna 560 from Nicolet, with 32 scans, in the mid-red: 4000

The results for microstructure and composition determination of polybutadiene portion in styrene-butadiene copolymers are presented in the following section. Such characterization was carried out using the indicated spectrometer and contrasted with the results obtained

A represents the absorbance at certain wave number, K is the extinction coefficient at certain wave number,

A*i* represents the absorbance at wave number *i*,

C*j* is the solution concentration of *j* unit, L is the path length for the laser light

K*ij* represents the extinction coefficient for *j* unit at wave number *i*,

a 400 cm-1. These samples also were characterized by H1 NMR.

C is the solution concentration, L is the path length for the laser light

(Escobar et al., 2000).

expressed as:

where:

as follows:

where:

windows.

by H1 NMR.

**5.1 Experimental** 

**5.2 Results and discussion** 

The structures corresponding to the polybutadiene isomers, which can be evaluated as it is detailed in the next section, are shown in Figure 7.

Fig. 7. Isomer structure for the polybutadiene

It is important to mention that the studied styrene-butadiene copolymers and the polybutadiene homopolymer were synthesized by anionic polymerization. Therefore, just three isomers are possible to obtain, the 1,4-cis; 1,4-trans and the 1,2-vinyl.

These isomers are present at different relative quantities, depending on the polymerization conditions. One key factor to modify the microstructure is the presence of chemical named microstructure modifier, which has an interaction with the counterion of the initiator, modifying the bond length and promotes the ion pair formation.

#### **5.2.1 Microstructure and composition**

Considering the Beer-Lambert law, and knowing the extinction coefficients for each isomer at specifics wave numbers: 698, 728, 910 and 966 cm-1, for the case of styrene-butadiene copolymers, it is possible to evaluate microstructure. The Beer-Lambert law can be expressed as:

$$\mathbf{A}\_i \mathbf{\bot} \,\Sigma \,\mathbf{K}\_{\vec{\eta}} \,\mathbf{C}\_{\vec{\eta}} \,\mathbf{L} \tag{3}$$

Thus, the equation system can be expressed as a matrix, which can be solved since L and K*ij* are constants. The reported (Huang, 1995) values for the extinction coefficient are shown in Table 1.


Table 1. Extinction coefficients for styrene and isomers of styrene-butadiene copolymers

FTIR – An Essential Characterization Technique for Polymeric Materials 209

1,2-vinyl

Styrene

1,4-cis

Fig. 9. Spectra of styrene-butadiene copolymer (A) and its hydrogenated product (B)

that polystyrene remained unsaturated during hydrogenation.

specific functional groups with time, during reactions.

development of new materials in the polymer science.

with time, to establish the end and/or beginning of a particular reaction.

**6. Conclusions**

The spectra shown in Figure 9 indicate that there was a noticeable reduction of the absorbance of 1,2-vinyl, then 1,4-trans followed by 1,4-cis and the absorbance corresponding to styrene remains constant. In previous work (Escobar et al, 2000) it was shown that kinetics of hydrogenation is higher for 1,2-vinyl bonds than for 1,4-trans and 1,4-cis bonds. The 1,2-vinyl bonds are pendant from the backbone and, therefore, they are more accessible to hydrogen. The FTIR is a useful characterization technique in this case, since it was possible to establish the hydrogenation selectivity toward the 1,2-vinyl bonds and showed

FTIR is a powerful and useful characterization method for polymers, and materials in general. This is an economic, short time characterization that allows to establish the chemical composition, microstructure, chemical interactions and even to follow variation of

All this features, makes the FTIR an essential characterization technique for polymers. From hopolymers, copolymers to nanocomposites and biocomposites, can be characterized, qualitative and quantitatively, giving the possibility to understand specific interactions and to establish mechanism of reaction, between materials. Thus, the FTIR has contributed to the

In the examples presented herein, the FTIR has permitted to follow specific functional groups

Fig. 8. Hydrogenation of styrene-butadiene copolymer

1,4-trans

The path length was constant (0.02 cm). Therefore, the matrix is:

$$\begin{array}{ccccccccc} \text{A}\_{698} = 7.7 \times 10^{3} & \text{C}\_{\text{cis}} & + & 3.06 \times 10^{-3} & \text{C}\_{\text{vinyl}} & + & 1 \times 10^{4} & \text{C}\_{\text{trans}} & + & 5.406 \times 10^{2} \text{ C}\_{\text{Si}}\\ \text{A}\_{728} = 1.102 \times 10^{2} \text{ C}\_{\text{cis}} & + & 1 \times 10^{3} & \text{C}\_{\text{vinyl}} & + & 1.4 \times 10^{4} & \text{C}\_{\text{trans}} & + & 7.6 \times 10^{4} & \text{C}\_{\text{Si}}\\ \text{A}\_{910} = 7.4 \times 10^{4} & \text{C}\_{\text{cis}} & + & 6.386 \times 10^{2} \text{ C}\_{\text{vinyl}} & + & 1.1 \times 10^{4} & \text{C}\_{\text{trans}} & + & 1.28 \times 10^{3} & \text{C}\_{\text{Sr}}\\ \text{A}\_{966} = 1.16 \times 10^{3} & \text{C}\_{\text{cis}} & + & 1.96 \times 10^{3} & \text{C}\_{\text{vinyl}} & + & 5.084 \times 10^{2} \text{ C}\_{\text{trans}} & + & 1 \times 10^{3} & \text{C}\_{\text{Sr}} \end{array}$$

Once it was established the equations, they were solved. Thus, several commercial samples were evaluated according with matrix. The obtained values are shown in Table 2.

As it can be seen from Table 2, there is a good agreement in the microstructure calculated by FTIR and H1NMR. Bearing in mind that microstructure and composition calculated by H1NMR are the real values, the higher deviation, for cis and trans values, was achieved when the styrene content was above 39 % w/w. The last is probably due for overtones for vinyl and trans isomers, which appear with such styrene content.


Table 2. Microstructure and composition of several commercial samples.

Nevertheless, there was a good correlation between the obtained results by both methods, corroborating that FTIR can be used with confidence, in order to evaluate composition and microstructure of styrene-butadiene copolymers. Nevertheless, it is important to mention that the use of the liquid cell is not infallible, since the path length can be modified when the cell is cleaned.

#### **5.2.2 Hydrogenation**

Once the method for microstructure by FTIR was established, the hydrogenation of butadiene-based copolymers can be traced. Disappearance of each isomer of butadiene can be recorded with time. Thus, it is possible to see if hydrogenation is selective toward certain isomer or not, and also it is possible to see if polystyrene portion is saturated or affected by hydrogenation.

The hydrogenation of elastomeric portion (polybutadiene) in the styrene-butadiene copolymers can be visualized as it is shown the Figure 8.

The spectra of styrene-butadiene copolymer and the hydrogenated counterpart are shown in the Figure 9.

Fig. 8. Hydrogenation of styrene-butadiene copolymer

Fig. 9. Spectra of styrene-butadiene copolymer (A) and its hydrogenated product (B)

The spectra shown in Figure 9 indicate that there was a noticeable reduction of the absorbance of 1,2-vinyl, then 1,4-trans followed by 1,4-cis and the absorbance corresponding to styrene remains constant. In previous work (Escobar et al, 2000) it was shown that kinetics of hydrogenation is higher for 1,2-vinyl bonds than for 1,4-trans and 1,4-cis bonds.

The 1,2-vinyl bonds are pendant from the backbone and, therefore, they are more accessible to hydrogen. The FTIR is a useful characterization technique in this case, since it was possible to establish the hydrogenation selectivity toward the 1,2-vinyl bonds and showed that polystyrene remained unsaturated during hydrogenation.

#### **6. Conclusions**

208 Infrared Spectroscopy – Materials Science, Engineering and Technology

A698 = 7.7 x 10-3 Ccis + 3.06 x10-3 Cvinyl + 1 x10-4 Ctrans + 5.406 x10-2 CSt A728 = 1.102 x 10-2 Ccis + 1 x10-3 Cvinyl + 1.4 x10-4 Ctrans + 7.6 x10-4 CSt A910 = 7.4 x 10-4 Ccis + 6.386 x10-2 Cvinyl + 1.1 x10-4 Ctrans + 1.28 x10-3 CSt A966 = 1.16 x 10-3 Ccis + 1.96 x10-3 Cvinyl + 5.084 x10-2 Ctrans + 1 x10-3 CSt Once it was established the equations, they were solved. Thus, several commercial samples

As it can be seen from Table 2, there is a good agreement in the microstructure calculated by FTIR and H1NMR. Bearing in mind that microstructure and composition calculated by H1NMR are the real values, the higher deviation, for cis and trans values, was achieved when the styrene content was above 39 % w/w. The last is probably due for overtones for

S-200 0 9.12 40.21 50.67 0 9.51 42.39 48.1 S-1110 16.15 9.12 38.95 51.93 15.43 9.43 40.11 50.44 S-1205 24.39 9.39 40.63 49.98 24.05 9.74 38.5 51.76 S-1322 30.43 8.59 40.18 51.22 29.72 8.87 42.68 48.44 S-1430 39.94 8.82 39.07 52.12 39.14 8.74 43.21 48.05 S-314 69.69 16.89 27.12 55.98 67.35 14.79 33.78 51.43 S-411 30.37 14.56 35.54 49.89 29.7 15.07 35.71 49.23 S-416 29.96 12.69 36.51 50.79 29.37 13.71 36.89 49.39 S-4318 33.17 13.44 36.12 50.44 32.04 14.05 37.29 49.39

Nevertheless, there was a good correlation between the obtained results by both methods, corroborating that FTIR can be used with confidence, in order to evaluate composition and microstructure of styrene-butadiene copolymers. Nevertheless, it is important to mention that the use of the liquid cell is not infallible, since the path length can be modified when the

Once the method for microstructure by FTIR was established, the hydrogenation of butadiene-based copolymers can be traced. Disappearance of each isomer of butadiene can be recorded with time. Thus, it is possible to see if hydrogenation is selective toward certain isomer or not, and also it is possible to see if polystyrene portion is saturated or affected by

The hydrogenation of elastomeric portion (polybutadiene) in the styrene-butadiene

The spectra of styrene-butadiene copolymer and the hydrogenated counterpart are shown in

% Trans

(H1NMR) % St % Vinyl % Cis % Trans

were evaluated according with matrix. The obtained values are shown in Table 2.

% Cis (H1NMR)

Table 2. Microstructure and composition of several commercial samples.

copolymers can be visualized as it is shown the Figure 8.

The path length was constant (0.02 cm). Therefore, the matrix is:

vinyl and trans isomers, which appear with such styrene content.

% Vinyl (H1NMR)

Sample % St

cell is cleaned.

hydrogenation.

the Figure 9.

**5.2.2 Hydrogenation** 

(H1NMR)

FTIR is a powerful and useful characterization method for polymers, and materials in general. This is an economic, short time characterization that allows to establish the chemical composition, microstructure, chemical interactions and even to follow variation of specific functional groups with time, during reactions.

All this features, makes the FTIR an essential characterization technique for polymers. From hopolymers, copolymers to nanocomposites and biocomposites, can be characterized, qualitative and quantitatively, giving the possibility to understand specific interactions and to establish mechanism of reaction, between materials. Thus, the FTIR has contributed to the development of new materials in the polymer science.

In the examples presented herein, the FTIR has permitted to follow specific functional groups with time, to establish the end and/or beginning of a particular reaction.

FTIR – An Essential Characterization Technique for Polymeric Materials 211

Choi, H., Bae, S., Park, J., Seo, K., Kim, C., Kim, B., Song, H., & Shin, H. (2004). Experimental

Davila-Rodriguez, J. L., Escobar-Barrios, V. A., Shirai, K. & Rangel-Mendez, J. R. (2009).

De Sarkar, M., De, P.P., & Bhowmick A.K., (1997). Thermoplastic elastomeric hydrogenated

Escobar-Barrios, V. A., Herrera-Najera, R., Petit, A. & Pla, F. (2000) Selective Hydrogenation

Fowler, P. A., Hughes, J. M. & Elias, R. M. (2006). Biocomposites: technology, environmental

Hepburn, C. (1982). *Polyurethane Elastomers*, Applied Science Publishers, London and New York Hirsch A (2002) Functionalization of single-walled carbon nanotubes, *Angewandte Chemie International Edition*, Vol.41, No.11, (June 2002) pp. 1853-1859, ISSN 1521-3773 Huang, D., Lin, Y. Y., & Tsiang, Ch. (1995). Synthesis of SBS thermoplastic block copolymers

Elnashar, M. M., (Ed.). (2010). *Biopolymers*, Sciyo, ISBN 978-953-307-109-1, Rijeka, Croatia Fidalgo, A., Ilharco L.M. (2005). The influence of the wet gels processing on the structure

study. *European Polymer Journal,* Vol. 36, pp. 1817-1834

(September 2006), No.12, pp.1781-1789, ISSN 1097-0010

*of Polymer Research*, Vol. 2, No. 2, pp. 91-98 ISSN 1022-9760

Academic Press , ISBN-10: 0124421016, New York, USA

and kinetics. *Journal of Applied Polymer Science,* Vol.66, No. 6, pp. 1151-1162 Dresselhaus, M., Dresselhaus, G., & Avouris, Ph. (2001). *Carbon Nanotubes. Synthesis,* 

pp. 5742-5744, ISSN 0003-6951

pp.718-726, ISSN 0022-1139

Heidelberg, Germany

229–235, ISSN 1387-1811

Kane, S. R., Ashby, P. D. & Pruitt, L. A.

1859572847, Shropshire. UK

2004) pp. 1091-1094, ISSN 0022-2461

ISSN 1552-4981

and theoretical studies on the structure of N-doped carbon nanotubes: Possibility of intercalated molecular N-2, *Applied Physics Letters*, Vol.85, No.23, (November 2004)

Synthesis of a chitin-based biocomposite for water treatment: Optimization for fluoride removal. *Journal of Fluorine Chemistry,* Vol.130, (August 2009), No.8,

styrene-butadiene elastomers: Optimization of reaction conditions, thermodynamics

*structure, properties and applications*, Springer, ISBN 3-540-41086-4, Verlag Berlin

of butadiene-styrene copolymers using a Ziegler-Natta type catalyst. Part 1. Kinetic

and properties of silica xerogels. *Microporous and Mesoporous Materials* Vol. 84, pp.

credentials and market forces. *Journal of the Science of Food and Agriculture,* Vol.86,

in cyclohexane in the presence of diethylether used as a structure modifier. *Journal* 

technique for hydrated polymer-on-polymer coatings. *Journal of Biomedical Materials Research Part B: Applied Biomaterials,* Vol.91B, No.2, (November 2009), pp.613-620,

reliable technique for removal hexavalent chromium in wastewater. *Bioresource* 

prepared by ethanol flames, *Journal of Materials Science,* Vol.39, No.3, (February

Synthesis methods and growth mechanisms*,* In: *Understanding carbon nanotubes. From basics to applications*, A. Loiseau, P. Launois, P. Petit, S. Roche, J.P. Salvetat

Koenig, J.L. (2001). *Infrared and Raman Spectroscopy of Polymers,* Rapra Technology, ISBN-10:

Lee, L.H., Ed. (1977). *Characterization of Metal and Polymer Surfaces: Polymer Surfaces* 

Li, H., Liu, T., Li, Z. & Deng, L. (2008). Low-cost supports used to immobilize fungi and

Liu, Y., Pan, C., & Wang, J. (2004) Raman spectra of carbon nanotubes and nanofibers

Loiseau, A., Blasé, X., Charlier, J., Gadelle, P., Journet, C., Laurent, Ch., & Peigney, A. (2006).

(Eds.), 49-130, Springer, ISBN 103-540-26922-3, Berlin Heidelberg, Germany

*Technology,* Vol.99, (May 2008), No.7, pp.2234-2241, ISSN 0960-8524

(2009). ATR-FTIR as a thickness measurement

In addition, FTIR was useful to elucidate about interactions between polymeric matrix (polyurethane) and biopolymer (chitin). However, it is necessary to obtain thinner samples in order to see specific interaction in the interface of materials.

Regarding the nanotubes, it was possible to establish carbon functional group present in the surface of them once these were chemically modified. This is important since this allows figuring out what kind of interactions with other materials could be taken place, and understanding the mechanism of such interactions.

In the case of the establishment of microstructure of styrene-butadiene copolymers, it was demonstrated the capacity of FTIR to be used with confidence as an alternative technique to primary technique like H1NMR. The FTIR gives very close values of the relative quantities of the different isomers to those obtained by NMR, besides the composition. Once the methodology was established, it was possible to follow the hydrogenation reaction, in order to see its selectivity toward the isomers.

### **7. Acknowledgments**

The authors are grateful to the Instituto Potosino de Investigacion Cientifica y Tecnologica, A.C. (IPICyT) and to Consejo Nacional de Ciencia y Tecnologia (CONACYT) through Fondos Mixtos CONACYT-State of Puebla (PUE-2004-C02-5), Fondos Mixtos CONACYT-State of San Luis Potosi (FMSLP-2009-C02-106795) and Fondos SEP-CONACYT (SEP-2004- C01-45764 and SEP-CB-2008-01-105920) for the economical support to carry out this work.

In addition, the authors appreciate the technical support of M.C. Dulce Partida-Gutierrez, M.C. Guillermo Vidriales Escobar and Ing. Daniel Ramirez-Gonzalez, the National Laboratory of Nanosciences and Nanotechnology (LINAN) and the National Laboratory of Agricola Biotechnology, Medical and Environmental (LANBAMA) of IPICYT.

#### **8. References**


In addition, FTIR was useful to elucidate about interactions between polymeric matrix (polyurethane) and biopolymer (chitin). However, it is necessary to obtain thinner samples

Regarding the nanotubes, it was possible to establish carbon functional group present in the surface of them once these were chemically modified. This is important since this allows figuring out what kind of interactions with other materials could be taken place, and

In the case of the establishment of microstructure of styrene-butadiene copolymers, it was demonstrated the capacity of FTIR to be used with confidence as an alternative technique to primary technique like H1NMR. The FTIR gives very close values of the relative quantities of the different isomers to those obtained by NMR, besides the composition. Once the methodology was established, it was possible to follow the hydrogenation reaction, in order

The authors are grateful to the Instituto Potosino de Investigacion Cientifica y Tecnologica, A.C. (IPICyT) and to Consejo Nacional de Ciencia y Tecnologia (CONACYT) through Fondos Mixtos CONACYT-State of Puebla (PUE-2004-C02-5), Fondos Mixtos CONACYT-State of San Luis Potosi (FMSLP-2009-C02-106795) and Fondos SEP-CONACYT (SEP-2004- C01-45764 and SEP-CB-2008-01-105920) for the economical support to carry out this work. In addition, the authors appreciate the technical support of M.C. Dulce Partida-Gutierrez, M.C. Guillermo Vidriales Escobar and Ing. Daniel Ramirez-Gonzalez, the National Laboratory of Nanosciences and Nanotechnology (LINAN) and the National Laboratory of

Ago, H., Kugler, T., Cacialli, S., Salaneck, W., Shaffer, M., Windle, A., & Friend, R. (1999). Work

Andrade-Espinosa, G, Escobar-Barrios, V.A. & Rangel-Mendez, J.R. (2010). Synthesis and

Bhattacharjee, S., Bhowmick, A. K., & Avasthi, B,N. (1993). Selective hydrogenation of

Bhargava, R., Wang, S., & Koening, J. L. (2003). FTIR Microspectroscopy of polymeric

Bower, D.I., Maddams, W.F. (1989) *The Vibrational Spectroscopy of Polymer,* Cambridge

Chen, J., Wu, S. (2004). Acid/base-treated activated carbons: characterization of functional

functions and surface functional groups of multiwall carbon nanotubes, *Journal of Physical Chemistry B,* Vol.103, No.38, (September 1999) pp. 8116-8121, ISSN 1520-6106 Alhakawati, M. S. & Banks, C. J. (2004). Removal of copper from aqueous solution by

Ascophyllum nodosum immobilised in hydrophilic polyurethane foam. *Journal of Environmental Management,* Vol.72, No.4, (September 2004), pp.195-204, ISSN 0301-4797

characterization of silica xerogels obtained via fast sol–gel process, *Colloid and* 

olefnic bonds in styrene-isoprene-styrene triblock copolymer by palladium acetate

systems, *Advances in Polymer Science,* Vol.163, pp. 137-191, Springer, Verlag Berlin

groups and metal adsorptive properties, *Langmuir,* Vol.20, No.6, (March 2004) pp.

Agricola Biotechnology, Medical and Environmental (LANBAMA) of IPICYT.

*Polymer Science,* Vol. 288, pp. 1697-1704, ISSN 1435-1536

University Press, ISBN 0-521-24633-4, Cambridge, UK

catalyst. *Polymer,* Vol. 49, No. 11, pp. 1971-1977

Heidelberg, Germany

2233-2242, ISSN 0743-7463

in order to see specific interaction in the interface of materials.

understanding the mechanism of such interactions.

to see its selectivity toward the isomers.

**7. Acknowledgments** 

**8. References** 


**10** 

Somayeh Mohamadi

*University College of Science, University of Tehran, Tehran* 

*Polymer group, School of Chemistry,* 

*Iran* 

**Preparation and Characterization** 

**of PVDF/PMMA/Graphene Polymer Blend** 

**Nanocomposites by Using ATR-FTIR Technique** 

With improvement in the human life, the requisite of the new materials with special properties for many different applications ranging from food packaging and consumer products to use as medical devices and in aerospace technologies can be sensed, strongly. Polymeric materials offer this opportunity to scientists and engineers for designing these new materials. In this regard, precise understanding of Structure and Properties

Recently, polymeric nanocomposites have opened a new research area and attracted strong attentions. The synthesis of polymer nanocomposites by inserting the nanometric inorganic compounds is an integral aspect of polymer nanotechnology (A. Lagashetty, 2005). These materials, depending upon the inorganic materials present, have particular and improved

Fourier transform infrared (FTIR) spectroscopy is a powerful and reliable technique that for many years has been an important tool for investigating chemical processes and structures. In the polymer fields, FTIR data is used in order to study characterization of chemical bonds, polymer microstructure, chain conformation, polymer morphology, crystallinity and

The combination of infrared spectroscopy with the theories of reflection has made advances in surface analysis possible. Attenuated Total Reflectance (ATR) spectroscopy is an innovative technique for proving chemical information of a sample surface and the ability to quantify newly formed species, based upon Fick's second law. The fundamentals of attenuated total reflection (ATR) spectroscopy date back to the initial work of Jacques Fahrenfort and N.J. Harrick, both of whom independently devised the theories of ATR spectroscopy and suggested a wide range of applications. The schematic showing ATR-FTIR configuration is illustrated in Fig. 1 (KS. Kwan, 1998). The penetration

**1. Introduction** 

Relationship (SPR) should be very crucial.

etc, consequently is useful in SPR studies.

depth, d, can be estimated as:

properties respect to pure polymers that invest their applications.


## **Preparation and Characterization of PVDF/PMMA/Graphene Polymer Blend Nanocomposites by Using ATR-FTIR Technique**

Somayeh Mohamadi *Polymer group, School of Chemistry, University College of Science, University of Tehran, Tehran Iran* 

## **1. Introduction**

212 Infrared Spectroscopy – Materials Science, Engineering and Technology

Mao, J., Won, S. W., Vijayaraghavan, K. & Yun, Y.-S. (2010). Immobilized citric acid-treated

Misra, A., Tyagi, P., Rai, P., & Misra, D. (2007). FTIR spectroscopy of multiwalled carbon

Ovejero, G., Sotelo, J., Romero, M., Rodríguez, A., Ocana, M., Rodríguez, G., & Garcia, J.

Perez-Aguilar NV, Muñoz-Sandoval E, Diaz-Flores PE, & Rangel-Mendez JR (2010)

Sawyer, L.C., Grubb, D.T., & Meyers, G.F. (2008). *Polymer Microscopy* (3rd. edition), Springer,

Sudha Bai, R., Abraham, T. E. (2003). Studies on chromium(VI) adsorption-desorption using

Terrones, M., Ajayan, P.M., Banhart, F., Blasé, X., Carroll, D.L., Charlier, J.C., Czerw, R.,

Vullo, D. L., Ceretti, H. M., Daniel, M. A., Ramirez, S. A. M. & Zalts, A. (2008). Cadmium,

*Technology,* Vol.99, No.13, (September 2008), pp.5574-5581, ISSN 0960-8524 Wang, H., Zhou, A., Peng, F., Yu, H., & Yang, J. (2007). Mechanism study on adsorption of

Zhang, J., Zou, H., Qing, Q., Yang, Y., Li, Q., Liu, Z., Guo, X., & Du, Z. (2003). Effect of

*Science*, Vol.316, No.2, ( December 2007) pp. 277-283, ISSN 0021-9797 Zawadski, J. (1980). IR Spectroscopic investigations of the mechanism of oxidation of

*Journal,* Vol.162, No.2, (August 2010), pp.662-668, ISSN 1385-8947

Vol.35, No.12, (October 2002) pp. 1105-1113, ISSN 0001-4842

ISBN 978-0-387-72627-4, Verlag Berlin Heidelberg, Germany

Perú, ISBN 9972-42-659-9, Lima, Perú

pp.17-26, ISSN 0960-8524

285, ISSN 0008-6223

(February 2010) pp. 467-480, ISSN 1388-0764

(December 2001), pp. 355-361, ISSN 0947-8396

(February 2006), pp.788-798, ISSN 0043-1354

bacterial biosorbents for the removal of cationic pollutants. *Chemical Engineering* 

nanotubes: A simple approach to study the nitrogen doping, *Journal of Nanoscience and Nanotechnology*, Vol.7, No.12, (December 2007) pp. 1820-1823, ISSN 1533-4880 Niyogi, S., Hamon, M., Hu, H., Zhao, B., Bhowmik, P., Sen, R., Itkis, M., & Haddon, R.

(2002). Chemistry of single-wall carbon nanotubes, *Accounts of Chemical Research*,

(2006). Multiwalled carbon nanotubes for liquid-phase oxidation. Functionalization, characterization and catalytic activity. *Industrial Engineering Chemistry Research*, Vol.45, No.7, (March 2003) pp. 2206-2212, ISSN 0888-5885 Pastor, A., (Ed.). (2004). *Quitina y Quitosano: obtención, caracterización y aplicaciones*, Programa

CYTED, CIAD, A.C., Fondo Editorial de la Pontificia Universidad Católica del

Adsorption of cadmium and lead onto oxidized nitrogen-doped multiwall carbon nanotubes in aqueous solution. *Journal of Nanoparticle Research*, Vol.12, No.2,

immobilized fungal biomass. *Bioresource Technology,* Vol.87, No.1, (March 2003),

Foley, B., Grobert, N., Kamalakaran, R., Kohler-Redlich, P., Ruhle, M., Seeger, T., & Terrones, H. (2002). N-doping and coalescence of carbon nanotubes: synthesis and electronic properties, *Applied Physics A Materials Science & Processing*, Vol.74, No.3,

zinc and copper biosorption mediated by Pseudomonas veronii 2E. *Bioresource* 

acidified multiwalled carbon nanotubes to Pb(II), *Journal of Colloid and Interface* 

carbonaceous films with HNO3 solution, *Carbon*, Vol.18, No.4, (April 2003) pp. 281-

chemical oxidation on the structure of single-walled carbon nanotubes, *Journal of Physical Chemistry B,* Vol.107, No.16, (March 2003) pp. 3712-3718, ISSN 1520-6106 Zhang, P. He, J., & Zhou, X. (2008). An FTIR standard addition method for quantification of bound styrene in its copolymers, *Polymer Testing,* vol. 27, pp. 153-157 ISSN 0142-9418 Zhang, Y., Banks, C. (2006). A comparison of the properties of polyurethane immobilised

Sphagnum moss, seaweed, sunflower waste and maize for the biosorption of Cu, Pb, Zn and Ni in continuous flow packed columns. *Water Research,* Vol.40, No.4, With improvement in the human life, the requisite of the new materials with special properties for many different applications ranging from food packaging and consumer products to use as medical devices and in aerospace technologies can be sensed, strongly. Polymeric materials offer this opportunity to scientists and engineers for designing these new materials. In this regard, precise understanding of Structure and Properties Relationship (SPR) should be very crucial.

Recently, polymeric nanocomposites have opened a new research area and attracted strong attentions. The synthesis of polymer nanocomposites by inserting the nanometric inorganic compounds is an integral aspect of polymer nanotechnology (A. Lagashetty, 2005). These materials, depending upon the inorganic materials present, have particular and improved properties respect to pure polymers that invest their applications.

Fourier transform infrared (FTIR) spectroscopy is a powerful and reliable technique that for many years has been an important tool for investigating chemical processes and structures. In the polymer fields, FTIR data is used in order to study characterization of chemical bonds, polymer microstructure, chain conformation, polymer morphology, crystallinity and etc, consequently is useful in SPR studies.

The combination of infrared spectroscopy with the theories of reflection has made advances in surface analysis possible. Attenuated Total Reflectance (ATR) spectroscopy is an innovative technique for proving chemical information of a sample surface and the ability to quantify newly formed species, based upon Fick's second law. The fundamentals of attenuated total reflection (ATR) spectroscopy date back to the initial work of Jacques Fahrenfort and N.J. Harrick, both of whom independently devised the theories of ATR spectroscopy and suggested a wide range of applications. The schematic showing ATR-FTIR configuration is illustrated in Fig. 1 (KS. Kwan, 1998). The penetration depth, d, can be estimated as:

Preparation and Characterization of PVDF/PMMA/Graphene

Giannetti, 2001, A. Lovinger, 1982 & N. S. Nalwa, 1995):

common form and is the most thermodynamically stable.

for designing of modification procedure.

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 215

The modification of graphene sheets via organic oligomeric and polymeric chains is a favorable way to promote the compatibility of these nanoparticles with polymeric media. So understanding of the functional groups present on the graphene surface should be very vital

Poly(vinylidene fluoride) (PVDF) is a semicrystalline engineering polymer with very good resistance to chemicals, oxidation, and UV radiation (J. H. Yen, 2006). PVDF is known for its polymorphism crystalline structure and complicated microstructure. It is one of the most widely studied polymers due to its non-linearity, piezo- and pyro-electricity (L. T. Vo, 2007 & K. Pramoda, 2005). PVDF can crystallize in at least five well-known crystalline phases (E.

α and δ with conformation of the alternating trans-gauche (TG+TG- ) which the α is the most

Scheme 2. α and δ form with alternating trans-gauche conformation (J. H. Yen, 2006)

Scheme 3. β form all trans planer zigzag conformation (J. H. Yen, 2006)

β with all trans (TTT) planer zigzag conformation is polar form and has been extensively studied for its potential applications. This form develops under mechanical deformation (K. Matsushige, 1980) (S. Ramasundaram, 2008) , growth from solution (J. Wang, 2003 & R. L. Miller, 1976), addition of metal salts (X. He, 2006 &W. A. Yee, 2007) ,melt crystallization at high pressures (D. Yang, 1987), application of a strong electric field (J. I. Scheinbeim, 1986), blending with carbonyl-containing polymers (C. Lbonard, 1988 & K. J. Kim, 1995), and recently, addition of nanoparticles (S. Ramasundaram, 2008, T. Ogoshi, 2005 & L. He, 2010). This structure provides some unique properties for PVDF piezo- and pyro-electric activity:

$$d = \frac{1}{2\pi n\_c \sigma (\sin^2 \theta - n\_x^2)^{1/2}} \tag{1}$$

where �� is the refractive index of the ATR crystal and �� is the ratio between the refractive indexes of the sample and the ATR crystal ( ��which both of them are assumed to be constant in the considered frequency range. � is the wave number and � is the incident angle. The penetration depth for PVDF as an example by rhe assumption of �� = 2.4, �� = 1.5 and � = 45*°* from 500 to 4000 cm-1 is approximately 0.5-4 µm (Y. Jung Park, 2005).

Scheme 1. Schematic illustration of ATR-FTIR configuration (KS. Kwan, 1998)

Graphene and graphite have recently attracted strong attention as versatile, environmentally friendly and available carbon materials which can be used as inexpensive filler in the composite materials (S. Stankovich, 2006, 2007 & L. Al-Mashat, 2010) .Crystalline graphites are used in polymeric systems in order to improve polymer properties such as thermal and electrical conductivity, IR absorption, flame retardancy, barrier resistance, electromagnetic shielding, lubrication and abrasion resistance. When the crystalline graphite is exfoliated to individual graphene sheets, the specific surface would be as large as 2600 m2. g-1 and novel electronic and mechanical properties appeared (Steurer, 2009). Actually graphene sheets are one-atom two-dimensional layers of sp2- network carbon that their fracture strength should be comparable to that of carbon nanotubes with similar types of defects (S. Stankovich, 2007). How to exfoliate the flakes of natural graphite was first described in a US patent in 1891 (Inagaki, 2004). An exfoliation phenomenon was studied mostly and occurs when the graphene layers are forced apart by the sudden vaporization or decomposition of intercalated species (E. H. L. Falcao, 2007).

However, the mentioned properties of graphene-polymer nanocomposites are strongly dependent on the uniformly dispersion in polymeric matrices which is affected by functional groups present on the graphene surface.

� ��������������

where �� is the refractive index of the ATR crystal and �� is the ratio between the refractive indexes of the sample and the ATR crystal ( ��which both of them are assumed to be constant in the considered frequency range. � is the wave number and � is the incident angle. The penetration depth for PVDF as an example by rhe assumption of �� = 2.4,

� ) �

from 500 to 4000 cm-1 is approximately 0.5-4 µm (Y. Jung Park, 2005).

�� (1)

� =

Scheme 1. Schematic illustration of ATR-FTIR configuration (KS. Kwan, 1998)

intercalated species (E. H. L. Falcao, 2007).

functional groups present on the graphene surface.

Graphene and graphite have recently attracted strong attention as versatile, environmentally friendly and available carbon materials which can be used as inexpensive filler in the composite materials (S. Stankovich, 2006, 2007 & L. Al-Mashat, 2010) .Crystalline graphites are used in polymeric systems in order to improve polymer properties such as thermal and electrical conductivity, IR absorption, flame retardancy, barrier resistance, electromagnetic shielding, lubrication and abrasion resistance. When the crystalline graphite is exfoliated to individual graphene sheets, the specific surface would be as large as 2600 m2. g-1 and novel electronic and mechanical properties appeared (Steurer, 2009). Actually graphene sheets are one-atom two-dimensional layers of sp2- network carbon that their fracture strength should be comparable to that of carbon nanotubes with similar types of defects (S. Stankovich, 2007). How to exfoliate the flakes of natural graphite was first described in a US patent in 1891 (Inagaki, 2004). An exfoliation phenomenon was studied mostly and occurs when the graphene layers are forced apart by the sudden vaporization or decomposition of

However, the mentioned properties of graphene-polymer nanocomposites are strongly dependent on the uniformly dispersion in polymeric matrices which is affected by

�� = 1.5 and � = 45*°*

The modification of graphene sheets via organic oligomeric and polymeric chains is a favorable way to promote the compatibility of these nanoparticles with polymeric media. So understanding of the functional groups present on the graphene surface should be very vital for designing of modification procedure.

Poly(vinylidene fluoride) (PVDF) is a semicrystalline engineering polymer with very good resistance to chemicals, oxidation, and UV radiation (J. H. Yen, 2006). PVDF is known for its polymorphism crystalline structure and complicated microstructure. It is one of the most widely studied polymers due to its non-linearity, piezo- and pyro-electricity (L. T. Vo, 2007 & K. Pramoda, 2005). PVDF can crystallize in at least five well-known crystalline phases (E. Giannetti, 2001, A. Lovinger, 1982 & N. S. Nalwa, 1995):

α and δ with conformation of the alternating trans-gauche (TG+TG- ) which the α is the most common form and is the most thermodynamically stable.

Scheme 2. α and δ form with alternating trans-gauche conformation (J. H. Yen, 2006)

β with all trans (TTT) planer zigzag conformation is polar form and has been extensively studied for its potential applications. This form develops under mechanical deformation (K. Matsushige, 1980) (S. Ramasundaram, 2008) , growth from solution (J. Wang, 2003 & R. L. Miller, 1976), addition of metal salts (X. He, 2006 &W. A. Yee, 2007) ,melt crystallization at high pressures (D. Yang, 1987), application of a strong electric field (J. I. Scheinbeim, 1986), blending with carbonyl-containing polymers (C. Lbonard, 1988 & K. J. Kim, 1995), and recently, addition of nanoparticles (S. Ramasundaram, 2008, T. Ogoshi, 2005 & L. He, 2010). This structure provides some unique properties for PVDF piezo- and pyro-electric activity:

Scheme 3. β form all trans planer zigzag conformation (J. H. Yen, 2006)

Preparation and Characterization of PVDF/PMMA/Graphene

(TEA) were bought from Merck Co. and used as received.

**2.2 Preparation of exfoliated graphite** 

between graphite layer planes to form GICs.

centrifuge drainage extraction was natural.

**2.3 Modification with methacrylic anhydride** 

mixture.

groups on the surface and edge of graphene sheets.

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 217

1000HD) was provided by Atofina Co. The methyl methacrylate (MMA) and Methacyilic anhydride were supplied by Aldrich Co. and MMA used after purification and distillation in order to remove inhibitors. Concentrated sulfuric acid and nitric acid with concentration of 63% were used as chemical oxidizer to prepare expanded graphite. The initiator, Benzoyl peroxide (BPO), dichloromethane (CH2Cl2), dimethylformamide (DMF), triethylamine

The method which was used in this study is as same as general procedure employed in industry for producing exfoliated graphite. Natural graphite powders as starting material were treated with a mixture of 4:1 sulfuric and nitric acid at 80°C for 24h to produce Graphite Intercalated Compounds (GIC)s. Graphite can accept many species into the gallery

Sulfuric acid is the most conventional intercalate for achieving a high degree of exfoliation of natural graphite and so it is used most commonly in the industry. Nitric acid was also added as an oxidant to generate some functional groups such as hydroxyl, carboxyl, epoxid

The suspension of acid-treated graphite was added to excess distilled water, and this was followed by centrifugation with 5000 rpm and washing with water until the pH of the

These particles dried at oven at 100°C for 12h and vacuumed oven at 80°C for 8h. The oxidized graphite was heat-treated at 1050 °C for 45s to obtain expanded graphite. Actually, In Chung's review, exfoliation phenomena were classified into reversible and irreversible ones. When the oxidized graphite was heated to around 300°C,it expanded to a fractional expansion of about 30 and upon cooling collapsed, again. This expansion–collapse

But if it is heated to a higher temperature (e.g., 1000°C), the intercalating compound and some of functional groups decompose completely and at the same time the host graphite flakes exfoliate up to about 300*×* in volume, particularly by rapid heating. This exfoliated graphite never returns to the original thickness upon cooling to room temperature, that is, we have irreversible exfoliation. The expanded graphite was immersed in absolute ethanol and sonicated for 30 min in order to exfoliate and break down to individual layers. The dispersion was filtered and dried at oven at 80°C for 8h and vacuumed oven at 80°C for 4h

The above Functionalized Graphite, which is called FG, was reacted with methacrylic anhydride to provide some organic groups and vinyl moieties on the surface of graphene. Actually, the functional groups which were introduced in the previous process could further react with methacrylic anhydride through hydroxyl groups. A 250-mL flask was charged with 100 mL of DMF, 10 mL TEA and 100 mg FG and after sonication for 30 min, a solution of containing 5 mL methacrylic anhydride and 90 mL DMF was added in to the

to remove residual moisture in the graphite particles (S. Mohamadi N. S.-S., 2011).

phenomenon was reversible, and was thus called reversible exfoliation.

γ and ε with T3G+T3G- conformation is also polar but less than β form.

Scheme 4. γ and ε crystalline form with T3G+T3G- conformation (J. H. Yen, 2006)

Subsequently due to specific chain conformation in crystal unit cell and providing the highest remnant polarization, β phase has attracted more attention than the others in pyroand piezoelectric applications (J.Jungnickel, 1996).

Poly methylmethacrylate (PMMA) can interact with graphene sheets by the interaction of delocalized π-bonds of graphene with π-bonds of PMMA. On the other hand, as reported in articles, PVDF/PMMA blend is a miscible system. Consequently, in attempt to achieve a homogenous dispersion of nanographene layers in PVDF matrix, the use of PMMA chains as a compatiblizer can be useful.

In addition, presence of PMMA chains in the close touch with PVDF molecules causes the formation of polar crystalline form in PVDF (J. Wang, 2003). Furthermore, previous studies on the CNTs indicated TTT molecular chain prefers to be absorbed on the CNT surface compared with TGTG' molecular chain, and the configuration in which H atoms and CNT surface are face-to-face are more stable than that where F atoms and CNT surface are faceto-face. Since in the PVDF, the negative charge transfer from H to C atom and the negative charge is accumulated around the F atoms, the interaction between the H atoms with positive charge in PVDF and C atoms with π oribital in CNT should be stronger (S. Yu, 2009)

FT-IR is a powerful and reliable technique for description of chemical characterization of graphene and also study of the structure and properties relationships in PVDF/PMMA/graphene polymer blend nanocomposite. The interactions between these three components including: PVDF, PMMA and graphene sheets can be revealed and described by using this technique. As well, exploration of PVDF chains conformations (crystalline structures) are affected by presence of graphene sheets and PMMA chains can be done which is very important in order to design the new material with special properties.

## **2. Experimental**

#### **2.1 Materials**

The graphite used in this study was natural graphite powders with the size of >150 μm, and bulk density of 1.65 gr/cm3 supplied by Iran Petrochemical Co. The PVDF pelletlet (Kynar® 1000HD) was provided by Atofina Co. The methyl methacrylate (MMA) and Methacyilic anhydride were supplied by Aldrich Co. and MMA used after purification and distillation in order to remove inhibitors. Concentrated sulfuric acid and nitric acid with concentration of 63% were used as chemical oxidizer to prepare expanded graphite. The initiator, Benzoyl peroxide (BPO), dichloromethane (CH2Cl2), dimethylformamide (DMF), triethylamine (TEA) were bought from Merck Co. and used as received.

## **2.2 Preparation of exfoliated graphite**

216 Infrared Spectroscopy – Materials Science, Engineering and Technology

γ and ε with T3G+T3G- conformation is also polar but less than β form.

Scheme 4. γ and ε crystalline form with T3G+T3G- conformation (J. H. Yen, 2006)

and piezoelectric applications (J.Jungnickel, 1996).

as a compatiblizer can be useful.

2009)

**2. Experimental** 

**2.1 Materials** 

Subsequently due to specific chain conformation in crystal unit cell and providing the highest remnant polarization, β phase has attracted more attention than the others in pyro-

Poly methylmethacrylate (PMMA) can interact with graphene sheets by the interaction of delocalized π-bonds of graphene with π-bonds of PMMA. On the other hand, as reported in articles, PVDF/PMMA blend is a miscible system. Consequently, in attempt to achieve a homogenous dispersion of nanographene layers in PVDF matrix, the use of PMMA chains

In addition, presence of PMMA chains in the close touch with PVDF molecules causes the formation of polar crystalline form in PVDF (J. Wang, 2003). Furthermore, previous studies on the CNTs indicated TTT molecular chain prefers to be absorbed on the CNT surface compared with TGTG' molecular chain, and the configuration in which H atoms and CNT surface are face-to-face are more stable than that where F atoms and CNT surface are faceto-face. Since in the PVDF, the negative charge transfer from H to C atom and the negative charge is accumulated around the F atoms, the interaction between the H atoms with positive charge in PVDF and C atoms with π oribital in CNT should be stronger (S. Yu,

FT-IR is a powerful and reliable technique for description of chemical characterization of graphene and also study of the structure and properties relationships in PVDF/PMMA/graphene polymer blend nanocomposite. The interactions between these three components including: PVDF, PMMA and graphene sheets can be revealed and described by using this technique. As well, exploration of PVDF chains conformations (crystalline structures) are affected by presence of graphene sheets and PMMA chains can be done which is very important in order to design the new material with special properties.

The graphite used in this study was natural graphite powders with the size of >150 μm, and bulk density of 1.65 gr/cm3 supplied by Iran Petrochemical Co. The PVDF pelletlet (Kynar® The method which was used in this study is as same as general procedure employed in industry for producing exfoliated graphite. Natural graphite powders as starting material were treated with a mixture of 4:1 sulfuric and nitric acid at 80°C for 24h to produce Graphite Intercalated Compounds (GIC)s. Graphite can accept many species into the gallery between graphite layer planes to form GICs.

Sulfuric acid is the most conventional intercalate for achieving a high degree of exfoliation of natural graphite and so it is used most commonly in the industry. Nitric acid was also added as an oxidant to generate some functional groups such as hydroxyl, carboxyl, epoxid groups on the surface and edge of graphene sheets.

The suspension of acid-treated graphite was added to excess distilled water, and this was followed by centrifugation with 5000 rpm and washing with water until the pH of the centrifuge drainage extraction was natural.

These particles dried at oven at 100°C for 12h and vacuumed oven at 80°C for 8h. The oxidized graphite was heat-treated at 1050 °C for 45s to obtain expanded graphite. Actually, In Chung's review, exfoliation phenomena were classified into reversible and irreversible ones. When the oxidized graphite was heated to around 300°C,it expanded to a fractional expansion of about 30 and upon cooling collapsed, again. This expansion–collapse phenomenon was reversible, and was thus called reversible exfoliation.

But if it is heated to a higher temperature (e.g., 1000°C), the intercalating compound and some of functional groups decompose completely and at the same time the host graphite flakes exfoliate up to about 300*×* in volume, particularly by rapid heating. This exfoliated graphite never returns to the original thickness upon cooling to room temperature, that is, we have irreversible exfoliation. The expanded graphite was immersed in absolute ethanol and sonicated for 30 min in order to exfoliate and break down to individual layers. The dispersion was filtered and dried at oven at 80°C for 8h and vacuumed oven at 80°C for 4h to remove residual moisture in the graphite particles (S. Mohamadi N. S.-S., 2011).

### **2.3 Modification with methacrylic anhydride**

The above Functionalized Graphite, which is called FG, was reacted with methacrylic anhydride to provide some organic groups and vinyl moieties on the surface of graphene. Actually, the functional groups which were introduced in the previous process could further react with methacrylic anhydride through hydroxyl groups. A 250-mL flask was charged with 100 mL of DMF, 10 mL TEA and 100 mg FG and after sonication for 30 min, a solution of containing 5 mL methacrylic anhydride and 90 mL DMF was added in to the mixture.

Preparation and Characterization of PVDF/PMMA/Graphene

Fig. 1. FTIR spectrums of FG (a) and MG (b).

significant amount of oxygen (Steurer, 2009).

groups came into sight.

illustrated in Fig. 1.

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 219

functional groups existed in the graphene sheets after heat treated of graphene oxide (FG) and functionalization with methacrylic anhydride, the related FTIR spectra has been

As can be seen in Fig. 1 (a) most of carboxyl groups in the graphene oxide has decomposed during heat-treatment of graphite oxide so that the peak at 1720 cm-1 disappeared. In fact, during the acid treatment various oxygen-functional groups were produced on the graphene surface. While, in the heat-treatment step, the oxygen containing of graphene was reduced but some introduced functional groups remained and still the particles can contain

In addition, the peaks at 2960 and 2930 cm-1 are related to the asymmetric and symmetric stretching vibration of pendant methyl groups. Symmetric vibrations are generally weaker than asymmetric vibrations since the former lead to less of a change in dipole moment. Actually, during sonication it may graphene flakes are broken and this pendant methyl

In the case of MG, (Fig. 1 (b)) appearance of a new peak at 1740 cm-1 which has been assigned to vibration of esteric carbonyl group is due to the formation of esteric linkages between hydroxyl groups of FG and methacrylic anhydride (Scheme 6). Modification of FG by methacrylic anhydride (MG), causes appearance of the new methyl groups which show the peaks at 2960 and 2930 cm-1. Furthermore, C=C band at 1580 cm-1 has become

This mixture was allowed to react at 90°C for 24h under magnetic stirring. After that the Modified Graphite, for simplicity it is called MG, were collected by filtration and followed by thorough washing with distilled water to remove any residual TEA and then washed with CH2Cl2, in order to remove any unreacted methacrylic anhydride. The washed particles were dried in oven and vacuum oven at 80°C for 8h, separately.

## **2.4 Preparation of PMMA/ graphene masterbatch**

The PMMA-graphene masterbatch (PMMA-G master) was prepared via polymerization of MMA with BPO as an initiator. 300 mg of MG was dispersed in 15 mL MMA via ultrasonication for 30 min. The polymerization was initiated by (1.1% wt) of BPO respect to monomer in flask at 90°C, until the polymerization was solidified, completely. Later then, the prepared masterbatch was kept in vacuum oven at 120°C to remove any remaining MMA. The content amount of graphite was kept in 2.5 wt%.

## **2.5 Preparation of PVDF/PMMA/graphene nanocomposite**

PVDF and PMMA-G master solutions were prepared separately by dissolving in DMF at 50°C with stirring for 1 day. Both of the resulting solutions were blended, so that the final concentration of mixture adjusted in 12% wt. on the weight of PVDF/PMMA-G master respect to solvent. The mixture was sonicated for about 30 min, stirred 2 days and spread on a well-cleaned glass slide of petridish. The prepared sample was carefully evaporated at 60°C in an oven for 20 h and then divided into three parts and annealed at three different temperatures of 50, 90 and 120°C in vacuum oven for 24 h. three samples with varying PVDF/PMMA-G master ratios of 80:20, 70:30 and 60:40 were prepared and analyzed. For convenience the samples were named as 80:20, 70:30, 60:40 and 70:30 No G, respectively (S. Mohamadi, 2011).

### **2.6 Characterization**

The Fourier transform infrared (FT-IR) analysis was performed on a Bruker Equinox55 Analyzer, equipped with a DTGS detector and a golden gate micro ATR from 600 –3500 cm-1. Scanning electron microscopic (SEM) and TEM images were obtained on a Zeiss CEM 902A (Oberkochen, Germany) and Philips-CM-120, Netherlands at an accelerating voltage of 120 KV, respectively. The thermal behavior was measured with a DSC Q100 from the TA instruments with the heating and cooling ramp of 10 ºC min-1 from room temperature to 250 ºC under argon flow. The X-ray diffraction (XRD) patterns of the samples were recorded by Philips, Netherlands advanced diffractometer using Cu(Kα) radiation (wavelength: 1.5405Ǻ) at room temperature in the range of 2θ from 4 to 70° with a scanning rate of 0.04°. S−1.

## **3. Results and discussion**

As reported by others in several literatures, presence of the relatively broad peak at 3427 cm -1 and also the peak at 1402 cm -1 in the FTIR spectra of graphite oxide indicate existence of hydroxyl groups. In addition, the peaks at 1720, 1640, 1580 and 1060 cm-1 can be assigned to the stretching vibration of carboxyl, carbonyl moiety of quinine, aromatic C=C bonds and epoxide groups, respectively (scheme 1) (C. Hontoria-Lucas, 1995) . To determine which

This mixture was allowed to react at 90°C for 24h under magnetic stirring. After that the Modified Graphite, for simplicity it is called MG, were collected by filtration and followed by thorough washing with distilled water to remove any residual TEA and then washed with CH2Cl2, in order to remove any unreacted methacrylic anhydride. The washed

The PMMA-graphene masterbatch (PMMA-G master) was prepared via polymerization of MMA with BPO as an initiator. 300 mg of MG was dispersed in 15 mL MMA via ultrasonication for 30 min. The polymerization was initiated by (1.1% wt) of BPO respect to monomer in flask at 90°C, until the polymerization was solidified, completely. Later then, the prepared masterbatch was kept in vacuum oven at 120°C to remove any remaining

PVDF and PMMA-G master solutions were prepared separately by dissolving in DMF at 50°C with stirring for 1 day. Both of the resulting solutions were blended, so that the final concentration of mixture adjusted in 12% wt. on the weight of PVDF/PMMA-G master respect to solvent. The mixture was sonicated for about 30 min, stirred 2 days and spread on a well-cleaned glass slide of petridish. The prepared sample was carefully evaporated at 60°C in an oven for 20 h and then divided into three parts and annealed at three different temperatures of 50, 90 and 120°C in vacuum oven for 24 h. three samples with varying PVDF/PMMA-G master ratios of 80:20, 70:30 and 60:40 were prepared and analyzed. For convenience the samples were named as 80:20, 70:30, 60:40 and 70:30 No G, respectively (S.

The Fourier transform infrared (FT-IR) analysis was performed on a Bruker Equinox55 Analyzer, equipped with a DTGS detector and a golden gate micro ATR from 600 –3500 cm-1. Scanning electron microscopic (SEM) and TEM images were obtained on a Zeiss CEM 902A (Oberkochen, Germany) and Philips-CM-120, Netherlands at an accelerating voltage of 120 KV, respectively. The thermal behavior was measured with a DSC Q100 from the TA instruments with the heating and cooling ramp of 10 ºC min-1 from room temperature to 250 ºC under argon flow. The X-ray diffraction (XRD) patterns of the samples were recorded by Philips, Netherlands advanced diffractometer using Cu(Kα) radiation (wavelength: 1.5405Ǻ) at room temperature in the range of 2θ from 4 to 70° with

As reported by others in several literatures, presence of the relatively broad peak at 3427 cm -1 and also the peak at 1402 cm -1 in the FTIR spectra of graphite oxide indicate existence of hydroxyl groups. In addition, the peaks at 1720, 1640, 1580 and 1060 cm-1 can be assigned to the stretching vibration of carboxyl, carbonyl moiety of quinine, aromatic C=C bonds and epoxide groups, respectively (scheme 1) (C. Hontoria-Lucas, 1995) . To determine which

particles were dried in oven and vacuum oven at 80°C for 8h, separately.

**2.4 Preparation of PMMA/ graphene masterbatch** 

MMA. The content amount of graphite was kept in 2.5 wt%.

**2.5 Preparation of PVDF/PMMA/graphene nanocomposite** 

Mohamadi, 2011).

**2.6 Characterization** 

a scanning rate of 0.04°. S−1.

**3. Results and discussion** 

functional groups existed in the graphene sheets after heat treated of graphene oxide (FG) and functionalization with methacrylic anhydride, the related FTIR spectra has been illustrated in Fig. 1.

As can be seen in Fig. 1 (a) most of carboxyl groups in the graphene oxide has decomposed during heat-treatment of graphite oxide so that the peak at 1720 cm-1 disappeared. In fact, during the acid treatment various oxygen-functional groups were produced on the graphene surface. While, in the heat-treatment step, the oxygen containing of graphene was reduced but some introduced functional groups remained and still the particles can contain significant amount of oxygen (Steurer, 2009).

In addition, the peaks at 2960 and 2930 cm-1 are related to the asymmetric and symmetric stretching vibration of pendant methyl groups. Symmetric vibrations are generally weaker than asymmetric vibrations since the former lead to less of a change in dipole moment. Actually, during sonication it may graphene flakes are broken and this pendant methyl groups came into sight.

In the case of MG, (Fig. 1 (b)) appearance of a new peak at 1740 cm-1 which has been assigned to vibration of esteric carbonyl group is due to the formation of esteric linkages between hydroxyl groups of FG and methacrylic anhydride (Scheme 6). Modification of FG by methacrylic anhydride (MG), causes appearance of the new methyl groups which show the peaks at 2960 and 2930 cm-1. Furthermore, C=C band at 1580 cm-1 has become

Preparation and Characterization of PVDF/PMMA/Graphene

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 221

Fig. 2. ATR-FTIR spectrum of pure PVDF film which was annealed at 50°C

Rajendran, 2010 & Garton, 1992).

conformation.

It should be pointed that since the β- and γ-phase are similar to each other in short segmental conformations the identification of crystal phase between β and γ in FTIR spectrum is still in dispute. On the basis of the data from literature the absorption band at 763 cm-1 is related to In-plane bending or rocking vibration in α phase (N. Betz, 1994 & S. Lanceros-Me ndez, 2001), the band at 840 cm-1 stretching in β or γ phase (G. Chi Chen, 1994 , V. Bharti, 1997, B. Mattsson, 1999 & M. Benz, 2002), the band at 1173 cm−1 is associated to the symmetrical stretching of –CF2 group (M. Rajendran, 2010). The band at 1234 cm−1 is related to the γ phase and 1453 cm-1 is assigned to in-plane bending or scissoring of CH2 group (M.

In addition, some irregularities of head-to-tail addition, leading to defect structures, can occur during polymerization for several reasons in polymers of the vinylidene class, sequence isomerism may occur. The transmission band at 677 cm−1 points to the presence of head-to-head and tail-to-tail configurations (B. Hilczer, 1998). Such defects are produced during the polymerization process and reduce the dipole moment of the all-trans

Fig. 3 shows ATR-FTIR spectra for pure PVDF film (a), G-PMMA master (b), 80:20 (c), 70:30 (d), 70:30 NoG (e), 60:40 (f) which were annealed at 50°C from 550-2000 cm-1. When MG was incorporated in PMMA matrix (G-PMMA master), the absorption originating from MG particles alone were hardly visible. G-PMMA master spectrum clearly shows several characteristic peaks at 1723, 1449 and 1142 cm-1 which are related to the presence of PMMA

Polymer- polymer interactions may also affect PVDF chain conformations. Specific interactions are very sensitive to the distance between the interacting groups and to their

chains, assigned to ester carbonyl (C=O), O–CH3 and C-O (ester bond) stretching.

Scheme 5. Graphite oxide (P. Steurer, 2009)

more broader in MG rather than FG may due to presence of aromatic and aliphatic double bonds.

ATR-FTIR spectrum of pure PVDF film which was annealed at 50°C, from 550-3500 cm-1 is also shown in Fig.2. The observed pattern originates from oscillations of large parts of the skeleton and/or the skeleton and attached functional groups. Below 1500 cm−1, most single bonds absorb at similar frequencies, and the vibrations couple.

Scheme 6. Modified graphen with methacrylic anhydride (S. Mohamadi, 2011)

more broader in MG rather than FG may due to presence of aromatic and aliphatic double

ATR-FTIR spectrum of pure PVDF film which was annealed at 50°C, from 550-3500 cm-1 is also shown in Fig.2. The observed pattern originates from oscillations of large parts of the skeleton and/or the skeleton and attached functional groups. Below 1500 cm−1, most single

Scheme 5. Graphite oxide (P. Steurer, 2009)

bonds absorb at similar frequencies, and the vibrations couple.

Scheme 6. Modified graphen with methacrylic anhydride (S. Mohamadi, 2011)

bonds.

Fig. 2. ATR-FTIR spectrum of pure PVDF film which was annealed at 50°C

It should be pointed that since the β- and γ-phase are similar to each other in short segmental conformations the identification of crystal phase between β and γ in FTIR spectrum is still in dispute. On the basis of the data from literature the absorption band at 763 cm-1 is related to In-plane bending or rocking vibration in α phase (N. Betz, 1994 & S. Lanceros-Me ndez, 2001), the band at 840 cm-1 stretching in β or γ phase (G. Chi Chen, 1994 , V. Bharti, 1997, B. Mattsson, 1999 & M. Benz, 2002), the band at 1173 cm−1 is associated to the symmetrical stretching of –CF2 group (M. Rajendran, 2010). The band at 1234 cm−1 is related to the γ phase and 1453 cm-1 is assigned to in-plane bending or scissoring of CH2 group (M. Rajendran, 2010 & Garton, 1992).

In addition, some irregularities of head-to-tail addition, leading to defect structures, can occur during polymerization for several reasons in polymers of the vinylidene class, sequence isomerism may occur. The transmission band at 677 cm−1 points to the presence of head-to-head and tail-to-tail configurations (B. Hilczer, 1998). Such defects are produced during the polymerization process and reduce the dipole moment of the all-trans conformation.

Fig. 3 shows ATR-FTIR spectra for pure PVDF film (a), G-PMMA master (b), 80:20 (c), 70:30 (d), 70:30 NoG (e), 60:40 (f) which were annealed at 50°C from 550-2000 cm-1. When MG was incorporated in PMMA matrix (G-PMMA master), the absorption originating from MG particles alone were hardly visible. G-PMMA master spectrum clearly shows several characteristic peaks at 1723, 1449 and 1142 cm-1 which are related to the presence of PMMA chains, assigned to ester carbonyl (C=O), O–CH3 and C-O (ester bond) stretching.

Polymer- polymer interactions may also affect PVDF chain conformations. Specific interactions are very sensitive to the distance between the interacting groups and to their

Preparation and Characterization of PVDF/PMMA/Graphene

PVDF-PMMA, PVDF-graphene and PMMA-graphene interactions.

these structures are due to the formation of γ crystals (Y. J. Park, 2005).

PMMA.

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 223

slighter for 70:30 NO G than polymer blend nanocomposites, it can be imagined this change is related to the specific interactions between PVDF and graphene surface in the nanocomposites. Also, it can be concluded GNPs increase the compatibility of PVDF and

On the other hand as mentioned above, presence of polymer-polymer interactions can also affect the polymer conformations. PVDF exhibits short bond sequences tt and tg; the probability of a gg bond is negligible because of steric hindrance. Energy calculations emphasize that PMMA conformation remains close to all trans conformation of PVDF. In our study, presence of graphene particles along with PMMA chains makes it complex due to

The surface morphology of the casted films was studied using SEM analysis. In each sample, the surface which is exposed to the air is examined. The distinguishable spherulitic structure on the top surface of pure PVDF film could be seen which had extended from 9 µm to about 15 µm with increasing the annealing temperature from 50 to 120°C. But in 70:30 some new elongated ribbon like crystals on the spherulitic structures can be observed which became more visible with increasing annealing temperature to 120°C (Fig. 4). Y. J. Park et al reported the oriented align crystalline PVDF lamella which has bamboo-like structure can be achieved when PVDF film casted from polar solvent such as DMF and DMSO and crystallize in the confined spaces of the specific mold at high temperature. In their view,

Fig. 4. The surface morphology of the casted 70:30 film were annealed at 50, 90 and 120°C.

Fig. 3. ATR-FTIR spectra of pure PVDF (a), G-PMMA master (b), 80:20 (c), 70:30 (d), 70:30 NoG (e), 60:40 (f) which were annealed at 50°C.

relative orientation: hydrogen bonding strength falls off rapidly when the atomic distance increases or when the bond is bent instead of linear. This infers that the efficiency of the contact between two unlike chains depends on their respective conformation (eventually tacticity) and flexibility. The chains are expected to adopt an optimate conformation for interacting (C. Lbonard, 1988).

To identify the nature of the interactions in the polymer blend nanocomposite, we choose the band at 1720 cm-1 which is due to the stretching vibration of the C=O group in PMMA as a characteristic peak of PMMA. This band shows a little shift to the higher frequencies and has become broader in both of the polymer blend with no graphen and nanocomposites. These changes can be ascribed by the interaction between PMMA and PVDF chains (I. S. Elashmawi, 2008 & M. M. Colemann, 1995).

The strong absorption peak which appeared at 1173 cm-1 is assigned to the symmetrical stretching of –CF2 groups, was choose as characteristic peak of PVDF in polymer blend nanocomposites. The precise consideration reveals shifting of this band to the higher frequencies in the synthesised samples comparing to pure PVDF. Since this shifting is

Fig. 3. ATR-FTIR spectra of pure PVDF (a), G-PMMA master (b), 80:20 (c), 70:30 (d), 70:30

relative orientation: hydrogen bonding strength falls off rapidly when the atomic distance increases or when the bond is bent instead of linear. This infers that the efficiency of the contact between two unlike chains depends on their respective conformation (eventually tacticity) and flexibility. The chains are expected to adopt an optimate conformation for

To identify the nature of the interactions in the polymer blend nanocomposite, we choose the band at 1720 cm-1 which is due to the stretching vibration of the C=O group in PMMA as a characteristic peak of PMMA. This band shows a little shift to the higher frequencies and has become broader in both of the polymer blend with no graphen and nanocomposites. These changes can be ascribed by the interaction between PMMA and PVDF chains (I. S.

The strong absorption peak which appeared at 1173 cm-1 is assigned to the symmetrical stretching of –CF2 groups, was choose as characteristic peak of PVDF in polymer blend nanocomposites. The precise consideration reveals shifting of this band to the higher frequencies in the synthesised samples comparing to pure PVDF. Since this shifting is

NoG (e), 60:40 (f) which were annealed at 50°C.

Elashmawi, 2008 & M. M. Colemann, 1995).

interacting (C. Lbonard, 1988).

slighter for 70:30 NO G than polymer blend nanocomposites, it can be imagined this change is related to the specific interactions between PVDF and graphene surface in the nanocomposites. Also, it can be concluded GNPs increase the compatibility of PVDF and PMMA.

On the other hand as mentioned above, presence of polymer-polymer interactions can also affect the polymer conformations. PVDF exhibits short bond sequences tt and tg; the probability of a gg bond is negligible because of steric hindrance. Energy calculations emphasize that PMMA conformation remains close to all trans conformation of PVDF. In our study, presence of graphene particles along with PMMA chains makes it complex due to PVDF-PMMA, PVDF-graphene and PMMA-graphene interactions.

The surface morphology of the casted films was studied using SEM analysis. In each sample, the surface which is exposed to the air is examined. The distinguishable spherulitic structure on the top surface of pure PVDF film could be seen which had extended from 9 µm to about 15 µm with increasing the annealing temperature from 50 to 120°C. But in 70:30 some new elongated ribbon like crystals on the spherulitic structures can be observed which became more visible with increasing annealing temperature to 120°C (Fig. 4). Y. J. Park et al reported the oriented align crystalline PVDF lamella which has bamboo-like structure can be achieved when PVDF film casted from polar solvent such as DMF and DMSO and crystallize in the confined spaces of the specific mold at high temperature. In their view, these structures are due to the formation of γ crystals (Y. J. Park, 2005).

Fig. 4. The surface morphology of the casted 70:30 film were annealed at 50, 90 and 120°C.

Preparation and Characterization of PVDF/PMMA/Graphene

Differential Scanning Calorimetery (DSC) results (Fig. 6).

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 225

It indicated with increasing annealing temperature the percentage of γ-phase on the surface increased while α-crystal decreased at 90° C and then didn't change significantly, at 120°C. In this regards, this new droplet like structure identities on the top of the films can be related to γ-phase. Of course, this result is in agreement with X-ray Diffraction (XRD) and

Fig. 6. X-ray diffraction pattern (A) and DSC thermograms of first and second heating (B) of

70:30 films annealed at 50, 90 and 120°C for 24h (S. Mohamadi, 2011)

We analyzed ATR-FTIR spectrum of 70:30 sample which were annealed at 50, 90 and 120° C (Fig. 5). From the previous reports on the spectral features of each crystal phase in PVDF, two IR absorptions band at 762 cm-1 and 1234 cm-1 as the representative of α- and γ-phase were selected to compare the peak intensities in each annealed samples.

Fig. 5. ATR-FTIR spectra of 70:30 were annealed at 50, 90 and 120° C.

We analyzed ATR-FTIR spectrum of 70:30 sample which were annealed at 50, 90 and 120° C (Fig. 5). From the previous reports on the spectral features of each crystal phase in PVDF, two IR absorptions band at 762 cm-1 and 1234 cm-1 as the representative of α- and γ-phase

were selected to compare the peak intensities in each annealed samples.

Fig. 5. ATR-FTIR spectra of 70:30 were annealed at 50, 90 and 120° C.

It indicated with increasing annealing temperature the percentage of γ-phase on the surface increased while α-crystal decreased at 90° C and then didn't change significantly, at 120°C. In this regards, this new droplet like structure identities on the top of the films can be related to γ-phase. Of course, this result is in agreement with X-ray Diffraction (XRD) and Differential Scanning Calorimetery (DSC) results (Fig. 6).

Fig. 6. X-ray diffraction pattern (A) and DSC thermograms of first and second heating (B) of 70:30 films annealed at 50, 90 and 120°C for 24h (S. Mohamadi, 2011)

Preparation and Characterization of PVDF/PMMA/Graphene

temperature and most of β and γ form convert to α form.

conformation chains as is illustrated in scheme:

TTTG+TTTG- (B) conformation chains

surface of graphene sheets.

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 227

the graphene sheets can act as micropattern molds for crystallization of PVDF. In other words, a well-defined feature of micropattern can be provided by the wrinkles on the

Crystallization results basically in the succession of two events: the primary nucleation of a new phase and then the three-dimensional growth of lamellae; these steps can be followed by lamellar thickening, fold surface smoothing, or reorganization into more perfect crystals. As reported by others, with increasing the annealing temperature in PVDF/PMMA blend film, the amount of α phase increases due to easier local internal chain rotation at higher

While in our study with increasing the annealing temperature the amount of α phase decreased and γ form increased. It can be said graphene sheets can stabilize the β- and γphase at elevated temperature due to restricting effect of graphene sheets on TT

Scheme 7. Schematic illustration of to restricting effect of graphene sheets on TT (A) and

On the basis of the reported data in literatures, WAXD data of neat PVDF contain 100, 020, 110, 021 reflections at 2θ= 17.9°, 18.4°, 20.1°, and 26.7° for α- and only 200/110 reflection at 2θ= 20.8° for β-crystalline form (A. Kaito, 2007, L.Yu, 2009 & G. Guerra, 1986). γ-crystal planes are known to have overlapping reflections with (020), (110) and (021) α-crystal planes and also 200/110 reflection of β-phase. The (100) peak of α- phase at 2θ= 17.9° is the only peak that doesn't have any overlapping with γ-phase. Also the sharp diffraction peak at 26.38° in 70:30 is related to the crystallographic plane of graphite with d-spacing of 3.37Ǻ, indicating that the graphite retained its crystalline structure in nanocomposite but there is a different degree of stacking order and disordered microstructure (K. P. Pramoda, 2010 & T. Ramanathan, 2007). Furthermore, the film which was annealed at 50°C shows the 100, 020, 110 reflections of α-phase and 200/110 reflection of β-crystal In XRD pattern of the films were annealed at 90°C, 200/100 reflection of β-crystal has become stronger, while the 100 peak of α has not changed considerably. It means with increasing temperature to 90°C, the growth of β-crystals is more favorable than α-crystals. Increasing the annealed temperature to the 120°C caused the substantial variation in the intensity of reflections. The decrease in the intensity of 200/100 reflection of β-crystal and increase in 020, 110 reflections of α- or γphase without any significant change in 100 reflection of α-crystal implies some of β-crystals convert to γ-crystals at 120°C. The DSC results also matched well with those of X-ray diffractions and indicated the existence of the different types of crystal structures. The new appeared endotherm in the film which was annealed at 177° C can be attributed to the melting of the new formed ribbon-like structures on the surface.

Actually, in our work, it can be imagined PVDF chains which are trapped between graphen sheets and are in close contact with PMMA chains, crystallize in the confined spaces that are provided by graphen sheets and PMMA chains. It is demonstrated that exfoliated graphenebased materials are often compliant, and when dispersed in a polymer matrix are typically not observed as rigid disks, but rather as bent or crumpled platelets (J. R. Potts, 2011). Consequently, as can be seen in TEM image of 70:30 sample (Fig. 7), the wrinkled surface of

Fig. 7. TEM image and Wrinkled surface of graphene sheets in 70:30 polymer blend nanocomposite (S. Mohamadi, 2011)

On the basis of the reported data in literatures, WAXD data of neat PVDF contain 100, 020, 110, 021 reflections at 2θ= 17.9°, 18.4°, 20.1°, and 26.7° for α- and only 200/110 reflection at 2θ= 20.8° for β-crystalline form (A. Kaito, 2007, L.Yu, 2009 & G. Guerra, 1986). γ-crystal planes are known to have overlapping reflections with (020), (110) and (021) α-crystal planes and also 200/110 reflection of β-phase. The (100) peak of α- phase at 2θ= 17.9° is the only peak that doesn't have any overlapping with γ-phase. Also the sharp diffraction peak at 26.38° in 70:30 is related to the crystallographic plane of graphite with d-spacing of 3.37Ǻ, indicating that the graphite retained its crystalline structure in nanocomposite but there is a different degree of stacking order and disordered microstructure (K. P. Pramoda, 2010 & T. Ramanathan, 2007). Furthermore, the film which was annealed at 50°C shows the 100, 020, 110 reflections of α-phase and 200/110 reflection of β-crystal In XRD pattern of the films were annealed at 90°C, 200/100 reflection of β-crystal has become stronger, while the 100 peak of α has not changed considerably. It means with increasing temperature to 90°C, the growth of β-crystals is more favorable than α-crystals. Increasing the annealed temperature to the 120°C caused the substantial variation in the intensity of reflections. The decrease in the intensity of 200/100 reflection of β-crystal and increase in 020, 110 reflections of α- or γphase without any significant change in 100 reflection of α-crystal implies some of β-crystals convert to γ-crystals at 120°C. The DSC results also matched well with those of X-ray diffractions and indicated the existence of the different types of crystal structures. The new appeared endotherm in the film which was annealed at 177° C can be attributed to the

Actually, in our work, it can be imagined PVDF chains which are trapped between graphen sheets and are in close contact with PMMA chains, crystallize in the confined spaces that are provided by graphen sheets and PMMA chains. It is demonstrated that exfoliated graphenebased materials are often compliant, and when dispersed in a polymer matrix are typically not observed as rigid disks, but rather as bent or crumpled platelets (J. R. Potts, 2011). Consequently, as can be seen in TEM image of 70:30 sample (Fig. 7), the wrinkled surface of

Fig. 7. TEM image and Wrinkled surface of graphene sheets in 70:30 polymer blend

nanocomposite (S. Mohamadi, 2011)

melting of the new formed ribbon-like structures on the surface.

the graphene sheets can act as micropattern molds for crystallization of PVDF. In other words, a well-defined feature of micropattern can be provided by the wrinkles on the surface of graphene sheets.

Crystallization results basically in the succession of two events: the primary nucleation of a new phase and then the three-dimensional growth of lamellae; these steps can be followed by lamellar thickening, fold surface smoothing, or reorganization into more perfect crystals. As reported by others, with increasing the annealing temperature in PVDF/PMMA blend film, the amount of α phase increases due to easier local internal chain rotation at higher temperature and most of β and γ form convert to α form.

While in our study with increasing the annealing temperature the amount of α phase decreased and γ form increased. It can be said graphene sheets can stabilize the β- and γphase at elevated temperature due to restricting effect of graphene sheets on TT conformation chains as is illustrated in scheme:

Scheme 7. Schematic illustration of to restricting effect of graphene sheets on TT (A) and TTTG+TTTG- (B) conformation chains

Preparation and Characterization of PVDF/PMMA/Graphene

2634.

1938, ISSN 0024-9297

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 229

Elashmawi, I. E., Hakeem, N.A., (2008), Effect of PMMA Addition on Characterization and

Falcao, E. H. L., Blair, R. G., Mack, J. J., Viculis, L. M., Kwon, C., Bendikov, M., Kaner, R. B.,

Garton, A., (1992) *Infrared Spectroscopy of Polymer Blends, Composites and Surfaces,* Hanser

Giannetti. E., (2001), Semi-crystalline fluorinated polymers, *Polymer International*, Vol: 50,

Grafting of PMMA Chains Through in-situ Polymerization. Macromolecular Science Part A:

Guerra, G., Karasz, F. E. , Macknight. W. J., (1986), On blends of poly(vinylidene fluoride)

He, L., Xu, Q., Hua, C., Song. R., (2010) Effect of multi-walled carbon nanotubes on

Hilczer, B., & Kulek. J., (1998) The Effect of Dielectric Heterogeneity on Pyroelectric

Hontoria-Lucas, C., Lopez-Peinado, A. J., DE D. Lopez-Gonzalez, J., Rojas-Cervantes, M. L.,

Inagaki, M., Kang, F., & Toyoda, M. (2004) Exfoliation of Graphite via Intercalation

J.Jungnickel, B. (1996). *Polymeric Materials Handbook,* J. C., Salamone, New York : CRC press

Kaito, A., Iwakura, Y., Hatakeyama, K., & Li, Y., (2007) Organization of Oriented Lamellar

Kim, K. J. , Cho, Y. J. , & Kim. Y. H. , (1995) Factors determining the formation of the β

Kwan, KS. (1998) The Role of Penetrant Structure in the Transport and Mechanical

and poly(vinyl fluoride), Macromolecules, Vol. 19, No.7, (March, 1986) pp. 1935-

crystallization, thermal, and mechanical properties of poly(vinylidene fluoride, Polymer Composite, Vol. 31, No. 5, ( April 2009), pp. 921-927, ISSN 0272-8397. He, X. & Yao, K., (2006), Crystallization mechanism and piezoelectric properties of solution-

derived ferroelectric poly(vinylidene fluoride) thin films, *Applied Physics letter*. Vol.

Response of PVDF, *IEEE Transactions on Dielectrics and Electrical Insulation*, Vol. 5,

& Martin-Aranda, R. M., (1995). Study of oxygen-containing groups in a series of graphite oxides: physical and chemical characterization , *Carbon*, Vol. 33, No. 11,

Compounds in: *Chemistry and Physics of Carbon A Series of Advances* , Radovic, L. R.,

Structures in a Miscible Crystalline/Crystalline Polymer Blend under Uniaxial Compression Flow near the Melting Temperature, *Macromolecules*, Vol. 40,

crystalline phase of poly(vinylidene fluoride) in poly(vinylidene fluoride) poly(methyl methacrylate) blends, *Vibrational Spectroscopy*, Vol. 9, No. 2.

Properties of a Thermoset Adhesive, Ph.D. Thesis, Faculty of the Virginia

compound, *Carbon*, Vol. 45, No. 6, pp. 1364-1369, ISSN 0008-6223.

Pure & Applied Chemistry, Vol. 48, pp. 577–582, ISSN 1060-1325.

Publisher, Munich, Germany, ISBN 1-56990-034-5

No. 1, ( February 2000), pp. 10-26, ISSN 1097-0126.

89, No. 11, pp. 112909-1 – 112902-3, ISSN 0003-6951.

pp. (1-65) Marcel Dekker, NewYork, ISBN 0-8247-4088-2.

(February, 2007), pp. 2751-2759, ISSN 0024-9297

(September 1994), pp. 147-159, ISSN 0924-2031.

Polytechnic Institute, Blacksburg, 285f.

No. 1, pp. 45–50, ISSN 1070-9878.

pp. 1585-1592 ISSN, 0008-6223.

Inc, . pp. 7115-7122.

Morphology of PVDF, *Polymer Engineering Science*, Vol. 48, pp. 895-901, ISSN 1548-

Dunn, B. S., & Wudl, F., (2007). Microwave exfoliation of a graphite intercalation

## **4. Conclusion**

In this study, modification of graphite was carried out via introduction of hydroxyl groups and then vinyl groups on the surface of graphite by oxidation and estrification reaction respectively. The modified graphene (MG) was used to prepare the PMMA-graphene as a master batch by in-situ polymerization and followed by solution blending with PVDF in different ratios. The series of prepared polymer blend nanocomposite films were annealed at three different temperatures of 50, 90 and 120°C, and characterized.

FTIR results for FG and MG confirmed presence of different oxygen containing function groups on the surface of FG and vinyl organic moieties on the MG sheets.

ATR-FTIR spectra revealed the specific interaction between PVDF and PMMA chains in polymer blend nanocomposites. The surface morphology of 70:30 film which was annealed at 120°C, some new elongated ribbon like crystals formed. From ATR-FTIR data of this sample the identity of these new structures on the film surface can be related to the formation of γ crystals.

## **5. Acknowledgment**

We are thankful to the Research Council of the University of Tehran. We gratefully acknowledge the kind assistance from Ms. Fotouhi from the thermal analysis laboratory of the University College of Science at University of Tehran for DSC analyses. The authors would also like to thank Dr. Saeed Sepehriseresht from Tehran Heart Center for TEM analyses and helpful recommendations.

## **6. References**


In this study, modification of graphite was carried out via introduction of hydroxyl groups and then vinyl groups on the surface of graphite by oxidation and estrification reaction respectively. The modified graphene (MG) was used to prepare the PMMA-graphene as a master batch by in-situ polymerization and followed by solution blending with PVDF in different ratios. The series of prepared polymer blend nanocomposite films were annealed at

FTIR results for FG and MG confirmed presence of different oxygen containing function

ATR-FTIR spectra revealed the specific interaction between PVDF and PMMA chains in polymer blend nanocomposites. The surface morphology of 70:30 film which was annealed at 120°C, some new elongated ribbon like crystals formed. From ATR-FTIR data of this sample the identity of these new structures on the film surface can be related to the

We are thankful to the Research Council of the University of Tehran. We gratefully acknowledge the kind assistance from Ms. Fotouhi from the thermal analysis laboratory of the University College of Science at University of Tehran for DSC analyses. The authors would also like to thank Dr. Saeed Sepehriseresht from Tehran Heart Center for TEM

Al-Mashat, L., Shin, K., Kalantar-zadeh, K., D. Plessis, J., H. Han, S., W. Kojima, R., B. Kaner,

Benz, M., Euler, W. B., & Gerory, O. J., (2002) The Role of Solution Phase Water on the

Betz, N., Le Moel, A., Balanzat, E., Ramillon, J.M., Lamotte, J., & Gallas, J. P., (1994) FTIR

Bharti, V., Kaura, & T., Nath, R., (1997) Ferroelectric Hysteresis in Simultaneously Stretched

Coleman, M. M., Painter. P.C., (1995) Hydrogen bonded polymer blends, *Progress in Polymer* 

*Insulation,* Vol 4, No. 6, (December 1997) pp. 738–741, ISSN 1070-9878. Chi Chen, G., Su, J., Fina, L. J., (1994) FTIR-ATR Studies of Drawing and Poling in Polymer

*Polymer Physics,* Vol. 32, pp.1493–1502, ISSN 1099-0518.

*Science*, Vol. 20, No. 1, pp.1- 59, ISSN 0079-6700.

R., Li, D., Gou, X., l J. Ippolito,S. & Wlodarski, W., (2010) Graphene/Polyaniline Nanocomposite for Hydrogen Sensing, Journal of Physical chemistry C, Vol. 114,

Deposition of Thin Films of Poly (vinylidene fluoride), *Mocromolecules*, Vol. 35, No.

Study of PVDF Irradiated by Means of Swift Heavy Ions, *Polymer Science Part B:* 

and Corona-Poled PVDF Films*, IEEE Transactions on Dielectrics and Electrical* 

Bilaminate Films, J. *Polymer Science: Part B, Polymer Physics*, Vol.32, pp.2065-2075,

three different temperatures of 50, 90 and 120°C, and characterized.

groups on the surface of FG and vinyl organic moieties on the MG sheets.

**4. Conclusion** 

formation of γ crystals.

**5. Acknowledgment** 

**6. References** 

analyses and helpful recommendations.

pp. 16168–16173, 1932-7447.

ISSN 1099-0518

7, pp. 2682-2688, ISSN 0024-9297.


Preparation and Characterization of PVDF/PMMA/Graphene

1985), PP. 1454-1458. ISSN 0024-9297.

1099-0488.

2187, ISSN 1099-0518.

282-286, ISSN 0028-0836.

ISSN 0008-6223.

521, ISSN 0947-7047.

8226, ISSN 0024-9297.

ISSN 0261-8028.

York.

2002) pp. 1496-1497, ISSN 0002-7863.

Polymer Blend Nanocomposites by Using ATR-FTIR Technique 231

Ramanathan, T., Stankovich, S., Dikin, D. A., Liu, H., Shen, H., Nguyen, S. T., & Brinson. L.

Ramasundaram, S., Yoon, S., Kim, K. J. , Park. C., (2008). Preferential Formation of

Scheinbeim, J. I., Newman, B. A., Sen. A., (1986), Field-induced crystallization in highly

Stankovich, S., Dikin, D. A., Dommett, G. H. B., Kohlhaas, K. M., Zimney, E. J., Stach, E. A.,

Stankovich, S., Dikin, D. A., Piner, R. D., Kohlhaas, K. A., Kleinhammes, A., Jia, Y., Wu, Y.,

Steurer, P., Wissert, R., Thomann, R., & Mülhaupt. R. (2009) Functionalized Graphenes and

Steurer. P., Wissert, R., Thomann, R., Mülhaupt. R., (2009), Functionalized Graphenes and

*Macromolecular Rapid Communication*. Vol. 30, pp. 316–327, ISNN 1521-3927. Ulaganathan, M., & Rajendran, S. ( 2010) Preparation and characterizations of PVAc/P(VdF-

Vo, L. T., & Giannelis. E. P., (2007) Compatibilizing Poly(vinylidene fluoride)/Nylon-6

Wang, J., Li, H., Liu, J., Duan, Y., Jiang, S., Yan. S., (2003), On the α → β Transition of

Yang, D., & Chen. Y., (1987), β-phase formation of poly(vinylidene fluoride) from the melt

Yee, W. A., Kotaki, M., Liu, Y., Lu. X., (2007) Stress-induced structural changes in

rotating disk, *Polymer*, Vol. 49, No. 19, pp. 4196-4203, ISSN 0032-3861. Yen, J. H., Amin-Sanayei, R., (2006). Polyvinylidene Fluoride, in: *Encyclopedia of Chemical* 

C., (2007), Graphitic nanofillers in PMMA nanocomposites—An investigation of particle size and dispersion and their influence on nanocomposite properties, Polymer Science : Part B: Polymer Physics, Vol. 45, No. 15, pp. 2097-2112, ISSN

Electroactive Crystalline Phases in Poly(vinylidene fluoride)/Organically Modified Silicate Nanocomposites, Polymer science: Part B: Polymer Physics, Vol. 46, 2173–

plasticized poly(vinylidene fluoride) films, *Macromolecules*, Vol. 19 (November,

& Piner, R. D. (2006) Graphene-based composite materials, *Nature*, Vol, 442, pp.

Nguyen, S. T., & Ruoff, R. S. (2007) Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide , *Carbon*, Vol. 45, pp. 1558-1565,

Thermoplastic Nanocomposites Based upon Expanded Graphite Oxide, *Macromolecular Rapid Communication*, Vol. 30, No. 4-5. pp. 316-327, ISSN 1521-3927.

Thermoplastic Nanocomposites Based upon Expanded Graphite Oxide,

HFP)-basedpolymer blend electrolytes, Ionics, Vol. 16, (December 2009) pp. 515-

Blends with Nanoclay. J. Macromolecules, Vol. 41, No. 23( August , 2007) pp. 8271-

Carbon-Coated Highly Oriented PVDF Ultrathin Film Induced by Melt Recrystallization, *Journal of American chemical society*, Vol. 125, No. 6, (November,

induced by quenching, *Journal of material science letter*, Vol. 6, No. 5, pp. 599-603,

electrospun polyvinylidene difluoride nanofibers collected using a modified

*Processing,Vol.1,* Lee. S, pp. (2379-2381), Tailor & Francis, ISBN 9780824755003, New


Lagashetty, A., & Venkataraman, A., (2005) Polymer Nanocomposites, *RESONANCE*, Vol.

Lanceros-Me ndez, S., Mano, J. F., Costa. A. M. (2001) FTIR AND DSC STUDIES OF

Lbonard, C., Halary, J. L., & Monnerie, L., (1988), Crystallization of poly(vinylidene

Matsushige, K., Nagata, K., Imada, S., & Takemura. T., (1980) The II-I crystal transformation

Mattsson, B., Ericson, H., Torell, L. M., Sundholm. F., (1999) Micro-Raman Investigations of

Miller, R. L., Raisoni,. J. J., (1976) Single crystals of poly(vinylidene fluoride), Polymer

Mohamadi, S., Sharifi-Sanjani, N. & Mahdavi. H., (2011) Functionalization of Graphene

Mohamadi. S., & Sharifi-Sanjani, N., (2011) Investigation of the crystalline structure of PVDF

Nalwa. N. S., (1995) *Ferroelectric Polymers*, Marcel Dekker, NewYork , INC, ISBN 0-8247-

Ogoshi, T., & Chujo, Y., (2005) Synthesis of poly(vinylidene fluoride) (PVdF)/silica hybrids

Park, Y. J., Kang , Y. S., Park. C. (2005) Micropatterning of semicrystalline poly(vinylidene

Potts, J. R., Dreyer, D. R., Bielawski, C. W., Ruoff. R. S., (2011), Graphene-based polymer

Pramoda, K. P., Linh, N.T.T. , Tang , P.S. , Tjiu, W.C. , Goh, S. H., He. C.B. , (2010) Thermo-

Pramoda, K., Mohamed, A., Phang, I. Y., & Liu. T., (2005) Crystal transformation and

MECHANICALLY DEFORMED B-PVDF FILMS*, Macromolecular Science: Physics*,

fluoride)-poly(methyl methacrylate) blends: analysis of the molecular parameters controlling the nature of poly(vinylidene fluoride) crystalline phase, *Macromolecules*, Vol: 21, No. 10, (October 1998) pp. 2988-2994, ISSN 0024-9297. Lovinger, A., (1982) Annealing of poly(vinylidene fluoride) and formation of a fifth phase,

of poly(vinylidene fluoride) under tensile and compressional stresses, *Polymer*, Vol.

PVDF-Based Proton-Conducting Membranes. *Polymer Science Part A: Polymer* 

science polymer physics, Vol. 14, No. 12 (March 2003), pp. 2325-2326, ISSN 1099-

in PVDF/PMMA/Graphene polymer blend nanocomposites, Polymer Composites,

having interpenetrating polymer network structure by using crystallization between PVdF chains, Polymer Science: Part A, Polymer Chemistry, Vol. 43, No. 16,

fluoride) (PVDF) solutions, European Polymer Journal, Vol. 41, pp. 1002–1012,

nanocomposites, Polymer, Vol. 53, No. 1, (November 2010), pp. 5-25, ISSN 0032-

mechanical properties of poly(vinylidene fluoride) modified graphite/poly(methyl methacrylate) nano composites, *Composite Science and Technlogy,* Vol. 70, No. 4,(

thermomechanical properties of poly(vinylidene fluoride)/clay nanocomposites, *Polymer International*, Vol. 54, No. 1, (April 2004)pp. 226-232, ISSN 1097-0126.

10, Num. 7, (July 2005) pp. 49-57.

Vol. B40, No. 3&4, pp. 517-527, ISSN 0022-2348.

*Macromolecules,* Vol. 15, No. 1, pp. 40-44, ISSN 0024-9297.

21, No. 12, (October 1979), PP. 1391-1397, ISSN 0032-3861.

*Chemistry,* Vol. 37, pp.3317-3327, ISSN 1099-0518.

Vol. 32, pp. 1451–1460, ISSN 0272-8397.

December 2009) pp. 578-583, ISSN 0266-3538.

pp. 3543-3550, ISSN 1099-0518.

0518.

9468-0.

3861.

ISSN 0014-3057.

Sheets via Chemically


**11** 

*Iran* 

**Reflectance IR Spectroscopy** 

Infrared spectroscopy is study of the interaction of radiation with molecular vibrations which can be used for a wide range of sample types either in bulk or in microscopic amounts over a wide range of temperatures and physical states. As was discussed in the previous chapters, an infrared spectrum is commonly obtained by passing infrared radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy (the energy at which any peak in an absorption spectrum appears

Aside from the conventional IR spectroscopy of measuring light transmitted from the sample, the reflection IR spectroscopy was developed using combination of IR spectroscopy with reflection theories. In the reflection spectroscopy techniques, the absorption properties

Reflectance techniques may be used for samples that are difficult to analyze by the conventional transmittance method. In all, reflectance techniques can be divided into two categories: *internal reflection* and *external reflection*. In internal reflection method, interaction of the electromagnetic radiation on the interface between the sample and a medium with a higher refraction index is studied, while external reflectance techniques arise from the radiation reflected from the sample surface. External reflection covers two different types of reflection: *specular* (regular) reflection and *diffuse* reflection. The former usually associated with reflection from smooth, polished surfaces like mirror, and the latter associated with the

 **Internal reflection Specular reflection Diffuse reflection** 

corresponds to the frequency of a vibration of a part of a sample molecule).

of a sample can be extracted from the reflected light.

Fig. 1. Illustration of different reflection phenomenon.

reflection from rough surfaces.

**1. Introduction** 

Zahra Monsef Khoshhesab

*Payame Noor University Department of Chemistry* 


## **Reflectance IR Spectroscopy**

## Zahra Monsef Khoshhesab

*Payame Noor University Department of Chemistry Iran* 

#### **1. Introduction**

232 Infrared Spectroscopy – Materials Science, Engineering and Technology

Yu, L., & Cebe. P., (2009) Effect of nanoclay on relaxation of poly(vinylidene fluoride)

Yu, S., Zheng, W., Yu, W., Zhang, Y., Jiang, Q., & Zhao. Z., (2009) Formation Mechanism of

24, (August 2009) pp. 2520–2532, ISSN 1099-0518.

*Macromolecules,* Vol. 42, No. 22, pp. 8870, ISSN 0024-9297.

nanocomposites, Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, No.

β-Phase in PVDF/CNT Composite Prepared by the Sonication Method,

Infrared spectroscopy is study of the interaction of radiation with molecular vibrations which can be used for a wide range of sample types either in bulk or in microscopic amounts over a wide range of temperatures and physical states. As was discussed in the previous chapters, an infrared spectrum is commonly obtained by passing infrared radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy (the energy at which any peak in an absorption spectrum appears corresponds to the frequency of a vibration of a part of a sample molecule).

Aside from the conventional IR spectroscopy of measuring light transmitted from the sample, the reflection IR spectroscopy was developed using combination of IR spectroscopy with reflection theories. In the reflection spectroscopy techniques, the absorption properties of a sample can be extracted from the reflected light.

Reflectance techniques may be used for samples that are difficult to analyze by the conventional transmittance method. In all, reflectance techniques can be divided into two categories: *internal reflection* and *external reflection*. In internal reflection method, interaction of the electromagnetic radiation on the interface between the sample and a medium with a higher refraction index is studied, while external reflectance techniques arise from the radiation reflected from the sample surface. External reflection covers two different types of reflection: *specular* (regular) reflection and *diffuse* reflection. The former usually associated with reflection from smooth, polished surfaces like mirror, and the latter associated with the reflection from rough surfaces.

Fig. 1. Illustration of different reflection phenomenon.

Reflectance IR Spectroscopy 235

 Iev = I0 exp (-Z/P) (1) Where z is the distance normal to the optical interface, dp is the penetration depth (path

The depth of penetration, dp, is defined as the distance from the IRE- sample boundary where the intensity of the evanescent wave decays to 1/e (37%) of its original value), is

Where λ is the wavelength of the radiation, n1 is the refractive index of the IRE (ATR crystal), and n2 refractive index of the sample, and θ is the angle of incidence. Figure 3 illustrates the evanescent wave formed at the internal reflection element- sample interface.

An ATR spectrum can be obtained by measuring the interaction of the evanescent wave with the sample. If an absorbing material is placed in contact with ATR crystal, the evanescent wave will be absorbed by the sample and its intensity is reduced (attenuated) in regions of the IR spectrum where the sample absorbs, thus, less intensity can be reflected (attenuated total reflection). The resultant attenuated radiation as a function of wavelength produces an ATR spectrum which is similar to the conventional absorption spectrum except for the band intensities at longer wavelengths (Figure 4). This difference is due to the dependency of the penetration depth (dp) on wavelength: at longer wavelength, the evanescent wave penetrates deeper into the sample, thus, the absorption bands at longer wavelengths are relatively more intense than those shorter wavelengths. This results in greater absorption on the longer wavelength side of an absorption band, contributing to

Additionally, compared to the transmission spectrum, small differences may be seen in an ATR spectrum which arises from dispersion effects (variation of refractive index of a material with change of wavelength). An anomalous dispersion causes the refractive index and the penetration depth changes through an absorption band. For instance the effect of dispersion on penetration depth for cocaine is demonstrated in Figure 5. As can be seen, penetration depth changes strongly at wavelength in which the dispersion is the highest.

dp = λ /{ 2π n1 [sin2 θ − (n2/n1)2]1/2 } (2)

length), and I0 is the intensity at z = 0.

Fig. 3. Penetration of evanescent wave into the sample.

band distortion and band broadening.

given by Eq. (2):

## **2. Reflectance methods**

Upon interaction of electromagnetic radiation with a sample surface, depending on the characteristic of the surface and its environment, the light may undergo three types of reflection, *internal reflection, specular reflection* and *diffuse reflection*. In practice, all three types of reflections can occur at the same time, although with different contributions. Specular reflection is defined as light reflected from a smooth surface such as a mirror (any irregularities in the surface are small compared to λ) at a definite angle whereas diffuse reflection produced by rough surfaces that tend to reflect light in all directions.

### **2.1 Internal reflectance spectroscopy (IRS)**

Internal reflectance Spectroscopy (IRS) date back to the initial work of Jacques fahrenfort and N.J.Harrrick [1, 2] that independently devised the theories of IRS spectroscopy and suggested a wide range of applications. Internal reflection Spectroscopy is often termed as attenuated total reflection (ATR) spectroscopy. ATR became a popular spectroscopic technique in the early 1960s.

Attenuated total reflection spectroscopy utilizes total internal reflection phenomenon[3]. An internal reflection occurs when a beam of radiation enters from a more dense medium (with a higher refractive index, n1) into a less-dense medium (with a lower refractive index, n2), the fraction of the incident beam reflected increases as the angle of incidence rises. When the angle of incidence is greater than the critical angle θc (where is a function of refractive index of two media), all incident radiations are completely reflected at the interface, results in *total internal reflection* (Figure 2). In ATR spectroscopy a crystal with a high refractive index and excellent IR transmitting properties is used as internal reflection element (IRE, ATR crystal) and is placed in close contact with the sample (Figure 3). The beam of radiation propagating in IRE undergoes total internal reflection at the interface IRE- sample, provided the angle of incidence at the interface exceeds the critical angle θc. Total internal reflection of the light at the interface between two media of different refractive index creates an "*evanescent wave*" that penetrates into the medium of lower refractive index [3].

Fig. 2. Illustration of total internal reflection.

The evanescent field is a non-transverse wave along the optical surface, whose intensity decreases with increasing distance into the medium, normal to its surface, therefore, the field exists only the vicinity of the surface. The exponential decay evanescent wave can be expressed by Eq. (1):

Upon interaction of electromagnetic radiation with a sample surface, depending on the characteristic of the surface and its environment, the light may undergo three types of reflection, *internal reflection, specular reflection* and *diffuse reflection*. In practice, all three types of reflections can occur at the same time, although with different contributions. Specular reflection is defined as light reflected from a smooth surface such as a mirror (any irregularities in the surface are small compared to λ) at a definite angle whereas diffuse

Internal reflectance Spectroscopy (IRS) date back to the initial work of Jacques fahrenfort and N.J.Harrrick [1, 2] that independently devised the theories of IRS spectroscopy and suggested a wide range of applications. Internal reflection Spectroscopy is often termed as attenuated total reflection (ATR) spectroscopy. ATR became a popular spectroscopic

Attenuated total reflection spectroscopy utilizes total internal reflection phenomenon[3]. An internal reflection occurs when a beam of radiation enters from a more dense medium (with a higher refractive index, n1) into a less-dense medium (with a lower refractive index, n2), the fraction of the incident beam reflected increases as the angle of incidence rises. When the angle of incidence is greater than the critical angle θc (where is a function of refractive index of two media), all incident radiations are completely reflected at the interface, results in *total internal reflection* (Figure 2). In ATR spectroscopy a crystal with a high refractive index and excellent IR transmitting properties is used as internal reflection element (IRE, ATR crystal) and is placed in close contact with the sample (Figure 3). The beam of radiation propagating in IRE undergoes total internal reflection at the interface IRE- sample, provided the angle of incidence at the interface exceeds the critical angle θc. Total internal reflection of the light at the interface between two media of different refractive index creates an "*evanescent wave*"

The evanescent field is a non-transverse wave along the optical surface, whose intensity decreases with increasing distance into the medium, normal to its surface, therefore, the field exists only the vicinity of the surface. The exponential decay evanescent wave can be

reflection produced by rough surfaces that tend to reflect light in all directions.

**2. Reflectance methods** 

technique in the early 1960s.

**2.1 Internal reflectance spectroscopy (IRS)** 

that penetrates into the medium of lower refractive index [3].

Fig. 2. Illustration of total internal reflection.

expressed by Eq. (1):

$$\mathbf{I}\_{\rm ev} = \mathbf{I}\_0 \exp\left(\mathbf{-}\mathbf{Z}/\mathbf{P}\right) \tag{1}$$

Where z is the distance normal to the optical interface, dp is the penetration depth (path length), and I0 is the intensity at z = 0.

The depth of penetration, dp, is defined as the distance from the IRE- sample boundary where the intensity of the evanescent wave decays to 1/e (37%) of its original value), is given by Eq. (2):

$$\mathbf{dp} = \lambda \left/ \left\{ 2\mathbf{n} \text{ n} \left[ \sin^2 \theta - (\mathbf{n}\_2/\mathbf{n}\_1)^2 \right]^{1/2} \right\} \right. \tag{2}$$

Where λ is the wavelength of the radiation, n1 is the refractive index of the IRE (ATR crystal), and n2 refractive index of the sample, and θ is the angle of incidence. Figure 3 illustrates the evanescent wave formed at the internal reflection element- sample interface.

Fig. 3. Penetration of evanescent wave into the sample.

An ATR spectrum can be obtained by measuring the interaction of the evanescent wave with the sample. If an absorbing material is placed in contact with ATR crystal, the evanescent wave will be absorbed by the sample and its intensity is reduced (attenuated) in regions of the IR spectrum where the sample absorbs, thus, less intensity can be reflected (attenuated total reflection). The resultant attenuated radiation as a function of wavelength produces an ATR spectrum which is similar to the conventional absorption spectrum except for the band intensities at longer wavelengths (Figure 4). This difference is due to the dependency of the penetration depth (dp) on wavelength: at longer wavelength, the evanescent wave penetrates deeper into the sample, thus, the absorption bands at longer wavelengths are relatively more intense than those shorter wavelengths. This results in greater absorption on the longer wavelength side of an absorption band, contributing to band distortion and band broadening.

Additionally, compared to the transmission spectrum, small differences may be seen in an ATR spectrum which arises from dispersion effects (variation of refractive index of a material with change of wavelength). An anomalous dispersion causes the refractive index and the penetration depth changes through an absorption band. For instance the effect of dispersion on penetration depth for cocaine is demonstrated in Figure 5. As can be seen, penetration depth changes strongly at wavelength in which the dispersion is the highest.

Reflectance IR Spectroscopy 237

Two configurations of ATR accessory are available. In a single bounce–ATR, a single internal reflection occurs using a prism whereas Multi-Bounce ATR, undergoes multiple internal reflections (up to 25) using special prisms, as shown in Figure 6. In a multiple internal reflection cell, the effective path length of the sampling surface is the product of the number of reflections at measurement surface and penetration depth. In practice, multiple internal reflections (in Multi-Bounce ATR technique) produce more intensive spectra by multiple reflections and, hence it is useful for weak absorbers, while the single bounce–ATR

(a)

(b) Fig. 6. Schematic representation of total internal reflection with (a) Single reflection, and (b)

There are different designs of ATR cell including traditional ATR, horizontal ATR and cylindrical ATR for solids and liquids samples. In the traditional ATR, a thin sample is clamped against the vertical face of the crystal. This design has been replaced by more modern designs, horizontal and cylindrical designs. In horizontal ATR (HATR), the crystal is a parallel-side plate (typically 5 cm by 1 cm) with the upper surface exposed (Figure 6 (b)). The number of reflections at each surface of the crystal depends on length and thickness

The traditional design is used for continuous surface such as sheets and horizontal ATR (HATR) cells are suitable for liquids and pastes as well as soft powder and sheets films.

The ATR crystals are made from materials that have a very high refractive index and low solubility in water. Table 1 summarizes some of the commonly materials used in ATR crystals. Amongst these materials, diamond, zinc selenide (ZnSe) and germanium (Ge) are

crystal as well as the angle of incidence (usually between five and ten).

Application of cylindrical ATR (Figure 7) cell is limited to mobile fluids.

is suitable for strong absorbers.

Multiple reflections.

**2.1.1 ATR crystals** 

Fig. 4. Representation of ATR and Transmittance IR spectrum of Cocaine [4].

Fig. 5. Penetration depth and refractive index for cocaine at carbonyl absorbance band [4].

Sometimes an empirical so-called ATR correction is applied to compensate across the spectrum for linear wavelength increase, which is termed as Eq.(3):

$$
\mathbb{R}\_{\text{corr}} \sim \mathbb{R} / \lambda \tag{3}
$$

Other differences may occur due to the surface effects between the sample and internal reflection element (IRE crystal). For instance, the degree of physical contact between IRE and the sample influences the sensitivity of an ATR spectrum. Since the evanescent wave only propagates 2-15 μm beyond the surface of the crystal, thus, an intimate contact of the IRE with the sample is essential.

Fig. 4. Representation of ATR and Transmittance IR spectrum of Cocaine [4].

Fig. 5. Penetration depth and refractive index for cocaine at carbonyl absorbance band [4].

spectrum for linear wavelength increase, which is termed as Eq.(3):

with the sample is essential.

Sometimes an empirical so-called ATR correction is applied to compensate across the

 Rcorr ~ R /λ (3) Other differences may occur due to the surface effects between the sample and internal reflection element (IRE crystal). For instance, the degree of physical contact between IRE and the sample influences the sensitivity of an ATR spectrum. Since the evanescent wave only propagates 2-15 μm beyond the surface of the crystal, thus, an intimate contact of the IRE Two configurations of ATR accessory are available. In a single bounce–ATR, a single internal reflection occurs using a prism whereas Multi-Bounce ATR, undergoes multiple internal reflections (up to 25) using special prisms, as shown in Figure 6. In a multiple internal reflection cell, the effective path length of the sampling surface is the product of the number of reflections at measurement surface and penetration depth. In practice, multiple internal reflections (in Multi-Bounce ATR technique) produce more intensive spectra by multiple reflections and, hence it is useful for weak absorbers, while the single bounce–ATR is suitable for strong absorbers.

Fig. 6. Schematic representation of total internal reflection with (a) Single reflection, and (b) Multiple reflections.

There are different designs of ATR cell including traditional ATR, horizontal ATR and cylindrical ATR for solids and liquids samples. In the traditional ATR, a thin sample is clamped against the vertical face of the crystal. This design has been replaced by more modern designs, horizontal and cylindrical designs. In horizontal ATR (HATR), the crystal is a parallel-side plate (typically 5 cm by 1 cm) with the upper surface exposed (Figure 6 (b)). The number of reflections at each surface of the crystal depends on length and thickness crystal as well as the angle of incidence (usually between five and ten).

The traditional design is used for continuous surface such as sheets and horizontal ATR (HATR) cells are suitable for liquids and pastes as well as soft powder and sheets films. Application of cylindrical ATR (Figure 7) cell is limited to mobile fluids.

#### **2.1.1 ATR crystals**

The ATR crystals are made from materials that have a very high refractive index and low solubility in water. Table 1 summarizes some of the commonly materials used in ATR crystals. Amongst these materials, diamond, zinc selenide (ZnSe) and germanium (Ge) are

Reflectance IR Spectroscopy 239

clamp. Powders and soft pliable films can be used without any additional preparation and highly crystalline solids should be ground before application. It is an ideal method for liquids and oils, because the contact between the crystal and a liquid is inherently close and hence

As a conclusion, ATR is a non-destructive technique for a variety of materials including soft solid materials, liquids, powders, gels, pastes, surface layers, polymer films, samples solutions after evaporation of the solvent. It is an ideal technique for thick and dark colored materials which often absorb too much energy to be measured by IR transmission. Despite of these advantages, lack of a good contact between the sample and IRE can lead to nonaccurate results. Also, there are a few IRE crystals to be compatible with the samples

The incident radiation focused onto the sample may be directly reflected by the sample surface, giving rise to specular reflection, and it may also undergo multiple reflections at the sample, resulting in diffuse reflection. In external reflectance techniques, the radiation

Specular reflectance techniques basically involve a mirror-like reflection from the sample surface that occurs when the reflection angle equals the angle of incident radiation. It is used for samples that are reflective (smooth surface) or attached to a reflective backing. Thus, specular techniques provide a reflectance measurement for reflective materials, and a *reflection–absorption* (*transflectance*) measurement for the surface films deposited on, or

The reflectance spectra differ from those recorded in transmission, they appear as "derivative-like" bands. These spectra can be converted into absorption one by using of Kramers-Kronig transformation (K-K transformation) that is available in most spectrometer software package. Figure 10 depicts specular reflectance spectrum of oil on surface of

In absorption-reflection measurement, one fraction of the radiation is reflected on the upper interface and contributes towards the spectrum via specular reflection. Another part of the radiation penetrates the surface film and is reflected by the reflective surface, thus, the light

without requiring the high pressure clamp, liquids are applied directly onto the crystal.

properties, especially from pH point of view.

reflected from a surface is evaluated (Figure 8).

**2.2 Specular reflectance spectroscopy** 

Fig. 8. Illustration of external reflection.

pressed against reflective surfaces (Figure 9).

machined steel cylinder.

Fig. 7. Schematic representation cylindrical ATR cell.


Table 1. Materials used as ATR crystals

the most common materials used ATR crystals. Diamond can be used for a wide range of samples, because it is resistant to scratching and abrasion and can tolerate a wide pH range as well as strong oxidants and reductants. Also, due to its ideal properties, it often used as a protective film for ATR crystals such as zinc sulfide (ZnS). However, high cost of diamond causes that its application is limited. Ge with a higher refractive index is suitable for anlayzing of samples with high refractive index. However, ZnSe preferred for all routine applications, but because of its low resistivity towards surface etching, reactivity with complexing agents such as ammonia, and damaging with strong acids and alkalis, its application is limited. AMTIR with similar refractive is a good alternative for ZnSe crystal, when involve strong acids.

#### **2.1.2 Application of ATR spectroscopy**

ATR technique is used commonly in the near –infrared for obtaining absorption spectra of thin films and opaque materials. However, ATR spectra can be obtained using dispersive IR instruments, but the higher–quality spectra are obtained using FTIR spectrometers.

ATR is one of the most versatile sampling techniques that requires little or no sample preparation for most samples. It only requires that the sample is placed in intimate contact with the IRE crystal, which achieved by pressing the solid onto the crystal with a high pressure

Material Wave range (cm-1) Refractive index (at 1000 cm-1)

the most common materials used ATR crystals. Diamond can be used for a wide range of samples, because it is resistant to scratching and abrasion and can tolerate a wide pH range as well as strong oxidants and reductants. Also, due to its ideal properties, it often used as a protective film for ATR crystals such as zinc sulfide (ZnS). However, high cost of diamond causes that its application is limited. Ge with a higher refractive index is suitable for anlayzing of samples with high refractive index. However, ZnSe preferred for all routine applications, but because of its low resistivity towards surface etching, reactivity with complexing agents such as ammonia, and damaging with strong acids and alkalis, its application is limited. AMTIR with similar refractive is a good alternative for ZnSe crystal,

ATR technique is used commonly in the near –infrared for obtaining absorption spectra of thin films and opaque materials. However, ATR spectra can be obtained using dispersive IR

ATR is one of the most versatile sampling techniques that requires little or no sample preparation for most samples. It only requires that the sample is placed in intimate contact with the IRE crystal, which achieved by pressing the solid onto the crystal with a high pressure

instruments, but the higher–quality spectra are obtained using FTIR spectrometers.

2.2 2.41 2.4 4.0 2.65 1.74 2.15 2.37 2.5

45000-2500; 1800 -< 200

17,000 - 950 20,000- 650

5,5 00 -870 10,000-450 25,000-1800 25,000-1800 20,000-350 11,000-750

Fig. 7. Schematic representation cylindrical ATR cell.

Zinc Sulfide (ZnS) Zinc Selenide (ZnSe)

Germanium (Ge) Cd telluride (CdTe) Saphire (Al2O3) Cubic Zirconia (ZrO2) KRS-5 (TlI2/TlBr2) AMTIR (As/Ge/Se glass)

Table 1. Materials used as ATR crystals

when involve strong acids.

**2.1.2 Application of ATR spectroscopy** 

Diamond

clamp. Powders and soft pliable films can be used without any additional preparation and highly crystalline solids should be ground before application. It is an ideal method for liquids and oils, because the contact between the crystal and a liquid is inherently close and hence without requiring the high pressure clamp, liquids are applied directly onto the crystal.

As a conclusion, ATR is a non-destructive technique for a variety of materials including soft solid materials, liquids, powders, gels, pastes, surface layers, polymer films, samples solutions after evaporation of the solvent. It is an ideal technique for thick and dark colored materials which often absorb too much energy to be measured by IR transmission. Despite of these advantages, lack of a good contact between the sample and IRE can lead to nonaccurate results. Also, there are a few IRE crystals to be compatible with the samples properties, especially from pH point of view.

## **2.2 Specular reflectance spectroscopy**

The incident radiation focused onto the sample may be directly reflected by the sample surface, giving rise to specular reflection, and it may also undergo multiple reflections at the sample, resulting in diffuse reflection. In external reflectance techniques, the radiation reflected from a surface is evaluated (Figure 8).

Fig. 8. Illustration of external reflection.

Specular reflectance techniques basically involve a mirror-like reflection from the sample surface that occurs when the reflection angle equals the angle of incident radiation. It is used for samples that are reflective (smooth surface) or attached to a reflective backing. Thus, specular techniques provide a reflectance measurement for reflective materials, and a *reflection–absorption* (*transflectance*) measurement for the surface films deposited on, or pressed against reflective surfaces (Figure 9).

The reflectance spectra differ from those recorded in transmission, they appear as "derivative-like" bands. These spectra can be converted into absorption one by using of Kramers-Kronig transformation (K-K transformation) that is available in most spectrometer software package. Figure 10 depicts specular reflectance spectrum of oil on surface of machined steel cylinder.

In absorption-reflection measurement, one fraction of the radiation is reflected on the upper interface and contributes towards the spectrum via specular reflection. Another part of the radiation penetrates the surface film and is reflected by the reflective surface, thus, the light

Reflectance IR Spectroscopy 241

known "*Kubelka–Munk* "reflection,because they developed a theory on the radiation

Light incident onto a solid sample may be partly reflected regularly (specular reflection) by the sample surface, partly scattered diffusely, and partly penetrates into the sample. The latter part may be absorbed within the particles or be diffracted at grain boundaries, giving rise to diffusely scattered light in all directions. Diffuse reflectance spectroscopy associated with the reflected lights which are produced by diffuse scattering (Figure 11). Since regular reflection distorts the DRS spectra, thus, the regular reflection component should be eliminated in diffuse reflectance measurement. The DRIFTS accessory is designed to

In diffuse reflectance spectroscopy, there is no linear relation between the reflected light intensity (band intensity) and concentration, in contrast to traditional transmission spectroscopy in which the band intensity is directly proportional to concentration. Therefore, quantitative analyzes by DRIFTS are rather complicated. The empirical Kubelka - Munk equation relates the intensity of the reflected radiation to the concentration that can

 ƒ(R) =(1 − R)2/2R = c/k= K/s (4) where ƒ(R) is Kubelka–Munk function, R is the absolute reflectance of layer ( the ratio of the sample diffuse reflectance spectrum and a non-absorbing reference (normally KBr or KCl)), both measured at indefinite depth. K is the absorption coefficient, s is the diffusion (scattering) coefficient which is proportional to the fraction of diffused light, k is molar absorption coefficient (proportional to the fractional of transmitted light) and c is sample concentration. The reflectance, R, indicates the sample is thick enough that no radiation reaches the back surface (indefinite thickness). Appling Kubelka–Munk function on the reflectance spectrum produces a spectrum (corrected spectrum) that resembling the transmission spectrum (Figure 12). Also the corrected spectrum demonstrates a linear

In the case of R < 0.01, the simpler function log 1/R is often used for measuring the diffuse reflectance. Since such small R value is usually found in the near-IR region, therefore, an alternative relationship between concentration and reflected intensity in near–IR is defined

be used for quantitative evaluation. The Kubelka-Munk equation is defined as:

relationship between band intensity and the sample concentration.

as:

transport in scattering media [5].

eliminate the specularly reflected radiation.

Fig. 11. Representation of diffuse reflectance.

Fig. 9. Illustration of specular reflectance.

Fig. 10. Specular reflectance spectrum of oil on surface of machined steel cylinder.

passes through the surface layer twice-to and from the reflective surface, leading to increase the intensity of the reflectance spectrum as compared to the normal transmission. The effective path length depends on the angle of incidence, therefore, for thin films, a grazing angle of incidence as high as 80◦- 85◦ from normal incidence should be used, and for thick films an angle close to normal incidence is applied.

The most common applications of this technique are evaluation of surfaces such as: coating, thin films, contaminated metal surface.

#### **2.3 Diffuse Reflectance Spectroscopy (DRS)**

In diffuse reflectance spectroscopy, the electromagnetic radiation reflected by roughened surfaces is collected and analyzed. When this technique is applied in (FT) IR region, it is termed as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is also

**Specular reflection Reflection-Absorption** 

(Transflectance)

Fig. 10. Specular reflectance spectrum of oil on surface of machined steel cylinder.

passes through the surface layer twice-to and from the reflective surface, leading to increase the intensity of the reflectance spectrum as compared to the normal transmission. The effective path length depends on the angle of incidence, therefore, for thin films, a grazing angle of incidence as high as 80◦- 85◦ from normal incidence should be used, and for thick

The most common applications of this technique are evaluation of surfaces such as: coating,

In diffuse reflectance spectroscopy, the electromagnetic radiation reflected by roughened surfaces is collected and analyzed. When this technique is applied in (FT) IR region, it is termed as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is also

Fig. 9. Illustration of specular reflectance.

films an angle close to normal incidence is applied.

**2.3 Diffuse Reflectance Spectroscopy (DRS)** 

thin films, contaminated metal surface.

known "*Kubelka–Munk* "reflection,because they developed a theory on the radiation transport in scattering media [5].

Light incident onto a solid sample may be partly reflected regularly (specular reflection) by the sample surface, partly scattered diffusely, and partly penetrates into the sample. The latter part may be absorbed within the particles or be diffracted at grain boundaries, giving rise to diffusely scattered light in all directions. Diffuse reflectance spectroscopy associated with the reflected lights which are produced by diffuse scattering (Figure 11). Since regular reflection distorts the DRS spectra, thus, the regular reflection component should be eliminated in diffuse reflectance measurement. The DRIFTS accessory is designed to eliminate the specularly reflected radiation.

Fig. 11. Representation of diffuse reflectance.

In diffuse reflectance spectroscopy, there is no linear relation between the reflected light intensity (band intensity) and concentration, in contrast to traditional transmission spectroscopy in which the band intensity is directly proportional to concentration. Therefore, quantitative analyzes by DRIFTS are rather complicated. The empirical Kubelka - Munk equation relates the intensity of the reflected radiation to the concentration that can be used for quantitative evaluation. The Kubelka-Munk equation is defined as:

$$f(\mathbf{R\_w}) = (1 - \mathbf{R\_w})2/2\mathbf{R\_w} = \mathbf{c/k} = \mathbf{K/s} \tag{4}$$

where ƒ(R) is Kubelka–Munk function, R is the absolute reflectance of layer ( the ratio of the sample diffuse reflectance spectrum and a non-absorbing reference (normally KBr or KCl)), both measured at indefinite depth. K is the absorption coefficient, s is the diffusion (scattering) coefficient which is proportional to the fraction of diffused light, k is molar absorption coefficient (proportional to the fractional of transmitted light) and c is sample concentration. The reflectance, R, indicates the sample is thick enough that no radiation reaches the back surface (indefinite thickness). Appling Kubelka–Munk function on the reflectance spectrum produces a spectrum (corrected spectrum) that resembling the transmission spectrum (Figure 12). Also the corrected spectrum demonstrates a linear relationship between band intensity and the sample concentration.

In the case of R < 0.01, the simpler function log 1/R is often used for measuring the diffuse reflectance. Since such small R value is usually found in the near-IR region, therefore, an alternative relationship between concentration and reflected intensity in near–IR is defined as:

Reflectance IR Spectroscopy 243

mid-IR, diffuse reflectance is very weak in this region, as a consequent, in mid-IR, diffuse

In diffuse reflectance spectroscopy, diffusely scattered light can be directly, collected from material in a sampling cup or, alternatively, collected by using an abrasive sampling pad. In mid-IR, the diffusely reflected light from sample is generally collected by large ellipsoidal mirrors, which cover as much area above the sample as possible. In near-IR, diffuse reflectance spectra are usually measured by an integrating sphere, described by Ulbrich, (Figure 14). The inner surface of "Ulbrich sphere" is coated by strongly scattering, nonabsorbing powder. After repeated reflection, all radiations reach the detector. Thus, with

In mid-IR region, the inner surface of diffuse scattering sphere is treated with gold vapor to guarantee a high degree of reflection, while in near- IR region, Ulbrich sphere consists of spectralon (a thermostatic resin) which is applied as white standard due to its high degree of

DRIFTS is a versatile technique for analyzing nontransparent samples, powders, roughened surfaces and coating. It offers the advantages of easy sample preparation and applicability to analyze samples at elevated temperature and pressure [6]. Consideration spectrum obtained for an organic material by transmission IR using KBr pellet and DRIFTS reveals the capacity of DRIFTS compared to transmission (Figure 12,13). Both spectra show similar spectrum that corresponds to absorption bands of the sample, but in the spectrum recorded in transmission mode an intensive band appeared at 3500 cm-1 which is related to the water present in the KBr pellet. This example, demonstrates DRIFTS combines the advantages of

reflectance could only be measured by FT-IR spectrometer (DRIFTS).

Ulbrich sphere the entire radiation reflected by the sample is integrated.

easy sample preparation with the absence of water band in the spectrum.

Fig. 13. Transmission spectrum of 1,2-bis (diphenyl phosphino) ethane, KBr pellet [6].

reflection.

Fig. 12. DRIFT spectrum of 1,2-bis (diphenyl phosphino) ethane, pure powder [6].

$$\log\left(1/\mathbb{R}\_{\circ}\right) = \mathbf{k'c} \tag{5}$$

where k´ is a constant.

In Kubelka-Munk equation it is assumed that s is independent of wavelength and the sample is weakly absorbing. The former condition is achieved by proper sample preparation and the latter by dilution of strong absorbing samples with non-absorbing substrate powder (such as KBr or KCl). Therefore, to obtain reproducible results, particle size, sample packing and dilution should be carefully controlled, especially for quantitative analysis.

#### **2.3.1 Application of diffuse reflectance spectroscopy**

Diffuse reflectance technique is used for powders and solid samples having rough surface such as paper, cloth. In diffuse reflectance technique, particles size, homogeneity, and packing density of powdered samples play important role on the quality of spectrum. A sample with smaller particle size having narrow size distribution is preferred. Thus, in order to obtain a qualified spectrum, the sample should be ground into smaller size.

In this method,the sample can be analyzed either directly in bulk form or as dispersions in IR transparent matrices such as KBr and KCl. Sometimes, a thin film of KBr powder placed on the sample surface to improve the quality of the spectrum. Dilution of analyte in a nonabsorbing matrix increases the proportion of diffuse reflectance in the reflected light. Typically, the solid sample is diluted homogeneously to 5 to 10% by weight in KBr. The spectra of diluted samples are similar to those obtained from pellets when plotted in units such as log 1/R (R is the reflectance) or Kubelka- Munk units.

Diffuse reflectance measurement in near-IR is more common than in mid-IR. Because nonabsorbing scattering substrates are rare in mid-IR, and also more efficient scattering occurs at shorter wavelengths (near-IR). Additionally, due to lower efficiency of the scattering in

Fig. 12. DRIFT spectrum of 1,2-bis (diphenyl phosphino) ethane, pure powder [6].

and dilution should be carefully controlled, especially for quantitative analysis.

to obtain a qualified spectrum, the sample should be ground into smaller size.

**2.3.1 Application of diffuse reflectance spectroscopy** 

such as log 1/R (R is the reflectance) or Kubelka- Munk units.

In Kubelka-Munk equation it is assumed that s is independent of wavelength and the sample is weakly absorbing. The former condition is achieved by proper sample preparation and the latter by dilution of strong absorbing samples with non-absorbing substrate powder (such as KBr or KCl). Therefore, to obtain reproducible results, particle size, sample packing

Diffuse reflectance technique is used for powders and solid samples having rough surface such as paper, cloth. In diffuse reflectance technique, particles size, homogeneity, and packing density of powdered samples play important role on the quality of spectrum. A sample with smaller particle size having narrow size distribution is preferred. Thus, in order

In this method,the sample can be analyzed either directly in bulk form or as dispersions in IR transparent matrices such as KBr and KCl. Sometimes, a thin film of KBr powder placed on the sample surface to improve the quality of the spectrum. Dilution of analyte in a nonabsorbing matrix increases the proportion of diffuse reflectance in the reflected light. Typically, the solid sample is diluted homogeneously to 5 to 10% by weight in KBr. The spectra of diluted samples are similar to those obtained from pellets when plotted in units

Diffuse reflectance measurement in near-IR is more common than in mid-IR. Because nonabsorbing scattering substrates are rare in mid-IR, and also more efficient scattering occurs at shorter wavelengths (near-IR). Additionally, due to lower efficiency of the scattering in

where k´ is a constant.

log (1/R) = k´c (5)

mid-IR, diffuse reflectance is very weak in this region, as a consequent, in mid-IR, diffuse reflectance could only be measured by FT-IR spectrometer (DRIFTS).

In diffuse reflectance spectroscopy, diffusely scattered light can be directly, collected from material in a sampling cup or, alternatively, collected by using an abrasive sampling pad. In mid-IR, the diffusely reflected light from sample is generally collected by large ellipsoidal mirrors, which cover as much area above the sample as possible. In near-IR, diffuse reflectance spectra are usually measured by an integrating sphere, described by Ulbrich, (Figure 14). The inner surface of "Ulbrich sphere" is coated by strongly scattering, nonabsorbing powder. After repeated reflection, all radiations reach the detector. Thus, with Ulbrich sphere the entire radiation reflected by the sample is integrated.

In mid-IR region, the inner surface of diffuse scattering sphere is treated with gold vapor to guarantee a high degree of reflection, while in near- IR region, Ulbrich sphere consists of spectralon (a thermostatic resin) which is applied as white standard due to its high degree of reflection.

DRIFTS is a versatile technique for analyzing nontransparent samples, powders, roughened surfaces and coating. It offers the advantages of easy sample preparation and applicability to analyze samples at elevated temperature and pressure [6]. Consideration spectrum obtained for an organic material by transmission IR using KBr pellet and DRIFTS reveals the capacity of DRIFTS compared to transmission (Figure 12,13). Both spectra show similar spectrum that corresponds to absorption bands of the sample, but in the spectrum recorded in transmission mode an intensive band appeared at 3500 cm-1 which is related to the water present in the KBr pellet. This example, demonstrates DRIFTS combines the advantages of easy sample preparation with the absence of water band in the spectrum.

Fig. 13. Transmission spectrum of 1,2-bis (diphenyl phosphino) ethane, KBr pellet [6].

**12** 

*México* 

**Evaluation of Graft Copolymerization** 

**Polymers by Means Infrared Spectroscopy** 

José Luis Rivera-Armenta1, Cynthia Graciela Flores-Hernández1, Ruth Zurisadai Del Angel-Aldana1, Ana María Mendoza-Martínez1,

Infrared spectroscopy (IR spectroscopy) is a technique based on the vibrations of atoms of a molecule. An IR spectrum is commonly obtained by passing IR radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy. The energy at which any peak in an absorption spectrum appears corresponds to the

For a molecule to show IR absorptions, it must possess specific feature: electric dipole moment of molecule must change during the movement that means changes in molecular dipoles which are associated with vibrations and rotations. The atoms in molecules can move relative to one another. This is a description of stretching and bending movements that are collectively referred to as vibrations. Vibrations can involve either change in bond length (stretching) or bond angle (bending). Some bonds can stretch in-phase (symmetrical

The IR is divided into three regions; the near-, mid- and far- IR. The mid-IR is the most common region to identification and study of organic compounds based on fundamental

IR spectroscopy is a popular method for characterizing polymers. This spectroscopy may used to identify the composition of polymers, to monitor polymerization processes, to characterize polymer structure, to examine polymer surface, and to investigate polymer degradation processes. There are several reports of use of IR spectroscopy to evaluate grafting of acrylic monomers onto natural materials as carboxymethyl cellulose (CMC) and chicken feathers (CF) (Martínez et al, 2003, 2005, Vasile et al, 2004 , Zohuriann-Mehr et al,

frequency of a vibration of a part of a sample molecule (Bickford, 2008).

stretching) or out-of-plane (asymmetric stretching) (Bickford, 2008).

vibrations and associated rotational-vibrational structure.

**1. Introduction** 

2005, Joshi and Sinha, 2006).

Carlos Velasco-Santos2 and Ana Laura Martínez-Hernández2

**of Acrylic Monomers Onto Natural** 

*1Instituto Tecnológico de Ciudad Madero,* 

*2Instituto Tecnológico de Querétaro, Departamento de Metal-Mecánica* 

*División de Estudios de Posgrado e Investigación* 

Fig. 14. Integrating Sphere.

The most disadvantage of this technique is difficulty of quantitative analyses. Since the diffusion coefficient strongly depends on sample preparation, thus the reflectance intensity is influenced by the sample preparation. A same sample may produce different spectra in different experiments.

### **3. References**


## **Evaluation of Graft Copolymerization of Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy**

José Luis Rivera-Armenta1, Cynthia Graciela Flores-Hernández1, Ruth Zurisadai Del Angel-Aldana1, Ana María Mendoza-Martínez1, Carlos Velasco-Santos2 and Ana Laura Martínez-Hernández2

> *1Instituto Tecnológico de Ciudad Madero, División de Estudios de Posgrado e Investigación 2Instituto Tecnológico de Querétaro, Departamento de Metal-Mecánica México*

#### **1. Introduction**

244 Infrared Spectroscopy – Materials Science, Engineering and Technology

The most disadvantage of this technique is difficulty of quantitative analyses. Since the diffusion coefficient strongly depends on sample preparation, thus the reflectance intensity is influenced by the sample preparation. A same sample may produce different spectra in

[1] J. Fahrentfort, Attenuated total reflection- A new principles for the production of useful

[2] NJ. Harrick, Internal Reflection Spectroscopy. New York, John Wiley & Sons, Int.,1967. [3] FM. Maribeli, "Principles, Theory, and Practice of Internal Reflection Spectroscopy,"

[4] CV. Koulis, JA. Reffner, B.A. John, AM. Bibby, Comparison of transmission and internal reflection infrared spectra of cocaine. J. Forensic Science,46(4),(2001) 822.

[6] T. Armaroli, T.becue and S. Gautier, Oil, Gas Science and technology – Rev.IFP, 59 (2004)

infrared reflection spectra of organic compounds. Molecular Spectroscopy (Proceeding IV Int. Meeting, Bologna, 1959), 2, Mangini A, editor, Londan:

Internal Reflection Spectroscopy-Theory and Applications, ed. FM. Mirabella, Jr.,

Fig. 14. Integrating Sphere.

different experiments.

2, 215.

Pergamon, 1962, 437.

Marcel Dekker, Inc. New York 1993.

[5] P.Kubleka and F. Munk, Z.Phsysik, 12( 1931) 593.

**3. References** 

Infrared spectroscopy (IR spectroscopy) is a technique based on the vibrations of atoms of a molecule. An IR spectrum is commonly obtained by passing IR radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy. The energy at which any peak in an absorption spectrum appears corresponds to the frequency of a vibration of a part of a sample molecule (Bickford, 2008).

For a molecule to show IR absorptions, it must possess specific feature: electric dipole moment of molecule must change during the movement that means changes in molecular dipoles which are associated with vibrations and rotations. The atoms in molecules can move relative to one another. This is a description of stretching and bending movements that are collectively referred to as vibrations. Vibrations can involve either change in bond length (stretching) or bond angle (bending). Some bonds can stretch in-phase (symmetrical stretching) or out-of-plane (asymmetric stretching) (Bickford, 2008).

The IR is divided into three regions; the near-, mid- and far- IR. The mid-IR is the most common region to identification and study of organic compounds based on fundamental vibrations and associated rotational-vibrational structure.

IR spectroscopy is a popular method for characterizing polymers. This spectroscopy may used to identify the composition of polymers, to monitor polymerization processes, to characterize polymer structure, to examine polymer surface, and to investigate polymer degradation processes. There are several reports of use of IR spectroscopy to evaluate grafting of acrylic monomers onto natural materials as carboxymethyl cellulose (CMC) and chicken feathers (CF) (Martínez et al, 2003, 2005, Vasile et al, 2004 , Zohuriann-Mehr et al, 2005, Joshi and Sinha, 2006).

Evaluation of Graft Copolymerization of

Fig. 1. Carboxymethyl cellulose structure.

fibers (Joshi and Sinha, 2006) (Martínez et al., 2003, 2005).

reactions.

redox initiation.

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 247

Carboxymethyl cellulose (CMC) is a very important cellulose derivative and it is known by its superabsorbent properties. It is anionic polymer water soluble. It is produced by reaction of alkali cellulose and monochloroacetic sodium salt under strict reaction conditions (Klemm et al, 1998). Byproducts as sodium chloride and sodium glicolate are obtained in reaction whose are removed obtaining CMC sodium salt highly purified. Higher swelling capacity of CMC can be reached and controlled by addition of crosslinking agent, thermal treatment or ionic state conversion. The conversion of some hydroxyl groups from cellulose in hydrophobic substituents diminishes hydrogen bonding decreasing crystallinity and increasing water solubility. Multiple applications of Na-CMC in several areas of food industry made it and almost indispensable material. Other applications are: laundry detergent for instance, mainly as a viscosity modifier or thickener, as a soil suspension to deposit onto cotton and other cellulosic fabrics (Klemm et al, 1998). Other use of CMC is as a lubricant in non-volatile eye drops. These properties on depend of preparation method, degree of polymerization and degree of substitution (DS), that means how much of hydroxylic groups are displaced for carboxymethyl groups in anhydroglucose (AGU) structure. The more useful DS value is from 0.7 to 1.5. Due these CMC is an interesting material and properties can be modify by means graft

Modification of natural protein through grafting have become in a widely used path and some works have carry out studies with acrylic monomers as ethylacrylate, acrylic acid, methacrylic acid and methyl methacrylate (Mostafa, 1995; Athawale and Rathi, 1997; Jia and Yong, 2006; Zampano, et al 2009), which carried out graft of natural fibers and proteins. However, just few of works report grafting of hydroxyethyl methacrylate onto naturals

Graft copolymerization of acrylic monomers onto natural polymers is one of the most useful paths to modify physicochemical properties on order to add new properties to final copolymer with minimal. Acrylic monomers are the most grafted monomers in this kind of research works; being acrylonitrile, acrylic acid, methyl methacrylate more studied species. Redox initiation is an efficient method frequently used to obtain graft copolymers. Polymers with OH groups can react with ceric ion or an oxidant agent to form polymer radical capable to initiate copolymerization. In a redox initiation frequently there is a minimum degree of homopolymerization, due only polymer radicals can be formed. However, limitation of this method is only useful with polymers with functional groups; there are reports of polymers with amides, urethane and nitrile groups by means

Now a days have been an increasing interest in obtation and use of polymeric materials using naturals sources from a diversity of systems, getting atention such biopolymers in research areas as medicine, electronics, textil, corrosion and nanotechnology among others. This rising interest is due variety of properties that offers on depending of chemical structure and source (Martínez et al, 2008).

Use of natural fibers is an research area that allows obtain materials for cotidiane applications, using more resistence materials and with outstanding properties and specially materials that are environmental friendly. Keratin is a natural protein which can be found in wool, hair, claws, horns or nails, and is the main component in birds' feathers, represents from 5% to 7% of the body weight of chickens. Keratin is durable and resistant to organic solvents and chemically unreactive, which give benefits when is exposure to environmental conditions, thinking in industrial applications.

Chicken feathers (CF) are more abundant material of keratin in nature. Birds' feathers characterize by a complex branched structure formed by keratinic filaments that grow in a unique mechanism in cylindric feather follicles. This branched structure is a distinctive characteristic in feathers morphology and its origin is biologic evolution (Xu et al., 2001, 2003).

CF are considered residues of a byproduct of poultry, corresponding to more than 5 million tons around the world (Barone et al 2005). Aminoacids content of CF on depend of breed, feeding and environment of study animals. The aminoacids present in CF are mainly aspartic acid, glutamic acid, arginine, proline, glycine, phenylalanine, alanine, cystine, isoleucine, among others (Schmidt 1998) (see table 1).


Table 1. Aminoacid content in keratin fiber from chicken feather (Martínez et al 2005).

Now a days have been an increasing interest in obtation and use of polymeric materials using naturals sources from a diversity of systems, getting atention such biopolymers in research areas as medicine, electronics, textil, corrosion and nanotechnology among others. This rising interest is due variety of properties that offers on depending of chemical

Use of natural fibers is an research area that allows obtain materials for cotidiane applications, using more resistence materials and with outstanding properties and specially materials that are environmental friendly. Keratin is a natural protein which can be found in wool, hair, claws, horns or nails, and is the main component in birds' feathers, represents from 5% to 7% of the body weight of chickens. Keratin is durable and resistant to organic solvents and chemically unreactive, which give benefits when is exposure to environmental

Chicken feathers (CF) are more abundant material of keratin in nature. Birds' feathers characterize by a complex branched structure formed by keratinic filaments that grow in a unique mechanism in cylindric feather follicles. This branched structure is a distinctive characteristic in feathers morphology and its origin is biologic evolution (Xu et al., 2001,

CF are considered residues of a byproduct of poultry, corresponding to more than 5 million tons around the world (Barone et al 2005). Aminoacids content of CF on depend of breed, feeding and environment of study animals. The aminoacids present in CF are mainly aspartic acid, glutamic acid, arginine, proline, glycine, phenylalanine, alanine, cystine,

Functional group Aminoacid Content (as % mole)

Glutamic acid 7

Glycine 11

Alanine 4

Valine 9

Leucine 6 Tyrosine 1

Serine 16

Negatively charged Aspartic acid 5

Positively charged Arginine 5 Conformationally special Proline 12

Hydrophobic Phenylalanine 4

Cystine 7

Isoleucine 5

Hydrophilic Threonine 4

Table 1. Aminoacid content in keratin fiber from chicken feather (Martínez et al 2005).

structure and source (Martínez et al, 2008).

conditions, thinking in industrial applications.

isoleucine, among others (Schmidt 1998) (see table 1).

2003).

Carboxymethyl cellulose (CMC) is a very important cellulose derivative and it is known by its superabsorbent properties. It is anionic polymer water soluble. It is produced by reaction of alkali cellulose and monochloroacetic sodium salt under strict reaction conditions (Klemm et al, 1998). Byproducts as sodium chloride and sodium glicolate are obtained in reaction whose are removed obtaining CMC sodium salt highly purified. Higher swelling capacity of CMC can be reached and controlled by addition of crosslinking agent, thermal treatment or ionic state conversion. The conversion of some hydroxyl groups from cellulose in hydrophobic substituents diminishes hydrogen bonding decreasing crystallinity and increasing water solubility. Multiple applications of Na-CMC in several areas of food industry made it and almost indispensable material. Other applications are: laundry detergent for instance, mainly as a viscosity modifier or thickener, as a soil suspension to deposit onto cotton and other cellulosic fabrics (Klemm et al, 1998). Other use of CMC is as a lubricant in non-volatile eye drops. These properties on depend of preparation method, degree of polymerization and degree of substitution (DS), that means how much of hydroxylic groups are displaced for carboxymethyl groups in anhydroglucose (AGU) structure. The more useful DS value is from 0.7 to 1.5. Due these CMC is an interesting material and properties can be modify by means graft reactions.

Fig. 1. Carboxymethyl cellulose structure.

Modification of natural protein through grafting have become in a widely used path and some works have carry out studies with acrylic monomers as ethylacrylate, acrylic acid, methacrylic acid and methyl methacrylate (Mostafa, 1995; Athawale and Rathi, 1997; Jia and Yong, 2006; Zampano, et al 2009), which carried out graft of natural fibers and proteins. However, just few of works report grafting of hydroxyethyl methacrylate onto naturals fibers (Joshi and Sinha, 2006) (Martínez et al., 2003, 2005).

Graft copolymerization of acrylic monomers onto natural polymers is one of the most useful paths to modify physicochemical properties on order to add new properties to final copolymer with minimal. Acrylic monomers are the most grafted monomers in this kind of research works; being acrylonitrile, acrylic acid, methyl methacrylate more studied species. Redox initiation is an efficient method frequently used to obtain graft copolymers. Polymers with OH groups can react with ceric ion or an oxidant agent to form polymer radical capable to initiate copolymerization. In a redox initiation frequently there is a minimum degree of homopolymerization, due only polymer radicals can be formed. However, limitation of this method is only useful with polymers with functional groups; there are reports of polymers with amides, urethane and nitrile groups by means redox initiation.

Evaluation of Graft Copolymerization of

X1= CFF without modification

**2.3 CMC graft copolymers 2.3.1 Initiator preparation** 

**2.3.2 Grafting reaction** 

X3= container + hydrolyzed sample

X4= residual polymer after hydrolysis (X3- X2)

This solution was stored under refrigeration in an amber bottle.

determinated by weight differences according with next formulae:

**2.4 IR Spectroscopy characterization of graft copolymers** 

Where:

Where:

W0= CMC weight

W1= graft copolymer weight

ranging from 4000 to 600 cm-1.

**3. Results and discussion** 

different grafting yield of HEMA onto CMC.

X2 = container

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 249

It was necessary preparation of Ce (IV) initiator solution. The required amount of cerium ammonium nitrate salt (0.1, 0.25 and 0.5 M), was dissolved in 100 mL of 1M HNO3 solution.

For graft reaction 5 g of CMC were placed in the reactor with distilled water and HEMA was added in selected amount (3 levels: 0.2, 0.3 or 0.4 M), under constant stirring at 70 º C, the initiator was added and reaction was carried out for 3 hours. Once the reaction ends, it was necessary to neutralize the mixture with a NaOH 10% w/w solution, copolymer was precipitated with acetone, and then the material was milled and washed with 90% methanol solution, then dried. After, the material was subjected to a Soxhlet extraction with methanol, in order to extracting the material that has not been in the copolymer. Grafting yield was

IR analysis was carried out to evaluate structural changes of CMC and CFF and its grafted copolymers, by means main functional groups signals. IR spectra were recorded with a Perkin-Elmer Spectrum One Fourier Transform IR spectrophotometer, using an Attenuated Total Reflactance (ATR) accessory, with ZnSe plate, using 12 scans and resolution of 4 cm-1,

Results will be presented in sections following next order. First, the effect of HEMA concentration and time reaction over grafting yield onto CFF and HEMA and CAN concentration over grafting yield onto CMC. Next, IR spectra are presented for ungrafted and grafted CFF, as well as a comparison of IR spectra for different grafting yield of CFF. Also for CMC grafted and ungrafted are presented the IR spectra and comparison to

Grafting yield = ((W1 – W0)/W0)\* 100 (2)

Several natural polymers as chitin, cellulose, functionalized cellulose and natural fibers are some of most studied natural polymers in graft copolymerization using redox system as initiator, being cerium ion one with more reports.

Infrared spectroscopy is a useful tool to identify functional groups through vibrational frequencies in polymers to evaluate changes in structure This research was focused in graft copolymerization of Hydroxyethyl methacrylate (HEMA) onto chicken feathers fibers (CFF) and carboxymethyl cellulose (CMC), evaluating effect of reaction conditions (time reaction, monomer concentration, initiator concentrations) on grafting yield and probe presence of HEMA in copolymers by means Infrared Spectroscopy (IR).

## **2. Methodology**

## **2.1 Raw materials**

Chicken Feathers (CF) were obtained from a local slaughterhouse. A clean up procedure was carried out before use them for reaction, CF were washed several times with ethanol and dried at room temperature to have them clean white, sanitized and odor-free. Manual procedure by cutting to separate fibers from barbs and barbules was carried out. Chicken feather fiber (CFF) was used in graft copolymerization reaction.

CMC supplied by Sigma-Aldrich with 0.7 degree of substitution was used to graft reactions, without further purification process, same as all other reactives: Hydroxyethyl methacrylate (HEMA) (98%) and malic acid, potassium permanganate (KMnO4), hydrochloric acid (HCl), methanol, ethanol, sulfuric acid (H2SO4), Cerium ammonium nitrate were from Sigma-Aldrich. Distilled water was used as reaction medium and finally to wash the homopolymer residues.

## **2.2 CFF graft copolymers**

To carry out the grafting reaction procedure proposed by Martinez-Hernandez et al. (2003) was used. In this process the following reagents were used: 0.5 g CFF, distilled water, malic acid (0.005M), sulfuric acid (0.01M), KMnO4 (0.003M) and HEMA monomer in 3 different levels (0.025, 0.05 and 0.075 M).

The substances were mixed at a temperature of 60 ° C, under constant magnetic stirring. The reactions were carried out at three different times: 2, 3 and 4 h. Once the reaction time passed, proceeded to filter and wash the reaction product with hot water and methanol in order to remove residual monomer and homopolymer. Grafting yield was evaluated after a hydrolysis to reaction product, to determinate HEMA amount grafted in CFF. Hydrolysis procedure was: 1g of CFF grafted was swamped in a 6M HCl solution at 130°C for 24 hours in a soxhlet extractor. Once the time was over, hydrolysis residue was dried and weight was determinate, this weight correspond to grafted HEMA, due CFF decompose in hydrolysis process.

Grafted polymer percentage was determinate according with next formulae (Martínez-Hernández et al, 2003; Gupta and Sahoo, 2001):

$$\% \text{Gradted PHEMA} = \left( \mathbf{X}\_4 / \mathbf{X}\_1 \right) \text{\*100} \tag{1}$$

Where:

248 Infrared Spectroscopy – Materials Science, Engineering and Technology

Several natural polymers as chitin, cellulose, functionalized cellulose and natural fibers are some of most studied natural polymers in graft copolymerization using redox system as

Infrared spectroscopy is a useful tool to identify functional groups through vibrational frequencies in polymers to evaluate changes in structure This research was focused in graft copolymerization of Hydroxyethyl methacrylate (HEMA) onto chicken feathers fibers (CFF) and carboxymethyl cellulose (CMC), evaluating effect of reaction conditions (time reaction, monomer concentration, initiator concentrations) on grafting yield and probe presence of

Chicken Feathers (CF) were obtained from a local slaughterhouse. A clean up procedure was carried out before use them for reaction, CF were washed several times with ethanol and dried at room temperature to have them clean white, sanitized and odor-free. Manual procedure by cutting to separate fibers from barbs and barbules was carried out. Chicken

CMC supplied by Sigma-Aldrich with 0.7 degree of substitution was used to graft reactions, without further purification process, same as all other reactives: Hydroxyethyl methacrylate (HEMA) (98%) and malic acid, potassium permanganate (KMnO4), hydrochloric acid (HCl), methanol, ethanol, sulfuric acid (H2SO4), Cerium ammonium nitrate were from Sigma-Aldrich. Distilled water was used as reaction medium and finally to wash the homopolymer

To carry out the grafting reaction procedure proposed by Martinez-Hernandez et al. (2003) was used. In this process the following reagents were used: 0.5 g CFF, distilled water, malic acid (0.005M), sulfuric acid (0.01M), KMnO4 (0.003M) and HEMA monomer in 3 different

The substances were mixed at a temperature of 60 ° C, under constant magnetic stirring. The reactions were carried out at three different times: 2, 3 and 4 h. Once the reaction time passed, proceeded to filter and wash the reaction product with hot water and methanol in order to remove residual monomer and homopolymer. Grafting yield was evaluated after a hydrolysis to reaction product, to determinate HEMA amount grafted in CFF. Hydrolysis procedure was: 1g of CFF grafted was swamped in a 6M HCl solution at 130°C for 24 hours in a soxhlet extractor. Once the time was over, hydrolysis residue was dried and weight was determinate, this weight correspond to grafted HEMA, due CFF decompose in hydrolysis

Grafted polymer percentage was determinate according with next formulae (Martínez-

%Grafted PHEMA = (X4 /X1 ) \*100 (1)

initiator, being cerium ion one with more reports.

**2. Methodology 2.1 Raw materials** 

residues.

process.

**2.2 CFF graft copolymers** 

levels (0.025, 0.05 and 0.075 M).

Hernández et al, 2003; Gupta and Sahoo, 2001):

HEMA in copolymers by means Infrared Spectroscopy (IR).

feather fiber (CFF) was used in graft copolymerization reaction.

X1= CFF without modification X2 = container X3= container + hydrolyzed sample X4= residual polymer after hydrolysis (X3- X2)

## **2.3 CMC graft copolymers**

### **2.3.1 Initiator preparation**

It was necessary preparation of Ce (IV) initiator solution. The required amount of cerium ammonium nitrate salt (0.1, 0.25 and 0.5 M), was dissolved in 100 mL of 1M HNO3 solution. This solution was stored under refrigeration in an amber bottle.

## **2.3.2 Grafting reaction**

For graft reaction 5 g of CMC were placed in the reactor with distilled water and HEMA was added in selected amount (3 levels: 0.2, 0.3 or 0.4 M), under constant stirring at 70 º C, the initiator was added and reaction was carried out for 3 hours. Once the reaction ends, it was necessary to neutralize the mixture with a NaOH 10% w/w solution, copolymer was precipitated with acetone, and then the material was milled and washed with 90% methanol solution, then dried. After, the material was subjected to a Soxhlet extraction with methanol, in order to extracting the material that has not been in the copolymer. Grafting yield was determinated by weight differences according with next formulae:

$$\text{Graifying yield} = ((\text{W}\_1 - \text{W}\_0) / \text{W}\_0)^\* \, 100 \tag{2}$$

Where:

W0= CMC weight W1= graft copolymer weight

## **2.4 IR Spectroscopy characterization of graft copolymers**

IR analysis was carried out to evaluate structural changes of CMC and CFF and its grafted copolymers, by means main functional groups signals. IR spectra were recorded with a Perkin-Elmer Spectrum One Fourier Transform IR spectrophotometer, using an Attenuated Total Reflactance (ATR) accessory, with ZnSe plate, using 12 scans and resolution of 4 cm-1, ranging from 4000 to 600 cm-1.

## **3. Results and discussion**

Results will be presented in sections following next order. First, the effect of HEMA concentration and time reaction over grafting yield onto CFF and HEMA and CAN concentration over grafting yield onto CMC. Next, IR spectra are presented for ungrafted and grafted CFF, as well as a comparison of IR spectra for different grafting yield of CFF. Also for CMC grafted and ungrafted are presented the IR spectra and comparison to different grafting yield of HEMA onto CMC.

Evaluation of Graft Copolymerization of

**3.2 CMC-g-HEMA copolymer** 

grafting yields values of CMC.

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 251

A B

The graft copolymerization of HEMA onto CMC had differences compared with HEMA-g-CFF: Redox initiator system used was CAN and was conducted at 3 different concentrations; HEMA concentrations values were higher 0.05, 0.1 and 0.15M and reaction time was constant in 3 hours. Table 3 resumes the effect of HEMA and CAN concentrations over

CAN [M] HEMA [M] Grafting yield 0.1 0.05 69 % 0.1 0.1 207 % 0.1 0.15 249 % 0.25 0.05 88 % 0.25 0.1 214 % 0.25 0.15 275 % 0.5 0.05 82 % 0.5 0.1 179 % 0.5 0.15 397 %

Table 3. Effect of CAN and HEMA concentration on grafting yield over CMC

Graft copolymerization of HEMA onto CMC present a similar behavior as grating of HEMA onto CFF, grafting yield increased continuously with increase in concentration of HEMA when CAN concentration was constant, and reaches a maximum value with 0.15M at 3 CAN concentrations. This behavior could be explained by the fact of that an increase of HEMA

Fig. 2. SEM micrograph of CFF unmodified and CFF-g-HEMA

### **3.1 CFF-g-HEMA copolymer**

Effect of reaction conditions over graft yield were studied. Table 2 shows effect of time reaction and HEMA concentration on grafting yield of CFF, initiators concentration was constant. It can observe that an HEMA concentration increase causes an increase of grafting yield value at 3 reaction times studied. That increase is due the amount of free radicals formed in reaction system, and CFF main component is keratin which posses several pendant functional groups, for like –NH2, -COOH, -SH and –OH, which can form active sites where HEMA can be grafted. This behavior was also reported by Martínez et al (2003) that observed a maximum graft yield value. A higher HEMA concentration value was studied (1 M), but a fiber saturation was observed and also homopolymerization predominated over graft reaction which is not convenient for this study. Scanning Electron Microscopy shows the HEMA cover the CFF structure growing poly HEMA chains (figure 2) on fiber surface of CFF, but CFF reached a saturation of active sites and then homopolymerization happens.


The increase on graft yield can be also attributed that keratin forms a charge transfer complex with HEMA molecules, so it is possible increase monomer activity at higher concentrations of HEMA which leads homopolymerization.

Other reports (Joshi and Sinha, 2006) indicate an increase of monomer concentration increases graft yield, until a maximum and then decreases the obtained percentage. The difference is attributed to graft yield diminish due the saturation of available active sites in CFF. Also on depend of initiator system.

In the other hand, it can observe that when reaction time increase the graft yield has a maximum value at 3 h and then decrease at 4 h. this behavior is attributed that at higher reaction time the keratin pendant groups open and diffusion of HEMA molecules into the structure occurs, allowing the grafting on keratin structure not only on surface of CFF. However, when reaction time is high, a denaturalization of keratin can occurs, due the acidic medium where reaction is carried out, leading to HEMA homopolymerization. Other reason of this decrease in grafting yield is a reduction in number of free radicals available for grafting as the reaction proceeds, creating a saturation of active sites.

Fig. 2. SEM micrograph of CFF unmodified and CFF-g-HEMA

## **3.2 CMC-g-HEMA copolymer**

250 Infrared Spectroscopy – Materials Science, Engineering and Technology

Effect of reaction conditions over graft yield were studied. Table 2 shows effect of time reaction and HEMA concentration on grafting yield of CFF, initiators concentration was constant. It can observe that an HEMA concentration increase causes an increase of grafting yield value at 3 reaction times studied. That increase is due the amount of free radicals formed in reaction system, and CFF main component is keratin which posses several pendant functional groups, for like –NH2, -COOH, -SH and –OH, which can form active sites where HEMA can be grafted. This behavior was also reported by Martínez et al (2003) that observed a maximum graft yield value. A higher HEMA concentration value was studied (1 M), but a fiber saturation was observed and also homopolymerization predominated over graft reaction which is not convenient for this study. Scanning Electron Microscopy shows the HEMA cover the CFF structure growing poly HEMA chains (figure 2) on fiber surface of CFF, but CFF reached a saturation of active sites and then

Time/hr HEMA conc./ M Grafting yield 2 0.075 85% 2 0.05 70% 2 0.025 26% 3 0.075 92% 3 0.05 83% 3 0.025 57% 4 0.075 76% 4 0.05 38% 4 0.025 35%

Table 2. Effect of time reaction and HEMA concentration on grafting yield over CFF

concentrations of HEMA which leads homopolymerization.

for grafting as the reaction proceeds, creating a saturation of active sites.

CFF. Also on depend of initiator system.

The increase on graft yield can be also attributed that keratin forms a charge transfer complex with HEMA molecules, so it is possible increase monomer activity at higher

Other reports (Joshi and Sinha, 2006) indicate an increase of monomer concentration increases graft yield, until a maximum and then decreases the obtained percentage. The difference is attributed to graft yield diminish due the saturation of available active sites in

In the other hand, it can observe that when reaction time increase the graft yield has a maximum value at 3 h and then decrease at 4 h. this behavior is attributed that at higher reaction time the keratin pendant groups open and diffusion of HEMA molecules into the structure occurs, allowing the grafting on keratin structure not only on surface of CFF. However, when reaction time is high, a denaturalization of keratin can occurs, due the acidic medium where reaction is carried out, leading to HEMA homopolymerization. Other reason of this decrease in grafting yield is a reduction in number of free radicals available

**3.1 CFF-g-HEMA copolymer** 

homopolymerization happens.

The graft copolymerization of HEMA onto CMC had differences compared with HEMA-g-CFF: Redox initiator system used was CAN and was conducted at 3 different concentrations; HEMA concentrations values were higher 0.05, 0.1 and 0.15M and reaction time was constant in 3 hours. Table 3 resumes the effect of HEMA and CAN concentrations over grafting yields values of CMC.


Table 3. Effect of CAN and HEMA concentration on grafting yield over CMC

Graft copolymerization of HEMA onto CMC present a similar behavior as grating of HEMA onto CFF, grafting yield increased continuously with increase in concentration of HEMA when CAN concentration was constant, and reaches a maximum value with 0.15M at 3 CAN concentrations. This behavior could be explained by the fact of that an increase of HEMA

Evaluation of Graft Copolymerization of

attributed to vibration of C-S group.

Fig. 4. IR spectrum of CFF ungrafted.

96.00

%T

100.00

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 253

carboxymethyl cellulose ( Okieimen and Ogbeifun, 1996; Vasile et al , 2004), ethyl cellulose

Figure 4 shows IR spectrum of CFF, which main component is keratin, a mixture of aminoacids as serine, proline, glycine, valine, cysteine, and others. The main vibrations attributed to CFF structure were identified according to wavenumber. The region around 3300 cm-1 corresponds to range of amide bands, figure shows a peak in 3297 cm-1associated with ordered regions of NH group of amide A -helix conformation, peak in 2945 cm-1 is assigned to the assimetric vibration of CH group od metyl, the strong band at 1715 cm-1 is matched with vibration of amide I of -sheet conformation, band at 1650 cm-1 assigned to C=O group of amide I -helix conformation, peaks at 1520, 1449 and 1243 cm-1 attributed to in plane bending of NH group corresponding to -sheet conformation, bending of CH3 group and CN group of amide III respectively; vibrations on 1136, 1074 and 1023 cm-1 corresponds to asigned to vibrations of C-C group; and finally a peak around 700 cm-1

In figure 5 presents IR spectrum of PHEMA which presents signals at 3394 cm-1 attributed to vibracion of OH group, 2956 cm-1 from antisymetric vibration of CH2 and CH3, 2925 cm-1 symetric vibration of CH2, CH3, 2855 cm-1 symetric vibration of CH2, 1720 of stretching C=O, a small shoulder around 1652 from stretching C=C, 1369 cm-1 deformation of CH2, CH3, a shoulder at 1260 cm-1 from C=O stretiching, 1164 vibration of C-O-C, 1048 stretching of CO(H), and 771 CH2 coupled with skeletal stretching. This is according for reports of IR

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0

cm-1

Figure 6 shows IR spectra of CFF and CFF grafted. It can observe that main differences are IR spectrum of CFF grafted show peaks at 1720, 1160 and a shoulder around 1260 cm-1,

spectrum of PHEMA (Prachayawarakorn and Boonsawat, 2007).

(Kang et al 2006), which make it a powerfull tool in graft reactions.

concentration lead to the accumulation of monomer molecules in close proximity to CMC backbone. There are reports of modified polysacharides grafted with HEMA that found with CAN concentration of 0.2 M around 200% attributed the primary radicals attack the monomer instead of reacting with backbone polymer (Joshi and Sinha 2006). In this research higher CAN concentration studied was 0.5 M and higher grafting yield was reached with this concentration (397%). Also lower CAN concentrations (0.05 M) were tried in this research work, but not good results in grafting yield was obtained

In the other hand, about effect of CAN concentration over grafting yield, results show the initial increase in grafting yield with increase in initiator concentration levels off and decrease with further increase in CAN concentration. It would seem that termination of graft copolymerization would proceed by the reation of the growing graft polymer chain with ceric ions, the reation point is OH gruops of anhydroglucose, which form a complex with ceric ion. This complex may dissociate, and giving rise to free radical sites onto the polysacharides backbone and these radicals initiate the graft copolymerization (Zohuriaan-Mehr et al 2005).

Figure 3 shows SEM micrographs of CMC unmodified and CMC-g-HEMA. It is clearly seen that CMC morphology was totally modified by grafting of HEMA. CMC shows a porous surface which is covered by grafting of HEMA resulting in a homogeneous surface, which suggests that HEMA chains formed are long.

Fig. 3. SEM micrograph of CMC unmodified (A) and CMC-g-HEMA (B).

#### **3.3 IR of CFF-g-HEMA copolymer**

IR spectroscopy is a usefull technique to evaluate if a graft reaction is carry out and also to evaluate grafting yields on graft copolymers. Several research works used IR with this purpose for keratin (Martínez et al 2003; Martínez et al 2008; Kavitha et al 2005), starch (Meshram et al 2009), modified starch (Cao et al 2002), chitosan (Mun et al 2008), carboxymehtyl chitosan (Joshi and Sinha, 2006), cellulose (Zampano et al 2009),

concentration lead to the accumulation of monomer molecules in close proximity to CMC backbone. There are reports of modified polysacharides grafted with HEMA that found with CAN concentration of 0.2 M around 200% attributed the primary radicals attack the monomer instead of reacting with backbone polymer (Joshi and Sinha 2006). In this research higher CAN concentration studied was 0.5 M and higher grafting yield was reached with this concentration (397%). Also lower CAN concentrations (0.05 M) were tried in this

In the other hand, about effect of CAN concentration over grafting yield, results show the initial increase in grafting yield with increase in initiator concentration levels off and decrease with further increase in CAN concentration. It would seem that termination of graft copolymerization would proceed by the reation of the growing graft polymer chain with ceric ions, the reation point is OH gruops of anhydroglucose, which form a complex with ceric ion. This complex may dissociate, and giving rise to free radical sites onto the polysacharides backbone and these radicals initiate the graft copolymerization (Zohuriaan-

Figure 3 shows SEM micrographs of CMC unmodified and CMC-g-HEMA. It is clearly seen that CMC morphology was totally modified by grafting of HEMA. CMC shows a porous surface which is covered by grafting of HEMA resulting in a homogeneous surface, which

research work, but not good results in grafting yield was obtained

Fig. 3. SEM micrograph of CMC unmodified (A) and CMC-g-HEMA (B).

IR spectroscopy is a usefull technique to evaluate if a graft reaction is carry out and also to evaluate grafting yields on graft copolymers. Several research works used IR with this purpose for keratin (Martínez et al 2003; Martínez et al 2008; Kavitha et al 2005), starch (Meshram et al 2009), modified starch (Cao et al 2002), chitosan (Mun et al 2008), carboxymehtyl chitosan (Joshi and Sinha, 2006), cellulose (Zampano et al 2009),

Mehr et al 2005).

suggests that HEMA chains formed are long.

**3.3 IR of CFF-g-HEMA copolymer** 

carboxymethyl cellulose ( Okieimen and Ogbeifun, 1996; Vasile et al , 2004), ethyl cellulose (Kang et al 2006), which make it a powerfull tool in graft reactions.

Figure 4 shows IR spectrum of CFF, which main component is keratin, a mixture of aminoacids as serine, proline, glycine, valine, cysteine, and others. The main vibrations attributed to CFF structure were identified according to wavenumber. The region around 3300 cm-1 corresponds to range of amide bands, figure shows a peak in 3297 cm-1associated with ordered regions of NH group of amide A -helix conformation, peak in 2945 cm-1 is assigned to the assimetric vibration of CH group od metyl, the strong band at 1715 cm-1 is matched with vibration of amide I of -sheet conformation, band at 1650 cm-1 assigned to C=O group of amide I -helix conformation, peaks at 1520, 1449 and 1243 cm-1 attributed to in plane bending of NH group corresponding to -sheet conformation, bending of CH3 group and CN group of amide III respectively; vibrations on 1136, 1074 and 1023 cm-1 corresponds to asigned to vibrations of C-C group; and finally a peak around 700 cm-1 attributed to vibration of C-S group.

Fig. 4. IR spectrum of CFF ungrafted.

In figure 5 presents IR spectrum of PHEMA which presents signals at 3394 cm-1 attributed to vibracion of OH group, 2956 cm-1 from antisymetric vibration of CH2 and CH3, 2925 cm-1 symetric vibration of CH2, CH3, 2855 cm-1 symetric vibration of CH2, 1720 of stretching C=O, a small shoulder around 1652 from stretching C=C, 1369 cm-1 deformation of CH2, CH3, a shoulder at 1260 cm-1 from C=O stretiching, 1164 vibration of C-O-C, 1048 stretching of CO(H), and 771 CH2 coupled with skeletal stretching. This is according for reports of IR spectrum of PHEMA (Prachayawarakorn and Boonsawat, 2007).

Figure 6 shows IR spectra of CFF and CFF grafted. It can observe that main differences are IR spectrum of CFF grafted show peaks at 1720, 1160 and a shoulder around 1260 cm-1,

Evaluation of Graft Copolymerization of

peaks according with grafting yield.

**3.4 IR of CMC-g-HEMA copolymer** 

et al 2004).

Approximate

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 255

assignments, cm-1 Functional groups Approximate

PHEMA CFF (Martínez 2005)

<sup>3390</sup> <sup>3290</sup> (NH) Amide A, -

2950 a (CH3, CH2) 3080 a (NH) Amide B 2920 s (CH3, CH2) 2930 a (CH3)

<sup>2850</sup>(CH2) 1700 (C=O) Amide I -

<sup>1720</sup>(C=O) ester group 1650 (C=O) Amide I -

1650 (C=C) stretching 1525 (N-H) Amide II -

1370 (CH3, CH2) 1450 (C-H3) 1260 C=O stretching 1230 (CN) Amide III 1160 (C-O-C) 1170 (C-C) 1050 (CO-H) stretching 1075 (C-C) 770 (CH2) stretching 685 (C-S)

In figure 7 present IR spectra of CFF grafted with 26 and 92% grafting yield, to evaluate effect of grafting on main assignments. It is evident when graft yield increase assignations attributed to PHEMA are more evident in spectrum (3390 and 1720, 2850 and 1150 cm-1), and signals attributed to NH and OH groups disappear because there are sites where graft reaction is carrying out. There are several reports about use of IR spectroscopy for evaluate graft reactions on keratin from chicken feathers ( Martínez et al 2003, Martínez et al 2008 and Kavitha et al 2005). They use IR spectra for identify main functional gruops of keratin and of acrylic monomers in ungrafted and grafted copolymers, in addition to evaluate changes in

IR spectra of the CMC is presented in figure 8. We can notice the characteristic broad band attributed to OH stretching vibration at 3360 cm-1 due the CMC has a degree of substitution of 0.7, in average 2.3 OH groups of anhydroglucose ring are present in structure, peak at 2920 cm-1 due stretching of C-H, peak at 1620 cm-1 a strong absorption band that confirm the presence of carboxy group (COO-), 1420 and 1320 cm-1 are assigned to –CH2 scissoring and hydroxyl group bending vibration respectively, signal at 1060 cm-1 is due to >CH-O-CH2 stretching vibration and vibrations of the ether groups at 1060, 1110 cm-1. It is worth to remark that CMC used in this work was as sodium salt. The assignments are according with reports of IR spectroscopy studies of CMC (Bono et al 2009, Heydarzadeh et al 2009, Vasile

Table 4. IR assignments of main functional groups of ungrafted CFF and PHEMA.

assignments, cm-1 Functional groups

helix

sheet

helix

sheet

Fig. 5. IR spectrum of PHEMA

Fig. 6. IR spectra of ungrafted CFF (black) and CFF grafted (blue).

attributed to groups C=O and OH respectively, from HEMA structure, and which does not appear in CFF spectrum, which can indicate the graft reaction is carried out. CFF posses functional groups as –NH2, -SH nd -OH where graft reaction can carry out, signals from NH (3290, 3080 and 1525 cm-1) changed which make it suppose that graft reaction is taking place between NH groups and free radical of HEMA.

A comparison of main assignments from functional groups of ungrafted CFF and PHEMA are sumarized in table 4.

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0

cm-1

Fig. 5. IR spectrum of PHEMA

70.00

%T

90.0

%T

100.00

Fig. 6. IR spectra of ungrafted CFF (black) and CFF grafted (blue).

between NH groups and free radical of HEMA.

are sumarized in table 4.

attributed to groups C=O and OH respectively, from HEMA structure, and which does not appear in CFF spectrum, which can indicate the graft reaction is carried out. CFF posses functional groups as –NH2, -SH nd -OH where graft reaction can carry out, signals from NH (3290, 3080 and 1525 cm-1) changed which make it suppose that graft reaction is taking place

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0 cm-1

A comparison of main assignments from functional groups of ungrafted CFF and PHEMA


Table 4. IR assignments of main functional groups of ungrafted CFF and PHEMA.

In figure 7 present IR spectra of CFF grafted with 26 and 92% grafting yield, to evaluate effect of grafting on main assignments. It is evident when graft yield increase assignations attributed to PHEMA are more evident in spectrum (3390 and 1720, 2850 and 1150 cm-1), and signals attributed to NH and OH groups disappear because there are sites where graft reaction is carrying out. There are several reports about use of IR spectroscopy for evaluate graft reactions on keratin from chicken feathers ( Martínez et al 2003, Martínez et al 2008 and Kavitha et al 2005). They use IR spectra for identify main functional gruops of keratin and of acrylic monomers in ungrafted and grafted copolymers, in addition to evaluate changes in peaks according with grafting yield.

## **3.4 IR of CMC-g-HEMA copolymer**

IR spectra of the CMC is presented in figure 8. We can notice the characteristic broad band attributed to OH stretching vibration at 3360 cm-1 due the CMC has a degree of substitution of 0.7, in average 2.3 OH groups of anhydroglucose ring are present in structure, peak at 2920 cm-1 due stretching of C-H, peak at 1620 cm-1 a strong absorption band that confirm the presence of carboxy group (COO-), 1420 and 1320 cm-1 are assigned to –CH2 scissoring and hydroxyl group bending vibration respectively, signal at 1060 cm-1 is due to >CH-O-CH2 stretching vibration and vibrations of the ether groups at 1060, 1110 cm-1. It is worth to remark that CMC used in this work was as sodium salt. The assignments are according with reports of IR spectroscopy studies of CMC (Bono et al 2009, Heydarzadeh et al 2009, Vasile et al 2004).

Evaluation of Graft Copolymerization of

CMC.

%T

yield.

%T

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 257

Fig. 9. IR spectra of CMC ungrafted (black) and with 69% grafting yield (blue)

Effect of grafting yield on IR assignments was evaluated in figure 10. It can observe that main peaks attributed to PHEMA (2940, 1720, 1270 and 1060 cm-1) increase according with graft yield, which makes sense due the PHEMA is an higher proportion compared with

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0 cm-1

Fig. 10. IR spectra of CMC grafted with 69% (red), 249 % (black) and 397% (blue) grafting

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0 cm-1

Fig. 7. IR spectra of CFF-g-HEMA with 26 (blue) and 92% (black) graft yield

Fig. 8. IR spectrum of CMC

Figure 9 presents a comparative of IR spectra of CMC ungrafted and grafted with HEMA. Before, the main assignments for PHEMA were disscused. The CMC grafted present appearance of peak at 2930 cm-1 due antisymetric stretching CH3, 1715 cm-1 from stretching vibration of C=O and peaks at 1410, 1250, 1150 and 902 cm-1 assigned to bending vibration of CH3, CH2, stretching of C=O, stretching of CO-H and =CH2 groups respectively, all those signals are from PHEMA structure, and also peaks attributed to CMC structure as 3340, 1650 and broad peak at 1050 cm-1, which are from OH, COO- and C-O-C from CMC, which is indicative the graft reaction was carried out.

Fig. 7. IR spectra of CFF-g-HEMA with 26 (blue) and 92% (black) graft yield

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0 cm-1

Figure 9 presents a comparative of IR spectra of CMC ungrafted and grafted with HEMA. Before, the main assignments for PHEMA were disscused. The CMC grafted present appearance of peak at 2930 cm-1 due antisymetric stretching CH3, 1715 cm-1 from stretching vibration of C=O and peaks at 1410, 1250, 1150 and 902 cm-1 assigned to bending vibration of CH3, CH2, stretching of C=O, stretching of CO-H and =CH2 groups respectively, all those signals are from PHEMA structure, and also peaks attributed to CMC structure as 3340, 1650 and broad peak at 1050 cm-1, which are from OH, COO- and C-O-C from CMC, which is

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0

cm-1

Fig. 8. IR spectrum of CMC

90.00

%T

%T

100.00

indicative the graft reaction was carried out.

Fig. 9. IR spectra of CMC ungrafted (black) and with 69% grafting yield (blue)

Effect of grafting yield on IR assignments was evaluated in figure 10. It can observe that main peaks attributed to PHEMA (2940, 1720, 1270 and 1060 cm-1) increase according with graft yield, which makes sense due the PHEMA is an higher proportion compared with CMC.

Fig. 10. IR spectra of CMC grafted with 69% (red), 249 % (black) and 397% (blue) grafting yield.

Evaluation of Graft Copolymerization of

0032-3861.

Republic of Germany.

178 ISSN 0957-4352.

11, ISSN 1618-7229.

ISSN 0008-6215.

ISSN 0141-3910.

3969, ISSN 0014-3057.

(September 2001),pp 233-242, ISSN 0969-0239.

2006), pp 789-798, ISSN 1026-1265.

2006), pp 2198-2204, ISSN 0032-3861.

2008), pp 184-191, ISSN 1432-8917.

(January 2008), pp 389-395, ISSN 1381-5148.

Acrylic Monomers Onto Natural Polymers by Means Infrared Spectroscopy 259

Gupta, K.C., Sahoo, S., (2001) Graft copolymerization of 4-vinylpyridine onto cellulose using

Heydarzadeh, H.D., Najafpour, G.D., Nazari-Moghaddam, A.A., (2009), Catalyst-free

Joshi, J.M., Sinha, V.K., (2006), Graft copolymerization of 2-hydroxyethylmethacrylate onto

Kang, H., Liu, W., He, B., Shen, D., Ma, L., Huang, Y., (2006), Synthesis of amphiphilic ethyl

Klemm, D., Philipp, B., Heinze, T., Heinze, U., Wagenknecht, W., (1998). *Comprehensive* 

Kavitha, A., Boopalan, K., Radhakrishnan, G., Sankaran, S., Das, B., Sastry, T., (2005),

Martínez-Hernández, A.L., Santiago-Valtierra A. L., Alvarez-Ponce M. J., (2008), Chemical

Martínez-Hernández, A.L., Velasco-Santos, C., de Icaza, M., Castaño, V.M., (2005),

Martínez-Hernández, A.L., Velasco-Santos, C., de Icaza, M., Castaño, V.M., (2003), Grafting

Meshram, M.W., Patil, V.V., Mhaske, S.T., Thorat, B.N., (2009) Graft copolymers of starch

Mostafa, M., Graft polymerization of methacrylic acid on starch and hydrolyzed starches.

Mun, G.A., Zauresh, S., Nurkeeva, S.A., Dergunov, I.K., Nam, T.P., Maimakov, E.M.,

Vol. 42, No. 12, (December 2005), pp 1703-1713, ISSN 1060-1325.

methacrylate. *European Polymer Journal*, Vol. 43, No. 9, (September 2007) pp 3963-

Co (III) acetylacetonate complex in aqueous medium. *Cellulose*, Vol. 8, No. 3,

conversion of alkali cellulose to fine carboxymethyl cellulose at mild conditions. *World Applied Science Journal,* Vol. 6, No. 4, (2009 ), pp 564-569, ISSN 1818-4952. Jia, Z.,Yong, Y., (2006); Surface Modification of Poly Acrylic Fibers (PAC) via Grafting of

Soybean Protein Isolates (SPI). *Iranian Polymer Journal*, Vol. 15, No. 10, (October

carboxymethyl chitosan using CAN as an initiator. *Polymer,* Vol*.* 47, No. 6, (March

cellulose grafting poly (acrylic acid) copolymers and their self-assembly morphologies in water. *Polymer,* Vol. 47, No. 23, (October 2006), pp 7927-7934, ISSN

*cellulose chemistry volume 2,*Wiley-VCH, ISBN 3-527-29489-9, Weinheim, Federal

Preparation of Feather Keratin Hydrolyzate‐Gelatin Composites and Their Graft Copolymers. *Journal of Macromolecular Science Part A: Pure and Applied Chemistry,*

modification of keratin biofibres by graft polymerisation of methyl methacrylate using redox initiation. *Materials Research Innovations,* Vol. 12, No. 4, (December

Microstructural characterization of keratin fibers from chicken feathers. *International Journal Environmental and Pollution,* Vol. 23, No. 2, (MES 2005), pp 162-

of Methyl methacrylate onto natural keratin. *e-polymers,* No. 016, (April 2003), pp 1-

and its application in textiles. *Carbohydrate Polymers,* 75, 1, (January 2009), pp 71-78,

*Polymer Degradation and Stability,* (1995), Vol. 50, No. 2, (March 1995), pp 189-194,

Shaikhutdinov, Lee, S.Ch., Park, K., (2008), Studies on graft copolymerization of 2 hydroxyethyl acrylate onto chitosan. *Reactive and Functional Polymers,* Vol. 68, No. 1,

## **4. Concluding remarks**

From results presented in this research work it can be conclude that is possible graft HEMA onto CFF and CMC using different initiator system, and that grafting yield obtain on depends of kind of initiator system, being higher with CAN initiator system than KMnO4 malic acid. SEM microgrpah give evidence that the grafting reaction takes place on CFF and CMC surface until a saturation of active sites, and then homopolymerization happens.

One of the most common applications of IR spectroscopy is to the identification of organic compounds. In polymers, may be used to identify the composition, to monitor polymerization process, to characterize polymer structure. In present research, IR spectroscopy showed to be a usefull toll to evaluate if the graft reaction takes place. It was possible identify main functional groups of CFF and CMC ungrafted, and PHEMA. The IR spectra of grafted copolymers presented assignments due to ungrafted materials and PHEMA giving evidence that the graft reaction carried out. Futhermore, it was possible evaluate the changes in peaks of the grafted materials according with grafting yield, increasing signals attributed to PHEMA.

## **5. Acknowledgment**

The financial support from Dirección General de Educación Superior Tecnológica (DGEST) from Public Education Secretary (SEP) through projects 3616.10-P and 2203-09-P for development of this research work is gratefully acknowledged. The authors would like to thank to Pollos Villafranca for provide chicken feathers, and the micrography department of CFATA of Universidad Nacional Autónoma de México campus Juriquilla, especially to Alicia Del Real López for technical assistance.

## **6. References**


From results presented in this research work it can be conclude that is possible graft HEMA onto CFF and CMC using different initiator system, and that grafting yield obtain on depends of kind of initiator system, being higher with CAN initiator system than KMnO4 malic acid. SEM microgrpah give evidence that the grafting reaction takes place on CFF and CMC surface until a saturation of active sites, and then homopolymerization happens.

One of the most common applications of IR spectroscopy is to the identification of organic compounds. In polymers, may be used to identify the composition, to monitor polymerization process, to characterize polymer structure. In present research, IR spectroscopy showed to be a usefull toll to evaluate if the graft reaction takes place. It was possible identify main functional groups of CFF and CMC ungrafted, and PHEMA. The IR spectra of grafted copolymers presented assignments due to ungrafted materials and PHEMA giving evidence that the graft reaction carried out. Futhermore, it was possible evaluate the changes in peaks of the grafted materials according with grafting yield,

The financial support from Dirección General de Educación Superior Tecnológica (DGEST) from Public Education Secretary (SEP) through projects 3616.10-P and 2203-09-P for development of this research work is gratefully acknowledged. The authors would like to thank to Pollos Villafranca for provide chicken feathers, and the micrography department of CFATA of Universidad Nacional Autónoma de México campus Juriquilla, especially to

Athawale, V.D., Rathi, S.C., (1997), Effect of chain length of the alkyl group of alkyl

Barone, J.R., Schmidt, W.F., Liebner, C.F., (2005). Thermally processed keratin films. *Journal* 

Bickford, M., (2008). *Characterization and Analysis of Polymers.* John Wiley and Sons. ISBN

Bono, A., Ying, P.H., Yan, F.Y., Muei, C.L., Sarbatly, R., Krishnaiah, (2009), Synthesis and

Elizalde Peña, E., Flores Ramírez, N., Luna Barcenas, G., Vásquez García, S. R., Arámbula

978-0-470-23300-9, Hoboken, New Jersey, United States.

methacrylates on graft polymerization onto starch using ceric ammonium nitrate as initiator. *European Polymer Journal,* vol. 33, No. 7, (July 1997), pp 1067-1071, ISSN

*of Applied Polymer Science*, Vol. 97, No. 4, (august 2005), pp 1644-1651, ISSN 1097-

Characterization of Carboxymethyl Cellulose from Palm Kernel Cake. *Advances in Natural and Applied Sciences,* Vol. 3, No. 2, (January 2009), pp 5-11, ISSN 1995-0772. Cao, Y., Qing, X., Sun, J., Zhou, F., Lin, S., (2002), Graft copolymerization of acrylamide onto

carboxymethyl starch. *European Polymer Journal,* vol. 38, No. 9, (September 2002), pp

Villa, G., Garcia Gaitan, B., Rutiaga-Quiñones, J.G., González-Hernández, J. (2007). Syntesis and characterization of chitosan-g-glycidyl methacrylate with methyl

**4. Concluding remarks** 

increasing signals attributed to PHEMA.

Alicia Del Real López for technical assistance.

1921-1924, ISSN 0014-3057.

**5. Acknowledgment** 

**6. References** 

0014-3057.

4628.

methacrylate. *European Polymer Journal*, Vol. 43, No. 9, (September 2007) pp 3963- 3969, ISSN 0014-3057.


**13** 

*Spain* 

*University Carlos III of Madrid* 

**Applications of FTIR on Epoxy Resins –** 

**Process, Phase Separation and Water Uptake** 

Epoxy resins are a family of thermosetting materials widely used as adhesives, coatings and matrices in polymer composites because of the low viscosity of the formulations, good insulating properties of the final material even at high temperatures and good chemical and thermal resistance (May, 1988). Epoxy thermosets can be described as 3D polymer networks formed by the chemical reaction between monomers (*"curing"*). This 3D covalent network structure determines the properties of thermosetting polymers: unlike thermoplastics, this kind of polymers does not melt, and once the network has been formed the material cannot be reprocessed. Maybe one of the main advantages of epoxy thermosets is that the starting monomers have low viscosity so that complex geometries can be easily shaped and fixed after curing the monomers. Thus the formation of the network via chemical reaction is a key

Epoxy formulations usually include more than one component, although there are different crosslinking mechanisms involving either chemical reaction between one single type of monomer (homopolymerization) or two kinds of monomers with different functional groups. In both cases, a common constituent is always found: the epoxy monomer. The main feature of the epoxy monomer is the oxirane functional group, which is a three member ring formed between two carbon atoms and an oxygen, as shown in Figure 1. This atomic arrangement shows enhanced reactivity when compared with common ethers because of its high strain. Due to the different electronegativity of carbon and oxygen, the carbon atoms of the ring are electrophilic. Thus epoxies can undergo ring opening reactions towards nucleophiles. The polarity of the oxirane ring makes possible detection by IR

There are mainly two families of epoxies: the glycidyl epoxies and non-glycidyl epoxies (also called aliphatic or cycloaliphatic epoxy resins). The absence of aromatic rings in aliphatic epoxies makes them UV resistant and suitable for outdoor applications and also reduces viscosity. The most common epoxy monomers of each family are diglycidylether of bisphenol A (known as DGEBA) and 3,4-Epoxycyclohexyl-3'4'-epoxycyclohexane carboxylate (ECC) respectively and their structures are given in Figure 2 (a, b). Cycloaliphatic resins are usually found in the form of pure chemicals with a definite

**1. Introduction** 

spectroscopy.

aspect in this kind of materials.

María González González, Juan Carlos Cabanelas and Juan Baselga

**Identification, Monitoring the Curing** 


## **Applications of FTIR on Epoxy Resins – Identification, Monitoring the Curing Process, Phase Separation and Water Uptake**

María González González, Juan Carlos Cabanelas and Juan Baselga *University Carlos III of Madrid Spain* 

## **1. Introduction**

260 Infrared Spectroscopy – Materials Science, Engineering and Technology

Okieimen, F.E., Ogbeifun, D.E., (1996), Graft copolymerization of modified cellulose:

Prachayawarakorn, J., Boonsawat, K., (2007), Physical, chemical and dyeing properties of

Schmidt, W.F., (1998), Innovative feather utilization strategies. *Proceedings of National Poultry Waste Management conference,* pp 276-282, Springdale, Ar., October 19-22, 1998. Vasile, C., Bumbu, G.G., Dumitriu, R.P., Staikos, G., (2004), Comparative study of the

Xu, X. Zhou, Z., Prum, R.O., (2001), Branched integumental structures in Sinornithosaurus

Xu, X., Zhou, Z., Wong, X., Kuang, X. Zhang, F., Du, X., (2003), Four-winged dinosaurs from China. *Nature,* Vol. 421, No. 6921, pp 335-340, (January 2003), ISSN 0028-0836. Zampano, G., Bertoldo, M., Bronco, S., (2009) Poly(ethyl acrylate) surface-initiated ATRP

Zohuriaan-Mehr, J.M., Pourjavadi, A., Sadeghi, M., (2005), Modified CMC: Part1-Optimized

Vol. 14, No. 2, (February 2005), pp 131-138, ISSN 1026-1265.

(September 2006), pp 2960-2966, ISSN 1097-4628.

2004) pp 1209-1215, ISSN 0014-3057.

(January 2009), pp 22-31, ISSN 0008-6215.

0014-3057.

ISSN 0028-0836.

grafting of methyl acrylate, ethyl acrylate and ethyl methacrylate on carboxymethyl cellulose. *European Polymer Journal,* Vol. 32, No. 3, (March 1996), pp 311-315, ISSN

bomboyxmori silks grafted by 2-hydroxyethyl methacrylate. *Journal of Applied Polymer Science,* Vol. 106, No. 3., (November 2007), pp 1526-1534, ISSN 1097-4628. Radhakumary, C., Nair, P. D., Mathew, S., Nair, C.P., (2006), HEMA-Grafted Chitosan for

Dialysis Membrane Applications. *Journal of Applied Polymer Science,*Vol. 101, No 5,

behavior of carboxymethyl cellulose-g-poly(N-isopropylacrylamide) copolymers and their equivalent physical blends. *European polymer Journal,* Vol. 40, No. 6, (June

and the origin of feathers. *Nature,* Vol. 410, No. 6825, pp 200-204, (March 2001),

grafting from Wood pulp cellulose fibers. *Carbohydrate Polymers*, Vol. 75, No. 1,

Synthesis of Carboxymethyl cellulose-g-Polyacrylonitrile. *Iranian Polymer Journal,* 

Epoxy resins are a family of thermosetting materials widely used as adhesives, coatings and matrices in polymer composites because of the low viscosity of the formulations, good insulating properties of the final material even at high temperatures and good chemical and thermal resistance (May, 1988). Epoxy thermosets can be described as 3D polymer networks formed by the chemical reaction between monomers (*"curing"*). This 3D covalent network structure determines the properties of thermosetting polymers: unlike thermoplastics, this kind of polymers does not melt, and once the network has been formed the material cannot be reprocessed. Maybe one of the main advantages of epoxy thermosets is that the starting monomers have low viscosity so that complex geometries can be easily shaped and fixed after curing the monomers. Thus the formation of the network via chemical reaction is a key aspect in this kind of materials.

Epoxy formulations usually include more than one component, although there are different crosslinking mechanisms involving either chemical reaction between one single type of monomer (homopolymerization) or two kinds of monomers with different functional groups. In both cases, a common constituent is always found: the epoxy monomer. The main feature of the epoxy monomer is the oxirane functional group, which is a three member ring formed between two carbon atoms and an oxygen, as shown in Figure 1. This atomic arrangement shows enhanced reactivity when compared with common ethers because of its high strain. Due to the different electronegativity of carbon and oxygen, the carbon atoms of the ring are electrophilic. Thus epoxies can undergo ring opening reactions towards nucleophiles. The polarity of the oxirane ring makes possible detection by IR spectroscopy.

There are mainly two families of epoxies: the glycidyl epoxies and non-glycidyl epoxies (also called aliphatic or cycloaliphatic epoxy resins). The absence of aromatic rings in aliphatic epoxies makes them UV resistant and suitable for outdoor applications and also reduces viscosity. The most common epoxy monomers of each family are diglycidylether of bisphenol A (known as DGEBA) and 3,4-Epoxycyclohexyl-3'4'-epoxycyclohexane carboxylate (ECC) respectively and their structures are given in Figure 2 (a, b). Cycloaliphatic resins are usually found in the form of pure chemicals with a definite

Applications of FTIR on Epoxy Resins –

Neckers. 2001; Yagci & Reetz, 1998).

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 263

Wang et al., 2003) or homopolymerized via a cationic mechanism induced by UV radiation (Crivello, 1995; Crivello & Fan,1991; Crivello & Liu, 2000; Hartwig et al., 2003; Wang &

The chemical reactivity of the epoxies enables using a wide variety of molecules as curing agents depending on the process and required properties. The commonly used curing agents for epoxies include amines, polyamines, polyamides, phenolic resins, anhydrides, isocyanates and polymercaptans. The choice of both the resin and the hardener depends on the application, the process selected, and the properties desired. It is worthy to note that the reaction mechanism, the curing kinetics and the glass transition temperature (Tg) of the final material are also dependent on the molecular structure of the hardener. As it has been previously mentioned, amines are the best performance curing agents for diglycidylethertype epoxies. Aliphatic diamines like m-xylylenediamine or 1,2-trans-cyclohexyldiamine can be used for curing from room to moderate temperatures (Paz-Abuin, 1997a, 1997b, 1998), although the glass transition temperature of the material is also moderate. For high Tg materials aromatic amines, like 4,4´- methylen- bis (3- chloro- 2,6- diethylaniline) or 4,4´ diaminodiphenyl sulphone (Blanco et al., 2004; Girard-Reydet et al., 1999; Marieta et al.,

2003; Siddhamalli, 2000a) are used, although high curing temperatures are needed.

The curing process is the set of chemical reactions that leads to the formation of a highly crosslinked 3D network. For epoxy/amine the chemical process that leads to network

Fig. 3. Epoxy-amine reaction scheme. k1 and k2 correspond to the non catalyzed kinetic constants for the addition of primary and secondary amines respectively. k'1 and k'2

The reaction between monomers leads to the formation of the network and there are two important points during this process: gelation and vitrification. During the first stage the primary amino groups transform sequentially in secondary and tertiary amino groups. If R1 and R2 blocks contain a second reactive group (oxirane and amino, respectively), addition of more molecules proceeds at the ends of the branched molecule as well as with fresh monomers. Therefore, during the chemical reaction, both molecular weight and polydispersity increase until one single macromolecule is formed. At this point, if temperature is high enough, the behavior of the system changes irreversibly from liquid-like to rubber-like: the reactive system becomes a gel. According to the Flory-Stockmayer's theory of gelation (Flory, 1953) the extent of reaction at this point can be determined using

**1.1 Curing process. Gelation and vitrification. Conversion degree** 

formation can be described according to the scheme:

correspond to the catalyzed processes.

the expression:

Fig. 1. Oxirane ring

Fig. 2. Chemical structures of common epoxy resins: a) 2,2-Bis[4-(glycidyloxy)phenyl] propane (DGEBA); b) 3,4-Epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ECC); c) DGEBA oligomer, n = 0.2 typically.

molecular mass. But DGEBA-based resins are synthesized via the addition of epichlorohydrine and bisphenol A so oligomers with a relatively narrow distribution of polymerization degrees are obtained instead; their chemical structure is presented in Figure 2 (c) where *n* is typically 0.2. DGEBA oligomers typically contain a certain amount of hydroxyl groups, that play an important catalytic role in the kinetics of the curing process, providing a higher viscosity which is dependent on *n*. In addition, all of them have at least two oxirane functional groups, so they can finally lead to the 3D network. The nature and functionality of the epoxy monomer will determine its reactivity as well as the properties and performance of the final material.

Despite of having the same main functional group, the reactivity of both families of epoxies is completely different as a consequence of the structure of the molecules. It is worthy to note that the linkage between the aromatic ring and the oxygen (ether) in DGEBA has a strong electron-withdrawing effect that makes the oxirane group highly reactive towards nucleophilic compounds (like amines), unlike the cyclohexyl group in aliphatic epoxies which is reactive towards Lewis acids like anhydrides (Mark, 2004). Additionally, a protecting effect of axial and equatorial protons of the cyclohexyl ring against nucleophilic attack has been proposed as an explanation of the characteristic low reactivity of the oxirane ring in these aliphatic epoxies (Soucek et al., 1998). This way, the best performance and the highest crosslinking degree for DGEBA-based resins is achieved when cured via an addition mechanism with diamines (either aliphatic or aromatic), whilst cycloaliphatic epoxies are commonly cured with anhydrides (Barabanova et al., 2008; Chen et al., 2002; Tao et al., 2007;

Fig. 2. Chemical structures of common epoxy resins: a) 2,2-Bis[4-(glycidyloxy)phenyl] propane (DGEBA); b) 3,4-Epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ECC);

molecular mass. But DGEBA-based resins are synthesized via the addition of epichlorohydrine and bisphenol A so oligomers with a relatively narrow distribution of polymerization degrees are obtained instead; their chemical structure is presented in Figure 2 (c) where *n* is typically 0.2. DGEBA oligomers typically contain a certain amount of hydroxyl groups, that play an important catalytic role in the kinetics of the curing process, providing a higher viscosity which is dependent on *n*. In addition, all of them have at least two oxirane functional groups, so they can finally lead to the 3D network. The nature and functionality of the epoxy monomer will determine its reactivity as well as the properties

Despite of having the same main functional group, the reactivity of both families of epoxies is completely different as a consequence of the structure of the molecules. It is worthy to note that the linkage between the aromatic ring and the oxygen (ether) in DGEBA has a strong electron-withdrawing effect that makes the oxirane group highly reactive towards nucleophilic compounds (like amines), unlike the cyclohexyl group in aliphatic epoxies which is reactive towards Lewis acids like anhydrides (Mark, 2004). Additionally, a protecting effect of axial and equatorial protons of the cyclohexyl ring against nucleophilic attack has been proposed as an explanation of the characteristic low reactivity of the oxirane ring in these aliphatic epoxies (Soucek et al., 1998). This way, the best performance and the highest crosslinking degree for DGEBA-based resins is achieved when cured via an addition mechanism with diamines (either aliphatic or aromatic), whilst cycloaliphatic epoxies are commonly cured with anhydrides (Barabanova et al., 2008; Chen et al., 2002; Tao et al., 2007;

Fig. 1. Oxirane ring

c) DGEBA oligomer, n = 0.2 typically.

and performance of the final material.

Wang et al., 2003) or homopolymerized via a cationic mechanism induced by UV radiation (Crivello, 1995; Crivello & Fan,1991; Crivello & Liu, 2000; Hartwig et al., 2003; Wang & Neckers. 2001; Yagci & Reetz, 1998).

The chemical reactivity of the epoxies enables using a wide variety of molecules as curing agents depending on the process and required properties. The commonly used curing agents for epoxies include amines, polyamines, polyamides, phenolic resins, anhydrides, isocyanates and polymercaptans. The choice of both the resin and the hardener depends on the application, the process selected, and the properties desired. It is worthy to note that the reaction mechanism, the curing kinetics and the glass transition temperature (Tg) of the final material are also dependent on the molecular structure of the hardener. As it has been previously mentioned, amines are the best performance curing agents for diglycidylethertype epoxies. Aliphatic diamines like m-xylylenediamine or 1,2-trans-cyclohexyldiamine can be used for curing from room to moderate temperatures (Paz-Abuin, 1997a, 1997b, 1998), although the glass transition temperature of the material is also moderate. For high Tg materials aromatic amines, like 4,4´- methylen- bis (3- chloro- 2,6- diethylaniline) or 4,4´ diaminodiphenyl sulphone (Blanco et al., 2004; Girard-Reydet et al., 1999; Marieta et al., 2003; Siddhamalli, 2000a) are used, although high curing temperatures are needed.

#### **1.1 Curing process. Gelation and vitrification. Conversion degree**

The curing process is the set of chemical reactions that leads to the formation of a highly crosslinked 3D network. For epoxy/amine the chemical process that leads to network formation can be described according to the scheme:

Fig. 3. Epoxy-amine reaction scheme. k1 and k2 correspond to the non catalyzed kinetic constants for the addition of primary and secondary amines respectively. k'1 and k'2 correspond to the catalyzed processes.

The reaction between monomers leads to the formation of the network and there are two important points during this process: gelation and vitrification. During the first stage the primary amino groups transform sequentially in secondary and tertiary amino groups. If R1 and R2 blocks contain a second reactive group (oxirane and amino, respectively), addition of more molecules proceeds at the ends of the branched molecule as well as with fresh monomers. Therefore, during the chemical reaction, both molecular weight and polydispersity increase until one single macromolecule is formed. At this point, if temperature is high enough, the behavior of the system changes irreversibly from liquid-like to rubber-like: the reactive system becomes a gel. According to the Flory-Stockmayer's theory of gelation (Flory, 1953) the extent of reaction at this point can be determined using the expression:

Applications of FTIR on Epoxy Resins –

therefore heterogeneous.

1996; Rajagopalan et al., 2000).

**2. Epoxy resins and FTIR** 

in this region:

characterize the growth of the nascent structures.

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 265



Morphology development in modified thermosets takes place essentially between the "cloud point" conversion (beginning of the phase separation) and the gel point conversion (Bucknall & Partridge, 1986; Inoue, 1995; Mezzenga et al., 2000a), although it keeps evolving up to the vitrification of the system. Thermodynamics is the driving force for RIPS, but diffusion kinetics between phases is the controlling factor from the gel point on (Kiefer et al.,

As a consequence of phase separation particles or domains of very small size and different refraction index appear. When they are big enough they become light scatterers and the mixture becomes cloudy in the visible range. But the size of domains plays with the wavelenght, so IR radiation can also be used to determine the onset of phase separation and

For in-situ monitoring processes such as curing, phase separation or even ageing, the

Mid infrared spectroscopy has been widely used for characterization of organic compounds and plenty of reliable information and spectra libraries can easily be found. Both qualitative and quantitative information can be obtained by this technique, although its use in epoxy systems is quite restricted because of the location and intensity of the oxirane ring absorptions. Two characteristic absorptions of the oxirane ring are observed in the range between 4000 cm-1 and 400 cm-1. The first one, at 915 cm-1, is attributed to the C-O deformation of the oxirane group, although some works done by Dannenberg (Dannenberg & Harp, 1956) showed that this band does not correspond exclusively to this deformation but also to some other unknown process. The second band is located at 3050 cm-1 approximately and is attributed to the C-H tension of the methylene group of the epoxy ring. This band is not very useful since its intensity is low and it is also very close to the strong O-H absorptions; but in low polymerization degree epoxy monomers it can be used

Near IR is far more useful for epoxies. nIR spectrum covers the overtones of the strong vibrations in mIR and combination bands. In this range, fewer and less overlapped bands are observed so it has been used by several authors (Mijovic & Andjelic, 1995; Poisson et al., 1996; Xu & Schlup, 1998) for monitoring the curing reaction. The intensity of the bands in this region is much lower than in the mid range, allowing the use of thicker and undiluted samples to get good quality data. There are two absorptions related with the oxirane group

a. 4530 cm-1: It corresponds to a combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching at 2725 cm-1 (Chicke et al., 1993).

system. Segregation generates the final two-phase morphology.

interpretation of the spectra and the assignment of the bands are critical.

as a qualitative indicative of the presence of epoxy groups.

$$\alpha\_{\text{gel}} \cdot \mathcal{J}\_{\text{gel}} = \frac{1}{(f\_c - 1)(f\_a - 1)}$$

Where *gel* and *gel* are epoxy and amine conversions at the gel point and *fe* and *fa* are the functionality of the epoxy and amine components respectively (*fe* = 2 and *fa* = 4 typically, so under stoichiometric conditions, gel point appears at *ge l = gel* = 0.57) . Gelation usually has no effect on the curing kinetics.

Common diamines with relatively small molecular volume act as crosslinking points of the 3D network (since each diamine has four active hydrogen atoms, they can be visualized as points in space from which four chains emerge, each of them connecting other points of the network). As the reaction proceeds, along with molecular weight, the crosslinking degree of the system increases, and so the viscosity and the glass transition temperature (Tg). In those processes in which curing temperature is not very high, Tg of the reacting mixture may reach the curing temperature value; then, molecular mobility becomes severely restricted so diffusion of reactants controls the kinetics and the reaction rate decreases dramatically. At this point, the reaction becomes almost stopped and the properties of the material (at room temperature) depend on the extent of the reaction achieved. Unlike gelation, vitrification is a reversible process, so when heating above Tg the reaction is reactivated and higher conversions are attained. Postcuring processes, which are designed to allow volume and internal stresses relaxation, make use also of this chemical reactivation and have deep effects on the mechanical performance of these systems.

The extent of reaction is very commonly determined by differential scanning calorimetry (DSC) as the ratio between the heat released by the reaction at each moment and the total heat released. Although this procedure is useful, accuracy at high conversions is low and problems arise when monitoring fast reactions. Additionally, DSC only provides an overall conversion degree being impossible to independently determine epoxy and amine conversions. On the contrary, infrared spectroscopy allows a very accurate determination of both conversions by band integration of the corresponding IR signals (epoxy and amino) being low the integration error and allowing more accurate values at high conversions. Considering the reaction mechanism, we can define the extent of reaction in terms of epoxy groups () and in terms of N-H bonds (β) from the areas of the oxirane ring and the N-H absorptions respectively:

$$
\alpha\_{\varepsilon} = \frac{A\_{\varepsilon}(0) - A\_{\varepsilon}(t)}{A\_{\varepsilon}(0)} \quad \beta\_{N-H} = \frac{A\_{N-H}(0) - A\_{N-H}(t)}{A\_{N-H}(0)} \tag{1}
$$

In equation (1) subscript "e" indicates epoxy, "N-H" amine, "0" initial and "t" indicates a certain reaction time. Although epoxy and amino groups have absorptions in the mid-range, more accurate results are obtained working in the near range

#### **1.2 Epoxy blends. Reaction induced phase separation**

The main drawback of epoxy thermosets is its brittleness. To solve this problem, they are commonly modified with reinforcements of different nature (elastomers, thermoplastics, inorganic particles), geometry (particles, fibers, platelets) and size (micro and nano) which provide additional mechanical energy absorption mechanisms. The dispersion of a second phase can be obtained using mainly two strategies (Pascault et al., 2002):


Morphology development in modified thermosets takes place essentially between the "cloud point" conversion (beginning of the phase separation) and the gel point conversion (Bucknall & Partridge, 1986; Inoue, 1995; Mezzenga et al., 2000a), although it keeps evolving up to the vitrification of the system. Thermodynamics is the driving force for RIPS, but diffusion kinetics between phases is the controlling factor from the gel point on (Kiefer et al., 1996; Rajagopalan et al., 2000).

As a consequence of phase separation particles or domains of very small size and different refraction index appear. When they are big enough they become light scatterers and the mixture becomes cloudy in the visible range. But the size of domains plays with the wavelenght, so IR radiation can also be used to determine the onset of phase separation and characterize the growth of the nascent structures.

## **2. Epoxy resins and FTIR**

264 Infrared Spectroscopy – Materials Science, Engineering and Technology

( )( ) 1 1 *gel gel e a f f*

functionality of the epoxy and amine components respectively (*fe* = 2 and *fa* = 4 typically, so

Common diamines with relatively small molecular volume act as crosslinking points of the 3D network (since each diamine has four active hydrogen atoms, they can be visualized as points in space from which four chains emerge, each of them connecting other points of the network). As the reaction proceeds, along with molecular weight, the crosslinking degree of the system increases, and so the viscosity and the glass transition temperature (Tg). In those processes in which curing temperature is not very high, Tg of the reacting mixture may reach the curing temperature value; then, molecular mobility becomes severely restricted so diffusion of reactants controls the kinetics and the reaction rate decreases dramatically. At this point, the reaction becomes almost stopped and the properties of the material (at room temperature) depend on the extent of the reaction achieved. Unlike gelation, vitrification is a reversible process, so when heating above Tg the reaction is reactivated and higher conversions are attained. Postcuring processes, which are designed to allow volume and internal stresses relaxation, make use also of this chemical reactivation and have deep effects

The extent of reaction is very commonly determined by differential scanning calorimetry (DSC) as the ratio between the heat released by the reaction at each moment and the total heat released. Although this procedure is useful, accuracy at high conversions is low and problems arise when monitoring fast reactions. Additionally, DSC only provides an overall conversion degree being impossible to independently determine epoxy and amine conversions. On the contrary, infrared spectroscopy allows a very accurate determination of both conversions by band integration of the corresponding IR signals (epoxy and amino) being low the integration error and allowing more accurate values at high conversions. Considering the reaction mechanism, we can define the extent of reaction in terms of epoxy groups () and in terms of

N-H bonds (β) from the areas of the oxirane ring and the N-H absorptions respectively:

 

0 0

*A A*

In equation (1) subscript "e" indicates epoxy, "N-H" amine, "0" initial and "t" indicates a certain reaction time. Although epoxy and amino groups have absorptions in the mid-range,

The main drawback of epoxy thermosets is its brittleness. To solve this problem, they are commonly modified with reinforcements of different nature (elastomers, thermoplastics, inorganic particles), geometry (particles, fibers, platelets) and size (micro and nano) which provide additional mechanical energy absorption mechanisms. The dispersion of a second

0 0 ( ) () ( ) () ( ) ( ) *e e NH NH <sup>e</sup> N H e N H A At A A t*

 (1)

 

under stoichiometric conditions, gel point appears at

on the mechanical performance of these systems.

more accurate results are obtained working in the near range

phase can be obtained using mainly two strategies (Pascault et al., 2002):

**1.2 Epoxy blends. Reaction induced phase separation** 

Where

*gel* and 

no effect on the curing kinetics.

1

*gel* are epoxy and amine conversions at the gel point and *fe* and *fa* are the

*ge l =* 

*gel* = 0.57) . Gelation usually has

For in-situ monitoring processes such as curing, phase separation or even ageing, the interpretation of the spectra and the assignment of the bands are critical.

Mid infrared spectroscopy has been widely used for characterization of organic compounds and plenty of reliable information and spectra libraries can easily be found. Both qualitative and quantitative information can be obtained by this technique, although its use in epoxy systems is quite restricted because of the location and intensity of the oxirane ring absorptions. Two characteristic absorptions of the oxirane ring are observed in the range between 4000 cm-1 and 400 cm-1. The first one, at 915 cm-1, is attributed to the C-O deformation of the oxirane group, although some works done by Dannenberg (Dannenberg & Harp, 1956) showed that this band does not correspond exclusively to this deformation but also to some other unknown process. The second band is located at 3050 cm-1 approximately and is attributed to the C-H tension of the methylene group of the epoxy ring. This band is not very useful since its intensity is low and it is also very close to the strong O-H absorptions; but in low polymerization degree epoxy monomers it can be used as a qualitative indicative of the presence of epoxy groups.

Near IR is far more useful for epoxies. nIR spectrum covers the overtones of the strong vibrations in mIR and combination bands. In this range, fewer and less overlapped bands are observed so it has been used by several authors (Mijovic & Andjelic, 1995; Poisson et al., 1996; Xu & Schlup, 1998) for monitoring the curing reaction. The intensity of the bands in this region is much lower than in the mid range, allowing the use of thicker and undiluted samples to get good quality data. There are two absorptions related with the oxirane group in this region:

a. 4530 cm-1: It corresponds to a combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching at 2725 cm-1 (Chicke et al., 1993).

Applications of FTIR on Epoxy Resins –

DGEBA

HDGEBA

DGEBA

HDGEBA

Mijovic et al., 1995).

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 267

2965- 2873 StretchingC-H of CH2 and CH aromatic and aliphatic

**Resin Band (cm-1) Assignment** 

3057 Stretching of C-H of the oxirane ring

3052 Stretching of C-H of the oxirane ring

2937- 2862 StretchingC-H of CH2 and CH 1448 Deformation C-H of CH2 and CH3 1368 Deformation CH3 of C-(CH3)2 1098 Stretching C-O-C of ethers 909 Stretching C-O of oxirane group 846 Stretching C-O-C of oxirane group

Table 1. Characteristic bands of DGEBA and HDGEBA in the mid IR.

**Resin Band (cm-1) Assignment** 

6072 First overtone of terminal CH2 stretching mode

4623 Overtone of C-H stretching of the aromatic ring

6060 First overtone of terminal CH2 stretching mode

Table 2. Characteristic bands of DGEBA and HDGEBA in the near IR. (George et al., 1991;

5239 Combination assymetric stretching and bending of O-H

<sup>4526</sup>Combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching

5840- 5734 Overtones of -CH and -CH2 stretching

5244 Combination asymmetric stretching and bending of O-H

<sup>4531</sup>Combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching

5988- 5889 Overtones of -CH and -CH2 stretching

4066 Stretching C-H of aromatic ring

1608 Stretching C=C of aromatic rings

1509 Stretching C-C of aromatic 1036 Stretching C-O-C of ethers 915 Stretching C-O of oxirane group 831 Stretching C-O-C of oxirane group

3500 O-H stretching

772 Rocking CH2

759 Rocking CH2

7099 O-H overtone

7028 O-H overtone

3500 O-H stretching

Anyway, this band is sufficiently separated from others and is suitable for quantitative analysis (Mijovic et al., 1995; Paz-Abuín et al., 1997a; Poisson et al., 1996; Xu & Schlup, 1998).

b. 6070 cm-1: First overtone of terminal CH2 stretching mode (Musto et al., 2000). This band interferes with the aromatic C-H stretching overtone at 5969 cm-1 (Xu et al., 1996), so in case there are aromatic rings in the structure (i.e. DGEBA) is not suitable for quantification.

### **2.1 Characterizing epoxy resins by IR**

Characterization of epoxies involves much more than the location of the oxirane ring bands. There are many epoxy resins with different structures, different polimerization degrees...etc. IR spectroscopy can be used to characterize the nature of the epoxy. Figure 4 shows the mIR and nIR spectra of two similar epoxy resins: Diglycidylether of bisphenol A (DGEBA) and its hydrogenated derivative (HDGEBA).

Fig. 4. FTIR spectra of DGEBA and HDGEBA in the medium and near ranges

The difference between both resins is the absence of aromatic rings in HDGEBA, which conditions both the properties (Tg, viscosity...etc) and reaction rate towards amines. Table 1 shows the assignation of bands for both resins in the mid range. The C-O deformation band is centered at 915 cm-1 in DGEBA and at 909 cm-1 in HDGEBA. C-H stretching of terminal oxirane group is observed in both cases at 3050 cm-1. The broad band at 3500 cm-1 is assigned to O-H stretching of hydroxyl groups, revealing the presence of dimers or high molecular weight species. There are also bands corresponding to the ether linkage located at 1000-1100 cm-1 in both cases. In HDGEBA no signals corresponding to neither aromatic rings nor double bonds are observed, so these two epoxies can be easily distinguished through these bands.

Spectra in the near range are shown in Figure 4 also and assignments in Table 2. The combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching is centered at 4531 cm-1 in DGEBA and at 4526 cm-1 in HDGEBA. The region from 4000 to 4500 cm-1 contains the overtones from the fingerprint of the molecule.

The hydroxyl bands are sometimes useful for characterization although its quantitative use is very limited. Their presence is associated to the use of oligomers of low polymerization

b. 6070 cm-1: First overtone of terminal CH2 stretching mode (Musto et al., 2000). This band interferes with the aromatic C-H stretching overtone at 5969 cm-1 (Xu et al., 1996), so in case there are aromatic rings in the structure (i.e. DGEBA) is not suitable for

Characterization of epoxies involves much more than the location of the oxirane ring bands. There are many epoxy resins with different structures, different polimerization degrees...etc. IR spectroscopy can be used to characterize the nature of the epoxy. Figure 4 shows the mIR and nIR spectra of two similar epoxy resins: Diglycidylether of bisphenol A (DGEBA) and

Fig. 4. FTIR spectra of DGEBA and HDGEBA in the medium and near ranges

The difference between both resins is the absence of aromatic rings in HDGEBA, which conditions both the properties (Tg, viscosity...etc) and reaction rate towards amines. Table 1 shows the assignation of bands for both resins in the mid range. The C-O deformation band is centered at 915 cm-1 in DGEBA and at 909 cm-1 in HDGEBA. C-H stretching of terminal oxirane group is observed in both cases at 3050 cm-1. The broad band at 3500 cm-1 is assigned to O-H stretching of hydroxyl groups, revealing the presence of dimers or high molecular weight species. There are also bands corresponding to the ether linkage located at 1000-1100 cm-1 in both cases. In HDGEBA no signals corresponding to neither aromatic rings nor double bonds are observed, so these two epoxies can be easily distinguished through these bands.

Spectra in the near range are shown in Figure 4 also and assignments in Table 2. The combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching is centered at 4531 cm-1 in DGEBA and at 4526 cm-1 in HDGEBA. The region

The hydroxyl bands are sometimes useful for characterization although its quantitative use is very limited. Their presence is associated to the use of oligomers of low polymerization

from 4000 to 4500 cm-1 contains the overtones from the fingerprint of the molecule.

1998).

quantification.

**2.1 Characterizing epoxy resins by IR** 

its hydrogenated derivative (HDGEBA).

Anyway, this band is sufficiently separated from others and is suitable for quantitative analysis (Mijovic et al., 1995; Paz-Abuín et al., 1997a; Poisson et al., 1996; Xu & Schlup,


Table 1. Characteristic bands of DGEBA and HDGEBA in the mid IR.


Table 2. Characteristic bands of DGEBA and HDGEBA in the near IR. (George et al., 1991; Mijovic et al., 1995).

Applications of FTIR on Epoxy Resins –

corresponding to organic bonds appear.

usually weak.

the secondary amines show one single band.

Fig. 6. mIR and nIR spectra of metaxylylenediamine

diamines in both ranges are shown in Figure 6.

absorption bands at ≈ 3050 cm-1 or at 900 cm-1.

**3.1 Monitoring the curing process** 

**3. Curing process** 

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 269



The quantitative use of these bands is limited because of its position in the spectra: the N-H stretching is very close to the strong O-H absorption band (minimal amount of water perturbs its area), while the deformation band is located in the region where many signals

In the near range, the bands of amines are well defined and intense. There are also differences between the absorptions of primary and secondary amines. Primary amines N-H stretching first overtone is composed of two bands (symmetric and antisymmetric) located between 6897 cm-1 and 6452 cm-1, being the symmetric more intense. For secondary amines there is a single band. When both species coexist, this band cannot be used because the two bands overlap. Combination of N-H stretching and bending is observed at around 4900-5000 cm-1, and it can be used for quantitative purposes. (Weyer & Lo, 2002). Example spectra of

As shown in Figure 3, curing of epoxy resins with diamines can be described as a two step reaction: Firstly an epoxy group reacts with a primary amine yielding a secondary amine,

Considering these chemical reactions, the process can be monitored through the evolution of concentration of epoxy groups, primary amines or in some extent, secondary amines. The concentration of species is quantitatively related to the area of the absorption band only in the linear region, where Lambert Beer's law is satisfied. Taking this into account, changes in concentration of epoxy groups may be determined by mIR measuring the area of the

which in the second step reacts with another epoxy group yielding a tertiary amine

degree, as shown in Figure 2. In the mid range, quantification of OH is quite difficult because of the shape and overlapping of the band at around 3500 cm-1. In the near range, the absorption of the first O-H overtone is located at around 7000 cm-1, and although it has been used for quantification, no good results were obtained because of its weakness.

3,4-Epoxycyclohexyl-3'4'-epoxycyclohexane carboxylate (ECC) is probably the most common cycloaliphatic epoxy. The oxirane ring is in this case located in a six-member aliphatic cycle (Figure 2), shifting its absorptions in the mid range towards lower wave numbers, so the main absorption band is located at 790 cm-1 (Figure 5). This band has been used for quantitative analysis of photochemical reactions (Hartwig et al., 2002; Kim et al., 2003). Apart from the oxirane absorptions this resin shows the bands corresponding to the stretching C-O-C of ethers (1100 cm-1) and the C=O stretching (1730 cm-1) of esters, which can be useful for identification.

Fig. 5. FTIR spectra of ECC in the mid and near ranges

In the near range (Figure 5), it is worthy to note that the oxirane combination band (bending+ stretching) usually located at around 4530 cm-1 for common epoxies cannot be observed in the nIR spectrum of ECC probably because it may be overlapped with C-H combination bands. Neither the C=O second overtone (usually located at around 5100-5200 cm-1) is clearly observed. The main features observed in the spectrum are only the overtones of C-H and CH2 stretching bands. Although near infrared spectroscopy does not provide much useful information for this resin, it can still be used for monitoring the curing process through the evolution of the bands assigned to the curing agent (M. Gonzalez et al., 2011).

#### **2.2 Characterizing diamine hardeners by IR**

Among all the curing agents used to obtain epoxy thermosets, this chapter will be focused on one specific type: diamines. Their high reactivity is attributed to the high nucleophilicity of the nitrogen atom of the amino group although it is conditioned by its chemical structure. For instance, aliphatic diamines such as ethylene diamine, show a very high reactivity, while substituted aromatic amines like 4,4´-methylene-bis(3-chloro-2,6-diethylaniline) show lower reactivity because of the electronic effects of the aromatic ring and the susbstituents.

The amino group shows well defined absorptions both in the mid and in the near infrared ranges. The main absorptions in the mid range are stretching and deformation of N-H bonds. These bands also reflect some differences between primary and secondary amines:


The quantitative use of these bands is limited because of its position in the spectra: the N-H stretching is very close to the strong O-H absorption band (minimal amount of water perturbs its area), while the deformation band is located in the region where many signals corresponding to organic bonds appear.

Fig. 6. mIR and nIR spectra of metaxylylenediamine

In the near range, the bands of amines are well defined and intense. There are also differences between the absorptions of primary and secondary amines. Primary amines N-H stretching first overtone is composed of two bands (symmetric and antisymmetric) located between 6897 cm-1 and 6452 cm-1, being the symmetric more intense. For secondary amines there is a single band. When both species coexist, this band cannot be used because the two bands overlap. Combination of N-H stretching and bending is observed at around 4900-5000 cm-1, and it can be used for quantitative purposes. (Weyer & Lo, 2002). Example spectra of diamines in both ranges are shown in Figure 6.

## **3. Curing process**

268 Infrared Spectroscopy – Materials Science, Engineering and Technology

degree, as shown in Figure 2. In the mid range, quantification of OH is quite difficult because of the shape and overlapping of the band at around 3500 cm-1. In the near range, the absorption of the first O-H overtone is located at around 7000 cm-1, and although it has been

3,4-Epoxycyclohexyl-3'4'-epoxycyclohexane carboxylate (ECC) is probably the most common cycloaliphatic epoxy. The oxirane ring is in this case located in a six-member aliphatic cycle (Figure 2), shifting its absorptions in the mid range towards lower wave numbers, so the main absorption band is located at 790 cm-1 (Figure 5). This band has been used for quantitative analysis of photochemical reactions (Hartwig et al., 2002; Kim et al., 2003). Apart from the oxirane absorptions this resin shows the bands corresponding to the stretching C-O-C of ethers (1100 cm-1) and the C=O stretching (1730 cm-1) of esters, which

In the near range (Figure 5), it is worthy to note that the oxirane combination band (bending+ stretching) usually located at around 4530 cm-1 for common epoxies cannot be observed in the nIR spectrum of ECC probably because it may be overlapped with C-H combination bands. Neither the C=O second overtone (usually located at around 5100-5200 cm-1) is clearly observed. The main features observed in the spectrum are only the overtones of C-H and CH2 stretching bands. Although near infrared spectroscopy does not provide much useful information for this resin, it can still be used for monitoring the curing process through the evolution of the bands assigned to the curing agent (M. Gonzalez et al., 2011).

Among all the curing agents used to obtain epoxy thermosets, this chapter will be focused on one specific type: diamines. Their high reactivity is attributed to the high nucleophilicity of the nitrogen atom of the amino group although it is conditioned by its chemical structure. For instance, aliphatic diamines such as ethylene diamine, show a very high reactivity, while substituted aromatic amines like 4,4´-methylene-bis(3-chloro-2,6-diethylaniline) show lower

The amino group shows well defined absorptions both in the mid and in the near infrared ranges. The main absorptions in the mid range are stretching and deformation of N-H bonds. These bands also reflect some differences between primary and secondary amines:

reactivity because of the electronic effects of the aromatic ring and the susbstituents.

used for quantification, no good results were obtained because of its weakness.

can be useful for identification.

Fig. 5. FTIR spectra of ECC in the mid and near ranges

**2.2 Characterizing diamine hardeners by IR** 

#### **3.1 Monitoring the curing process**

As shown in Figure 3, curing of epoxy resins with diamines can be described as a two step reaction: Firstly an epoxy group reacts with a primary amine yielding a secondary amine, which in the second step reacts with another epoxy group yielding a tertiary amine

Considering these chemical reactions, the process can be monitored through the evolution of concentration of epoxy groups, primary amines or in some extent, secondary amines. The concentration of species is quantitatively related to the area of the absorption band only in the linear region, where Lambert Beer's law is satisfied. Taking this into account, changes in concentration of epoxy groups may be determined by mIR measuring the area of the absorption bands at ≈ 3050 cm-1 or at 900 cm-1.

Applications of FTIR on Epoxy Resins –

secondary amines into tertiary amines.

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 271

region the overtones of both primary and secondary amines overlap, so an initial decrease is observed, followed by an increase and a shift towards lower wavenumbers (because of the generation of secondary amines) and a final decrease consequence of the transformation of

For quantitative analysis, changes in concentration of epoxy and primary amines can be directly determined from the integration of the bands at ≈ 4530 cm-1 and at ≈ 4900 cm-1 respectively, and the epoxy () and primary amine (β) conversion degrees can be calculated as shown in eq (1). This fact opens the possibility of using complex models in which the concentration of all species (primary, secondary, tertiary amine and epoxy) can be considered during the curing process and kinetic parameters for the different steps of the reaction can be obtained. In Figure 8 typical conversion-time profiles for both and β at different temperatures are shown. After an initial fast increase in conversion a "plateau" region is reached, corresponding to the diffusion controlled stage (vitrification). As it is shown, the "plateau" for the primary amine conversion is often achieved at conversions

very close to 1, indicating that during curing the primary amine is fully consumed.

Fig. 8. Epoxy () and primary amine (β) conversions at different temperatures for

usually bands corresponding to overtones of the resin skeleton are used.

cm-1 (Kradjel & Lee, 2008; Mijovic and Andjelic, 1995; M. Gonzalez et al., 2011).

Shrinkage during curing or initial sample thermostatting can lead to major errors in epoxy and primary amino bands integration. To avoid this difficulty, it is useful to normalize the integrated areas to a characteristic band not changing during curing. For this purpose,

Curing cycloaliphatic epoxies with amines is not common because of the low reactivity of the system even at high temperatures. Anyway, its thermal curing with some complex amines (like poly (3-aminopropylmethylsiloxane)) has been reported. Determining conversion in these systems by IR is not an easy task, since the combination bands of the epoxy group in the near range overlap with other bands. Nevertheless, it is possible a semiquantitative approach considering the primary amine combination band at ≈ 4900 cm-1 and at longer reaction times (when primary amine is exhausted) progress of the reaction can be qualitatively followed from the primary and secondary amine combination band at 6530

HDGEBA/poly(3-aminopropylmethyl)siloxane.

Nevertheless, following curing by IR is not always easy, because the epoxy band at higher wavenumbers shows low sensitivity to changes in concentration as a consequence of its intrinsic low intensity and the 900 cm-1 band may be affected by the uncleanliness of the region where it is located. This may induce some uncertainty at the final stages of reaction when the concentration of epoxy groups is small. On the other hand, the quantification of primary and secondary amines in epoxy/amine reactive systems is not possible since the band corresponding to primary amines overlaps both with the band corresponding to secondary amines and the one corresponding to hydroxyl groups, which are species appearing as a consequence of the advance of the chemical reaction. Despite all these facts, mIR has been successfully used for monitoring the epoxy amine chemical reaction in several cases (Nikolic et al., 2010).

Fortunately nIR can be safely used for quantitatively monitor the chemical reaction (Paz-Abuin et al., 1997a, 1997b; Mijovic & Andjelic 1995a; Mijovic et al. 1995). In this region we can find well defined bands free of overlapping related with the epoxy and primary amine: the combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching (≈ 4530 cm-1) and the combination band of NH stretching and bending (≈ 4900-5000 cm-1). In Figure 7, a typical spectral evolution on cure can be observed.

Fig. 7. Time evolution of FTnIR spectra during the isothermal curing at 70 ºC of the stoichiometric HDGEBA/poly(3-aminopropylmethyl)siloxane system.

The reaction mechanism indicates that the epoxy concentration decreases, and this is observed in the spectra as the decrease of the band centered at ≈ 4530 cm-1 and also of the weak overtone of terminal CH2 at ≈ 6060 cm-1. The primary amine combination band decreases too (≈ 4900 cm-1), and once it is exhausted it can be observed that there are still epoxy groups in the reaction media, which will react with the previously formed secondary amines up to vitrification or until the reaction is completed. The band correponding to O-H overtones (≈ 7000 cm-1) also increases during curing as a consequence of the oxirane ringopening, although this band is not suitable for quantification because of the low signal/noise ratio. The behavior of the band located at ≈ 6500 cm-1 is more complex: in this

Nevertheless, following curing by IR is not always easy, because the epoxy band at higher wavenumbers shows low sensitivity to changes in concentration as a consequence of its intrinsic low intensity and the 900 cm-1 band may be affected by the uncleanliness of the region where it is located. This may induce some uncertainty at the final stages of reaction when the concentration of epoxy groups is small. On the other hand, the quantification of primary and secondary amines in epoxy/amine reactive systems is not possible since the band corresponding to primary amines overlaps both with the band corresponding to secondary amines and the one corresponding to hydroxyl groups, which are species appearing as a consequence of the advance of the chemical reaction. Despite all these facts, mIR has been successfully used for monitoring the epoxy amine chemical reaction in several

Fortunately nIR can be safely used for quantitatively monitor the chemical reaction (Paz-Abuin et al., 1997a, 1997b; Mijovic & Andjelic 1995a; Mijovic et al. 1995). In this region we can find well defined bands free of overlapping related with the epoxy and primary amine: the combination band of the second overtone of the epoxy ring stretching with the fundamental C-H stretching (≈ 4530 cm-1) and the combination band of NH stretching and bending (≈ 4900-5000 cm-1). In Figure 7, a typical spectral evolution on cure can be observed.

Fig. 7. Time evolution of FTnIR spectra during the isothermal curing at 70 ºC of the

The reaction mechanism indicates that the epoxy concentration decreases, and this is observed in the spectra as the decrease of the band centered at ≈ 4530 cm-1 and also of the weak overtone of terminal CH2 at ≈ 6060 cm-1. The primary amine combination band decreases too (≈ 4900 cm-1), and once it is exhausted it can be observed that there are still epoxy groups in the reaction media, which will react with the previously formed secondary amines up to vitrification or until the reaction is completed. The band correponding to O-H overtones (≈ 7000 cm-1) also increases during curing as a consequence of the oxirane ringopening, although this band is not suitable for quantification because of the low signal/noise ratio. The behavior of the band located at ≈ 6500 cm-1 is more complex: in this

stoichiometric HDGEBA/poly(3-aminopropylmethyl)siloxane system.

cases (Nikolic et al., 2010).

region the overtones of both primary and secondary amines overlap, so an initial decrease is observed, followed by an increase and a shift towards lower wavenumbers (because of the generation of secondary amines) and a final decrease consequence of the transformation of secondary amines into tertiary amines.

For quantitative analysis, changes in concentration of epoxy and primary amines can be directly determined from the integration of the bands at ≈ 4530 cm-1 and at ≈ 4900 cm-1 respectively, and the epoxy () and primary amine (β) conversion degrees can be calculated as shown in eq (1). This fact opens the possibility of using complex models in which the concentration of all species (primary, secondary, tertiary amine and epoxy) can be considered during the curing process and kinetic parameters for the different steps of the reaction can be obtained. In Figure 8 typical conversion-time profiles for both and β at different temperatures are shown. After an initial fast increase in conversion a "plateau" region is reached, corresponding to the diffusion controlled stage (vitrification). As it is shown, the "plateau" for the primary amine conversion is often achieved at conversions very close to 1, indicating that during curing the primary amine is fully consumed.

Fig. 8. Epoxy () and primary amine (β) conversions at different temperatures for HDGEBA/poly(3-aminopropylmethyl)siloxane.

Shrinkage during curing or initial sample thermostatting can lead to major errors in epoxy and primary amino bands integration. To avoid this difficulty, it is useful to normalize the integrated areas to a characteristic band not changing during curing. For this purpose, usually bands corresponding to overtones of the resin skeleton are used.

Curing cycloaliphatic epoxies with amines is not common because of the low reactivity of the system even at high temperatures. Anyway, its thermal curing with some complex amines (like poly (3-aminopropylmethylsiloxane)) has been reported. Determining conversion in these systems by IR is not an easy task, since the combination bands of the epoxy group in the near range overlap with other bands. Nevertheless, it is possible a semiquantitative approach considering the primary amine combination band at ≈ 4900 cm-1 and at longer reaction times (when primary amine is exhausted) progress of the reaction can be qualitatively followed from the primary and secondary amine combination band at 6530 cm-1 (Kradjel & Lee, 2008; Mijovic and Andjelic, 1995; M. Gonzalez et al., 2011).

Applications of FTIR on Epoxy Resins –

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 273

Fig. 9. Typical evolution of concentration of primary (A1), secondary (A2) and tertiary amine (A3) groups during curing. Reactive system: DGEBA/m-Xylylenediamine at 80ºC. (Used with permission from (González, M.; Kindelán, M.; Cabanelas, J.C.; Baselga,J.

1

 <sup>0</sup> 1 1

1 1 [ ] ´ [ ] *<sup>d</sup> OH K K dt E*

*d Bd OH B KK dt dt E*

 

2 1 10

1 10

**3.2.2 Reactivity ratio between primary and secondary amines** 

[ ] ´ ´[ ] *K kE K kE* 

<sup>0</sup>

[ ] R ´

 

2 2

Some kinetic models assume that the reactivity of primary and secondary amines is the same. Considering that primary amines have two reactive hydrogen atoms, equal reactivity yields R = 0.5. Nevertheless, in most of epoxy amine systems higher reactivity of primary amines (R<0.5) has been experimentally observed (Matejka, 2000; Paz-Abuin et al.,1997a, 1997b, 1998; Liu et al., 2004; Varley et al., 2006). This behavior is not surprising since the addition of the epoxy molecule to a primary amine causes an steric hindrance. On the other hand, the chemical nature of the new substituent usually decreases the nucleophilic character (and thus the reactivity) of the nitrogen atom in the amine group due to a negative inductive effect.

Paz-Abuín et al. developed a method for quantifying the reactivity ratio from the concentration-time plots of amines (Paz-Abuin et al., 1997a). Considering the classical reaction mechanism, applying the condition for maximum to *[A2]*, it is obtained that

1 1

0

(2)

(3)

[ ]

 

1 1 ´ <sup>R</sup> ´ *k k k k*

0

*Macromolecular Symposia,* Vol.200. Copyright (2003)).

2

 

where:

## **3.2 Modeling kinetics**

Curing kinetics is a key aspect in epoxy systems, since it determines the time spam available for shaping, storing... As in most chemically reactive systems, reaction rate is temperature dependent.

Several models have been developed for epoxy/amine kinetics through the study of model compounds (Schechter et al., 1956). An acceleration of reaction in the presence of OH groups was observed and explained considering a third order reaction mechanism (Smith, 1961). Horie and coworkers proposed a model in 1970 (Horie et al., 1970) considering the catalysis of both initial OH (due to DGEBA oligomers and impurities) and OH generated during chemical reaction (autocatalysis) which has been used and validated in many epoxy/amine systems at different temperatures (Simon et al., 2000; Vyazovkin & Sbirrazouli, 1996; Cole et al., 1991; Riccardi et al., 1984). Later on, modifications to the model have been introduced, for example considering the different reactivity of hydrogens belonging to primary and secondary amines and the possible homopolymerization reactions between epoxy groups under certain conditions (Cole et al., 1991; Riccardi & Williams, 1986). Thus, following the evolution of concentration of the different species during curing is useful for modeling epoxy/amine systems.

The commonly accepted kinetic scheme for epoxy-amine reactions considers two reaction paths: a non-catalyzed and an autocatalyzed path. The autocatalysis is attributed to the formation of complex between generated or initially present hydroxyl groups, amino groups and epoxy groups. A simple reaction mechanism is presented in Figure 3 although it can be improved considering some equilibrium reactions for the complexes formation (Ehlers et al., 2007). With appropriate mass balances it is possible to set out rate equations that can be fitted to experimental data to extract the relevant kinetic parameters

## **3.2.1 Determining concentrations during curing**

Considering the reaction scheme, the concentration of epoxy groups, primary, secondary and tertiary amine, as well as hydroxyl groups can be determined through the following mass balances:

$$\begin{aligned} [A\_1] = [A\_1]\_0 - [A\_2] - [A\_3] \ & \vdots & [E] = [E]\_0 - [A\_2] - 2[A\_3] \ & \vdots & [OH] = [OH]\_0 + [E]\_0 - [E] \end{aligned} $$

If initial concentrations of epoxy and primary amine are known, the concentration of all species at each instant can be determined from the conversion data obtained by nIR:

$$\begin{array}{cccc} [E] = [E]\_0 \left( 1 - \alpha \right) & ; & [A\_1] = [A\_1]\_0 \left( 1 - \beta \right) & ; & [A\_2] = [E]\_0 \left( \beta B - \alpha \right) & ; & [A\_3] = [E]\_0 \left( \alpha - \beta \frac{B}{2} \right) \end{array}$$

Where *B=2[A1]0/[E]0* is the ratio between the initial concentration of primary amine and epoxy. A typical variation of all these species with time is presented in Figure 9.

Assuming that the reactivity ratio between primary and secondary amines (R) is independent of the reaction path, the kinetic equations for the epoxy and primary amine conversion are:

Fig. 9. Typical evolution of concentration of primary (A1), secondary (A2) and tertiary amine (A3) groups during curing. Reactive system: DGEBA/m-Xylylenediamine at 80ºC. (Used with permission from (González, M.; Kindelán, M.; Cabanelas, J.C.; Baselga,J. *Macromolecular Symposia,* Vol.200. Copyright (2003)).

$$\frac{d\alpha}{dt} = \frac{\mathcal{B}}{2}\frac{d\beta}{dt} + \mathcal{R}\left(\mathcal{B}\beta - \alpha\right)\left(1 - \alpha\right)\left(K\_1 + K\_1'\left(\frac{\left[OH\right]\_0}{\left[E\right]\_0} + \alpha\right)\right) \tag{2}$$

$$\frac{d\beta}{dt} = (1 - \beta)(1 - \alpha)\left(K\_1 + K\_1' \left(\frac{[OH]\_0}{[E]\_0} + \alpha\right)\right) \tag{3}$$

where:

272 Infrared Spectroscopy – Materials Science, Engineering and Technology

Curing kinetics is a key aspect in epoxy systems, since it determines the time spam available for shaping, storing... As in most chemically reactive systems, reaction rate is temperature

Several models have been developed for epoxy/amine kinetics through the study of model compounds (Schechter et al., 1956). An acceleration of reaction in the presence of OH groups was observed and explained considering a third order reaction mechanism (Smith, 1961). Horie and coworkers proposed a model in 1970 (Horie et al., 1970) considering the catalysis of both initial OH (due to DGEBA oligomers and impurities) and OH generated during chemical reaction (autocatalysis) which has been used and validated in many epoxy/amine systems at different temperatures (Simon et al., 2000; Vyazovkin & Sbirrazouli, 1996; Cole et al., 1991; Riccardi et al., 1984). Later on, modifications to the model have been introduced, for example considering the different reactivity of hydrogens belonging to primary and secondary amines and the possible homopolymerization reactions between epoxy groups under certain conditions (Cole et al., 1991; Riccardi & Williams, 1986). Thus, following the evolution of concentration of the different species during curing is useful for modeling

The commonly accepted kinetic scheme for epoxy-amine reactions considers two reaction paths: a non-catalyzed and an autocatalyzed path. The autocatalysis is attributed to the formation of complex between generated or initially present hydroxyl groups, amino groups and epoxy groups. A simple reaction mechanism is presented in Figure 3 although it can be improved considering some equilibrium reactions for the complexes formation (Ehlers et al., 2007). With appropriate mass balances it is possible to set out rate equations that can be

Considering the reaction scheme, the concentration of epoxy groups, primary, secondary and tertiary amine, as well as hydroxyl groups can be determined through the following

1 10 2 3 02 3 0 0 [ ] [ ] [ ] [ ];[] [] [ ] [ ];[ ] [ ] [] [] *A A A A E E A A OH OH E E* 2

If initial concentrations of epoxy and primary amine are known, the concentration of all

<sup>0</sup> 1 10 2 0 3 0 1 1 <sup>2</sup> [] [] ; [ ] [ ] ; [ ] [] ; [ ] [] *<sup>B</sup> EE A A A E B A E*

Where *B=2[A1]0/[E]0* is the ratio between the initial concentration of primary amine and

Assuming that the reactivity ratio between primary and secondary amines (R) is independent of the reaction path, the kinetic equations for the epoxy and primary amine

   

species at each instant can be determined from the conversion data obtained by nIR:

epoxy. A typical variation of all these species with time is presented in Figure 9.

fitted to experimental data to extract the relevant kinetic parameters

**3.2.1 Determining concentrations during curing** 

**3.2 Modeling kinetics** 

epoxy/amine systems.

mass balances:

conversion are:

dependent.

$$\begin{aligned} K\_1 &= k\_1 [E]\_0 \\ K'\_1 &= k'\_1 [E]\_0^2 \end{aligned} \qquad \qquad \qquad \qquad \quad \mathbf{R} = \frac{k\_2}{k\_1} = \frac{k'\_2}{k'\_1}$$

#### **3.2.2 Reactivity ratio between primary and secondary amines**

Some kinetic models assume that the reactivity of primary and secondary amines is the same. Considering that primary amines have two reactive hydrogen atoms, equal reactivity yields R = 0.5. Nevertheless, in most of epoxy amine systems higher reactivity of primary amines (R<0.5) has been experimentally observed (Matejka, 2000; Paz-Abuin et al.,1997a, 1997b, 1998; Liu et al., 2004; Varley et al., 2006). This behavior is not surprising since the addition of the epoxy molecule to a primary amine causes an steric hindrance. On the other hand, the chemical nature of the new substituent usually decreases the nucleophilic character (and thus the reactivity) of the nitrogen atom in the amine group due to a negative inductive effect.

Paz-Abuín et al. developed a method for quantifying the reactivity ratio from the concentration-time plots of amines (Paz-Abuin et al., 1997a). Considering the classical reaction mechanism, applying the condition for maximum to *[A2]*, it is obtained that

Applications of FTIR on Epoxy Resins –

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 275

Fig. 11. and as a function of curing time for DGEBA / m-XDA at 60 ºC. Lines: fitting to

() () ( ) ( ) *i i*

were O(ti) and C(ti) are the observed and calculated values of (or ) respectively. (Used with permission from (González, M.; Kindelán, M.; Cabanelas, J.C.; Baselga,J. *Macromolecular* 

Phase separation in blends involves the development of a second phase which usually has a different refractive index, and can be detected by the appearance of turbidity. Most studies analyze the so-called "cloud point" (instant when the sample is no longer transparent) measuring visible transmittance. Particles scatter light when its size is similar to the wavelength of the incident radiation; since infrared radiation ranges from 780 nm to 15 μm (from 780 nm to 1.1 μm for the near range and from 1.1 μm to 15 μm for the mid range), the onset of phase separation can be detected using nIR or mIR although with less accuracy (Bhargava et al, 1999) than using visible light. Therefore, IR measurements give delayed values of the cloud point. This fact is clear, and even more sophisticated techniques like SAXS may give information of the incipient phase separation process, but there is an advantage for IR: it provides additional chemical information during phase separation. Because of its longer wavelength, mIR is rarely used for characterizing phase separation phenomena, although it is used for characterizing other systems containing particles of

*Ot Ct r t O t* 

*i*

*i*

eqs (1) and (2). Below: weighted residuals calculated as:

**4. Phase separation influence on IR spectra** 

bigger size or to avoid interferences due to the color of the systems.

*Symposia,* Vol.200. Copyright (2003)).

*R=[A1]/[A2].* Thus, R value can determined from the concentration curves as the ratio of the concentration of primary amine and secondary amine at the maximum of secondary amine concentration. Usually the R value is below 0.5, i.e. it shows the higher reactivity of primary amines. If R is not very low, the uncertainty in the determination of the maximum is small and the R value can be precisely determined.

#### **3.2.3 Solving kinetic equations**

Rate constants can be obtained solving the rate equations (2) and (3) mentioned above. Since both equations are interdependent, two approaches for solving them may be used:


$$\mathcal{A} = \frac{[A\_2]}{[E]\_0}$$

Thus a single equation in terms of the derivative of epoxy conversion is obtained, so that the global kinetic constants can be obtained as the intercept and slope of the linear fit at low conversions (far from the diffusion controlled region) of the expression:

$$\frac{\frac{da}{dt}}{(1-a)\left(\lambda + R\left(B - 2\lambda - a\right)\right)} = K\_1 + K\_1'\left(\frac{[OH]\_0}{[E]\_0} + a\right).$$

A typical example of the linearization method is presented in Figure 10.

Fig. 10. Determination of K1 and K´1.


*R=[A1]/[A2].* Thus, R value can determined from the concentration curves as the ratio of the concentration of primary amine and secondary amine at the maximum of secondary amine concentration. Usually the R value is below 0.5, i.e. it shows the higher reactivity of primary amines. If R is not very low, the uncertainty in the determination of the maximum is small

Rate constants can be obtained solving the rate equations (2) and (3) mentioned above. Since

2 0 [ ] [ ] *A E*

*dt OH K K R B E*


0

[ ] ´ [ ]

1 1

Thus a single equation in terms of the derivative of epoxy conversion is obtained, so that the global kinetic constants can be obtained as the intercept and slope of the linear fit at low

both equations are interdependent, two approaches for solving them may be used:

<sup>0</sup> 1 2

 

and the R value can be precisely determined.


conversions (far from the diffusion controlled region) of the expression:

A typical example of the linearization method is presented in Figure 10.

*d*

**3.2.3 Solving kinetic equations** 

Fig. 10. Determination of K1 and K´1.

Gonzalez et al., 2003).

Fig. 11. and as a function of curing time for DGEBA / m-XDA at 60 ºC. Lines: fitting to eqs (1) and (2). Below: weighted residuals calculated as:

$$r(t\_i) = \frac{O(t\_i) - C(t\_i)}{\sqrt{O(t\_i)}}$$

were O(ti) and C(ti) are the observed and calculated values of (or ) respectively. (Used with permission from (González, M.; Kindelán, M.; Cabanelas, J.C.; Baselga,J. *Macromolecular Symposia,* Vol.200. Copyright (2003)).

### **4. Phase separation influence on IR spectra**

Phase separation in blends involves the development of a second phase which usually has a different refractive index, and can be detected by the appearance of turbidity. Most studies analyze the so-called "cloud point" (instant when the sample is no longer transparent) measuring visible transmittance. Particles scatter light when its size is similar to the wavelength of the incident radiation; since infrared radiation ranges from 780 nm to 15 μm (from 780 nm to 1.1 μm for the near range and from 1.1 μm to 15 μm for the mid range), the onset of phase separation can be detected using nIR or mIR although with less accuracy (Bhargava et al, 1999) than using visible light. Therefore, IR measurements give delayed values of the cloud point. This fact is clear, and even more sophisticated techniques like SAXS may give information of the incipient phase separation process, but there is an advantage for IR: it provides additional chemical information during phase separation. Because of its longer wavelength, mIR is rarely used for characterizing phase separation phenomena, although it is used for characterizing other systems containing particles of bigger size or to avoid interferences due to the color of the systems.

Applications of FTIR on Epoxy Resins –

information on dimensional changes of the specimens.

bonded respectively (Musto et al., 2002).

be calculated as:

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 277

and many different techniques have been used: from the fast and easy gravimetry to more complex techniques such as NMR spectroscopy (Zhou & Lucas 1999) or fluorescence (Mikes et al., 2003). Also infrared spectroscopy has been widely used. IR shows an advantage when compared with gravimetry: it is not only an accurate technique for determining water concentration, but also provides information at the molecular level about the interactions between water molecules and the thermoset structure and can be used to provide

Water has three active vibration modes in infrared corresponding to the stretching of O-H bond (≈ 3800- 3600 cm-1 in liquid state) and bending (≈ 1650-1590 cm-1 in liquid state). The position of the bands of this molecule is particularly sensitive towards interactions like hydrogen bonding, which originates displacements towards lower wavenumbers (< 3600 cm-1), enabling the distinction between free water, hydrogen bonded and intramolecular hydrogen bonding (Socrates, 1994). When absorbed in epoxy resins, two types of water are found: highly mobile free water molecules (≈ 3600 cm-1) and water bounded to specific sites through hydrogen bonding (≈ 3300 cm-1) (Blanco et al., 2006; Cotugno et al., 2005; Grave et al, 1998). Signals can also be observed in the near infrared range: at 5215 cm-1 resulting from the combination of asymmetric stretching and bending and in the range 7800 - 6000 cm-1 hydroxyl vibrations are also found. The latter band can be deconvoluted into three peaks centered at 7075, 6820 and 3535 cm-1 attributed to free water, self-associated and hydrogen

Infrared spectroscopy in both ranges has been used to monitor water uptake and diffusion coefficients have been determined using Fick's law. The band located at 5215 cm-1 can be used to quantify water, although it must be normalized for sample thickness. The band at higher wavenumbers is used to determine the kind of interactions between water and network (Mijovic & Zhang, 2003; Cabanelas et al., 2003), but not with quantitative purposes since it is superimposed on the O-H overtone of the resin (Musto et al., 2000). To overcome the thickness variation it is possible to normalize water signal with a reference band invariant against the presence of water (for example, the band at 4623 cm-1, corresponding to aromatic rings of DGEBA). This peak, in principle should not change by the water ingress, only by the volume change due to swelling. In this way, the fractional absorbed water can

> 5215 0 5215 4623 0 4623

, , , , , ,

*A A*

*A*

Ingress of water swells the specimens changing its dimensions. The volume changes related with swelling have been characterized measuring a reference band and using the following

0 4623

3 2 0

<sup>1</sup> ( ) *t*

0 0 5215

*W A A*

*t*

*t t*

*W A*

expression that can be easily derived from Lambert-Beer law (Cabanelas et al. 2003).

*V t A V A* 

0

Turbidity is observed in IR spectra as an increase of baseline. This parameter can be used to follow the phase separation process in a region where no bands exist, i.e 6300 cm-1. Alternatively, this method has also been used to follow compatibilization of initially immiscible systems. As an example Cabanelas et al. (Cabanelas et al., 2005) studied the compatibilization process and phase separation of a third component in reactive blends based on DGEBA and poly(3-aminopropylmethylsiloxane) modified with PMMA. As shown in Figure 12, the initial decrease of the baseline was related with the compatibilization between DGEBA and the silicone hardener and the subsequent increase was related with the onset of phase separation of the thermoplastic modifier. IR can also provide information about the interactions between the modifier and the thermosetting matrix. Typical cured epoxy thermosets present a variety of OHN, OHNH and OHOH hydrogen bonds. In the presence of PMMA intramolecular interactions become redistributed since the carbonyl groups of PMMA interact with the initially present and newly formed OH groups as it is shown by the presence of a carbonyl-OH hydrogen bonding band centered at 3500 cm-1 (Blanco et al., 2009). These changes in IR spectra may be related with the miscibility in complex systems.

Fig. 12. Baseline from FTIR spectra at 6300 cm-1 as a function of epoxy conversion of DGEBA/PAMS for different weight concentrations of PMMA as modifier (Adapted with permission from (Cabanelas, J.C.; Serrano, B.; Baselga, J. *Macromolecules,* Vol.38, No.3, (2005),). Copyright (2005) American Chemical Society).

#### **5. Water uptake**

One of the main drawbacks of epoxy resins is its high water uptake. Water deteriorates thermomechanical properties (Tg, modulus, yield strength, toughness...), and adhesion, it induces chemical degradation of the network and also generates stresses because of swelling (Nogueira et al., 2001; Cotugno et al., 2001; Blanco et al., 2006; Ji et al., 2006; Xiao & Shanahan, 2008). Significant efforts have been done to elucidate the interactions of water with epoxy/amine networks and the diffusion mechanisms operating during water uptake,

Turbidity is observed in IR spectra as an increase of baseline. This parameter can be used to follow the phase separation process in a region where no bands exist, i.e 6300 cm-1. Alternatively, this method has also been used to follow compatibilization of initially immiscible systems. As an example Cabanelas et al. (Cabanelas et al., 2005) studied the compatibilization process and phase separation of a third component in reactive blends based on DGEBA and poly(3-aminopropylmethylsiloxane) modified with PMMA. As shown in Figure 12, the initial decrease of the baseline was related with the compatibilization between DGEBA and the silicone hardener and the subsequent increase was related with the onset of phase separation of the thermoplastic modifier. IR can also provide information about the interactions between the modifier and the thermosetting matrix. Typical cured epoxy thermosets present a variety of OHN, OHNH and OHOH hydrogen bonds. In the presence of PMMA intramolecular interactions become redistributed since the carbonyl groups of PMMA interact with the initially present and newly formed OH groups as it is shown by the presence of a carbonyl-OH hydrogen bonding band centered at 3500 cm-1 (Blanco et al., 2009). These changes in IR spectra may be

Fig. 12. Baseline from FTIR spectra at 6300 cm-1 as a function of epoxy conversion of DGEBA/PAMS for different weight concentrations of PMMA as modifier (Adapted with permission from (Cabanelas, J.C.; Serrano, B.; Baselga, J. *Macromolecules,* Vol.38, No.3,

One of the main drawbacks of epoxy resins is its high water uptake. Water deteriorates thermomechanical properties (Tg, modulus, yield strength, toughness...), and adhesion, it induces chemical degradation of the network and also generates stresses because of swelling (Nogueira et al., 2001; Cotugno et al., 2001; Blanco et al., 2006; Ji et al., 2006; Xiao & Shanahan, 2008). Significant efforts have been done to elucidate the interactions of water with epoxy/amine networks and the diffusion mechanisms operating during water uptake,

(2005),). Copyright (2005) American Chemical Society).

**5. Water uptake** 

related with the miscibility in complex systems.

and many different techniques have been used: from the fast and easy gravimetry to more complex techniques such as NMR spectroscopy (Zhou & Lucas 1999) or fluorescence (Mikes et al., 2003). Also infrared spectroscopy has been widely used. IR shows an advantage when compared with gravimetry: it is not only an accurate technique for determining water concentration, but also provides information at the molecular level about the interactions between water molecules and the thermoset structure and can be used to provide information on dimensional changes of the specimens.

Water has three active vibration modes in infrared corresponding to the stretching of O-H bond (≈ 3800- 3600 cm-1 in liquid state) and bending (≈ 1650-1590 cm-1 in liquid state). The position of the bands of this molecule is particularly sensitive towards interactions like hydrogen bonding, which originates displacements towards lower wavenumbers (< 3600 cm-1), enabling the distinction between free water, hydrogen bonded and intramolecular hydrogen bonding (Socrates, 1994). When absorbed in epoxy resins, two types of water are found: highly mobile free water molecules (≈ 3600 cm-1) and water bounded to specific sites through hydrogen bonding (≈ 3300 cm-1) (Blanco et al., 2006; Cotugno et al., 2005; Grave et al, 1998). Signals can also be observed in the near infrared range: at 5215 cm-1 resulting from the combination of asymmetric stretching and bending and in the range 7800 - 6000 cm-1 hydroxyl vibrations are also found. The latter band can be deconvoluted into three peaks centered at 7075, 6820 and 3535 cm-1 attributed to free water, self-associated and hydrogen bonded respectively (Musto et al., 2002).

Infrared spectroscopy in both ranges has been used to monitor water uptake and diffusion coefficients have been determined using Fick's law. The band located at 5215 cm-1 can be used to quantify water, although it must be normalized for sample thickness. The band at higher wavenumbers is used to determine the kind of interactions between water and network (Mijovic & Zhang, 2003; Cabanelas et al., 2003), but not with quantitative purposes since it is superimposed on the O-H overtone of the resin (Musto et al., 2000). To overcome the thickness variation it is possible to normalize water signal with a reference band invariant against the presence of water (for example, the band at 4623 cm-1, corresponding to aromatic rings of DGEBA). This peak, in principle should not change by the water ingress, only by the volume change due to swelling. In this way, the fractional absorbed water can be calculated as:

$$\frac{\Delta W\_t}{W\_0} = \frac{\left(\frac{A\_{t,S215}}{A\_{t,4623}}\right) - \left(\frac{A\_{0,S215}}{A\_{0,4623}}\right)}{\left(\frac{A\_{0,S215}}{A\_{0,4623}}\right)}$$

Ingress of water swells the specimens changing its dimensions. The volume changes related with swelling have been characterized measuring a reference band and using the following expression that can be easily derived from Lambert-Beer law (Cabanelas et al. 2003).

$$\frac{\Delta V(t)}{V\_0} = \left(\frac{A\_0}{A\_t}\right)^{\frac{\lambda}{\lambda}} - 1$$

Applications of FTIR on Epoxy Resins –

0021-8995.

54-62, ISSN 0032-3888.

ISSN 0003-7028.

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 279

Barabanova, A.I.; Shevnin, P.L.; Pryakhina, T.A.; Bychko, K.A.; Kazantseva, V.V.; Zavin,

Blanco, I.; Cicala, G.; Motta, O.; Recca, A. (2004). Influence of a selected hardener on the

Blanco, M.; Lopez, M.; Fernandez, R.; Martin, L.; Riccardi, C.C.; Mondragon, I. (2009).

Bucknall, C.B.; Partridge, I.K. (1986). Phase separation in crosslinked resins containing

Cabanelas, J.C.; Prolongo, S.G.; Serrano, B.; Bravo, J.; Baselga, J. (2003). Water absorption in

Cabanelas, J.C.; Serrano, B.; Baselga, J. (2005). Development of cocontinuous morphologies

*Macromolecules,* Vol.38, No.3, (February 2005), pp. 961-970, ISSN 0024-9297. Chen, J.S.; Ober, C.K.; Poliks, M.D. (2002). Characterization of Thermally Reworkable

Chike, K.E.; Myrick M.L.; Lyon, R.E.; Angel S.M. (1993). Raman ans near infrared studies of

Cole, K.C. ; Hecher, J.J.; Noel, D. (1991). A new approach to modeling the cure kinetics of

*Macromolecules,* Vol.24, No.11, (May 1991), pp. 3098-3110, ISSN 0024-9297. Cotugno, S.; Mensitieri, G.; Musto,P.; Sanguigno, L. (2005). Molecular interactions in and

Cotugno, S.; Larobina, D.; Mensitieri, G.; Musto, J.; Ragosta, G. (2001). A novel spectroscopic

system. *Polymer,* Vol.42, No.15, (July 2001), pp. 6431-6438, ISSN 0032-3861.

(February 2003), pp. 801-811, ISSN 0024-9297.

*Technology,* Vol.143, No.SI, (2003), pp. 311-315, ISSN 0924-0136.

*Polymer,* Vol.43, No.1, (January 2002), pp. 131-139, ISSN 0032-3861.

*Science,* Vol.94, No. 1, (September 2004), pp. 361-371, ISSN 1097-4628. Blanco, I.; Cicala, G.; Costa, M.; Recca, A. (2006). Development of an epoxy system

(November 2007), pp. 808-819, ISSN 0965-545X.

Vol.97, No.3, (June 2009), pp. 969-978, ISSN 1388-6150.

B.G.; Vygodskii, Y.S.; Askadskii, A.A.; Philippova, O.E.; Khokhlov, A.R. (2008). Nanocomposites Based on Epoxy Resin and Silicon Dioxide Particles. *Nanocomposites Based on Epoxy Resin and Silicon Dioxide Particles,* Vol. 50, No. 7,

phase separation in epoxy/thermoplastic polymer blends. *Journal of Applied Polymer* 

characterized by low water absorption and high thermomechanical performances. *Journal of Applied Polymer Science,* Vol.100, No.6, (June 2006), pp. 4880–4887, ISSN

Thermoplastic-modified epoxy resins cured with different functionalities amine mixtures. Kinetics and miscibility study. *Journal of Thermal Analysis and Calorimetry,* 

polymeric modifiers. *Polymer Engineering Science,* Vol.26, No.1, (January 1986), pp.

polyaminosiloxane-epoxy thermosetting polymers. *Journal of Materials Processing* 

in initially heterogeneous thermosets blended with poly(methyl methacrylate).

Thermosets: Materials for Environmentally Friendly Processing and Reuse.

an epoxy resin. *Applied Spectroscopy,* Vol.47, No.10, (October 1993), pp. 1631-1635,

epoxy amine thermosetting resins 2. Application to a typical system based on bis[4- (diglycidylamino)phenyl]methane and bis(2-aminophenyl) sulphone.

transport properties of densely cross-linked networks: A time-resolved FT-IR spectroscopy investigation of the epoxy/H2O system. *Macromolecules,* Vol.38, No.3,

approach to investigate transport processes in polymers: the case of water-epoxy

The small volume changes due to swelling are prone to large errors if determined by usual means (for example, with a caliper). Figure 13 shows good correlations between the fractional volume change during water uptake in an epoxy resin as measured gravimetrically, by FTIR or measuring the change on dimensions of the specimens.

Fig. 13. Comparison of volume change determined by n-FTIR, (V/V0)IR, and measured with a micrometer, (left) or by gravimetry (right), for fully cured DGEBA/ Poly(aminopropylsiloxane) with 0.381 mm thickness.

## **6. Conclusion**

The curing and ageing of epoxy resins are complex phenomena of the prime importance in industry. FTIR appears to be a valuable tool for both qualitative analysis and quantification of these processes. It has been shown how to extract relevant information from spectra to identify typical components of resins and hardeners. Following time variations of specific bands allows extracting relevant kinetic parameters to get more insight about the specific reaction mechanism of curing process. Inspection of subtle changes in baseline can be correlated with both, miscibilization or phase separation processes. Detailed analysis of OH bands allows extracting information about intermolecular interactions within the components of the resin. And, finally, water uptake can be easily quantified and both diffusion coefficients and dimensional changes can be measured with less error than other common methods.

## **7. Acknowledgment**

Authors wish to thank Ministerio de Ciencia e Innovación (Spain) for partial funding under project MAT2010-17091.

## **8. References**

Bhargava, R.; Wang, S.Q.; Koenig, J.L. (1999). Studying polymer-dispersed liquid-crystal formation by FTIR spectroscopy. 2. Phase separation and ordering. *Macromolecules,*  Vol.32, No.26, (December 1999), pp.8989-8995, ISSN 0024-9297.

The small volume changes due to swelling are prone to large errors if determined by usual means (for example, with a caliper). Figure 13 shows good correlations between the fractional volume change during water uptake in an epoxy resin as measured

gravimetrically, by FTIR or measuring the change on dimensions of the specimens.

Fig. 13. Comparison of volume change determined by n-FTIR, (V/V0)IR, and measured

The curing and ageing of epoxy resins are complex phenomena of the prime importance in industry. FTIR appears to be a valuable tool for both qualitative analysis and quantification of these processes. It has been shown how to extract relevant information from spectra to identify typical components of resins and hardeners. Following time variations of specific bands allows extracting relevant kinetic parameters to get more insight about the specific reaction mechanism of curing process. Inspection of subtle changes in baseline can be correlated with both, miscibilization or phase separation processes. Detailed analysis of OH bands allows extracting information about intermolecular interactions within the components of the resin. And, finally, water uptake can be easily quantified and both diffusion coefficients and

Authors wish to thank Ministerio de Ciencia e Innovación (Spain) for partial funding under

Bhargava, R.; Wang, S.Q.; Koenig, J.L. (1999). Studying polymer-dispersed liquid-crystal

formation by FTIR spectroscopy. 2. Phase separation and ordering. *Macromolecules,* 

dimensional changes can be measured with less error than other common methods.

Vol.32, No.26, (December 1999), pp.8989-8995, ISSN 0024-9297.

with a micrometer, (left) or by gravimetry (right), for fully cured DGEBA/

Poly(aminopropylsiloxane) with 0.381 mm thickness.

**6. Conclusion** 

**7. Acknowledgment** 

project MAT2010-17091.

**8. References** 


Applications of FTIR on Epoxy Resins –

ISSN 0032-3861.

CRC Press, ISBN, Boca Ratón.

111-119, ISSN 1022-1360.

6150.

ISSN 0014-3057.

ISBN, New Jersey.

0824776909, New York.

1279-1288, ISSN 0024-9297.

(April 1995), pp.2787-2796, ISSN 0024-9297.

1995), pp. 2797-2806, ISSN 0024-9297.

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 281

Kiefer, J.; Hilborn, J.G.; Hedrick, J.L. (1996). Chemically induced phase separation: A new

Kim, Y.M.; Kostanski, L.K.; Mac Gregor, J.F. (2003). Photopolymerization of 3,4-

Kradjel, C.; Lee, K.A. (2008). *NIR analysis of polymers, in Handbook of near infrared analysis,* 

Liu, H.; Uhlherr, A.; Varley, R.J.; Bannister, M.K. (2004). Influence of substituents on the

*Polymer Chemistry,* Vol.42, No.13, (July 2004), pp. 3143–3156, ISSN 1099-0518. González, M.; Kindelán, M.; Cabanelas, J.C.; Baselga,J. (2003). Modelling auto-acceleration in

Gonzalez, M.; Cabanelas, J.C.; Pozuelo, J.; Baselga, J.(2011). Preparation of cycloaliphatic

Marieta, C.; Remiro, G.; Garmendia, G.; Harismendy, I.; Mondragón, I. (2003). AFM

Mark, H.F. (2004). *Encyclopaedia of polymer science and technology,* Vol. 9*,* Wiley and Sons,

Matejka, L. (2000). Amine cured epoxide networks: Formation, structure, and properties. *Macromolecules,* Vol.33, No. 10, (May 2000), pp. 3611-3619, ISSN 0024-9297. May, C.A. (1988). *Epoxi resins, Chemistry and Technology* (2nd)*,* Marcel Dekker, ISBN

Mezzenga, R.; Boogh, L.; Manson, J.A.E. (2000). A thermodynamic model for thermoset

Mijovic, J.; Zhang, H. (2003). Local dynamics and molecular origin of polymer network-

Mijovic, J.; Andjelic, S. (1995). A study of reaction kinetics by near infrared spectroscopy 1.

Mijovic, J.; Andjelic, S.; Yee, C.F.W.; Bellucci, F.; Nicolais, L. (1995). A study of reaction

*Physics,* Vol.38, No.14, (July 2000), pp.1893-1902, ISSN 1099-0488.

polymer blends with reactive modifiers. *Journal of Polymer Science Part B: Polymer* 

water interactions as studied by broadband dielectric relaxation spectroscopy, FTIR, and molecular simulations. *Macromolecules,* Vol.36, No.4, (February 2003), pp.

Comprehensive analysis of a model epoxy/amine. *Macromolecules,* Vol.28, No.8,

kinetics by near infrared spectroscopy 2. Comparison with dielectric spectroscopy of model and multifunctional epoxy/amine. *Macromolecules,* Vol.28, No.8, (April

No.25, (1996), pp. 5715-5725, ISSN 0032-3861.

technique for the synthesis of macroporous epoxy networks. *Polymer,* Vol.37,

epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate and tri (ethylene glycol) methyl vinyl ether. *Polymer,* Vol. 44, No.18, (August 2003), pp.5103-5109,

kinetics of epoxy/aromatic diamine resin systems. *Journal of Polymer Science Part A:* 

DGEBA/diamine systems. *Macromolecular Symposia,* Vol.200, (September 2003), pp.

epoxy hybrids with non-conventional amine-curing agents. *Journal of Thermal Analysis and Calorimetry,* Vol.103, No.2, (February 2011), pp. 717-723, ISSN 1388-

approach toward understanding morphologies in toughened thermosetting matrices. *European Polymer Journal,* Vol.39, No.10, (October 2003), pp. 1965-1973,


Crivello, J.V.; Fan, M. (1991). Novel platinum-containing initiators for ring-opening

Crivello, J.V.; Liu, S. (2000). Photoinitiated cationic polymerization of epoxy alcohol

Crivello, J.V. (1993). In: *Ring-Opening Polymerization,* DJ Brunelle, pp. 157-159, Ed., Hanser,

Ehlers J.E.; Rondan, N.G.; Huynh, L.K.; Pham, H.; Marks, M.; Truong, T.N. (2007).

George, G.A.; Clarke, P.C.; Jhon, N.S.; Friend, G. (1991). Real time monitoring of the cure

*Polymer Science,* Vol.42, No.3, (February 1991), pp. 643-657, ISSN 0021-8995. Girard-Reydet, E.; Sautereau, H.; Pascault, J.P. (1999). Use of block copolymers to control the

Gonzalez-Benito, J.; Bravo, J.; Mikes, J.; Baselga, J. (2003). Fluorescence labels to monitor

Grave, C.; McEwan, I.; Pethrick, R.A. (1998). Influence of stoichiometric ratio on water

Hartwig, A.; Schenider, B.; Lühring, A. (2002). Influence of moisture on the photochemically

Hartwig, A.; Koschek, K.; Luhring, A.; Schorsch, O. (2003). Cationic polymerization of a

Inoue, T. (1995). Reaction-induced phase decomposition in polymer blends. *Progress in* 

Ji, W.; Hu, J.; Zhang, J.; Cao, C. (2006). Reducing the water absorption in epoxy coatings by

*Polymer Science,* Vol.20, No.1, (1995), pp.119-153, ISSN 0079-6700.

*Polymer,* Vol.43, No.15, (July 2003), pp. 4243-4250, ISSN 0032-3861 .

*Macromolecules*, Vol.40, No.12, (2007), pp.4370-4377, ISSN 0024-9297. Dannenberg, H.; Harp, W.R. (1956). Determination of cure and analysis of cured epoxy resins. *Analytical Chemistry,* Vol.28, No.1, (1956), pp.81-90, ISSN 0003-2700. Flory, P.J. (1953). *Principles of Polymer Chemistry,* Cornell University Press, ISBN 0801401348,

(December 1991), pp. 1853-1863, ISSN 0887-624X.

No.7, (March 1999), pp.1677-1687, ISSN 0032-3861.

(September 1998), pp. 2369-2376, ISSN 0021-8995.

2000), pp. 389-401, ISSN 0887-624X.

ISBN 3446162933, Munich.

Ithaca.

659, ISSN 0032-3861.

0449-296X.

pp. 3731–3739, ISSN 0010-938X.

polymerizations. *Journal of Polymer Science A: Polyme Chemistry,* Vol.29, No.13,

monomers. *Journal of Polymer Science A: Polymer Chemistry,* Vol.38, No.3, (February

Theoretical study on mechanisms of the epoxy-amine curing reaction.

reaction of a TGDDM/DDS epoxy resin using fiber optic FT-IR. *Journal of Applied* 

morphologies and properties of thermoplastic/thermoset blends. *Polymer,* Vol.40,

water absorption in epoxy resins. *Polymer,* Vol.44, No.3, (February 2003), pp. 653–

absorption in epoxy resins. *Journal of Applied Polymer Science,* Vol.69, No.12,

induced polymerisation of epoxy groups in different chemical environment.

cycloaliphatic diepoxide with latent initiators in the presence of structurally different diols. *Polymer,* Vol.44, No.10, (May 2003), pp. 2853-2858, ISSN 0032-3861. Horie, K.; Hiura, H.; Sawada, M.; Mitta, I.; Kambe, H. (1970). Calorimetic investigation of

polymerization reactions 3: Curing reaction of epoxides with amines. *Journal of Polymer Science Part A-1- Polymer Chemistry,* Vol.8, No.6, (1970), pp. 1357-&, ISSN

silane monomer incorporation. *Corrosion Science,* Vol.48, No.11, (November 2006),


Applications of FTIR on Epoxy Resins –

855, ISSN 0272-8397.

Chischester, England

pp. 1867-1873, ISSN 0024-9297.

pp. 6202-6205, ISSN 0024-9297.

1352.

1837.

3861.

Vol.2, No.1, (1961), pp. 95-108, ISSN 0032-3861.

No.4, (January 2007), pp. 1470-1479, ISSN 0014-3057.

Vol.99, No.6 (March 2006), pp. 3288-3299, ISSN 0021-8995.

Identification, Monitoring the Curing Process, Phase Separation and Water Uptake 283

Schechter, L.; Wynstra, J.; Kurkjy, R. (1956). Glycidyl Ether Reactions with Alcohols,

Simon, S.L.; McKenna, G.B.; Sindt, O. (2000). Modelling the evolution of the dynamic

Socrates, G. (1994). *Infrared Characteristic Group Frequencies,* Wiley & Sons, ISBN 0471942308,

Soucek, M.D.; Abu-Shanab, O.L.; Anderson, C.D.; Wu, S. (1998). Kinetic Modeling of the

Tao Z, Yang S, Chen J, Fan L. (2007). Synthesis and characterization of imide ring and

Varley, R.J.; Liu, W.; Simon, G.P. (2006). Investigation of the reaction mechanism of different

Vyazovkin, S.; Sbirrazuoli, N. (1996). Mechanism and kinetics of epoxy-amine cure studied

Wang, F.; Neckers, D.C. (2001). Photopolymerization of Epoxides with Platinum(II)

Wang, Z.; Xie, M.; Zhao, Y.; Yu, Y.; Fang, S. (2003). Synthesis and properties of novel liquid

Xiao, G.Z.; Shanahan, M.E.R. (1998). Swelling of DGEBA/DDA epoxy resin during

Xu, L.S.; Schlup, J.R. (1998). Etherification versus amine addition during epoxy resin amine

Xu, L.; Fu, J.H.; Schlup, J.R. (1996). In situ near-infrared spectroscopic investigation of the

*Science,* Vol.67, No.5, (January 1998), pp. 895-90, ISSN 0021-8995.

Phenols, Carboxylic Acids, and Acid Anhydrides. *Journal of Industrial and Engineering Chemistry,* Vol.48, No.1, (January 1956), pp. 86-93, ISSN 1226-086X. Siddhamalli, S.K. (2000). Toughening of epoxy/polycaprolactone composites via reaction

induced phase separation. *Polymer Composites,* Vol.21, No.5, (October 2000), pp.846-

mechanical properties of a commercial epoxy during cure after gelation. *Journal of Applied Polymer Science,* Vol.76, No.4, (April 200), pp. 495-508, ISSN 0021-8995. Smith, I.T. (1961). The mechanism of the crosslinking of epoxide resins by amines. *Polymer,* 

Crosslinking Reaction of Cycloaliphatic Epoxides with Carboxyl Functionalized Acrylic Resins: Hammett Treatment of Cycloaliphatic Epoxides. *Macromolecular Chemistry and Physics,* Vol.199, No.6, (December 1998), pp. 1035-1042, ISSN 1022-

siloxane-containing cycloaliphatic epoxy resins. *European Polymer Journal,* Vol.43,

epoxy resins using a phosphorus-based hardener. *Journal of Applied Polymer Science,* 

by differential scanning calorimetry. *Macromolecules,* Vol.29, No.6, (March 1996),

Bis(acetylacetonato)/Silane Catalysts. *Macromolecules,* Vol.34, No.18, (August 2001),

ester-free reworkable cycloaliphatic diepoxides for electronic packaging application. *Polymer,* Vol.44, No.4, (January 2003), pp.923-929, ISSN 0032-3861. Weyer, L. and Lo, S.-C. (2002). *Spectra-Structure Correlations in the Near-infrared, In Handbook* 

*of Vibrational Spectroscopy, Vol. 3,* Wiley and Sons, ISBN 0471988472, UK, pp. 1817-

hygrothermal ageing. *Polymer,* Vol.39, No.14, (June 1998), pp. 3253-3260, ISSN 0032-

cure: An in situ study using near-infrared spectroscopy. *Journal of Applied Polymer* 

kinetics and mechanisms of reactions between phenyl glycidyl ether (PGE) and


Mikes, F.; Baselga, J.; Paz-Abuin, S. (2002). Fluorescence probe-label methodology for in situ

Musto, P.; Mascia, L.; Ragosta, G.; Scarinzi, G.; Villano, P. (2000). The transport of water in a

Musto, P.; Ragosta, G.; Mensitier, G. (2002). Time-resolved FTIR/FTNIR spectroscopy:

Nogueira, P.; Ramirez, C.; Torres, A.; Abad, M.J.; Cano, J.; Lopez-bueno, I.; Barral,L. (2001).

Pascault, J.P., Sautereau, H.; Verdu, J.; Williams, R.J.J. (2002). *Thermosetting Polymer,* Marcel

Paz-Abuin, S.; Lopez-Quintela, A.; Varela, M.; Pellin, M.P.; Prendes, P. (1997). Method for

Paz-Abuin, S.; Pellin, M.P.; Paz-Pazos, M.; Lopez-Quintela, A. (1997). Influence of the

Paz-Abuín, S.; Lopez-Quintela, A.; Varela, M.; Pellín, M.P.; Prendes, P. (1998).

Poisson, N.; Lachenal, G.; Sautereau, H. (1996). Near- and mid-infrared spectroscopy studies

Rajagopalan, G.; Gillespie, J.W.; McKnight, S.H. (2000). Diffusion of reacting epoxy and

Riccardi, C.C.; Williams, R.J.J. (1986). A kinetic scheme for an amine-epoxy reaction with

Riccardi, C.C.; Addabo, H.E.; Williams, R.J.J. (1984). Curing reaction of epoxy resins with

*Polymer,* Vol. 41, No.2, (January 2000), pp.565-574, ISSN 0032-3057.

membranes. *E-Polymers,* Article Nº 017 (April 2002), ISSN 1618-7229. Nikolic, G.; Zlatkovic, S.; Cakic, M.; Cakic, S.; Lacnjevac, C; Rajic, Z. (2010). Fast Fourier

(December 2002), pp. 2393–2404, ISSN 0014-3057.

pp. 684-696, ISSN 1424-8220.

3795-3804, ISSN 0032-3861.

pp.237-247, ISSN 0924-2031.

3445-3456, ISSN 0024-9297.

ISSN 0021-8995.

(October 2000), pp. 7723-7733, ISSN 0032-3861.

ISSN 0887-624X.

Dekker Inc, ISBN 0824706706, New York.

ISSN 0021-8995.

monitoring network forming reactions. *European Polymer Journal,* Vol.38, No.12,

tetrafunctional epoxy resin by near-infrared Fourier transform spectroscopy.

powerful tools to investigate diffusion processes in polymeric films and

Transform IR Characterization of Epoxy GY Systems Crosslinked with Aliphatic and Cycloaliphatic EH Polyamine Adducts. *Sensors,* Vol.10, No.1, (January 2010),

Effect of water sorption on the structure and mechanical properties of an epoxy resin system*. Journal of Applied Polymer Science,* Vol.80, No.1, (April 2001), pp. 71-80,

determination of the ratio of rate constants, secondary to primary amine, in epoxyamine systems. *Polymer,* Vol.38, No.12, (June 1997), pp. 3117-3120, ISSN 0032-3861.

reactivity of amine hydrogens and the evaporation of monomers on the cure kinetics of epoxy-amine: kinetic questions. *Polymer,* Vol.38, No.15, (July 1997), pp.

Autoacceleration and inhibition: Free volume. Epoxy-amine kinetics. *Journal of Polymer Science A: Polymer Chemistry*, Vol.36, No.6, (April 1998), pp. 1001–1016,

of an epoxy reactive system. *Vibrational Spectroscopy,* Vol.12, No.2, (October 1996),

amine monomers in polysulfone: a diffusivity model. *Polymer,* Vol.41, No.21,

simultaneous. *Journal of Applied Polymer Science,* Vol.32, No.2, (August 1986), pp.

diamines. *Journal of Applied Polymer Science,* Vol.32, No.8, (1984), pp. 2841-2492,


**14** 

*Italy* 

**Use of FTIR Analysis to Control the** 

Liberata Guadagno1,2,\* and Marialuigia Raimondo1

*1Dipartimento di Ingegneria Industriale, Università di Salerno, Fisciano (SA)* 

*Università di Salerno, Fisciano (SA)* 

**Self-Healing Functionality of Epoxy Resins** 

*2Nano-Mates – Research Centre for NANOMAterials and nanoTEchnology,* 

In recent years polymer composites are increasingly used to replace the traditional metal alloys in structural applications, ranging from civil infrastructure to high performance vehicles such as racing cars and military aircraft. This popularity is due to their lower weight, as well as to a continuous improvement of their performance aided in recent years by nanotechnology. However, limited storage stability and reliability are critical for polymer composites designed for structural applications [1-4]. In fact, in service, they are subject to damage due to microcracks that are produced in the structure under the action of various kinds of stresses, for example: a) mechanical vibrations or various types of mechanical stresses, b) sudden temperature changes, c) irradiation by electromagnetic radiation causing direct or indirect rupture of chemical bonds (UV light, γ rays, etc.), d) intentional or inadvertent contact with chemical substances that adversely affect the structure, e) various factors which in combination can contribute to compromising the integrity of the structure. Internal damage is difficult to detect and, once developed, even more difficult to repair. This critical point is a real problem in the field of aeronautic vehicles. In fact, for large components, such as parts of primary structures, several non-destructive damage detection techniques have been developed including ultrasonic, infrared thermography, x-ray tomography, and computerized vibro-thermography. This technology has helped to detect damage but repair of this damage has been limited to reinforced patch bonding and/or bolting. Actually, durability and reliability are still problematic in the field of these structural materials; in fact, in order to achieve the mechanical strength required for many structural applications, highly cross-linked polymeric materials are necessary. The trade off for this gain in mechanical strength is that the resulting materials tend to be brittle and are

therefore more prone to developing cracks trough normal usage, ultimately failing.

In addition to conventional methods for damage detection, and common repair methods, there has recently been the development of self-healing composites which are expected to significantly extend the life of polymeric components by autonomically healing micro-cracks

**1. Introduction** 

 \*

Corresponding Author

multifunctional aromatic amines. *Industrial and Engineering Chemistry Research,*  Vol.35, No. 3, (March 1996), pp. 963-972, ISSN 0888-5885.


## **Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins**

Liberata Guadagno1,2,\* and Marialuigia Raimondo1

*1Dipartimento di Ingegneria Industriale, Università di Salerno, Fisciano (SA) 2Nano-Mates – Research Centre for NANOMAterials and nanoTEchnology, Università di Salerno, Fisciano (SA) Italy* 

## **1. Introduction**

284 Infrared Spectroscopy – Materials Science, Engineering and Technology

Yagci, Y.; Reetz, I. (1998). Externally stimulated initiator systems for cationic polymerization.

Zhou, J.; Lucas, J.P. (1999). Hygrothermal effects of epoxy resin. Part I: the nature of water in epoxy. *Polymer,* Vol.40, No.20, (June 1999), pp. 5505–5512, ISSN 0032-3861.

Vol.35, No. 3, (March 1996), pp. 963-972, ISSN 0888-5885.

0079-6700.

multifunctional aromatic amines. *Industrial and Engineering Chemistry Research,* 

*Progress in Polymer Science,* Vol.23, No.8, (December 1998), pp. 1485-1538, ISSN

In recent years polymer composites are increasingly used to replace the traditional metal alloys in structural applications, ranging from civil infrastructure to high performance vehicles such as racing cars and military aircraft. This popularity is due to their lower weight, as well as to a continuous improvement of their performance aided in recent years by nanotechnology. However, limited storage stability and reliability are critical for polymer composites designed for structural applications [1-4]. In fact, in service, they are subject to damage due to microcracks that are produced in the structure under the action of various kinds of stresses, for example: a) mechanical vibrations or various types of mechanical stresses, b) sudden temperature changes, c) irradiation by electromagnetic radiation causing direct or indirect rupture of chemical bonds (UV light, γ rays, etc.), d) intentional or inadvertent contact with chemical substances that adversely affect the structure, e) various factors which in combination can contribute to compromising the integrity of the structure. Internal damage is difficult to detect and, once developed, even more difficult to repair. This critical point is a real problem in the field of aeronautic vehicles. In fact, for large components, such as parts of primary structures, several non-destructive damage detection techniques have been developed including ultrasonic, infrared thermography, x-ray tomography, and computerized vibro-thermography. This technology has helped to detect damage but repair of this damage has been limited to reinforced patch bonding and/or bolting. Actually, durability and reliability are still problematic in the field of these structural materials; in fact, in order to achieve the mechanical strength required for many structural applications, highly cross-linked polymeric materials are necessary. The trade off for this gain in mechanical strength is that the resulting materials tend to be brittle and are therefore more prone to developing cracks trough normal usage, ultimately failing.

In addition to conventional methods for damage detection, and common repair methods, there has recently been the development of self-healing composites which are expected to significantly extend the life of polymeric components by autonomically healing micro-cracks

<sup>\*</sup> Corresponding Author

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 287

properties of the manufactured materials. Since 2003, a large number of parameters were investigated by the team from the University of Illinois and other research groups. In a recent paper, the dissolution properties, initial polymerization kinetics, chemical stabilities, and thermal stabilities were analyzed for three catalysts: Grubbs' first (G1) and second generation (G2) catalysts and Hoveyda–Grubbs' second generation catalyst (HG2) [30]. Ruthenium-based catalysts are reported as exhibiting great functional group tolerance, as well as greatly enhanced air and water stability [30,31]. However, thermolytic decomposition can limit the usefulness of these ruthenium systems in self-healing composites based on epoxy resins [32-33]. This is a crucial aspect for self-healing systems in aeronautic applications because, even if linked to the cost reduction from a process aspect, resins have been developed with low temperature manufacturing (under 100 °C), yet the

In fact, we generally need a glass transition temperature after wet aging of 110 °C minimum, and a curing temperature equal to or less than 100 °C is not enough. To achieve this goal it is necessary to make a post cure with a temperature that could be as high as 180 °C. For this reason, we have carried out our investigation in the temperature range of 150–180 °C. It is worth noting that there are critical issues in the use of epoxy precursors in conjunction with Grubbs metathesis catalysts, because in self healing composites based on epoxy resins, ruthenium systems give rise to a reaction with the oxirane rings of the epoxy precursors, and, therefore, the metathesis catalyst was not subsequently able to promote the polymerization of the reactive monomer, thus losing the self-healing ability. Such phenomena could strongly limit the use in the practice, in self-healing composite materials, of Grubbs' catalysts. In particular, in this chapter, we show that selecting the appropriate curing cycle as well as the specific chemical formulation, the catalyst remains intact in the formed epoxy matrix during the curing process, and is thus capable of subsequently performing its catalytic activity of the polymerization of the reactive monomer consisting in the cyclic olefin (5-ethylidene-2-norbornene –ENB-), when the latter comes out of a microcapsule affected by a crack. Studies on the choice of an appropriate curing cycle for a different self-healing formulation have already been reported in previous papers [32-35]. Another drawback to overcome in the employment of the ROMP catalysts is related to the local availability of the catalyst particles. In particular, the catalyst for the metathesis reaction is embedded in the precursors of the epoxy matrix in the form of solid particles, i.e. powders with different morphology and crystallographic modifications [30]. In practice, the effective concentration of the catalyst depends on the availability of the aforesaid particles at the level of the fracture and on the rate of dissolution of the catalyst in the reactive monomer (healing agent) within the polymer matrix. Even with high concentrations of catalyst particles exposed at the level of the fracture, the effective concentration of the catalyst could be relatively low because of limited rates of dissolution of the catalyst. The rate of dissolution of the catalyst depends not only on the chemical nature of the various components, but also on morphological and structural characteristics of the catalyst particles. It has been found in practice that the presence of the catalyst in the form of crystalline powders has some critical aspects relating to the uniform availability of the catalyst in all the zones in which a micro-crack can potentially develop, compromising the

Figure 1 illustrates as, independently from the catalyst amount, large crystals of catalyst particles can cause loss of catalytic activity in many zones. In addition, a very effective self-

problem of the material treatment at high temperature is not resolved.

effectiveness of the self-healing process [35].

whenever and wherever they develop [5-15]. For structural applications, self-healing systems are of great interest because they would allow to overcome not only some difficulties connected to damage diagnosis, but also the following appropriate interventions to restore the material functionality. Airlines for example see polymeric composites' potential to cut fuel costs and save on maintenance very attractive, therefore, are willing to entertain the idea to put into action self-healing composites in the development of different materials for aeronautical applications. The challenge facing materials scientists is to assure these systems must be able to stem fatigue damage and preserve their integrity, increase their life span, reduce maintenance costs and provide safety during use. A lot of strategies were formulated up to now in the development of self-healing materials [5,16–27].

One of the strategies to manufacture a thermosetting self-repair material seems to be the storage of healing agents inside composites that restore the strength of the materials after damage.

The first demonstration of self-healing in an engineered material, an epoxy matrix, occurred in 2001 [5] by a team from the University of Illinois (USA). This self-healing system for thermosetting materials is very interesting also in terms of design. The concept was based on the introduction of a microencapsulated healing agent and suspended catalyst phase in a polymer matrix. Since that time, advancement has been made in the field following this conceptual approach, while at the same time alternative concepts have emerged in the scientific literature. Microencapsulation and thermally reversible networks [28] are the two main strategies used for the development of self-healing thermosetting materials. Microencapsulation is the first and most studied self-healing concept. The initial self-healing epoxy system developed by White et al. involves the incorporation of a microencapsulated healing agent, dicyclopentadiene (DCPD) and a dispersed solid catalyst, bis(tricyclohexylphosphine) benzylidine ruthenium (IV) dichloride (called Grubbs' catalyst) in an epoxy-amine network. In these systems, an approaching crack ruptures embedded microcapsules releasing a polymerizer agent into the crack plane through capillary action. Polymerization of the healing agent is triggered by contact with the embedded catalyst, bonding the crack faces. In these systems the efficiency of self-repair function, in terms of trigger, speed and yield, is related to ring-opening metathesis polymerization of the healing agent by appropriate catalysts. The healing agent is a microencapsulated liquid monomer that must include a long shelf life, prompt deliverability, high reactivity, and low volume shrinkage upon polymerization [29]. The monomer most often used as the healing agent for the manufacture of these first ingenious systems is dicyclopentadiene (DCPD) [5,29]. Very recently, however, blends of DCPD/5-ethylidene-2-norbornene (DCPD/ENB) or DCPD/ 5 norbornene-2carboxylic acid have also been proposed [30]. Thermosetting auto-repair polymers, which have been proposed so far, include Grubbs' first-generation catalyst (G1); [5,17,29-30] and currently, the possibility of applying other ruthenium catalysts for ringopening metathesis polymerization-based self-healing applications is being evaluated. This system is a challenge for epoxy structural composites: however, some drawbacks have to be re-evaluated in order to be fully applied to advanced applications. These mainly regard the thermal stability of the Grubbs' catalyst inside the epoxy resin during the curing cycle and the impossibility to utilize primary amines as hardeners, since they can poison the catalyst.

This last drawback forces the use of compounds which have not been fully explored in literature as hardener agents, especially with regard to the cure behavior and mechanical

whenever and wherever they develop [5-15]. For structural applications, self-healing systems are of great interest because they would allow to overcome not only some difficulties connected to damage diagnosis, but also the following appropriate interventions to restore the material functionality. Airlines for example see polymeric composites' potential to cut fuel costs and save on maintenance very attractive, therefore, are willing to entertain the idea to put into action self-healing composites in the development of different materials for aeronautical applications. The challenge facing materials scientists is to assure these systems must be able to stem fatigue damage and preserve their integrity, increase their life span, reduce maintenance costs and provide safety during use. A lot of strategies

One of the strategies to manufacture a thermosetting self-repair material seems to be the storage of healing agents inside composites that restore the strength of the materials after

The first demonstration of self-healing in an engineered material, an epoxy matrix, occurred in 2001 [5] by a team from the University of Illinois (USA). This self-healing system for thermosetting materials is very interesting also in terms of design. The concept was based on the introduction of a microencapsulated healing agent and suspended catalyst phase in a polymer matrix. Since that time, advancement has been made in the field following this conceptual approach, while at the same time alternative concepts have emerged in the scientific literature. Microencapsulation and thermally reversible networks [28] are the two main strategies used for the development of self-healing thermosetting materials. Microencapsulation is the first and most studied self-healing concept. The initial self-healing epoxy system developed by White et al. involves the incorporation of a microencapsulated healing agent, dicyclopentadiene (DCPD) and a dispersed solid catalyst, bis(tricyclohexylphosphine) benzylidine ruthenium (IV) dichloride (called Grubbs' catalyst) in an epoxy-amine network. In these systems, an approaching crack ruptures embedded microcapsules releasing a polymerizer agent into the crack plane through capillary action. Polymerization of the healing agent is triggered by contact with the embedded catalyst, bonding the crack faces. In these systems the efficiency of self-repair function, in terms of trigger, speed and yield, is related to ring-opening metathesis polymerization of the healing agent by appropriate catalysts. The healing agent is a microencapsulated liquid monomer that must include a long shelf life, prompt deliverability, high reactivity, and low volume shrinkage upon polymerization [29]. The monomer most often used as the healing agent for the manufacture of these first ingenious systems is dicyclopentadiene (DCPD) [5,29]. Very recently, however, blends of DCPD/5-ethylidene-2-norbornene (DCPD/ENB) or DCPD/ 5 norbornene-2carboxylic acid have also been proposed [30]. Thermosetting auto-repair polymers, which have been proposed so far, include Grubbs' first-generation catalyst (G1); [5,17,29-30] and currently, the possibility of applying other ruthenium catalysts for ringopening metathesis polymerization-based self-healing applications is being evaluated. This system is a challenge for epoxy structural composites: however, some drawbacks have to be re-evaluated in order to be fully applied to advanced applications. These mainly regard the thermal stability of the Grubbs' catalyst inside the epoxy resin during the curing cycle and the impossibility to utilize primary amines as hardeners, since they can poison the catalyst. This last drawback forces the use of compounds which have not been fully explored in literature as hardener agents, especially with regard to the cure behavior and mechanical

were formulated up to now in the development of self-healing materials [5,16–27].

damage.

properties of the manufactured materials. Since 2003, a large number of parameters were investigated by the team from the University of Illinois and other research groups. In a recent paper, the dissolution properties, initial polymerization kinetics, chemical stabilities, and thermal stabilities were analyzed for three catalysts: Grubbs' first (G1) and second generation (G2) catalysts and Hoveyda–Grubbs' second generation catalyst (HG2) [30]. Ruthenium-based catalysts are reported as exhibiting great functional group tolerance, as well as greatly enhanced air and water stability [30,31]. However, thermolytic decomposition can limit the usefulness of these ruthenium systems in self-healing composites based on epoxy resins [32-33]. This is a crucial aspect for self-healing systems in aeronautic applications because, even if linked to the cost reduction from a process aspect, resins have been developed with low temperature manufacturing (under 100 °C), yet the problem of the material treatment at high temperature is not resolved.

In fact, we generally need a glass transition temperature after wet aging of 110 °C minimum, and a curing temperature equal to or less than 100 °C is not enough. To achieve this goal it is necessary to make a post cure with a temperature that could be as high as 180 °C. For this reason, we have carried out our investigation in the temperature range of 150–180 °C. It is worth noting that there are critical issues in the use of epoxy precursors in conjunction with Grubbs metathesis catalysts, because in self healing composites based on epoxy resins, ruthenium systems give rise to a reaction with the oxirane rings of the epoxy precursors, and, therefore, the metathesis catalyst was not subsequently able to promote the polymerization of the reactive monomer, thus losing the self-healing ability. Such phenomena could strongly limit the use in the practice, in self-healing composite materials, of Grubbs' catalysts. In particular, in this chapter, we show that selecting the appropriate curing cycle as well as the specific chemical formulation, the catalyst remains intact in the formed epoxy matrix during the curing process, and is thus capable of subsequently performing its catalytic activity of the polymerization of the reactive monomer consisting in the cyclic olefin (5-ethylidene-2-norbornene –ENB-), when the latter comes out of a microcapsule affected by a crack. Studies on the choice of an appropriate curing cycle for a different self-healing formulation have already been reported in previous papers [32-35]. Another drawback to overcome in the employment of the ROMP catalysts is related to the local availability of the catalyst particles. In particular, the catalyst for the metathesis reaction is embedded in the precursors of the epoxy matrix in the form of solid particles, i.e. powders with different morphology and crystallographic modifications [30]. In practice, the effective concentration of the catalyst depends on the availability of the aforesaid particles at the level of the fracture and on the rate of dissolution of the catalyst in the reactive monomer (healing agent) within the polymer matrix. Even with high concentrations of catalyst particles exposed at the level of the fracture, the effective concentration of the catalyst could be relatively low because of limited rates of dissolution of the catalyst. The rate of dissolution of the catalyst depends not only on the chemical nature of the various components, but also on morphological and structural characteristics of the catalyst particles. It has been found in practice that the presence of the catalyst in the form of crystalline powders has some critical aspects relating to the uniform availability of the catalyst in all the zones in which a micro-crack can potentially develop, compromising the effectiveness of the self-healing process [35].

Figure 1 illustrates as, independently from the catalyst amount, large crystals of catalyst particles can cause loss of catalytic activity in many zones. In addition, a very effective self-

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 289

tris[(dimethylamino) methyl] (Trade name Ancamine K54). This hardener agent was already used for self-healing formulations [5]. The catalyst, Hoveyda-Grubbs' first generation (Dichloro(o-isopropoxyphenylmethylene)(tricyclo-hexylphosphine)ruthenium(II) (catalyst HG1) also obtained from Aldrich was used to manufacture the epoxy matrix. It was dispersed at molecular level into the epoxy matrix. The healing agents used in this work were dicyclopentadiene (DCPD), which was obtained from Acros Organics, and 5 ethylidene-2-norbornene (ENB), which was obtained from Sigma-Aldrich. Figure 2 shows

the chemical structures of compounds used in this work.

Fig. 2. Chemical structures of compounds used for the self-healing system.

Fig. 1. SEM micrograph of the section surface of the self-healing epoxy specimen in which we can observe the empty geometrical places left by the catalyst in the epoxy matrix (see arrow)

healing system also needs the study and control of the capsule dimensions that act as reserve of healing agent.

Another aim of this chapter is therefore to study a process for the development of a selfhealing composite, which does not have the drawbacks mentioned above with respect to the catalyst morphology. This purpose is achieved by a process for the manufacture of a selfhealing composite material comprising a preliminary step of dispersing at molecular level the catalyst in an epoxy mixture containing healing agent in nanometric vessels. This solution allows to obtain the catalyst dispersed at molecular level and also able to react with nano-encapsulated healing agent.

## **2. Materials**

T*he epoxy matrix composite* was prepared by mixing an epoxy (Bisphenol A diglycidyl ether - Acronym BADGE) with a reactive diluent (1,4-Butanediol-diglycidyl ether- Acronym BDDGE) which was used in small percentage to reduce the viscosity of the material, to improve handling and ease of processing and to optimize consequently performance properties. These resins, both containing an epoxy, were obtained by Sigma-Aldrich. *The curing agent* investigated for this study is an anionic initiator Phenol, 2,4,6-

Fig. 1. SEM micrograph of the section surface of the self-healing epoxy specimen in which we can observe the empty geometrical places left by the catalyst in the epoxy matrix (see

healing system also needs the study and control of the capsule dimensions that act as

Another aim of this chapter is therefore to study a process for the development of a selfhealing composite, which does not have the drawbacks mentioned above with respect to the catalyst morphology. This purpose is achieved by a process for the manufacture of a selfhealing composite material comprising a preliminary step of dispersing at molecular level the catalyst in an epoxy mixture containing healing agent in nanometric vessels. This solution allows to obtain the catalyst dispersed at molecular level and also able to react with

T*he epoxy matrix composite* was prepared by mixing an epoxy (Bisphenol A diglycidyl ether - Acronym BADGE) with a reactive diluent (1,4-Butanediol-diglycidyl ether- Acronym BDDGE) which was used in small percentage to reduce the viscosity of the material, to improve handling and ease of processing and to optimize consequently performance properties. These resins, both containing an epoxy, were obtained by Sigma-Aldrich. *The curing agent* investigated for this study is an anionic initiator Phenol, 2,4,6-

arrow)

**2. Materials** 

reserve of healing agent.

nano-encapsulated healing agent.

tris[(dimethylamino) methyl] (Trade name Ancamine K54). This hardener agent was already used for self-healing formulations [5]. The catalyst, Hoveyda-Grubbs' first generation (Dichloro(o-isopropoxyphenylmethylene)(tricyclo-hexylphosphine)ruthenium(II) (catalyst HG1) also obtained from Aldrich was used to manufacture the epoxy matrix. It was dispersed at molecular level into the epoxy matrix. The healing agents used in this work were dicyclopentadiene (DCPD), which was obtained from Acros Organics, and 5 ethylidene-2-norbornene (ENB), which was obtained from Sigma-Aldrich. Figure 2 shows the chemical structures of compounds used in this work.

Fig. 2. Chemical structures of compounds used for the self-healing system.

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 291

samples) show that the largest part are characterized by a spherical form with average

The efficiency of the microencapsulation process for the above described formulation was not evaluated because for this new formulation other experiments on the self-healing efficiency were carried out. This analysis does not require a hard-working procedure if we use the infrared spectroscopy; by way of example, hereafter, in the section "Methodologies" we report a procedure to analyze the efficiency of the microencapsulation process for

FTIR analysis can be performed to control the success of the microencapsulation process. For this investigation, all the microcapsule fractions were powdered in a mortar; a first small fraction was dried under vacuum and analyzed by FT/IR spectroscopy (red spectrum) (see Figure 4), while another fraction was directly treated with Grubbs' catalyst powder and again analyzed by FT/IR (brown spectrum). Mixing and powdered the fraction of

Fig. 4. FTIR spectra of microcapsules (blue spectrum), DCPD (green spectrum), of the microcapsules fraction dried under vacuum (red spectrum) and microcapsules fraction

treated with Grubbs' catalyst powder (brown spectrum).

diameter of about 500-600 nm.

**3. Methodologies** 

microcapsules only filled with DCPD.

**3.1 Efficiency of the microencapsulation process** 

## **2.1 Epoxy specimen manufacture**

## **2.1.1 Epoxy matrix**

Sample EBA was obtained by mixing BADGE with BDDGE diluent at a concentration of 90%: 10% (by wt) epoxide to diluent. Ancamine K54 was added at a concentration of 10:100 (by wt) hardener to mixture (BADGE and BDDGE).

## **2.1.2 Self-healing epoxy system**

*Self-healing epoxy system* was obtained by dispersing ENB/DCPD(5%)-filled nanocapsules at a concentration of 20 wt% into the epoxy matrix.

Healing efficiency was also measured by carefully controlled fracture experiments for both the virgin and the healed materials using a well established protocol [5].

### **2.1.3 Microcapsule manufacture**

The microcapsules, with the outer shell composed of poly(urea-formaldehyde) and the inner shell of ethylene maleic anhydride copolymer (EMA) were prepared by *in situ* polymerization in an oil-in-water emulsion in accord with a procedure already described in previous papers [5, 33]. The only change, with respect to the aforementioned synthesis procedure, consisted of using as healing agent a blend of 5ethylidene-2norbornene (95 wt%) and DCPD (5 wt%). According to such a procedure, a desired dimension range can be selected by a suitable variation of the process parameters during the synthesis stage, and/or with the use of molecular sieves. The capsules used to manufacture the self-healing system are shown in Fig. 3. The analysis of the capsule size distribution (obtained from a series of 20

Fig. 3. Scanning electron microscope image of micro and nanocapsules at magnification 5.00 K x

samples) show that the largest part are characterized by a spherical form with average diameter of about 500-600 nm.

The efficiency of the microencapsulation process for the above described formulation was not evaluated because for this new formulation other experiments on the self-healing efficiency were carried out. This analysis does not require a hard-working procedure if we use the infrared spectroscopy; by way of example, hereafter, in the section "Methodologies" we report a procedure to analyze the efficiency of the microencapsulation process for microcapsules only filled with DCPD.

## **3. Methodologies**

290 Infrared Spectroscopy – Materials Science, Engineering and Technology

Sample EBA was obtained by mixing BADGE with BDDGE diluent at a concentration of 90%: 10% (by wt) epoxide to diluent. Ancamine K54 was added at a concentration of 10:100

*Self-healing epoxy system* was obtained by dispersing ENB/DCPD(5%)-filled nanocapsules at

Healing efficiency was also measured by carefully controlled fracture experiments for both

The microcapsules, with the outer shell composed of poly(urea-formaldehyde) and the inner shell of ethylene maleic anhydride copolymer (EMA) were prepared by *in situ* polymerization in an oil-in-water emulsion in accord with a procedure already described in previous papers [5, 33]. The only change, with respect to the aforementioned synthesis procedure, consisted of using as healing agent a blend of 5ethylidene-2norbornene (95 wt%) and DCPD (5 wt%). According to such a procedure, a desired dimension range can be selected by a suitable variation of the process parameters during the synthesis stage, and/or with the use of molecular sieves. The capsules used to manufacture the self-healing system are shown in Fig. 3. The analysis of the capsule size distribution (obtained from a series of 20

Fig. 3. Scanning electron microscope image of micro and nanocapsules at magnification

the virgin and the healed materials using a well established protocol [5].

**2.1 Epoxy specimen manufacture** 

**2.1.2 Self-healing epoxy system** 

**2.1.3 Microcapsule manufacture** 

5.00 K x

(by wt) hardener to mixture (BADGE and BDDGE).

a concentration of 20 wt% into the epoxy matrix.

**2.1.1 Epoxy matrix** 

### **3.1 Efficiency of the microencapsulation process**

FTIR analysis can be performed to control the success of the microencapsulation process. For this investigation, all the microcapsule fractions were powdered in a mortar; a first small fraction was dried under vacuum and analyzed by FT/IR spectroscopy (red spectrum) (see Figure 4), while another fraction was directly treated with Grubbs' catalyst powder and again analyzed by FT/IR (brown spectrum). Mixing and powdered the fraction of

Fig. 4. FTIR spectra of microcapsules (blue spectrum), DCPD (green spectrum), of the microcapsules fraction dried under vacuum (red spectrum) and microcapsules fraction treated with Grubbs' catalyst powder (brown spectrum).

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 293

The transparent mixture containing the completely dissolved catalyst was then taken out of the oil bath and left to cool 50°C, and then the curing agent was added to it. The mixture thus obtained was cured in a two-stage process. The first stage was carried out at a temperature of 80°C for 3 hours, while the second stage was carried out according to three

Figs. 6a, 6b, 6c and 6d show FT/IR spectra of the cured material respectively after the intermediate stages, and the second stage carried out with the three variants stated above, to which ENB was then added in the same way as was done after the preliminary step of

We can observe that, in conjunction with different signals of the epoxy precursors (among which the C-O-C stretch at 1247 cm-1 and the symmetric stretch at 1039 cm-1 ) all the spectra

The presence in all cases of the peak at 966 cm-1, indicating formation of the metathesis product, proves that the catalytic activity of the HG1 catalyst within the epoxy matrix remained unchanged after the described treatments. It can be seen, in particular, from the FT/IR spectra that, after the treatment at 110°C, the mixture can be cured up to 170°C for 2 hours without compromising the catalytic activity. This high temperature increases the cross-linking degree, as we can deduce observing the signal at 916 cm-1 in the spectra of figs. 6a-6d. The very small intensity of the signal at 916 cm-1 in the spectrum of fig. 6a and the further progressive decrease up to the disappearance in the spectrum of fig. 6d indicates that almost the epoxy rings have reacted for a treatment up to 110 °C (the signals at 916 cm-1

Fig. 5. FT/IR spectrum of the solid film (metathesis product)

dissolution of the catalyst.

show the band at 966 cm-1.

variants, i.e. at the three different temperatures of 125, 150 and 170°C.

microcapsules (containing DCPD) with the Grubbs catalyst instantaneously initiates ROMP at room temperature. ROMP polymers can display a very rich microstructure. Depending on the monomer, different characteristics can be observed¸ among these cis/trans isomerism, tacticity, etc.. Cis/trans isomerism is present in all ROMP polymers and relatively easy to quantify using spectroscopic techniques. In fact, in the brown spectrum we can observe the strong and sharp signal at 968 cm-1 due to the carbon hydrogen bending vibration of a trans carbon double bond. This signal was already assigned to the absorption of trans poly(DCPD) fractions5,36. Using this procedure ROMP of DCPD primarily produces trans double bonds as observed for the ROMP of DCPD by Grubbs first generation catalyst. In figure 4 FT/IR spectra of DCPD and microcapsules, as obtained from the synthesis, were also reported for comparison. In particular, in the analyzed spectral range, a comparison between red and brown spectra shows many common signals due to poly(ureaformaldehyde) of the microcapsule wall. We can observe, in fact, some of the vibrational mode for the CH2 group (δsCH2 at 1465 cm-1 – in-plane bending or scissoring; ωCH2 and τCH2 between 1350-1150 cm-1 – out-of-plane bending or wagging and/or twisting; ρCH2 at 720 cm-1 – in-plane bending or rocking) and the carbonyl frequency (C=O) of the wall tertiary amides. This absorption occurs in the spectral range of 1680 – 1630 cm-1 as expected (the C=O absorption of amides occurs at lower frequencies than "normal" carbonyl absorption due to the resonance effect. The resonance effect increases the C=O bond length and reduces the frequency of absorption37).

The highlighted peak at 968 cm-1 characteristic of ring-opened poly(DCPD) is present only in the brown spectrum. It clearly evidences that the embedded DCPD is active in the metathesis reaction. As shown by the FT/IR analysis reported in figure 4, the amount of DCPD encapsulated is a sufficient quantity to activate the ROMP reaction [32 – see Supp. Inf.].

#### **3.2 Evaluation of the catalytic activity**

In the development of our self-healing epoxy resins, the evaluation of the catalytic activity was investigated for the epoxy matrix (formulation without nanocapsules).

The procedure adopted for preparing this mixture is as follows. The epoxy precursor (BADGE) was mixed mechanically with the reactive diluent (BDDGE) at a temperature of 90°C, maintained with an oil bath, and then the catalyst HG1 in the form of crystalline powder was added. The catalyst was dispersed at molecular level by mechanical agitation of the mixture maintained at 90°C for 90 minutes. To verify complete dispersion and dissolution of the catalyst, and that its catalytic activity remained unchanged, spectroscopic investigation was carried out. For this purpose, four drops of the mixture were deposited on a slide for light microscopy. Complete transparency, which is achieved when the catalyst is completely dissolved, can be verified by light microscopy with observation in transmission. Two drops of ENB were added to the aforesaid drops of mixtures. A thin solid film of metathesis product, whose FT/IR spectrum is shown in Fig. 5, formed immediately.

This spectrum shows a peak at 966 cm-1, which is an indication of the formation of the metathesis product and hence of the fact that the activity of the catalyst has not been compromised by the chemical nature of the oligomers, by the temperature and by the treatments of mechanical mixing.

microcapsules (containing DCPD) with the Grubbs catalyst instantaneously initiates ROMP at room temperature. ROMP polymers can display a very rich microstructure. Depending on the monomer, different characteristics can be observed¸ among these cis/trans isomerism, tacticity, etc.. Cis/trans isomerism is present in all ROMP polymers and relatively easy to quantify using spectroscopic techniques. In fact, in the brown spectrum we can observe the strong and sharp signal at 968 cm-1 due to the carbon hydrogen bending vibration of a trans carbon double bond. This signal was already assigned to the absorption of trans poly(DCPD) fractions5,36. Using this procedure ROMP of DCPD primarily produces trans double bonds as observed for the ROMP of DCPD by Grubbs first generation catalyst. In figure 4 FT/IR spectra of DCPD and microcapsules, as obtained from the synthesis, were also reported for comparison. In particular, in the analyzed spectral range, a comparison between red and brown spectra shows many common signals due to poly(ureaformaldehyde) of the microcapsule wall. We can observe, in fact, some of the vibrational mode for the CH2 group (δsCH2 at 1465 cm-1 – in-plane bending or scissoring; ωCH2 and τCH2 between 1350-1150 cm-1 – out-of-plane bending or wagging and/or twisting; ρCH2 at 720 cm-1 – in-plane bending or rocking) and the carbonyl frequency (C=O) of the wall tertiary amides. This absorption occurs in the spectral range of 1680 – 1630 cm-1 as expected (the C=O absorption of amides occurs at lower frequencies than "normal" carbonyl absorption due to the resonance effect. The resonance effect increases the C=O bond length

The highlighted peak at 968 cm-1 characteristic of ring-opened poly(DCPD) is present only in the brown spectrum. It clearly evidences that the embedded DCPD is active in the metathesis reaction. As shown by the FT/IR analysis reported in figure 4, the amount of DCPD

In the development of our self-healing epoxy resins, the evaluation of the catalytic activity

The procedure adopted for preparing this mixture is as follows. The epoxy precursor (BADGE) was mixed mechanically with the reactive diluent (BDDGE) at a temperature of 90°C, maintained with an oil bath, and then the catalyst HG1 in the form of crystalline powder was added. The catalyst was dispersed at molecular level by mechanical agitation of the mixture maintained at 90°C for 90 minutes. To verify complete dispersion and dissolution of the catalyst, and that its catalytic activity remained unchanged, spectroscopic investigation was carried out. For this purpose, four drops of the mixture were deposited on a slide for light microscopy. Complete transparency, which is achieved when the catalyst is completely dissolved, can be verified by light microscopy with observation in transmission. Two drops of ENB were added to the aforesaid drops of mixtures. A thin solid film of metathesis product, whose FT/IR spectrum is shown in Fig. 5,

This spectrum shows a peak at 966 cm-1, which is an indication of the formation of the metathesis product and hence of the fact that the activity of the catalyst has not been compromised by the chemical nature of the oligomers, by the temperature and by the

encapsulated is a sufficient quantity to activate the ROMP reaction [32 – see Supp. Inf.].

was investigated for the epoxy matrix (formulation without nanocapsules).

and reduces the frequency of absorption37).

**3.2 Evaluation of the catalytic activity** 

formed immediately.

treatments of mechanical mixing.

Fig. 5. FT/IR spectrum of the solid film (metathesis product)

The transparent mixture containing the completely dissolved catalyst was then taken out of the oil bath and left to cool 50°C, and then the curing agent was added to it. The mixture thus obtained was cured in a two-stage process. The first stage was carried out at a temperature of 80°C for 3 hours, while the second stage was carried out according to three variants, i.e. at the three different temperatures of 125, 150 and 170°C.

Figs. 6a, 6b, 6c and 6d show FT/IR spectra of the cured material respectively after the intermediate stages, and the second stage carried out with the three variants stated above, to which ENB was then added in the same way as was done after the preliminary step of dissolution of the catalyst.

We can observe that, in conjunction with different signals of the epoxy precursors (among which the C-O-C stretch at 1247 cm-1 and the symmetric stretch at 1039 cm-1 ) all the spectra show the band at 966 cm-1.

The presence in all cases of the peak at 966 cm-1, indicating formation of the metathesis product, proves that the catalytic activity of the HG1 catalyst within the epoxy matrix remained unchanged after the described treatments. It can be seen, in particular, from the FT/IR spectra that, after the treatment at 110°C, the mixture can be cured up to 170°C for 2 hours without compromising the catalytic activity. This high temperature increases the cross-linking degree, as we can deduce observing the signal at 916 cm-1 in the spectra of figs. 6a-6d. The very small intensity of the signal at 916 cm-1 in the spectrum of fig. 6a and the further progressive decrease up to the disappearance in the spectrum of fig. 6d indicates that almost the epoxy rings have reacted for a treatment up to 110 °C (the signals at 916 cm-1

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 295

Fig. 6c. FT/IR spectrum of the solid film (metathesis product) in the material cured up to 150 °C

Fig. 6d. FT/IR spectrum of the solid film (metathesis product) in the material cured up to 170 °C

Fig. 6a. FT/IR spectrum of the solid film (metathesis product) in the material cured up to 110 °C.

Fig. 6b. FT/IR spectrum of the solid film (metathesis product) in the material cured up to 125 °C

Fig. 6a. FT/IR spectrum of the solid film (metathesis product) in the material cured up to

Fig. 6b. FT/IR spectrum of the solid film (metathesis product) in the material cured up to

110 °C.

125 °C

Fig. 6c. FT/IR spectrum of the solid film (metathesis product) in the material cured up to 150 °C

Fig. 6d. FT/IR spectrum of the solid film (metathesis product) in the material cured up to 170 °C

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 297

Crack healing efficiency, η, (defined as the ability to recover fracture toughness) was evaluated for the sample cured up to 170 °C using a tapered double–cantilever beam (TDCB) [23,38] geometry. The healing efficiency η, calculated as the ratio of critical fracture loads for the healed and virgin samples, is obtained from data shown in Figure 8 where we report the Load-Displacement curves for a sample with 5% of Hoveyda-Grubbs I Catalyst and 20% of microcapsules (ENB+DCPD) filled, cured up to 170 °C. The healing efficiency is

Fig. 9 shows the sample geometry for getting quantitative results on the self-healing functionality. The figure also reports the morphology of an healed sample after a controlled

> **HG1(molecular dispersion) 5% Microcapsules 20% (ENB+DCPD 5%) - T= 170 °C**

**0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8**

**Displacement (mm)**

Fig. 8. Load-Displacement curves for Virgin (black curve) and Healed (red curve) sample.

**3.3 Healing efficiency of the self-healing sample** 

**Virgin Healed**

72 %.

damage.

**Load (kN)**

**0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16**

is characteristic of the epoxy groups and assigned to asymmetrical ring stretching in which the C-C bond is stretching during contraction of the C-O bond 37).

Self-healing epoxy system was obtained by dispersing ENB/DCPD(5% wt%)-filled nanocapsules at a concentration of 20 wt% into the epoxy matrix using for the curing process the same procedure above described for the epoxy matrix. Use of a mixture of ENB with low concentrations of DCPD as healing agent was used because It can greatly increase the degree of crosslinking of the metathesis product especially at extremely low temperatures. For example, by carrying out the ring opening metathesis reaction of an ENB/DCPD mixture (at 5% of DCPD) in the molar ratio 1:1000 (Hoveyda-Grubbs 1 catalyst/monomer) at a temperature of -53°C, the degree of crosslinking is found to be 57% (with a degree of conversion of 84%) after a reaction time of 7 hours. In similar conditions, the degree of crosslinking of ENB on its own is 10% with a degree of conversion of 100% [35].

A simplified scheme of the self-healing formulation is shown in Figure 7.

Fig. 7. Scheme of self-healing formulation which includes: a) capsules containing a polymerizer agent (DCPD and ENB); b) a catalyst for the polymerizer (HG1).

#### **3.3 Healing efficiency of the self-healing sample**

296 Infrared Spectroscopy – Materials Science, Engineering and Technology

is characteristic of the epoxy groups and assigned to asymmetrical ring stretching in which

Self-healing epoxy system was obtained by dispersing ENB/DCPD(5% wt%)-filled nanocapsules at a concentration of 20 wt% into the epoxy matrix using for the curing process the same procedure above described for the epoxy matrix. Use of a mixture of ENB with low concentrations of DCPD as healing agent was used because It can greatly increase the degree of crosslinking of the metathesis product especially at extremely low temperatures. For example, by carrying out the ring opening metathesis reaction of an ENB/DCPD mixture (at 5% of DCPD) in the molar ratio 1:1000 (Hoveyda-Grubbs 1 catalyst/monomer) at a temperature of -53°C, the degree of crosslinking is found to be 57% (with a degree of conversion of 84%) after a reaction time of 7 hours. In similar conditions, the degree of crosslinking of ENB on its own is 10% with a degree of

the C-C bond is stretching during contraction of the C-O bond 37).

A simplified scheme of the self-healing formulation is shown in Figure 7.

Fig. 7. Scheme of self-healing formulation which includes: a) capsules containing a polymerizer agent (DCPD and ENB); b) a catalyst for the polymerizer (HG1).

conversion of 100% [35].

Crack healing efficiency, η, (defined as the ability to recover fracture toughness) was evaluated for the sample cured up to 170 °C using a tapered double–cantilever beam (TDCB) [23,38] geometry. The healing efficiency η, calculated as the ratio of critical fracture loads for the healed and virgin samples, is obtained from data shown in Figure 8 where we report the Load-Displacement curves for a sample with 5% of Hoveyda-Grubbs I Catalyst and 20% of microcapsules (ENB+DCPD) filled, cured up to 170 °C. The healing efficiency is 72 %.

Fig. 9 shows the sample geometry for getting quantitative results on the self-healing functionality. The figure also reports the morphology of an healed sample after a controlled damage.

Use of FTIR Analysis to Control the Self-Healing Functionality of Epoxy Resins 299

[5] White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S., Kessler, M. R., Sriram, S. R.;

[11] Cho, S. H., Andersson, H.M.; White, S. R., Sottos, N. R., Braun, P. V. Adv Mater 2006,

[15] Toohey, K. S.; Hansen, C. J.; Lewis, J. A.; White, S. R.; Sottos, N. R. Adv Funct Mater

[16] Dry, C.; Corsaw, M.; Bayer, E. A comparison of internal self-repair with resin injection

[17] Van der Zwaag, S. editor*.* Self Healing Materials: An Alternative Approach to 20

[20] Dry, C.; Sottos, N. NASA ADS: Passive smart self-repair in polymer matrix composite

[24] Brown, E.N.; Kessler, M.R.; Sottos, N.R.; White, S.R. J. Microencapsulation

[26] Jones, A.S.; Rule, J.D.; Moore, J.S.; White, S.R.; Sottos, N.R. Chem Mater 2006;18:1312-

[29] Toohey, K.S.; Sottos, N.R.; Lewis, J.A.; Moore, J.S.; White, S.R. Self-healing materials

[30] Wilson, G.O.; Caruso, M.M.; Reimer, N.T.; White, S.R.; Sottos, N.R.; Moore, J.S.

[31] Grubbs, R.H. editor. Handbook of metathesis. Weinheim (Germany): Wiley- VCH; 2003. [32] Guadagno, L.; Longo, P.; Raimondo, M.; Naddeo, C.; Mariconda, A.; Vittoria, V.;

[33] Guadagno, L.; Longo, P.; Raimondo, M.; Naddeo, C.; Mariconda, A.; Sorrentino, A.;

Evaluation of ruthenium catalysts for ring-opening metathesis polymerizationbased selfhealing applications. Chem Mater 2008;20(10):3288–97 [supporting

Vittoria, V.; Iannuzzo, G.; Russo, S. Journal of Polymer Science Part B: Polymer

materials. In: Proc. SPIE Vol. 1916, p. 438-444, Smart Structures and Materials 1993:

Centuries of Materials Science (Springer Series in Materials) 2007.

[22] Motuku, M.; Vaidya, U.K.; Janowski, G.M. Smart Mater Struct 1999; 8(5):623.

[23] Brown, E.N.; Sottos, N.R.; White, S.R. Exp Mech 2002;42(4):372-379.

[25] Brown, E.N.; White, S.R.; Sottos, N.R. J Mater Sci 2004;39(5):1703-1710.

[27] Syret, J.A.; Becer, C.R.; Haddleton, D.M. Polym. Chem., 2010,1, 978-987. [28] Zhang, Y.; Broekhuis, A. A.; Picchioni, F. Macromolecules 42, 1906-1912 (2009).

with microvascular networks. Nature Mater 2007;6:581–5.

Iannuzzo, G.; Russo, S.; Composites Part: B 42, 296-301(2011).

Brown, E. N.; Viswanathan, S. Nature 2001, 409, 794-797. [6] Wu, D. Y.; Meure, S.; Solomon, D. Prog Polym Sci 2008, 33, 479-522. [7] Jud, K.; Kausch, H. H.; Williams, J. G. J Mater Sci 1981, 16, 204-210.

[10] Kessler, M. R.; Sottos, N. R., White, S. R., Composites Part A, 2003, 34, 743-753.

[13] Pang, J. W. C.; Jody, W. C.; Bond, I. P. Comp Sci Tech 2005, 65, 1791-1799. [14] Keller, M. W.; White, S. R.; Sottos, N. R. Adv Funct Mater 2007, *17*, 2399-2404.

[8] Dry, C.; Mcmillan, W. Smart Mater Struct 1997, 6, 35-39.

18, 997-1000.

2009, 19, 1399-1405.

2003;20(6):719-730.

1317.

information].

Physics, 48, 2413-2423 (2010).

[18] Dry, C. Int Patent WO/2007/005657 2007. [19] Dry, C. Comp Struct 1996;35(3):263-269.

Smart Materials, Vijay K. Varadan; Ed. [21] Dry, C.; Mcmillan, W. Smart Mater Struct 1997;6(1):35.

[9] Kessler, M. R.; White, S. R.; Composites Part A, 2001, 32, 683-699.

[12] Pang, J. W. C.; Bond, I. P. Composites Part A, 2004, 36, 183-188.

in repair of concrete. J Adh Sci Tech 2003;17(1):79-89.

Fig. 9. Samples for getting quantitative results on the self-healing functionality (the first two images on the top); SEM images of healed crack faces closed by means of the metathesis product inside a crack after a (TDCB) test (images on the bottom).

## **4. Conclusions**

We have formulated, prepared and characterized a multifunctional autonomically healing composite with a self-healing efficiency higher than 70 %. Carrying out the curing process in several stages at increasing temperature makes it possible to avoid deactivation of the catalyst. In fact, choosing a relatively low temperature in the first stage means that only the curing agent, and not the catalyst, reacts with the oxirane rings of the epoxy precursor. Therefore, the catalyst remains intact in the epoxy matrix that has formed and is thus able subsequently to perform its catalytic action of polymerization of the reactive monomer, when the latter is released from the vessel and interacts with the epoxy matrix containing the catalyst. Infrared Spectroscopy proves to be a useful way to identify metathesis product directly inside the epoxy resin during the curing reactions of epoxy formulations containing catalyst powder dispersed at molecular level.

## **5. References**


Fig. 9. Samples for getting quantitative results on the self-healing functionality (the first two images on the top); SEM images of healed crack faces closed by means of the metathesis

We have formulated, prepared and characterized a multifunctional autonomically healing composite with a self-healing efficiency higher than 70 %. Carrying out the curing process in several stages at increasing temperature makes it possible to avoid deactivation of the catalyst. In fact, choosing a relatively low temperature in the first stage means that only the curing agent, and not the catalyst, reacts with the oxirane rings of the epoxy precursor. Therefore, the catalyst remains intact in the epoxy matrix that has formed and is thus able subsequently to perform its catalytic action of polymerization of the reactive monomer, when the latter is released from the vessel and interacts with the epoxy matrix containing the catalyst. Infrared Spectroscopy proves to be a useful way to identify metathesis product directly inside the epoxy resin during the curing reactions of epoxy formulations containing

[1] Riefsnider, K. L.; Schulte, K.; Duke, J. C.; Long term failure behavior of composite materials. ASTM Special Technical Pubblications. 1983; 813, 136-159. [2] Osswald, T.; Menges, G. Mater. Sci. Polym. Eng. Munich: Hanser Publishers, 2003; 447-

[3] Chamis, C.C.; Sullivan, T. L. In situ ply strength: an initial assessment. Cleveland, OH,

[4] Wilson, D. J. K.; Wells, J. N.; Hay, D.; Owens, G. A.; Johnson, F. 18th International SAMPE Technical Conference, Washington, USA, 1986; 18, 242-253.

product inside a crack after a (TDCB) test (images on the bottom).

catalyst powder dispersed at molecular level.

NASA Lewis Research Center. 1987, 19.

**4. Conclusions** 

**5. References** 

519.


**15** 

*Japan* 

**Infrared Analysis of Electrostatic** 

Weimin Zhou, Huitan Fu and Takaomi Kobayashi\*

*Nagaoka University of Technology, Kamitomioka, Nagaoka* 

*Department of Materials Science and Technology,* 

**Layer-By-Layer Polymer Membranes Having** 

**Characteristics of Heavy Metal Ion Desalination** 

Layer-by-layer (LbL) self-assembly is a spontaneous and reversible organization process of interacting organic and polymeric components by their aggregation into ordered structures on substrate. LbL self-assembly process which occurs due to sequential adsorption of materials with complementary functional groups. Producing robust films and allow a precise control over the film thickness and its properties. This technique has been widely applied for the fabrication of multilayer films of organic and polymeric compounds, organic-inorganic hybrid structures, larger objects such as latex particles, and even purely inorganic thin films. By combining various functional materials for the formation of self-assembly multilayer, the LbL

It has been noticed that the LbL fabrication is usually guided by a driving force of hydrophobic interaction,[3] hydrogen-bond,[4] covalent bonding[5] and electrostatic interaction[6] between assembled compounds. Among those driving forces, electrostatic interaction between oppositely charged molecules becomes a very fascinating and attractive approach because of its simplicity and efficiency. The technique of alternate layer-by-layer assembly of cationic and anionic polyelectrolytes, generally referred to electrostatic selfassembly (ESA) as a firstly reported by Decher et al in 1991.[7] They took advantage of the charge-charge interaction between oppositely charged layers to create electrolytes multilayer. The polyelectrolyte conformation and layer interpenetration were due to an idealization of the surface charge reversal with each adsorption step which was based on the

ESA technique provided effective surface modification for thin film and separation membrane. [8, 9]Because the substrate surface contained negatively or positively charged due to the alternate deposition of polycation or polyanion, the transmission of charged particles like ions through self-assembly polyelectrolyte multilayer membrane might be affected by the charged surface. It was also showed that the presence of fixed charges at the surface of the multilayer resulted in Donnan exclusion of multivalent ions as well as the preferential transport of small

technique has led to a wide range of novel materials for various applications.[1,2]

electrostatically driven multilayer build-up depicted.

**1. Introduction** 

 \*

Corresponding Author


## **Infrared Analysis of Electrostatic Layer-By-Layer Polymer Membranes Having Characteristics of Heavy Metal Ion Desalination**

Weimin Zhou, Huitan Fu and Takaomi Kobayashi\* *Department of Materials Science and Technology, Nagaoka University of Technology, Kamitomioka, Nagaoka Japan* 

## **1. Introduction**

300 Infrared Spectroscopy – Materials Science, Engineering and Technology

[34] Guadagno, L.; Longo, P.; Raimondo, M.; Mariconda, A.; Naddeo, C.; Sorrentino, A.;

[35] Guadagno, L.; Raimondo, M.; Naddeo, C.; Mariconda, A.; Corvino, R.; Longo, P.;

[36] Dall'Asta, G.; Motroni, R.; Manetti, R.; Tosi, C.; Makromol Chem 1969, 130, 153-165. [37] *Spectrometric Identification Of Organic Compounds*, *6Th* Ed by *Robert M*. *Silverstein*, Francis

[38] Mostovoy, S.; Croseley, P.B.; Ripling, E.J. J. Mater. 2, 661–681 (1967).

(A1) Publication date: Jul. 1, 2010.

Publication date: May. 19, 2011.

X. Webster, John Wiley & Sons, Inc.

Vittoria, V.; Iannuzzo, G.; Russo, S.; Calvi, E. Publication number: US 2010168280

Vittoria, V.; Russo, S.; Iannuzzo, G. Publication number: US 2011118385 (A1)

Layer-by-layer (LbL) self-assembly is a spontaneous and reversible organization process of interacting organic and polymeric components by their aggregation into ordered structures on substrate. LbL self-assembly process which occurs due to sequential adsorption of materials with complementary functional groups. Producing robust films and allow a precise control over the film thickness and its properties. This technique has been widely applied for the fabrication of multilayer films of organic and polymeric compounds, organic-inorganic hybrid structures, larger objects such as latex particles, and even purely inorganic thin films. By combining various functional materials for the formation of self-assembly multilayer, the LbL technique has led to a wide range of novel materials for various applications.[1,2]

It has been noticed that the LbL fabrication is usually guided by a driving force of hydrophobic interaction,[3] hydrogen-bond,[4] covalent bonding[5] and electrostatic interaction[6] between assembled compounds. Among those driving forces, electrostatic interaction between oppositely charged molecules becomes a very fascinating and attractive approach because of its simplicity and efficiency. The technique of alternate layer-by-layer assembly of cationic and anionic polyelectrolytes, generally referred to electrostatic selfassembly (ESA) as a firstly reported by Decher et al in 1991.[7] They took advantage of the charge-charge interaction between oppositely charged layers to create electrolytes multilayer. The polyelectrolyte conformation and layer interpenetration were due to an idealization of the surface charge reversal with each adsorption step which was based on the electrostatically driven multilayer build-up depicted.

ESA technique provided effective surface modification for thin film and separation membrane. [8, 9]Because the substrate surface contained negatively or positively charged due to the alternate deposition of polycation or polyanion, the transmission of charged particles like ions through self-assembly polyelectrolyte multilayer membrane might be affected by the charged surface. It was also showed that the presence of fixed charges at the surface of the multilayer resulted in Donnan exclusion of multivalent ions as well as the preferential transport of small

<sup>\*</sup> Corresponding Author

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

15

Scheme 1. Chemical structures of CTAC and PAA.

*Preparation of composite multilayer on base membrane surface* 

a. Immerging the base membrane into the CTAC or TMAC solution. b. Washing away the loosely adsorbed cations by pure water.

procedure consisted of the following four steps:

c. Immerging into the PAA solution.

negative charge.

Cl

CH3

CH3

CH3

H3C CH2 N

Membranes Having Characteristics of Heavy Metal Ion Desalination 303

**CTAC PAA**

**2. Multilayer composite surfaces prepared by an electrostatic self-assembly** 

Aqueous solutions with 1g / L concentration of CTAC, tetramethyl ammonium chloride (TMAC) and PAA were prepared respectively and used as dipping solutions for the surface modification of base P(AN-*co*-AA) membrane. As shown in Scheme 2-1, the preparation

d. Washing out the excessive PAA anion adsorbed on membrane surface by pure water.

Scheme 2-1. Preparation procedure of ESA multi-layer on polymer membrane having

formed on the negatively charged base membrane surface.

By alternately immerging the base P(AN-*co*-AA) membrane into anion and cation solution as indicated by the four steps, ESA multiple layers of CTAC or TMAC and PAA could be

**of quaternary ammonium salt or tetramethyl ammonium chloride and polyacrylic acid onto poly (acrylonitrile-***co***-acrylic acid) membrane** 

CH2 CH

<sup>n</sup> CO

OH

monovalent ions.[8] Selective ion transport behaviour through ESA composite multilayer films was studied by Take[8] and Schelenoff.[9] The results suggested that highly specific ion separation was achieved and was affected by the charge and size of ion that pass through the composite multilayer. These researches made the polyelectrolytes multilayers interesting for the application of water desalination like heavy metal ion removal.

Generally, in LbL systems, infrared analysis method is more important method in studying chemical structures [**10, 11**] of the membranes, because the membranes were usually formed with the hydrogen bond, ion interactions and others which were difficultly investigated using other fundamental analysis methods such as 1H NMR, XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). In such infrared (IR) analysis, for example, Arrondo et al. used the infrared (IR) spectroscopy to the study of membrane proteins because the assignment of IR protein bands in H2O and in D2O, one of the more difficult points in protein IR spectroscopy.[12] Similarly, reflection-infrared spectroscopy is a powerful technique for characterizing protein and peptide-membrane interactions.[13] Mauntele et. al., used the IR to analyze lipid–protein interaction. In his case, lipid protein interactions play a key role in the stability and function of various membrane proteins. The FT-IR technique was used for a label free analysis of the global secondary structural changes and local changes in the tyrosine microenvironment.[14]

Furthermore, the FT-IR analytical technique was applied in analysis of surface of membranes interacted with other materials. For instance, Luo et. al., fabricated the SPES/Nano-TiO2 composite ultrafiltration membrane. In their studies, the TiO2 nanoparticle self-assembly on the SPES membrane surface was confirmed by X-ray photoelectron spectroscopy (XPS) and FT-IR spectrometer. When the nano-TiO2 was self-assembled on the SPES membrane the absorption peak at 1243 cm-1 attributes to the stretching vibration of the ether C-O-C bond in the SPES polymer shifted to 1239 cm-1. The result of FT-IR spectrometer effectively verified the self-assembly process of nano-TiO2 on the SPES membrane.[15]

In study of ionic exchange membrane, FT-IR method was usually used to analyze the chemical structures of formed membrane. Fang et. al., prepared novel anion exchange membranes based on the copolymer of methyl methacrylate, vinylbenzyl chloride and ethyl acrylate. The structure of membrane was mainly verified by measurement of FT-IR. This study remarkably showed the importance of FT-IR method in analysis of chemical structure of membranes.[16]

Ding et al., confirmed the thermal treatment of precursor composite poly(amic acid) tertiary amine salts (PAAS) membrane using FT-IR/ATR analysis at 150 °C, and obtained composite polyimide hollow fiber membranes. In their studies, the FT-IR spectroscopy effectively evaluated the imidization of the coating layer.[17]

These reports indicated that importance of IR spectroscopy in analysis of chemical structures of membranes and interactions of membranes with other materials. Accordingly, IR analysis could provide to verify the chemical structure of ESA membranes.

In this chapter, we introduce the ESA modified membranes by forming electrostatic alternate layers of cetyl trimethyl ammonium chloride (CTAC) and poly(acrylic acid) (PAA) onto charged copolyacrylonitrile membranes (Scheme 1). Along with surface characterization of the ESA multilayer, infrared analysis was carried out. Finally, we evaluated the adsorptive properties of ESA multilayer for Fe3+ and Fe2+, and ESA membranes with chitosan microspheres / PAA for removing of Cu2+.

## **CTAC PAA**

302 Infrared Spectroscopy – Materials Science, Engineering and Technology

monovalent ions.[8] Selective ion transport behaviour through ESA composite multilayer films was studied by Take[8] and Schelenoff.[9] The results suggested that highly specific ion separation was achieved and was affected by the charge and size of ion that pass through the composite multilayer. These researches made the polyelectrolytes multilayers interesting for

Generally, in LbL systems, infrared analysis method is more important method in studying chemical structures [**10, 11**] of the membranes, because the membranes were usually formed with the hydrogen bond, ion interactions and others which were difficultly investigated using other fundamental analysis methods such as 1H NMR, XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). In such infrared (IR) analysis, for example, Arrondo et al. used the infrared (IR) spectroscopy to the study of membrane proteins because the assignment of IR protein bands in H2O and in D2O, one of the more difficult points in protein IR spectroscopy.[12] Similarly, reflection-infrared spectroscopy is a powerful technique for characterizing protein and peptide-membrane interactions.[13] Mauntele et. al., used the IR to analyze lipid–protein interaction. In his case, lipid protein interactions play a key role in the stability and function of various membrane proteins. The FT-IR technique was used for a label free analysis of the global secondary structural changes and local

Furthermore, the FT-IR analytical technique was applied in analysis of surface of membranes interacted with other materials. For instance, Luo et. al., fabricated the SPES/Nano-TiO2 composite ultrafiltration membrane. In their studies, the TiO2 nanoparticle self-assembly on the SPES membrane surface was confirmed by X-ray photoelectron spectroscopy (XPS) and FT-IR spectrometer. When the nano-TiO2 was self-assembled on the SPES membrane the absorption peak at 1243 cm-1 attributes to the stretching vibration of the ether C-O-C bond in the SPES polymer shifted to 1239 cm-1. The result of FT-IR spectrometer

In study of ionic exchange membrane, FT-IR method was usually used to analyze the chemical structures of formed membrane. Fang et. al., prepared novel anion exchange membranes based on the copolymer of methyl methacrylate, vinylbenzyl chloride and ethyl acrylate. The structure of membrane was mainly verified by measurement of FT-IR. This study remarkably showed the importance of FT-IR method in analysis of chemical structure of membranes.[16]

Ding et al., confirmed the thermal treatment of precursor composite poly(amic acid) tertiary amine salts (PAAS) membrane using FT-IR/ATR analysis at 150 °C, and obtained composite polyimide hollow fiber membranes. In their studies, the FT-IR spectroscopy effectively

These reports indicated that importance of IR spectroscopy in analysis of chemical structures of membranes and interactions of membranes with other materials. Accordingly,

In this chapter, we introduce the ESA modified membranes by forming electrostatic alternate layers of cetyl trimethyl ammonium chloride (CTAC) and poly(acrylic acid) (PAA) onto charged copolyacrylonitrile membranes (Scheme 1). Along with surface characterization of the ESA multilayer, infrared analysis was carried out. Finally, we evaluated the adsorptive properties of ESA multilayer for Fe3+ and Fe2+, and ESA

IR analysis could provide to verify the chemical structure of ESA membranes.

membranes with chitosan microspheres / PAA for removing of Cu2+.

effectively verified the self-assembly process of nano-TiO2 on the SPES membrane.[15]

the application of water desalination like heavy metal ion removal.

changes in the tyrosine microenvironment.[14]

evaluated the imidization of the coating layer.[17]

Scheme 1. Chemical structures of CTAC and PAA.

## **2. Multilayer composite surfaces prepared by an electrostatic self-assembly of quaternary ammonium salt or tetramethyl ammonium chloride and polyacrylic acid onto poly (acrylonitrile-***co***-acrylic acid) membrane**

*Preparation of composite multilayer on base membrane surface* 

Aqueous solutions with 1g / L concentration of CTAC, tetramethyl ammonium chloride (TMAC) and PAA were prepared respectively and used as dipping solutions for the surface modification of base P(AN-*co*-AA) membrane. As shown in Scheme 2-1, the preparation procedure consisted of the following four steps:


Scheme 2-1. Preparation procedure of ESA multi-layer on polymer membrane having negative charge.

By alternately immerging the base P(AN-*co*-AA) membrane into anion and cation solution as indicated by the four steps, ESA multiple layers of CTAC or TMAC and PAA could be formed on the negatively charged base membrane surface.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

dissociated to react with CTAC by electrostatic attraction.

Membranes Having Characteristics of Heavy Metal Ion Desalination 305

dissociated carboxylic group at about 1645 cm-1 and 1720 cm-1 existed in the spectrum. In the spectrum (1), it was noticed that the absorption peak around 3500-2800 cm-1 became weaker, indicating that the dimmer band formed by hydrogen bonds was reduced due to the deposition of CTAC over membrane surface. Opposite to the increase of the C-H absorption strength at about 1454cm-1, the C=O absorption strength at about 1737 cm-1 was somewhat lower than that of -C≡N band strength at 2300 cm-1. From the obvious change of the membrane spectra after the deposition of CTAC layer, it was reasonable to say that the change of the chemical structure of LbL suggested the presence of part of carboxylic groups

Compared to the amphiphilic CTAC, TMAC behaved similar cationic property except for long alkyl chain on the quaternary ammonium site. The FT-IR spectra of the base P(AN-*co*-AA), P(AN-*co*-AA) with TMAC monolayer and with TMAC/PAA composite multilayer are shown in Figure 2-2. Apparently, the dimmer band in the range from 3500-2800 cm-1 changed alternately after the formation of TMAC layer on the P(AN-*co*-AA) base membrane surface and then the PAA layer over the P(AN-*co*-AA)/TMAC. Also, the ratio of >C=O group at about 1720 cm-1 to that of the dissociated carboxylic group at about 1645 cm-1

Fig. 2-2. Comparison of FT-IR spectra of the base P(AN-co-AA), P(AN-co-AA)/TMAC/PAA

multi- layer composites, the layer numbers.

#### *Confirmation of composite multilayer*

The surface nature of resultant membranes modified with CTAC/PAA and TMAC/PAA ESA multiple layers would change alternately due to the alternate deposition of CTAC or TMAC and PAA layer on the surface. In order to estimate the formation of CTAC (or TMAC) and PAA layers on P(AN-*co*-AA) base membrane, the increment of membrane weight due to the repeated construction of composite layer on membrane surface was measured. Herein, we listed the weight value for each deposited layer of CTAC, TMAC and PAA. Here, the value of [DL] (Deposited Layer)(mmol / g) was calculated from [DL] = Δm / Fw, where Δm was the corresponding weight increment of membrane induced by each deposition step and Fw were 319.5 g / mol, 109.5 g / mol and 72 g / mol for CTAC, TMAC and PAA, respectively. The obtained average value of [DL] was about 1.4 mmol / g for P(AN-*co*-AA)/CTAC/PAA system and 1.6 mmol / g for P(AN-*co*-AA)/TMAC/PAA system.

FT-IR spectroscopy was applied to investigate the chemical component of resultant membranes as to confirm the formation of alternate layers of anion and cation. Figure 2-1 shows IR spectra of base P(AN-*co*-AA) and P(AN-*co*-AA) membranes for LbL of the CTAC/PAA composite multilayer on the surface. Spectrum (0) was for P(AN-*co*-AA), (1) for P(AN-*co*-AA) with CTAC monolayer, (2) for P(AN-*co*-AA)/CTAC/PAA, (3) for P(AN-*co*-AA)/CTAC/PAA/CTAC and (4) for P(AN-*co*-AA)/CTAC/PAA/CTAC/PAA. In the spectrum (0), there was a wide and strong absorption peak appeared near 3500-2800cm-1, which was assigned to the dimmer formed by hydrogen bonds of carboxylic acids of the AA segments in the P(AN-*co*-AA). Also, the characteristic absorption peak assigned to

Fig. 2-1. Comparison of FT-IR spectra of the base P(AN-co-AA) and multi-layer composites of CTAC/PAA on the base P(AN-co-AA). The layer number of each spectrum was represented as 0, 1, 2, 3 and 4 for P(AN-co-AA), P(AN-co-AA)/CTAC, P(AN-co-AA)/CTAC/PAA, P(AN-co-AA)/CTAC/PAA/CTAC and P(AN-co-AA)/CTAC/PAA/CTAC/PAA, respectively.

The surface nature of resultant membranes modified with CTAC/PAA and TMAC/PAA ESA multiple layers would change alternately due to the alternate deposition of CTAC or TMAC and PAA layer on the surface. In order to estimate the formation of CTAC (or TMAC) and PAA layers on P(AN-*co*-AA) base membrane, the increment of membrane weight due to the repeated construction of composite layer on membrane surface was measured. Herein, we listed the weight value for each deposited layer of CTAC, TMAC and PAA. Here, the value of [DL] (Deposited Layer)(mmol / g) was calculated from [DL] = Δm / Fw, where Δm was the corresponding weight increment of membrane induced by each deposition step and Fw were 319.5 g / mol, 109.5 g / mol and 72 g / mol for CTAC, TMAC and PAA, respectively. The obtained average value of [DL] was about 1.4 mmol / g for P(AN-*co*-AA)/CTAC/PAA system and 1.6 mmol / g for P(AN-*co*-AA)/TMAC/PAA

FT-IR spectroscopy was applied to investigate the chemical component of resultant membranes as to confirm the formation of alternate layers of anion and cation. Figure 2-1 shows IR spectra of base P(AN-*co*-AA) and P(AN-*co*-AA) membranes for LbL of the CTAC/PAA composite multilayer on the surface. Spectrum (0) was for P(AN-*co*-AA), (1) for P(AN-*co*-AA) with CTAC monolayer, (2) for P(AN-*co*-AA)/CTAC/PAA, (3) for P(AN-*co*-AA)/CTAC/PAA/CTAC and (4) for P(AN-*co*-AA)/CTAC/PAA/CTAC/PAA. In the spectrum (0), there was a wide and strong absorption peak appeared near 3500-2800cm-1, which was assigned to the dimmer formed by hydrogen bonds of carboxylic acids of the AA segments in the P(AN-*co*-AA). Also, the characteristic absorption peak assigned to

**PAA**

**(0) P(AN-***co***-AA)**

**(1) P(AN-***co***-AA)/CTAC**

**(2) P(AN-***co***-AA)/CTAC/PAA**

**(3) P(AN-***co***-AA)/CTAC/PAA/CTAC**

**(4) P(AN-***co***-AA)/CTAC/PAA/CTAC/PAA**

**CTAC**

3500 3000 2500 2000 1500 1000

of CTAC/PAA on the base P(AN-co-AA). The layer number of each spectrum was represented as 0, 1, 2, 3 and 4 for P(AN-co-AA), P(AN-co-AA)/CTAC, P(AN-co-

AA)/CTAC/PAA, P(AN-co-AA)/CTAC/PAA/CTAC and P(AN-co-

AA)/CTAC/PAA/CTAC/PAA, respectively.

Fig. 2-1. Comparison of FT-IR spectra of the base P(AN-co-AA) and multi-layer composites

Wavenumber(cm-1)

*Confirmation of composite multilayer* 

system.

Transmittance (%)

dissociated carboxylic group at about 1645 cm-1 and 1720 cm-1 existed in the spectrum. In the spectrum (1), it was noticed that the absorption peak around 3500-2800 cm-1 became weaker, indicating that the dimmer band formed by hydrogen bonds was reduced due to the deposition of CTAC over membrane surface. Opposite to the increase of the C-H absorption strength at about 1454cm-1, the C=O absorption strength at about 1737 cm-1 was somewhat lower than that of -C≡N band strength at 2300 cm-1. From the obvious change of the membrane spectra after the deposition of CTAC layer, it was reasonable to say that the change of the chemical structure of LbL suggested the presence of part of carboxylic groups dissociated to react with CTAC by electrostatic attraction.

Compared to the amphiphilic CTAC, TMAC behaved similar cationic property except for long alkyl chain on the quaternary ammonium site. The FT-IR spectra of the base P(AN-*co*-AA), P(AN-*co*-AA) with TMAC monolayer and with TMAC/PAA composite multilayer are shown in Figure 2-2. Apparently, the dimmer band in the range from 3500-2800 cm-1 changed alternately after the formation of TMAC layer on the P(AN-*co*-AA) base membrane surface and then the PAA layer over the P(AN-*co*-AA)/TMAC. Also, the ratio of >C=O group at about 1720 cm-1 to that of the dissociated carboxylic group at about 1645 cm-1

Fig. 2-2. Comparison of FT-IR spectra of the base P(AN-co-AA), P(AN-co-AA)/TMAC/PAA multi- layer composites, the layer numbers.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

EDAMA)/PAA/CTAC system, respectively.

the following characterization of the membrane properties.

H3C H2C Br

H3C CH2 N 15 CH3

EDAMA) and electrolytes of CTAC and PAA.

Cl

**CTAC**

**P(AN-***co***-EDAMA)**

CH3 CH3

Scheme 3-1. Chemical structures of membrane materials P(AN-*co*-AA) and P(AA-*co*-

The metal-ion permselective properties were examined as follows: metal ion solutions containing Fe(NO3)3 and FeCl2 with 2 ppm concentration were prepared, respectively. Before and after the formation of ESA layers, filtration measurement of the metal ion solution through sample membranes was carried out by using an ultrafiltration cell (Amicon 8010, 50 ml volume). The metal-ion rejection (R) was defined by the following equation:

CH2 m

C CH3 <sup>n</sup> <sup>O</sup> <sup>C</sup> O CH2 N CH2 CH3 CH3

CH2 CH CN

*Multilayer desalination membranes* 

substrate.[18, 19]

Membranes Having Characteristics of Heavy Metal Ion Desalination 307

As presented in scheme 3-1, quaternized *N,N*-dimethylethyl ammonium bromide (EDAMA) was polymerized with AN for preparation of cationic base membrane. After that, AA and the quaternized EDAMA were used for their functional groups in order to provide charged sites for polyacrylonitrile base membranes. Also, Poly(acrylonitrile-*co*-acrylic acid) [P(AN*co*-AA)] membrane was used as a negatively charged base, while Poly(acrylonitrile-*co*-*N,N*dimethyethyl ammonium bromide P(AN-*co*-EDAMA) was used as a positively charged

Their base membranes of P(AN-*co*-AA) and P(AN-*co*-EDAMA) were prepared by phaseinversion of their respective dimethyl sulfoxide solutions in water.[20] The base membranes of P(AN-*co*-AA) or P(AN-*co*-EDAMA) were orderly dipped in positive CTAC and negative PAA aqueous solutions. After each deposition step, the resulting membranes were rinsed thoroughly with water. Such deposition process was repeated to obtain desired number of self-assembled layers. In this research, P(AN-*co-*AA) or P(AN-*co*-EDAMA) membranes with ESA layers are referred to as P(AN-*co*-AA)/CTAC/PAA system or as P(AN-*co*-

Before measuring the properties of membranes with ESA multilayer, sample membranes were dehydrated using freeze-drying method in order to preserve the sample structure for

> CH2 CH CN

CH2 m

**P(AN-***co***-AA)**

R= (1-ct / co)×100% (3-1)

CH2 CH n O C OH **PAA**

C CH3 <sup>n</sup> <sup>O</sup> <sup>C</sup> OH

**3. Preparation of multilayer composite surfaces prepared by electrostatic** 

**self-assembled technique for desalination of Fe3+, and Fe2+**

increased as the TMAC and PAA were deposited. The reason for this phenomena was same with the P(AN-*co*-AA)/CTAC/PAA system as induced by the fact that the deposition of TMAC layer over PAA layer enhanced the dissociation of carboxylic group which neutralized with TMAC molecules. FT-IR data also showed that the C-H absorption peak around 2920 cm-1 increased slightly as the number of TMAC layer increased. Moreover, characteristic absorption peak from TMAC at around 950 cm-1 appeared in the spectra of the resultant P(AN-*co*-AA) membrane with TMAC/PAA composites multilayer on the surface. Moreover, it was noticed that the strength of C-H absorption peak at 1454 cm-1 increased with the increase of composite layers on membrane surface, even through the changes were smaller as compared with P(AN-*co*-AA)/CTAC/PAA. All these changes confirmed the alternate construction of TMAC and PAA layers on base membrane surface which was induced by the electrostatic interaction between two neighboring layers.

Due to the electrostatic interaction between CTAC (or TMAC) and AA segments in the P(AN-*co*-AA) base membrane as well as between neighboring CTAC (or TMAC) and PAA layers, the composite multilayer of CTAC/PAA and TMAC/PAA successfully selfassembled. To reveal the alternate formation of self-assembly CTAC/PAA and TMAC/PAA multilayer qualitatively, the strength ratio of >C=O band at about 1737 cm-1 to 1654 cm-1 as shown in Figure 2-3 was calculated from FT-IR spectra. It was observed that the bond intensity of >C=O group decreased after the deposition of CTAC (or TMAC) layer as part of carboxylic groups of AA segment electrostatically associated with CTAC (or TMAC) the CTAC (or TMAC) layer, and then increased as the PAA layer was formed to be the top layer for the composite multilayer. This alternate change tendency revealed the dissociation extent change of carboxylic acid groups in base membrane or in each PAA layer and strongly confirmed the sequent adsorption of CTAC (or TMAC) and PAA layer on the negatively charged P(AN-*co*-AA) base membrane surface.

Fig. 2-3. Absorption ratio of observed IR bands for 1737cm-1, 1645 cm-1 C=O assigned to carboxylic acid and to the dissociated carboxylic acid in IR spectra obtained in P(AN-co-AA)/CTAC/PAA (▲)and P(AN-co-AA)/TMAC/PAA (●) multilayer composites. The layer numbers were the same with those described in Figure 2-2.

### **3. Preparation of multilayer composite surfaces prepared by electrostatic self-assembled technique for desalination of Fe3+, and Fe2+**

#### *Multilayer desalination membranes*

306 Infrared Spectroscopy – Materials Science, Engineering and Technology

increased as the TMAC and PAA were deposited. The reason for this phenomena was same with the P(AN-*co*-AA)/CTAC/PAA system as induced by the fact that the deposition of TMAC layer over PAA layer enhanced the dissociation of carboxylic group which neutralized with TMAC molecules. FT-IR data also showed that the C-H absorption peak around 2920 cm-1 increased slightly as the number of TMAC layer increased. Moreover, characteristic absorption peak from TMAC at around 950 cm-1 appeared in the spectra of the resultant P(AN-*co*-AA) membrane with TMAC/PAA composites multilayer on the surface. Moreover, it was noticed that the strength of C-H absorption peak at 1454 cm-1 increased with the increase of composite layers on membrane surface, even through the changes were smaller as compared with P(AN-*co*-AA)/CTAC/PAA. All these changes confirmed the alternate construction of TMAC and PAA layers on base membrane surface which was

Due to the electrostatic interaction between CTAC (or TMAC) and AA segments in the P(AN-*co*-AA) base membrane as well as between neighboring CTAC (or TMAC) and PAA layers, the composite multilayer of CTAC/PAA and TMAC/PAA successfully selfassembled. To reveal the alternate formation of self-assembly CTAC/PAA and TMAC/PAA multilayer qualitatively, the strength ratio of >C=O band at about 1737 cm-1 to 1654 cm-1 as shown in Figure 2-3 was calculated from FT-IR spectra. It was observed that the bond intensity of >C=O group decreased after the deposition of CTAC (or TMAC) layer as part of carboxylic groups of AA segment electrostatically associated with CTAC (or TMAC) the CTAC (or TMAC) layer, and then increased as the PAA layer was formed to be the top layer for the composite multilayer. This alternate change tendency revealed the dissociation extent change of carboxylic acid groups in base membrane or in each PAA layer and strongly confirmed the sequent adsorption of CTAC (or TMAC) and PAA layer on the

Fig. 2-3. Absorption ratio of observed IR bands for 1737cm-1, 1645 cm-1 C=O assigned to carboxylic acid and to the dissociated carboxylic acid in IR spectra obtained in P(AN-co-AA)/CTAC/PAA (▲)and P(AN-co-AA)/TMAC/PAA (●) multilayer composites. The layer

Layer number on membrane surface

induced by the electrostatic interaction between two neighboring layers.

negatively charged P(AN-*co*-AA) base membrane surface.

numbers were the same with those described in Figure 2-2.

Absor

ption ratio of 1737cm-1

/

As presented in scheme 3-1, quaternized *N,N*-dimethylethyl ammonium bromide (EDAMA) was polymerized with AN for preparation of cationic base membrane. After that, AA and the quaternized EDAMA were used for their functional groups in order to provide charged sites for polyacrylonitrile base membranes. Also, Poly(acrylonitrile-*co*-acrylic acid) [P(AN*co*-AA)] membrane was used as a negatively charged base, while Poly(acrylonitrile-*co*-*N,N*dimethyethyl ammonium bromide P(AN-*co*-EDAMA) was used as a positively charged substrate.[18, 19]

Their base membranes of P(AN-*co*-AA) and P(AN-*co*-EDAMA) were prepared by phaseinversion of their respective dimethyl sulfoxide solutions in water.[20] The base membranes of P(AN-*co*-AA) or P(AN-*co*-EDAMA) were orderly dipped in positive CTAC and negative PAA aqueous solutions. After each deposition step, the resulting membranes were rinsed thoroughly with water. Such deposition process was repeated to obtain desired number of self-assembled layers. In this research, P(AN-*co-*AA) or P(AN-*co*-EDAMA) membranes with ESA layers are referred to as P(AN-*co*-AA)/CTAC/PAA system or as P(AN-*co*-EDAMA)/PAA/CTAC system, respectively.

Before measuring the properties of membranes with ESA multilayer, sample membranes were dehydrated using freeze-drying method in order to preserve the sample structure for the following characterization of the membrane properties.

Scheme 3-1. Chemical structures of membrane materials P(AN-*co*-AA) and P(AA-*co*-EDAMA) and electrolytes of CTAC and PAA.

The metal-ion permselective properties were examined as follows: metal ion solutions containing Fe(NO3)3 and FeCl2 with 2 ppm concentration were prepared, respectively. Before and after the formation of ESA layers, filtration measurement of the metal ion solution through sample membranes was carried out by using an ultrafiltration cell (Amicon 8010, 50 ml volume). The metal-ion rejection (R) was defined by the following equation:

$$\mathbf{R} = (\mathbf{1} \cdot \mathbf{c}\_t \text{ / } \mathbf{c}\_o) \times 100\% \tag{3-1}$$

were Co is the ionic concentration in the feed solution and Ct is the ionic concentration in the permeate solution after a time *t.*

In order to evaluate the selective rejection property of the resultant membranes, the removal selectivity coefficient (SR) for Fe3+ and Fe2+ ions was calculated from the following equation:

$$\mathbf{S}\_{\mathbf{R}} = R\_{\mathrm{Fr}} ^{3\*} \Big/ \ R\_{\mathrm{Fr}} 2^\* \tag{3-2}$$

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

EDAMA) was confirmed by the FT-IR measurement.

effective for the metal ion removal application.

Membranes Having Characteristics of Heavy Metal Ion Desalination 309

**PAA**

**(0) P(AN-co-EDAMA)**

**(1) P(AN-co-EDAMA)/PAA (2) P(AN-co-EDAMA)/PAA/CTAC**

**(3) P(AN-co-EDAMA)/PAA/CTAC/PAA**

**(4) P(AN-co-EDAMA)/PAA/CTAC/PAA/CTAC**

**CTAC**

3500 3000 2500 2000 1500 1000 500

Fig. 3-1. FT-IR spectra of base membrane and membrane with self-assembled layers.

layer resulted that the carboxylic acid absorption peak became weaker relative to that of P(AN-*co*-EDAMA)/PAA. A similar tendency repeated itself in accordance with the selfassembly of the first cycle of PAA layer and CTAC layer. Hereby, the alternate self-assembly of CTAC and PAA onto the charged base membranes of P(AN-*co*-AA) and P(AN-*co*-

Since the alternate ESA layers on the membrane surface contained charged groups, it was expected that the permeate behaviour of metal ions through the modified membrane could be affected by the ESA layers formed on the membrane surface.[21, 22] Thus, the desalination of the ESA modified membranes was evaluated by testing the permeability for metal ion solution. Rejection values for metal ions of Fe3+ and Fe2+ of resultant membranes of the P(AN-*co*-AA)/CTAC/PAA system and P(AN-*co*-EDAMA)/PAA/CTAC system are shown in Figure 3-2 and Figure 3-3, respectively, with solid lines for Fe3+ and dotted lines for Fe2+. It could be seen that the membrane modified with ESA layers showed considerable high rejection for metal ion of Fe3+ and the rejection for Fe3+ was higher than for Fe2+. This may be induced by the smaller hydrated ion radius of Fe2+ than that of Fe3+ resulting from the fact that Fe2+ has a bigger ion radius of about 0.78 nm than the 0.64 nm of Fe3+. As revealed by both of the two systems, the metal-ion rejection was strongly related to the charge property of the outer layer on the modified membrane surface. It was noted interestingly that the metal ion rejection changed alternately and was higher in deposition cycle of 1, 3 and 5 for the P(AN-*co*-AA)/CTAC/PAA system, while higher in deposition cycle of 2, 4 and 6 for the P(AN-*co*-EDAMA)/PAA/CTAC system. It seemed that the deposition of CTAC layers having positively charged groups enhanced the rejection of membranes for metal ions, might be due to the electrostatic repulsion between metal ion and the CTAC layer on membrane surface. Therefore, it was considered that membrane having the LbL surface terminated by positively charged species, due to the self-assembly of CTAC layers might be

RFe3+ and RFe2+ were the rejection values of the resultant membranes for Fe3+ and Fe2+, respectively.

#### *IR measurement of membranes with ESA multilayer*

As presented in Scheme 3-2, the base membrane surface was modified alternately by layerby-layer formation of CTAC and PAA via ESA treatment. The structure of membrane formed in P(AN-co-AA)/CTAC/PAA system is shown in Figure 2-1. In this section, we mainly introduced the analysis of structure of membrane formed in P(AN-*co*-EDAMA)/PAA/CTAC system. In Figure 3-1, the FT-IR spectrum indicated that for the positively charged substrate of P(AN-*co*-EDAMA), the ESA treatment was performed in the deposition cycle order of PAA/CTAC/PAA/CTAC. As a result of the first deposition of the PAA layer, the band strength of the carboxylic group at about 1720 cm-1 became stronger, which confirmed the assembly of the PAA layer onto the positively charged base membrane. However, after the deposition of CTAC layer, the carboxylic groups bound on the CTAC

(b) P(AN-*co*-EDAMA)/PAA/CTAC system

Scheme 3-2. Illustration images of layer-by-layer assembling of CTAC and PAA on negatively charged P(AN-*co*-AA) membrane surface (a) and on positively charged P(AN-*co*-EDAMA) membrane surface (b).

were Co is the ionic concentration in the feed solution and Ct is the ionic concentration in the

In order to evaluate the selective rejection property of the resultant membranes, the removal selectivity coefficient (SR) for Fe3+ and Fe2+ ions was calculated from the following equation:

 SR= *RFe3+* / *RFe2+* (3-2) RFe3+ and RFe2+ were the rejection values of the resultant membranes for Fe3+ and Fe2+,

As presented in Scheme 3-2, the base membrane surface was modified alternately by layerby-layer formation of CTAC and PAA via ESA treatment. The structure of membrane formed in P(AN-co-AA)/CTAC/PAA system is shown in Figure 2-1. In this section, we mainly introduced the analysis of structure of membrane formed in P(AN-*co*-EDAMA)/PAA/CTAC system. In Figure 3-1, the FT-IR spectrum indicated that for the positively charged substrate of P(AN-*co*-EDAMA), the ESA treatment was performed in the deposition cycle order of PAA/CTAC/PAA/CTAC. As a result of the first deposition of the PAA layer, the band strength of the carboxylic group at about 1720 cm-1 became stronger, which confirmed the assembly of the PAA layer onto the positively charged base membrane. However, after the deposition of CTAC layer, the carboxylic groups bound on the CTAC

(a) P(AN-co-AA)/CTAC/PAA system

(b) P(AN-*co*-EDAMA)/PAA/CTAC system

negatively charged P(AN-*co*-AA) membrane surface (a) and on positively charged P(AN-*co*-

Scheme 3-2. Illustration images of layer-by-layer assembling of CTAC and PAA on

permeate solution after a time *t.*

EDAMA) membrane surface (b).

*IR measurement of membranes with ESA multilayer* 

respectively.

Fig. 3-1. FT-IR spectra of base membrane and membrane with self-assembled layers.

layer resulted that the carboxylic acid absorption peak became weaker relative to that of P(AN-*co*-EDAMA)/PAA. A similar tendency repeated itself in accordance with the selfassembly of the first cycle of PAA layer and CTAC layer. Hereby, the alternate self-assembly of CTAC and PAA onto the charged base membranes of P(AN-*co*-AA) and P(AN-*co*-EDAMA) was confirmed by the FT-IR measurement.

Since the alternate ESA layers on the membrane surface contained charged groups, it was expected that the permeate behaviour of metal ions through the modified membrane could be affected by the ESA layers formed on the membrane surface.[21, 22] Thus, the desalination of the ESA modified membranes was evaluated by testing the permeability for metal ion solution. Rejection values for metal ions of Fe3+ and Fe2+ of resultant membranes of the P(AN-*co*-AA)/CTAC/PAA system and P(AN-*co*-EDAMA)/PAA/CTAC system are shown in Figure 3-2 and Figure 3-3, respectively, with solid lines for Fe3+ and dotted lines for Fe2+. It could be seen that the membrane modified with ESA layers showed considerable high rejection for metal ion of Fe3+ and the rejection for Fe3+ was higher than for Fe2+. This may be induced by the smaller hydrated ion radius of Fe2+ than that of Fe3+ resulting from the fact that Fe2+ has a bigger ion radius of about 0.78 nm than the 0.64 nm of Fe3+. As revealed by both of the two systems, the metal-ion rejection was strongly related to the charge property of the outer layer on the modified membrane surface. It was noted interestingly that the metal ion rejection changed alternately and was higher in deposition cycle of 1, 3 and 5 for the P(AN-*co*-AA)/CTAC/PAA system, while higher in deposition cycle of 2, 4 and 6 for the P(AN-*co*-EDAMA)/PAA/CTAC system. It seemed that the deposition of CTAC layers having positively charged groups enhanced the rejection of membranes for metal ions, might be due to the electrostatic repulsion between metal ion and the CTAC layer on membrane surface. Therefore, it was considered that membrane having the LbL surface terminated by positively charged species, due to the self-assembly of CTAC layers might be effective for the metal ion removal application.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

formation of ESA layers onto charged base membrane surface.

circle of CTAC/PAA layers was formed onto membrane surface.

(a) 10 mol% and (b) 26 mol% of AA in the P(AN-*co*-AA)s were used.

(c) 10 mol% and (d) 15 mol% of EDAMA were used in the P(AN-*co*-EDAMA).

system and Fe3+/Fe2+ of the P(AN-*co*-EDAMA)/PAA/CTAC system.

Table 3. Selective removel coefficient (SR) for Fe3+ / Fe2+ of the P(AN-*co*-AA)/CTAC/PAA

Number of ESA

SR for Fe3+/Fe2+ of P(AN-*co*-AA) a)

SR for Fe3+/Fe2+ of P(AN-*co*-AA) b)

SR for Fe3+/Fe2+ of P(AN-*co*-EDAMA)c)

SR for Fe3+/Fe2+ of P(AN-*co*-EDAMA) d)

Membranes Having Characteristics of Heavy Metal Ion Desalination 311

base membrane was prepared by cast solution with a higher concentration of 15%, the rejection for Fe3+ and Fe2+ rose significantly as compared with that of membranes prepared with a concentration of 10%. This result was also induced by the higher charge density of base membrane prepared with higher concentration which consequently affected the

To study the charge density of PAA layer on membrane surface, we calculated the ratio of dissociated COO- group and COOH group by referring to the characteristic peaks at 1446 cm-1 and 1710 cm-1, respectively, revealed by FT-IR data in Figure 2-1 and Figure 3-1. For the P(AN-*co*-AA)/CTAC/PAA system, the value of [COO-]/[COOH] was 72% for base membrane of P(AN-*co*-AA), after the surface was converted with CTAC layer, the value became to be 78% and then decreased to 47 % due to the assembly of PAA layer onto membrane surface. Then, the value changed to be 83% and 49% consequently as the second

As the rejection of the resultant membranes for Fe3+ and Fe2+ was obviously different, it was considered that the resultant membranes were showed permselectivity for Fe3+ and Fe2+. Herein, the removal selectivity coefficient for Fe3+ and Fe2+ was calculated according to equation 3-2. The results are shown in Table 3 for the P(AN-*co*-AA)/CTAC/PAA system and the P(AN-*co*-EDAMA)/CTAC/PAA system. The results indicated that membrane modified with ESA layers of CTAC and PAA was capable of selectively removing Fe3+ and Fe2+ along with relatively high selective permeability. For the P(AN-*co*-AA)/CTAC/PAA system, as shown in Table 3-(a) and 3-(b), base membrane containing 10 mol% AA segment (denoted by a) resulted in ESA modified membrane having relative higher removal selectivity coefficients (SR) for metal ion pair of Fe3+ and Fe2+ as compared with base membrane containing 26 mol% AA segment. The results revealed that lower fraction of AA segments in the base membrane led to resultant membrane with lower selective

layers 1 2 3 4 5 6

1.54±0.02 1.68±0.04 1.75±0.07 1.61±0.03 2.10±0.06 1.62±0.03

1.64±0.03 1.42±0.01 1.53±0.04 1.36±0.05 1.75±0.06 1.37±0.06

1.07±0.04 1.32±0.05 1.28±0.05 1.52±0.02 1.13±0.06 1.69±0.03

1.07±0.03 1.42±0.02 1.15±0.05 1.82±0.06 1.33±0.04 2.10±0.04

Fig. 3-2. **a**) The effect of AA fraction contained in base membrane on the rejection of Fe3+ (—) and Fe2+ (…) for P(AN-co-AA)/CTAC/PAA system (cast solution concentration for base membrane: 15wt%; pH of metal ions solution: 7). **b**) The effect of P(AN-*co*-EDAMA) base membrane on the rejection of Fe3+ (— ) and Fe2+ (… ) for P(AN-*co*-EDAMA)/PAA/CTAC system. (EDAMA contained in base membrane: 2 mol%; pH of metal ions solution: 7).

Additionally, the change tendency of the rejection curves of the two systems for both metal ions behaved oppositely to each other as revealed by Figure 3-2. These indicated that P(AN*co*-AA) base membrane having COO- on the surface was negatively charged, while P(AN-*co*-EDAMA) base membrane having quatemary ammonium groups on the surface was negatively charged. Their different charge properties of base membrane surface resulted in different cycle order of self-assembled multilayer deposited on the base membrane surface, as performed in cycle of CTAC/PAA/CTAC/PAA for the former and PAA/CTAC/PAA/CTAC for the latter. The opposite change tendency of the permeability of resultant membranes further confirmed the influence of the ESA terminal layer on its permeability.

Furthermore, we examined the effect of base membrane structure on the rejection behavior of resultant membrane. For P(AN-*co*-AA)/CTAC/PAA system, the charged sites on P(AN*co*-AA) base membrane were different for membranes having different amounts of AA segments.

Three kinds of P(AN-*co*-AA) copolymers prepared with a AA fraction of 10 mol%, 15 mol% and 26 mol%,[23] were used as base membranes for ESA treatment. Then, the surfaces were modified with ESA layers of CTAC and PAA to study the effect of AA fractions contained in the base membrane on the removal properties of modified membranes. It was found that, when the base membrane used for self-assembly treatment contained higher AA mol%, the corresponding resultant membranes showed higher removal properties for both Fe3+ and Fe2+. This meant that base membrane which contained higher AA mol% had more ionized carboxyl charged sites on the surface. Then, there would be more CTAC and PAA adsorbed onto membrane surface during the alternate dipping process, leading to resultant membrane surface having higher charge density and then affecting the permeability for metal ions. For the P(AN-*co*-EDAMA)/PAA/CTAC system, by increasing the concentration of the cast solution, the effect of the base membrane structure on the permeability of ESA layer modified membranes was studied. As shown in Figure 3-2(b), when the P(AN-*co*-EDAMA)

Fig. 3-2. **a**) The effect of AA fraction contained in base membrane on the rejection of Fe3+ (—) and Fe2+ (…) for P(AN-co-AA)/CTAC/PAA system (cast solution concentration for base membrane: 15wt%; pH of metal ions solution: 7). **b**) The effect of P(AN-*co*-EDAMA) base membrane on the rejection of Fe3+ (— ) and Fe2+ (… ) for P(AN-*co*-EDAMA)/PAA/CTAC system. (EDAMA contained in base membrane: 2 mol%; pH of metal ions solution: 7).

Number of layers on membrane surface Number of layers on membrane surface

Rejection for Fe ions (%)

Additionally, the change tendency of the rejection curves of the two systems for both metal ions behaved oppositely to each other as revealed by Figure 3-2. These indicated that P(AN*co*-AA) base membrane having COO- on the surface was negatively charged, while P(AN-*co*-EDAMA) base membrane having quatemary ammonium groups on the surface was negatively charged. Their different charge properties of base membrane surface resulted in different cycle order of self-assembled multilayer deposited on the base membrane surface, as performed in cycle of CTAC/PAA/CTAC/PAA for the former and PAA/CTAC/PAA/CTAC for the latter. The opposite change tendency of the permeability of resultant membranes further confirmed the influence of the ESA terminal layer on its

Furthermore, we examined the effect of base membrane structure on the rejection behavior of resultant membrane. For P(AN-*co*-AA)/CTAC/PAA system, the charged sites on P(AN*co*-AA) base membrane were different for membranes having different amounts of AA

Three kinds of P(AN-*co*-AA) copolymers prepared with a AA fraction of 10 mol%, 15 mol% and 26 mol%,[23] were used as base membranes for ESA treatment. Then, the surfaces were modified with ESA layers of CTAC and PAA to study the effect of AA fractions contained in the base membrane on the removal properties of modified membranes. It was found that, when the base membrane used for self-assembly treatment contained higher AA mol%, the corresponding resultant membranes showed higher removal properties for both Fe3+ and Fe2+. This meant that base membrane which contained higher AA mol% had more ionized carboxyl charged sites on the surface. Then, there would be more CTAC and PAA adsorbed onto membrane surface during the alternate dipping process, leading to resultant membrane surface having higher charge density and then affecting the permeability for metal ions. For the P(AN-*co*-EDAMA)/PAA/CTAC system, by increasing the concentration of the cast solution, the effect of the base membrane structure on the permeability of ESA layer modified membranes was studied. As shown in Figure 3-2(b), when the P(AN-*co*-EDAMA)

**a**) **b**)

permeability.

Rejection for Fe ions (%)

segments.

base membrane was prepared by cast solution with a higher concentration of 15%, the rejection for Fe3+ and Fe2+ rose significantly as compared with that of membranes prepared with a concentration of 10%. This result was also induced by the higher charge density of base membrane prepared with higher concentration which consequently affected the formation of ESA layers onto charged base membrane surface.

To study the charge density of PAA layer on membrane surface, we calculated the ratio of dissociated COO group and COOH group by referring to the characteristic peaks at 1446 cm-1 and 1710 cm-1, respectively, revealed by FT-IR data in Figure 2-1 and Figure 3-1. For the P(AN-*co*-AA)/CTAC/PAA system, the value of [COO-]/[COOH] was 72% for base membrane of P(AN-*co*-AA), after the surface was converted with CTAC layer, the value became to be 78% and then decreased to 47 % due to the assembly of PAA layer onto membrane surface. Then, the value changed to be 83% and 49% consequently as the second circle of CTAC/PAA layers was formed onto membrane surface.

As the rejection of the resultant membranes for Fe3+ and Fe2+ was obviously different, it was considered that the resultant membranes were showed permselectivity for Fe3+ and Fe2+. Herein, the removal selectivity coefficient for Fe3+ and Fe2+ was calculated according to equation 3-2. The results are shown in Table 3 for the P(AN-*co*-AA)/CTAC/PAA system and the P(AN-*co*-EDAMA)/CTAC/PAA system. The results indicated that membrane modified with ESA layers of CTAC and PAA was capable of selectively removing Fe3+ and Fe2+ along with relatively high selective permeability. For the P(AN-*co*-AA)/CTAC/PAA system, as shown in Table 3-(a) and 3-(b), base membrane containing 10 mol% AA segment (denoted by a) resulted in ESA modified membrane having relative higher removal selectivity coefficients (SR) for metal ion pair of Fe3+ and Fe2+ as compared with base membrane containing 26 mol% AA segment. The results revealed that lower fraction of AA segments in the base membrane led to resultant membrane with lower selective


(a) 10 mol% and (b) 26 mol% of AA in the P(AN-*co*-AA)s were used.

(c) 10 mol% and (d) 15 mol% of EDAMA were used in the P(AN-*co*-EDAMA).

Table 3. Selective removel coefficient (SR) for Fe3+ / Fe2+ of the P(AN-*co*-AA)/CTAC/PAA system and Fe3+/Fe2+ of the P(AN-*co*-EDAMA)/PAA/CTAC system.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

OH-

H2O

Scheme 4-1. Chemical reaction of PAN treated by alkali solution.

weight of membrane (g), and was calculated by following equation.

CH2 CH

membrane being used.

Reflection (%)

Wevenumber (cm-1)

assigned to the carbonyl group from COO-

carboxyl groups resulting from the KOH treatment.

solution.

CN <sup>m</sup>

Membranes Having Characteristics of Heavy Metal Ion Desalination 313

<sup>C</sup> <sup>m</sup>

OH-

CH2 CH

O

**PAN**

**PAN treated by KOH for 4 hours** 

**PAN treated by KOH for 24 hours** 

existed in the FT-IR spectrum. So, it was

<sup>C</sup> <sup>m</sup>

<sup>+</sup> NH3

O-

H2O

NH2

original and residual ion concentrations remained in the ion solution were measured by Atomic absorption spectrophotometer (AA-6300, Shimadzu Ins). The metal ion adsorption capacity (10-6 g / g-memebrane) was defined as the amount of Cu2+ (g) adsorbed by unit

 *Cu*(II)(*adsorption capacity*) = *v(co-ce)/M* (4-1) where co is the initial concentration of Cu(II) (ppm/ml), Ce is the concentration at equilibrium, V is the volume (40 ml) of solution and M is the wet mass of resultant

4000 3000 2000 1000

Fig. 4-1. FT-IR spectra of original PAN membrane and membrane treated with KOH

In order to convent the CN groups to negatively charged COO- group on the PAN membrane surface, the chemical reaction (Scheme 4-1) was performed by using KOH solution. The FT-IR measurement was carried out to study the chemical composition of the PAN membrane surface before and after the KOH treatment. In the spectrum of the PAN membrane treated by KOH for 24 hours, the characteristic absorption peak for the CN group at about 2300 cm-1 became weaker while the characteristic peak assigned to carboxyl group in the range from 2800 cm-1 to 3700 cm-1 appeared stronger. Especially, the band at 1700 cm-1

concluded that the PAN membrane surface was negatively charged due to the formation of

CH2 CH

O

permeability for Fe3+ and Fe2+. This may be because that much of the electrolytes selfassembled onto membrane surface, which was enhanced by the higher charge density resulting from the higher fraction of AA segments in the base membrane. So the permeability of membrane for metal ion solution was weakened and then led to lower permselectivity.

Also, the base membrane used for ESA treatment contained 10 mol% AA segment, SRs for metal ion pair of Fe3+ and Fe2+ of the modified membranes terminated with CTAC, CTAC/PAA/CTAC and CTAC/PAA/CTAC/PAA/CTAC ESA layers were 1.54±0.02, 1.75±0.07 and 2.10±0.06, respectively.

While the SRs of membranes deposited with CTAC/PAA, CTAC/PAA/CTAC/PAA, CTAC/PAA/CTAC/PAA/CTAC/PAA ESA layers were 1.68±0.04, 1.61±0.03 and 1.62±0.03, respectively. It was reasonable to conclude that the resultant membrane was endued with higher selective removal property for Fe3+ and Fe2+ when the surface was self-assembly terminated with positive CTAC layer.

For P(AN-*co*-EDAMA)/PAA/CTAC system, the SRs for Fe3+ and Fe2+ ions are shown in Table 3-(c) and (d). It can obviously seen that base membrane prepared by 15 mol% cast solution of P(AN-*co*-EDAMA)(denoted by c) resulted in ESA modified membranes with higher SR as compared with base membrane prepared with 10 mol% cast solution (denoted by d). When the base membrane used for ESA treatment was prepared by 15wt% cast solution of P(AN-*co*-EDMA), SRs of the corresponding resultant membranes modified with PAA/CTAC, PAA/CTAC/PAA/CTAC and PAA/CTAC/PAA/CTAC and PAA/CTAC/PAA/CTAC/PAA/CTAC multilayer were 1.42±0.02, 1.82±0.06 and 2.10±0.04, respectively, while the SRs of membranes having CTAC/PAA, CTAC/PAA/CTAC/PAA, CTAC/PAA/CTAC/PAA/CTAC/PAA layers on the surface became 1.07±0.03, 1.15±0.05 and 1.33±0.04, respectively. These indicated that positively charged surface induced by the formation of self-assembly CTAC layers was capable of selectively removing Fe3+ and Fe2+ ions. From these results, it was found out that for the P(AN-*co*-AA)/CTAC/PAA system, showed higher selectivity but lower rejection for Fe3+ and Fe2+. While for the P(AN-*co*-EDAMA)/PAA/CTAC system, showed considerable high rejection for Fe3+ about 90% as well as a relative high permselectivity for Fe3+ and Fe2+. It was proposed that the membranes prepared in this experimental could be used for the selective desalination of solution containing Fe3+ and Fe2+.

## **4. Self-assembly functionalized membranes with chitosan microsphere / Polyacrylic acid layers and their application for metal ion removal of Cu2+**

*Modification of PAN to anionic charged base membrane for adsorption of Cu (II)* 

PAN membrane, which was used as the base membrane for ESA treatment, was prepared and then treated with 2M KOH solution for negatively charged PAN base membrane (Scheme 4-1)[19].

#### *Adsorption of Cu(II)*

The adsorption experiment for Cu(II) was performed using the static method and was carried out as follows. Immerging the sample membranes was carried out into CuSO4 solution with a concentration of 2 ppm for 40 min at room temperature and then, the

Scheme 4-1. Chemical reaction of PAN treated by alkali solution.

original and residual ion concentrations remained in the ion solution were measured by Atomic absorption spectrophotometer (AA-6300, Shimadzu Ins). The metal ion adsorption capacity (10-6 g / g-memebrane) was defined as the amount of Cu2+ (g) adsorbed by unit weight of membrane (g), and was calculated by following equation.

$$\text{Cu(II)} (adsorption\ capacity) = \upsilon (\mathbf{c}\_{\sigma^\*} \mathbf{c}\_d)\_{\text{/M}} \tag{4-1}$$

where co is the initial concentration of Cu(II) (ppm/ml), Ce is the concentration at equilibrium, V is the volume (40 ml) of solution and M is the wet mass of resultant membrane being used.

Wevenumber (cm-1)

312 Infrared Spectroscopy – Materials Science, Engineering and Technology

permeability for Fe3+ and Fe2+. This may be because that much of the electrolytes selfassembled onto membrane surface, which was enhanced by the higher charge density resulting from the higher fraction of AA segments in the base membrane. So the permeability of membrane for metal ion solution was weakened and then led to lower

Also, the base membrane used for ESA treatment contained 10 mol% AA segment, SRs for metal ion pair of Fe3+ and Fe2+ of the modified membranes terminated with CTAC, CTAC/PAA/CTAC and CTAC/PAA/CTAC/PAA/CTAC ESA layers were 1.54±0.02,

While the SRs of membranes deposited with CTAC/PAA, CTAC/PAA/CTAC/PAA, CTAC/PAA/CTAC/PAA/CTAC/PAA ESA layers were 1.68±0.04, 1.61±0.03 and 1.62±0.03, respectively. It was reasonable to conclude that the resultant membrane was endued with higher selective removal property for Fe3+ and Fe2+ when the surface was self-assembly

For P(AN-*co*-EDAMA)/PAA/CTAC system, the SRs for Fe3+ and Fe2+ ions are shown in Table 3-(c) and (d). It can obviously seen that base membrane prepared by 15 mol% cast solution of P(AN-*co*-EDAMA)(denoted by c) resulted in ESA modified membranes with higher SR as compared with base membrane prepared with 10 mol% cast solution (denoted by d). When the base membrane used for ESA treatment was prepared by 15wt% cast solution of P(AN-*co*-EDMA), SRs of the corresponding resultant membranes modified with PAA/CTAC, PAA/CTAC/PAA/CTAC and PAA/CTAC/PAA/CTAC and PAA/CTAC/PAA/CTAC/PAA/CTAC multilayer were 1.42±0.02, 1.82±0.06 and 2.10±0.04, respectively, while the SRs of membranes having CTAC/PAA, CTAC/PAA/CTAC/PAA, CTAC/PAA/CTAC/PAA/CTAC/PAA layers on the surface became 1.07±0.03, 1.15±0.05 and 1.33±0.04, respectively. These indicated that positively charged surface induced by the formation of self-assembly CTAC layers was capable of selectively removing Fe3+ and Fe2+ ions. From these results, it was found out that for the P(AN-*co*-AA)/CTAC/PAA system, showed higher selectivity but lower rejection for Fe3+ and Fe2+. While for the P(AN-*co*-EDAMA)/PAA/CTAC system, showed considerable high rejection for Fe3+ about 90% as well as a relative high permselectivity for Fe3+ and Fe2+. It was proposed that the membranes prepared in this experimental could be used for the selective desalination of solution

**4. Self-assembly functionalized membranes with chitosan microsphere / Polyacrylic acid layers and their application for metal ion removal of Cu2+** 

PAN membrane, which was used as the base membrane for ESA treatment, was prepared and then treated with 2M KOH solution for negatively charged PAN base membrane

The adsorption experiment for Cu(II) was performed using the static method and was carried out as follows. Immerging the sample membranes was carried out into CuSO4 solution with a concentration of 2 ppm for 40 min at room temperature and then, the

*Modification of PAN to anionic charged base membrane for adsorption of Cu (II)* 

permselectivity.

1.75±0.07 and 2.10±0.06, respectively.

terminated with positive CTAC layer.

containing Fe3+ and Fe2+.

(Scheme 4-1)[19]. *Adsorption of Cu(II)* 

Fig. 4-1. FT-IR spectra of original PAN membrane and membrane treated with KOH solution.

In order to convent the CN groups to negatively charged COO group on the PAN membrane surface, the chemical reaction (Scheme 4-1) was performed by using KOH solution. The FT-IR measurement was carried out to study the chemical composition of the PAN membrane surface before and after the KOH treatment. In the spectrum of the PAN membrane treated by KOH for 24 hours, the characteristic absorption peak for the CN group at about 2300 cm-1 became weaker while the characteristic peak assigned to carboxyl group in the range from 2800 cm-1 to 3700 cm-1 appeared stronger. Especially, the band at 1700 cm-1 assigned to the carbonyl group from COO existed in the FT-IR spectrum. So, it was concluded that the PAN membrane surface was negatively charged due to the formation of carboxyl groups resulting from the KOH treatment.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

O

O O

COOH

N

C S

Membranes Having Characteristics of Heavy Metal Ion Desalination 315

O CH2OH OH

NH2

O CH2OH OH

S C NH

NH <sup>n</sup>

O

O OH

COOH

O

O CH2OH OH

NH2

n

+

**Fluorescein-5-isothiocyanate Chitosan**

O CH2OH OH

NH2

O

**FITC-labeled chitosan** Scheme 4-3. Schematic illustration of the chemical synthesis of fluorescein isothiocyanate

Number of ESA layers on membrane surface

labelled (FITC) chitosan of free FTIC fluorescence signal in the washing medium.

Fig. 4-2. UV-absorption after each deposition of chitosan layer.

UV- absorption(%)

#### *Formation of the ESA multilayer of chitosan / PAA and chitosan microspheres / PAA*

As the surface of treated PAN membrane was negatively charged due to the KOH treatment, positively charged chitosan and negatively charged PAA were alternatively coated to form self-assembly layers onto the charged PAN membrane (Scheme 4-2). Also, chitosan polycation microspheres cross-liked by sulfate groups were used instead of chitosan to form electrostatical self-assembled layers onto the negatively charged base membrane. The deposition process of the alternate formation of chitosan or chitosan microspheres and PAA layers was repeated 4 times. In Scheme 4-2, for the first step, the cationic chitosan or chitosan microspheres electrostatically interacted with the negatively charged groups on the base membrane surface. The membrane surface was then positively charged due to the deposition of chitosan or chitosan microspheres. Then, for the second step, PAA was deposited over the first layer of chitosan or chitosan microspheres due to the electrostatic interaction between amino groups of the chitosan and carboxyl groups of the PAA. These steps were repeated to form multiple ESA layers on the base membrane surface.

Scheme 4-2. Illustration images of layer-by-layer assembling of chitosan/PAA and chitosan microsphere/PAA layers onto charged base membrane.

In order to confirm the fabrication of the multilayer of chitosan or chitosan microsphere and PAA layers. Here, the FTIC labeled-chitosan and PAA layers were alternatively deposited onto charged glass slide for UV-vis measurement (Scheme 4-3).

The FTIC-labeled-chitosan (FTIC-labeled chitosan microsphere)/PAA layers on flat glass after each deposited cycle was detected by UV absorption at 476 nm. The UV absorption results of the samples were represented in Figure 4-2 and Figure 4-3, the odd layers were chitosan (chitosan microsphere) outmost. As we can see from Figure 4-2 that for both of PAN/chitosan/PAA system and PAN/chitosan microsphere/PAA system, the value of UV-absorption increased almost linearly with the increasing cycle of deposition process

As the surface of treated PAN membrane was negatively charged due to the KOH treatment, positively charged chitosan and negatively charged PAA were alternatively coated to form self-assembly layers onto the charged PAN membrane (Scheme 4-2). Also, chitosan polycation microspheres cross-liked by sulfate groups were used instead of chitosan to form electrostatical self-assembled layers onto the negatively charged base membrane. The deposition process of the alternate formation of chitosan or chitosan microspheres and PAA layers was repeated 4 times. In Scheme 4-2, for the first step, the cationic chitosan or chitosan microspheres electrostatically interacted with the negatively charged groups on the base membrane surface. The membrane surface was then positively charged due to the deposition of chitosan or chitosan microspheres. Then, for the second step, PAA was deposited over the first layer of chitosan or chitosan microspheres due to the electrostatic interaction between amino groups of the chitosan and carboxyl groups of the PAA. These steps were repeated to form multiple ESA layers on the base membrane surface.

 **Chitosan solution**

**Polyanion (PAA) solution** 

**Repeat steps for desired number of layers**

> **Polyanion (PAA) layers**

Scheme 4-2. Illustration images of layer-by-layer assembling of chitosan/PAA and chitosan

In order to confirm the fabrication of the multilayer of chitosan or chitosan microsphere and PAA layers. Here, the FTIC labeled-chitosan and PAA layers were alternatively deposited

The FTIC-labeled-chitosan (FTIC-labeled chitosan microsphere)/PAA layers on flat glass after each deposited cycle was detected by UV absorption at 476 nm. The UV absorption results of the samples were represented in Figure 4-2 and Figure 4-3, the odd layers were chitosan (chitosan microsphere) outmost. As we can see from Figure 4-2 that for both of PAN/chitosan/PAA system and PAN/chitosan microsphere/PAA system, the value of UV-absorption increased almost linearly with the increasing cycle of deposition process

*Formation of the ESA multilayer of chitosan / PAA and chitosan microspheres / PAA* 

**Negatively charged membrane surface**

 **Polycation chitosan or chitosan microsphere layers**

 

microsphere/PAA layers onto charged base membrane.

onto charged glass slide for UV-vis measurement (Scheme 4-3).

**Fluorescein-5-isothiocyanate Chitosan**

#### **FITC-labeled chitosan**

Scheme 4-3. Schematic illustration of the chemical synthesis of fluorescein isothiocyanate labelled (FITC) chitosan of free FTIC fluorescence signal in the washing medium.

Fig. 4-2. UV-absorption after each deposition of chitosan layer.

n

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

Membranes Having Characteristics of Heavy Metal Ion Desalination 317

**(0) PAN treated by KOH**

**(1) PAN/C hitosan**

**(2) PA N/Chitosan/PAA (3) PAN /Chitosan/PAA/Chitosan**

**(4) PAN/Chitosan/PAA /Chitosan/PAA**

**(0) PAN treated by KOH**

**(1) PAN/Chitosan**

**(2) PAN/Chitosan/PAA (3) PAN/Chitosan/PAA/Chitosan**

**(4) PAN/Chitosan/PAA/Chitosan/PAA**

**PAA**

**Chitosan**

**PAA**

**Chitosan**

4000 3000 2000 1000

4000 3000 2000 1000

Fig. 4-4. FT-IR spectra of membranes with self-assembled layers.

Reflection(%)

Reflection (%)

(b) PAN/Chitosan microsphere/PAA system

Wavenumber(cm-1)

(a) PAN/Chitosan/PAA system

Wavenumber(cm-1)

Fig. 4-3. The effect of immerging time on UV-absorption.

which verified the alternate self-assembly of chitosan (chitosan microsphere) layers and PAA layers onto charged substrate. When the substrate was immersed into chitosan microsphere, there was a remarkable increase in UV-absorption proving that a certain amount of chitosan had deposited onto substrate surface. Also, the samples immersed into the solutions of chitosan (chitosan microsphere) and PAA for 30 mins showed higher UVabsorption, which indicated the increase of the deposited amount of chitosan (chitosan microsphere).

Then, we examined the effect of immerging time on the self-assembly of chitosan for PAN/chitosan/PAA system.

It was thought from data that the self-assembly deposition of chitosan layer onto substrate surface reached saturated state after 30 mins. So, each deposited step lasted for 40 mins as to effectively reversed the charge on the membrane surface.

#### *Characterization of ESA multilayer membranes by FT-IR*

The surface of membranes having alternate ESA multilayer was investigated by using reflection FT-IR spectroscopy. Results are shown in Figure 4-4(a) and Figure 4-4(b) for PAN/chitosan/PAA system and PAN/chitosan microspheres/PAA system, respectively. It was found that the characteristic band of CN group from the charged PAN base membrane at about 2300 cm-1 became weaker due to the deposition of ESA layers of chitosan or chitosan microgel and PAA. In addition, the characteristic band at about 1170 cm-1 corresponding to the primary alcoholic group of chitosan. The peak at about 1400 cm-1 was assigned to the stretching vibration of amide band from chitosan appeared in the spectra, especially for cycled 1 and 3, for which the surface was terminated with chitosan layer or chitosan microspheres layer. In the cases of cycle 2 and 4 for surfaces terminated with PAA layer, similar spectra with PAA were observed and it was believed to be resulted by the

which verified the alternate self-assembly of chitosan (chitosan microsphere) layers and PAA layers onto charged substrate. When the substrate was immersed into chitosan microsphere, there was a remarkable increase in UV-absorption proving that a certain amount of chitosan had deposited onto substrate surface. Also, the samples immersed into the solutions of chitosan (chitosan microsphere) and PAA for 30 mins showed higher UVabsorption, which indicated the increase of the deposited amount of chitosan (chitosan

Number of ESA layers on membrane surface

Then, we examined the effect of immerging time on the self-assembly of chitosan for

It was thought from data that the self-assembly deposition of chitosan layer onto substrate surface reached saturated state after 30 mins. So, each deposited step lasted for 40 mins as to

The surface of membranes having alternate ESA multilayer was investigated by using reflection FT-IR spectroscopy. Results are shown in Figure 4-4(a) and Figure 4-4(b) for PAN/chitosan/PAA system and PAN/chitosan microspheres/PAA system, respectively. It was found that the characteristic band of CN group from the charged PAN base membrane at about 2300 cm-1 became weaker due to the deposition of ESA layers of chitosan or chitosan microgel and PAA. In addition, the characteristic band at about 1170 cm-1 corresponding to the primary alcoholic group of chitosan. The peak at about 1400 cm-1 was assigned to the stretching vibration of amide band from chitosan appeared in the spectra, especially for cycled 1 and 3, for which the surface was terminated with chitosan layer or chitosan microspheres layer. In the cases of cycle 2 and 4 for surfaces terminated with PAA layer, similar spectra with PAA were observed and it was believed to be resulted by the

Fig. 4-3. The effect of immerging time on UV-absorption.

effectively reversed the charge on the membrane surface.

*Characterization of ESA multilayer membranes by FT-IR* 

microsphere).

UV- absorption (%)

PAN/chitosan/PAA system.

(a) PAN/Chitosan/PAA system

(b) PAN/Chitosan microsphere/PAA system

Fig. 4-4. FT-IR spectra of membranes with self-assembled layers.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

capability for Cu2+ ion of the resultant membrane at various pHs.

O

COO COO

HO NH2 HOH2C

Scheme 4-4. Illustration images of the chelation for Cu2+ by the assembled chitosan on

NH CH2OH <sup>3</sup>

O

H2O H2O H2O

Cu2+ Cu2+

NH3

O

O

HO NH2

uses of adsorption of such metal ions.

<sup>O</sup> <sup>O</sup>

HO CH2OH

negatively charged membrane surface.

H2O

HOH2C

*Adsorption capability for copper ion* 

Membranes Having Characteristics of Heavy Metal Ion Desalination 319

However, much less change in the surface roughness was observed for the membrane with chitosan/PAA ESA layers. The values of the surface roughness were 8.0 nm, 8.6 nm, 9.0 nm, 12.2 nm, and 13.7 nm for the cycled layers of 0, 1, 2, 3 and 4, respectively. Therefore, the introduction of the chitosan microspheres for the ESA multilayer could be effectively for roughing the base of membrane surfaces. Therefore, this approach could be attractive in the

It was known that chitosan could be applied for adsorption of metal ions since the amino group is capable of binding metal ions as served as coordination sites. For example, Juang et al.,[25] reported high selectivity for Cu2+ ion was observed by chitosan adsorbents. Chang et al[26] found that the chitosan-modified microsphere showed the largest equilibrium adsorption capacity at pH 5.5. Therefore, in the present work, we studied the binding

Here, the pH of the Cu2+ ion solution was adjusted using buffer solution of 0.2 mol / L CH3COOH and 0.2 mol / L CH3COONa. The assembled behaviour of chitosan onto negatively charged base membrane and binding behaviour of chitosan for copper ions is simply illustrated by Scheme 4-4. Figure 4-6 shows the effect of the pH of Cu2+ ion solution on the Cu2+ adsorption of resultant membranes with Chitosan microspheres/PAA multiplelayers. The adsorption amounts of the Cu2+ ion were plotted at the cycle number for the ESA multilayer formation. The odd layers were for the chitosan or chitosan microspheres layers exposed on the membrane surface outmost and evens were for PAA layers on the outmost. It could be seen that when the adsorption was carried out at pH 3, membranes terminated with PAA layers showed higher adsorption capacity rather than those with terminal chitosan or chitosan microspheres layers. But, when pH increased, the change regulation could not be seen at pH 7 in both (a) and (b) systems. In the case of the chitosan microspheres/PAA system, the values of the adsorption capacity were significantly decreased to be in the range of about 50 ppm / g-membrane. However, at pH 5 and 6, the values of the adsorption capacity increased with the increasing of the cycled number of the

coverage of PAA on membrane surface. It was also noted that the characteristic peak from chitosan became weaker, while the peak in the range of about 2500 cm-1 to 3750 cm-1 from the PAA layer became stronger, proving that the PAA layer converted the chitosan or chitosan microspheres layers on the surface. The upper change was repeated alternately along with the alternative deposition of chitosan (chitosan microspheres) layers and PAA layers onto charged PAN membrane as revealed by IR spectrum. These spectral changes proved the successful formation of alternate chitosan or chitosan microspheres layers and PAA layers onto charged PAN membrane surface. Furthermore, the obvious difference between the IR spectrum of PAN/Chitosan/PAA system and PAN/Chitosan microspheres/PAA system was observed in comparison of (a) and (b). For the PAN/Chitosan microsphers/PAA system, the disappearance of the peak of the CN group from the charged PAN base membrane was drastic due to the coverage of the ESA layers.

Surface roughness of the chitosan microspheres/PAA membranes were represented in Figure 4-5. As shown in Figure 4-5, the surface roughness of -1 and 0 which referred to membranes before and after NaOH treatment, indicated that the membrane surface became rougher due to the existence of hydrophilic carboxyl groups resulting from NaOH treatment. It was also apparent that the surface morphology was dramatically altered by the formation of the chitosan microspheres/PAA ESA multilayer. While the corresponding surface roughness values were 8.0 nm, 23.1 nm, 15.0 nm, 39.9 nm and 36.4 nm for the chitosan microspheres/PAA membranes with cycle layers of 0, 1, 2, 3 and 4, respectively.

Number of self-assembed layers on membrane surface

Fig. 4-5. Influence of numbers of self-assembled layers onto the membrane surface on the surface

However, much less change in the surface roughness was observed for the membrane with chitosan/PAA ESA layers. The values of the surface roughness were 8.0 nm, 8.6 nm, 9.0 nm, 12.2 nm, and 13.7 nm for the cycled layers of 0, 1, 2, 3 and 4, respectively. Therefore, the introduction of the chitosan microspheres for the ESA multilayer could be effectively for roughing the base of membrane surfaces. Therefore, this approach could be attractive in the uses of adsorption of such metal ions.

#### *Adsorption capability for copper ion*

318 Infrared Spectroscopy – Materials Science, Engineering and Technology

coverage of PAA on membrane surface. It was also noted that the characteristic peak from chitosan became weaker, while the peak in the range of about 2500 cm-1 to 3750 cm-1 from the PAA layer became stronger, proving that the PAA layer converted the chitosan or chitosan microspheres layers on the surface. The upper change was repeated alternately along with the alternative deposition of chitosan (chitosan microspheres) layers and PAA layers onto charged PAN membrane as revealed by IR spectrum. These spectral changes proved the successful formation of alternate chitosan or chitosan microspheres layers and PAA layers onto charged PAN membrane surface. Furthermore, the obvious difference between the IR spectrum of PAN/Chitosan/PAA system and PAN/Chitosan microspheres/PAA system was observed in comparison of (a) and (b). For the PAN/Chitosan microsphers/PAA system, the disappearance of the peak of the CN group from the charged PAN base membrane was drastic due to the coverage of the

Surface roughness of the chitosan microspheres/PAA membranes were represented in Figure 4-5. As shown in Figure 4-5, the surface roughness of -1 and 0 which referred to membranes before and after NaOH treatment, indicated that the membrane surface became rougher due to the existence of hydrophilic carboxyl groups resulting from NaOH treatment. It was also apparent that the surface morphology was dramatically altered by the formation of the chitosan microspheres/PAA ESA multilayer. While the corresponding surface roughness values were 8.0 nm, 23.1 nm, 15.0 nm, 39.9 nm and 36.4 nm for the chitosan microspheres/PAA membranes with cycle layers of 0, 1, 2, 3 and 4,

Fig. 4-5. Influence of numbers of self-assembled layers onto the membrane surface on the

Number of self-assembed layers on membrane surface

ESA layers.

respectively.

Surface roughness (nm)

surface

It was known that chitosan could be applied for adsorption of metal ions since the amino group is capable of binding metal ions as served as coordination sites. For example, Juang et al.,[25] reported high selectivity for Cu2+ ion was observed by chitosan adsorbents. Chang et al[26] found that the chitosan-modified microsphere showed the largest equilibrium adsorption capacity at pH 5.5. Therefore, in the present work, we studied the binding capability for Cu2+ ion of the resultant membrane at various pHs.

Here, the pH of the Cu2+ ion solution was adjusted using buffer solution of 0.2 mol / L CH3COOH and 0.2 mol / L CH3COONa. The assembled behaviour of chitosan onto negatively charged base membrane and binding behaviour of chitosan for copper ions is simply illustrated by Scheme 4-4. Figure 4-6 shows the effect of the pH of Cu2+ ion solution on the Cu2+ adsorption of resultant membranes with Chitosan microspheres/PAA multiplelayers. The adsorption amounts of the Cu2+ ion were plotted at the cycle number for the ESA multilayer formation. The odd layers were for the chitosan or chitosan microspheres layers exposed on the membrane surface outmost and evens were for PAA layers on the outmost. It could be seen that when the adsorption was carried out at pH 3, membranes terminated with PAA layers showed higher adsorption capacity rather than those with terminal chitosan or chitosan microspheres layers. But, when pH increased, the change regulation could not be seen at pH 7 in both (a) and (b) systems. In the case of the chitosan microspheres/PAA system, the values of the adsorption capacity were significantly decreased to be in the range of about 50 ppm / g-membrane. However, at pH 5 and 6, the values of the adsorption capacity increased with the increasing of the cycled number of the

Scheme 4-4. Illustration images of the chelation for Cu2+ by the assembled chitosan on negatively charged membrane surface.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

membranes.

Adsorption capacity for Cu2+ (10-6g/g)

**5. Conclusion** 

data of FT-IR spectra in detail.

was useful for introducing the functionality.

Membranes Having Characteristics of Heavy Metal Ion Desalination 321

Fig. 4-7. Comparison of the adsorption capacity for Cu2+ of the two kinds of resultant

adsorption capacity of membranes functionalized with chitosan microspheres/PAA ESA layers at pH 6 was several times higher than that of membranes modified with chitosan/PAA ESA layers. The reason for this might be due to the fact that even though the crossing-linking of chitosan in the chitosan microsphere significantly reduced the chelating sites. There were still large amounts of residual amino and dydroxyl groups retained in the microsphere which were active for chelation. So, it was suggested that the ESA multilayer of chitosan microsphers/PAA onto charged substrate prepared in the present paper showed great potential in functioning membranes with high removal capability of heavy metal ions.

Number of self-assembly layers on membrane surface

In the present chapter layer-by-layer electrostatic self-assembly technique was used for the preparation of functional polymeric membranes with metal ion removal capability. The fabrication process and properties of the resultant membranes were mentioned by using

A negatively charged P (AN-*co*-AA) membrane and positively charged P (AN-*co*-EDAMA) were used as the substrates for the ESA treatment. The desalination behaviour of the ESA multilayer modified resultant membranes was studied by evaluating the metal-ion removal properties for Fe3+ and Fe2+. It was found that the resultant membrane showed higher rejection for trivalent ion than divalent metal ion. Evidences indicated that the alternate layer-by-layer formation of the ESA multilayer on the base membrane surface effectively influenced the metal-ion removal of the resultant membranes, indicating that ESA technique

Fig. 4-6. Effect of the copper solution pH on the adsorption capacity of the resultant membranes of PAN/chitosan microsphere/PAA system.

ESA operation and then ranged in about 200-240 ppm / g-membrane. It was thought that the adsorption capacity of resultant membrane depended on the isoelectric point of chitosan, which was at about 6.3. At below the pH, the protonation of amine groups of chitosan could occur, while at higher pH the group behaved as non-protonated group. However, the binding ability of chitosan for copper ions was mainly due to the amine groups which were served as coordination sites for the sequestration of copper ion.[27] At lower pH, most of the amine groups in the chitosan segments was protonated and not available for copper uptaking by chelation. Thus, the values of the adsorption capacity decreased with decreasing pH. However, as the protonaed chitosan at low pH was able to bind anions by electrostatic attraction.[28] So, the resultant membranes showed a tendency with low adsorption capacity for Cu2+ ion at low pH region. But, at pH 7, the adsorption capacity became dramatically lower relative to roughness for the two systems. those at pH 5 and 6 for the chitosan microspheres/PAA system. It was considered that, at higher pH, the destruction of the self-assembled layers might have occurred, since the chitosan layers on membrane surface behaved as non-charged forms.

Especially the chitosan microspheres/PAA system demonstrated low adsorption at pH 7. This meant that the increase of the solution pH restrained the coordination of Cu2+ ions, which was induced by the destruction of the ESA multilayer at pH 7. However, at pH 5 and 6, partial amino group remained on the protonation form which was favorable for the stability of chitosan or chitosan microspheres layers on resultant membranes. Also, Figure 4-**7** was plotted for comparing the adsorption capacity of resultant membranes with chitosan/PAA ESA layers and chitosan microspheres/PAA ESA layers. It was seen that

Fig. 4-7. Comparison of the adsorption capacity for Cu2+ of the two kinds of resultant membranes.

adsorption capacity of membranes functionalized with chitosan microspheres/PAA ESA layers at pH 6 was several times higher than that of membranes modified with chitosan/PAA ESA layers. The reason for this might be due to the fact that even though the crossing-linking of chitosan in the chitosan microsphere significantly reduced the chelating sites. There were still large amounts of residual amino and dydroxyl groups retained in the microsphere which were active for chelation. So, it was suggested that the ESA multilayer of chitosan microsphers/PAA onto charged substrate prepared in the present paper showed great potential in functioning membranes with high removal capability of heavy metal ions.

## **5. Conclusion**

320 Infrared Spectroscopy – Materials Science, Engineering and Technology

Fig. 4-6. Effect of the copper solution pH on the adsorption capacity of the resultant

ESA operation and then ranged in about 200-240 ppm / g-membrane. It was thought that the adsorption capacity of resultant membrane depended on the isoelectric point of chitosan, which was at about 6.3. At below the pH, the protonation of amine groups of chitosan could occur, while at higher pH the group behaved as non-protonated group. However, the binding ability of chitosan for copper ions was mainly due to the amine groups which were served as coordination sites for the sequestration of copper ion.[27] At lower pH, most of the amine groups in the chitosan segments was protonated and not available for copper uptaking by chelation. Thus, the values of the adsorption capacity decreased with decreasing pH. However, as the protonaed chitosan at low pH was able to bind anions by electrostatic attraction.[28] So, the resultant membranes showed a tendency with low adsorption capacity for Cu2+ ion at low pH region. But, at pH 7, the adsorption capacity became dramatically lower relative to roughness for the two systems. those at pH 5 and 6 for the chitosan microspheres/PAA system. It was considered that, at higher pH, the destruction of the self-assembled layers might have occurred, since the chitosan layers on

Number of ESA layers on membrane surface

Especially the chitosan microspheres/PAA system demonstrated low adsorption at pH 7. This meant that the increase of the solution pH restrained the coordination of Cu2+ ions, which was induced by the destruction of the ESA multilayer at pH 7. However, at pH 5 and 6, partial amino group remained on the protonation form which was favorable for the stability of chitosan or chitosan microspheres layers on resultant membranes. Also, Figure 4-**7** was plotted for comparing the adsorption capacity of resultant membranes with chitosan/PAA ESA layers and chitosan microspheres/PAA ESA layers. It was seen that

membranes of PAN/chitosan microsphere/PAA system.

Adsorption capacity for Cu2+ (10-6g/g)

membrane surface behaved as non-charged forms.

In the present chapter layer-by-layer electrostatic self-assembly technique was used for the preparation of functional polymeric membranes with metal ion removal capability. The fabrication process and properties of the resultant membranes were mentioned by using data of FT-IR spectra in detail.

A negatively charged P (AN-*co*-AA) membrane and positively charged P (AN-*co*-EDAMA) were used as the substrates for the ESA treatment. The desalination behaviour of the ESA multilayer modified resultant membranes was studied by evaluating the metal-ion removal properties for Fe3+ and Fe2+. It was found that the resultant membrane showed higher rejection for trivalent ion than divalent metal ion. Evidences indicated that the alternate layer-by-layer formation of the ESA multilayer on the base membrane surface effectively influenced the metal-ion removal of the resultant membranes, indicating that ESA technique was useful for introducing the functionality.

Infrared Analysis of Electrostatic Layer-By-Layer Polymer

2444.

pp. 367-405.

pp. 495a.

*Chem. Eng*. Vol.19, pp 45-51.

*Phys. Chem. B.* Vol.112, pp. 2880–2887.

*Macromolecules* Vol.35, pp. 912–916.

Vol.354, pp 206-211.

1037-1043.

Membranes Having Characteristics of Heavy Metal Ion Desalination 323

Films and Successive Sulfonation. *J. Appl. Polym. Sci.* Vol.101, pp. 3587-3599. [11] Cakmak, G.; Zorlu, F.; Severcan, M. & Severcan, F. (2011). Screening of Protective Effect

[12] Arrondo, J. L. R.; & Goni, F. M. (1999). Structure and dynamics of membrane proteins as

[13] Li, J.; Wong, M. W.; Lin, L.; Bianchi, V.; Edwards, M. & Yip, C. M. (2011) IR

[14] Sukumaran, S.; Hauser, K.; Rauscher, A. & Mantele, W. (2005) Thermal stability of outer

[16] Xua, H. K.; Fanga, J.; Guo, M.; Lua, X. H.; Wei, X. L. & Tu, S. (2010) Novel anion

[17] Tang, B. B.; Wu, P. Y. & Siesler, H. W. (2008). In Situ Study of Diffusion and Interaction

[18] Ding, Y.; Bikson, B. & Nelson, J. K. (2002). Polyimide Membranes Derived from

[19] Kobayashi, T.; Fu, H. T.; Cui, Q. & Wang, H. Y. (2008). Multilayer Composite Surfaces

[20] Kobayashi, T.; Kumagai, K.; Nosaka, Y.; Miyama, H.; Fujii, N. & Tanzawa, H. (1991).

[21] Arica, M. Y.; Yilmaz, M. & Bayramoglu, G. (2007). Chitosan-grafted poly(hydroxyethyl

[22] Wang, D. X.; Su, M.; Yu, Z. Y.; Wang, X. L.; Ando, M. & Shintani, T. (2005). Separation

[23] Wang, X. L.; Tsuru, T.; Nakao, S. & Kimura, S. (1997). The electrostatic and steric-

Membranes. *J. Appl. Polym. Sci.* Vol.110, pp. 3234-3241.

immobilization. *J. Appl. Polym. Sci.* Vol.103, pp. 3084-3093.

of ions. *Desalination.* Vol.175, pp. 219-225.

membranes. *J. Membr. Sci.* Vol.135, pp. 19-32.

to study lipid–protein interaction. *FEBS Letters.* Vol.579, pp. 2546-2550. [15] Luo, M. L.; Wen, Q. Z.; Liu, J. L.; Liu, H. J. & Jia, Z. (2011) Fabrication of SPES/Nano-

Preirradiation Induced Grafting of Styrene/Divinylbenzene into Crosslinked PTFE

of Amifostine on Radiation-Induced Structural and Functional Variations in Rat Liver Microsomal Membranes by FT-IR Spectroscopy. *Anal. Chem.* Vol.83, pp. 2438–

studied by infrared spectroscopy. *Progress in Biophysics & Molecular Biology.* Vol.72,

Spectroscopy of Protein and Peptide-Membrane Interactions. *Biophys. J.* Vol.100,

membrane protein porin from *Paracoccus denitrificans*: FT-IR as a spectroscopic tool

TiO2 Composite Ultrafiltration Membrane and Its Anti-fouling Mechanism. *Chin. J.* 

exchange membrane based on copolymer of methyl methacrylate, vinylbenzyl chloride and ethyl acrylate for alkaline fuel cells. *Journal of Membrane Science.*

of Water and Mono- or Divalent Anions in a Positively Charged Membrane Using Two-Dimensional Correlation FT-IR/Attenuated Total Reflection Spectroscopy. *J.* 

Poly(amic acid) Salt Precursor Polymers. 2. Composite Membrane Preparation.

Prepared by an Electrostatic Self-Assembly Technique with Quaternary Ammonium Salt and Poly(acrylic acid) on Poly(acrylonitrile-co-acrylic acid)

Permeation behavior of dextrans by charged ultrafiltration membranes of polyacrylonitrile photografted with ionic monomers. *J. Appl. Polym. Sci.* Vol.43, pp.

methacrylate-co-glycidyl methacrylate) membranes for reversible enzyme

performance of a nanofiltration membrane influenced by species and concentration

hindrance model for the transport of charged solutes through nanofiltration

Membranes with chelation capability were prepared by the alternate self-assembled of chitosan layer and PAA layer on PAN membrane surface. Taking advantage of the adequate surface area and a large amount of loading sites for metal ions, chitosan microspheres were synthesized and used as cationic species for the construction of ESA multilayer. The result of adsorption experimental for Cu(II) showed that the layer-by-layer deposition of chitosan/PAA or chitosan microspheres/PAA on charged base membrane surface was functionally equipped it with chelating ability for Cu2+. Especially, because of the large internal porosities of the chitasan microspheres, resultant membranes with chitosan microspheres multilayer on the surface showed higher adsorption capacity for Cu2+ as compared with membranes modified by chitosan multilayer. This confirmed that fabrication of chitosan microsphere ESA multiplayer effectively improved the metal-ion uptake capability of membrane. It suggested that such membranes are expected to be applicable as a novel prospect for wastewater treatment.

#### **6. Acknowledgment**

We thank the supporting of Program for Project supported by forming a Hub for Human Resources Development and New Industry Creation Building a Sustainable Society Through Highly Interactive, Cooperative Educational Research with Pacific Rim Countries**.**

#### **7. References**


Membranes with chelation capability were prepared by the alternate self-assembled of chitosan layer and PAA layer on PAN membrane surface. Taking advantage of the adequate surface area and a large amount of loading sites for metal ions, chitosan microspheres were synthesized and used as cationic species for the construction of ESA multilayer. The result of adsorption experimental for Cu(II) showed that the layer-by-layer deposition of chitosan/PAA or chitosan microspheres/PAA on charged base membrane surface was functionally equipped it with chelating ability for Cu2+. Especially, because of the large internal porosities of the chitasan microspheres, resultant membranes with chitosan microspheres multilayer on the surface showed higher adsorption capacity for Cu2+ as compared with membranes modified by chitosan multilayer. This confirmed that fabrication of chitosan microsphere ESA multiplayer effectively improved the metal-ion uptake capability of membrane. It suggested that such membranes are expected to be applicable as

We thank the supporting of Program for Project supported by forming a Hub for Human Resources Development and New Industry Creation Building a Sustainable Society Through

[1] Zapotoczny, S.; Golonka, M. & Nowakowska, M. (2005). Novel Photoactive Polymeric

[2] Kweon, D. K.; Song, S. B. & Park, Y. Y. (2003). Preparation of water-soluble

[3] Lojou, E. & Bianco, P. (2004). Buildup of Polyelectrolyte-Protein Multilayer Assemblies

[4] Kozlovskaya, V.; Ok, S.; Sousa, S.; Libera, M. & Sukhishvili, S. A. (2003). Hydrogen-

[5] Kim, Y. S.; Koo, J. Y. & Kim, H. (2008). Interplay of Hydrogen-Bond and Coordinate

[6] Jiang, S. P.; Liu, Z. & Tian, Z. Q. (2006). Layer-by-Layer Self-Assembly of Composite

[7] Decher, G. (1997). Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites.

[8] Krasemann, L. & Tieke, B. Selective Ion Transport across Self-Assembled Alternating

[9] Farhat, T. R. & Schlenoff, J. B.; (2003). Doping-Controlled Ion Diffusion in Polyelectrolyte

[10] Li, J. Y.; Ichizuri, S.; Asanato, S.; Mutou, F.; Ikeda, S.; Iida, M.; Miura, T.; Oshima, A.;

chitosan/heparin complex and its application as wound healing accelerator.

on Gold Electrodes. Role of the Hydrophobic Effect. *Langmuir*. Vol.20, pp. 748-755.

Bonded Polymer Capsules Formed by Layer-by-Layer Self-Assembly.

Covalent-Bond Interactions in Self-Assembly of NH3 Molecules on the Si(001)

Polyelectrolyte – Nafion Membranes for Direct Methanol Fuel Cells. *Adv. Mater.*

Multilayers of Cationic and Anionic Polyelectrolytes. *Langmuir*. Vol.16, pp. 287-290.

Multilayers: Mass Transport in Reluctant Exchangers. *J. Am. Chem. Soc.* Vol.125, pp.

Tabata, Y. & Washio, M. (2006). Preparation of Ion Exchange Membranes by

Highly Interactive, Cooperative Educational Research with Pacific Rim Countries**.**

Multilayer Films*. Macromol. Rapid Commun,* Vol. 26, pp. 1049-1054.

a novel prospect for wastewater treatment.

*Biomaterials*, Vol.24, pp. 1595-1601.

*Macromolecules* Vol.36, pp. 8590-8592.

Vol.18, pp. 1068-1072.

4627-4636.

*Science.* Vol.277, pp. 1232-1237.

Surface. *Phys. Rev. Lett,* Vol.100, pp. 256105-1-256105-4.

**6. Acknowledgment** 

**7. References** 

Preirradiation Induced Grafting of Styrene/Divinylbenzene into Crosslinked PTFE Films and Successive Sulfonation. *J. Appl. Polym. Sci.* Vol.101, pp. 3587-3599.


**16** 

*1Italy* 

*2Switzerland* 

*1Politecnico di Torino 2ABB Corporate Research* 

**Infrared Spectroscopy as a** 

**Tool to Monitor Radiation Curing** 

Marco Sangermano1, Patrick Meier2 and Spiros Tzavalas2,\*

Photoinitiated polymerization of multifunctional monomers and oligomers is one of the most efficient methods to produce quasi-instantly highly cross-linked polymer networks. It has found a large number of commercial applications, mainly in the coating and printing industry. Among the advantages of this technology the high cure speed, the reduced energy consumption, and the absence of VOC emissions are the most remarkable. It is well known that the UV curing can be performed either by a radical or a cationic mechanism. The cationic photoinduced process presents some advantages compared to the radical one; in particular lack of inhibition by oxygen, lower shrinkage, good mechanical properties of the UV cured materials, and good adhesion properties to various substrates [Fouassier & Rabek, 1993]. The properties of a UV-cured material depend not only on the photocurable composition but also on its photopolymerization kinetics, it is very important to have access

One of the common features of all UV-curable systems is the rapidity at which the polymerization takes place under intense illumination, usually less than one second. Therefore it is difficult to accurately follow the kinetics of such ultrafast reactions, which is a prerequisite for a better understanding and control of the curing process. Moreover, evaluation of the kinetic parameters (rate of polymerization, kinetic chain length, propagation and termination rate constant) is essential in order to compare the reactivity of different photosensitive resins and assess the performance of novel photoinitiators and

Two types of analytical methods are currently used to study the kinetics of radiation curing: 1. Those based on discrete measurements of the physical or chemical modifications

2. Those based on the continuous monitoring in real time of some physical or chemical

to an analytical technique that will facilitate this purpose.

induced after a short exposure to UV light.

modifications induced by light.

**1. Introduction** 

monomers.

Corresponding Author

 \*


## **Infrared Spectroscopy as a Tool to Monitor Radiation Curing**

Marco Sangermano1, Patrick Meier2 and Spiros Tzavalas2,\* *1Politecnico di Torino 2ABB Corporate Research 1Italy 2Switzerland* 

## **1. Introduction**

324 Infrared Spectroscopy – Materials Science, Engineering and Technology

[24] Kobayashi, T.; Wang, H. Y. & Fujii, N. (1998). Molecular imprint membranes of

[25] Jang, R. S. & Shao, H. J. (2002). Effect of pH on Competitive Adsorption of Cu(II), Ni(II), and Zn(II) from Water onto Chitosan Beads. *Adsorption.* Vol.8, pp. 71-78. [26] Chang, Y. C.; Chang, S. W. & Chen, D. H. (2006). Magnetic chitosan nanoparticles:

[27] Dambies, L.; Guimon, C.; Yiacoumi, S. & Guibai. E. (2000). Characterization of metal ion

[28] Guibal, E. (2004). Interactions of metal ions with chitosan-based sorbents: a review.

*Separation and Purification Technology*. Vol.38, pp. 43-74.

Vol.365, pp. 81-88.

Vol.66, pp. 335-341.

Vol.77, pp. 203-214.

polyacrylonitrile copolymers with different acrylic acid segments. *Anal. Chim. Acta.*

Studies on chitosan binding and adsorption of Co(II) ions. *Reac. Func. Polym.*

interactions with chitosan by X-ray photoelectron spectroscopy. *Colloids Surf. A*.

Photoinitiated polymerization of multifunctional monomers and oligomers is one of the most efficient methods to produce quasi-instantly highly cross-linked polymer networks. It has found a large number of commercial applications, mainly in the coating and printing industry. Among the advantages of this technology the high cure speed, the reduced energy consumption, and the absence of VOC emissions are the most remarkable. It is well known that the UV curing can be performed either by a radical or a cationic mechanism. The cationic photoinduced process presents some advantages compared to the radical one; in particular lack of inhibition by oxygen, lower shrinkage, good mechanical properties of the UV cured materials, and good adhesion properties to various substrates [Fouassier & Rabek, 1993]. The properties of a UV-cured material depend not only on the photocurable composition but also on its photopolymerization kinetics, it is very important to have access to an analytical technique that will facilitate this purpose.

One of the common features of all UV-curable systems is the rapidity at which the polymerization takes place under intense illumination, usually less than one second. Therefore it is difficult to accurately follow the kinetics of such ultrafast reactions, which is a prerequisite for a better understanding and control of the curing process. Moreover, evaluation of the kinetic parameters (rate of polymerization, kinetic chain length, propagation and termination rate constant) is essential in order to compare the reactivity of different photosensitive resins and assess the performance of novel photoinitiators and monomers.

Two types of analytical methods are currently used to study the kinetics of radiation curing:


<sup>\*</sup> Corresponding Author

Infrared Spectroscopy as a Tool to Monitor Radiation Curing 327

In the RT-FTIR technique, the sample is simultaneously exposed to the polymerizing UVirradiation beam and to the analyzing IR beam (Fig. 1), which monitors the resulting drop of absorbance. In all the reported data the UV light was a medium pressure mercury lamp,

As an example the RT-FTIR spectra for a methacrylic resin at different irradiation time are shown in Figure 2 [Amerio et al. 2008]. It is evident that the band at 1630 cm-1 (C=C), attributed to the methacrylic functional group, decreases during irradiation of the sample with the UV beam. The decrease in intensity of the band at 1630 cm-1 is accompanied by an increase and shift to higher wavenumbers of the C=O oscillation mode related to the change

in mobility of the C=O bonds with the gradual opening of the C=C bonds.

Fig. 1. Set up for RT-FTIR monitoring of UV irradiation curing

2008]

Fig. 2. RT-FTIR spectroscopy during UV curing of a methacrylated system [Amerio et al.

with a broad UV spectra emission.

The first method, which provides quantitative and reliable information on the extent of cure, is a time-consuming technique. In addition, there is certain error included in the measurement owing to the post-polymerization reaction, which occurs during the lapse of time between the end of exposure and the measurements

Among the real-time (RT) techniques RT-FTIR spectroscopy is one of the most powerful analytical methods for monitoring UV-initiated curing processes, which proceed rather rapidly. RT-FTIR has several advantages over other real-time methods such as photo-DSC. The most important limitation of photo-DSC is its long response time, which makes it inadequate for the monitoring of fast polymerization reactions. In addition, using photo-DSC requires the knowledge on the theoretical enthalpy of reaction for the conversion of functional groups to be calculated using the heat release measured. Finally, the sample thickness is much higher than that in most practical applications (coatings, printing, etc), moreover, the thickness of the layer in the sample pan is poorly controlled.

In contrast, RT-FTIR spectroscopy allows a rapid and quantitative measurement of the conversion of specific reactive functional groups under variable conditions as light intensity, photoinitiator concentration, coating thickness, etc., which are closer matched to those in technical coating and printing processes. In the past, RT-FTIR spectroscopy has been successfully used to study the kinetics of photopolymerization reactions in dependence on the irradiation conditions and other experimental parameters, the reactivity of monomers and oligomers and the efficiency of newly developed photoinitiator systems.

One of the unique advantages of the IR technique is to permit an instant and precise evaluation of the amount of un-reacted groups (residual bonds or monomer groups), which remain trapped in the glassy polymer network. Its value is highly dependent on the monomer functionality as well as on the glass transition temperature, Tg, of the network. It should be emphasized that RT-FTIR spectroscopy has been proven to be very valuable for the precise determination both of the rate of polymerization and the amount of residual unreacted groups where the knowledge of the initial group content is not required.

## **2. Real time FTIR spectroscopy**

Real Time (RT-) FTIR spectroscopy permits not only to follow quantitatively the polymerization by monitoring the disappearance of the IR absorption characteristic of the polymerizable reactive groups (acrylates, methacrylates, epoxy rings, vinyl ether double bonds, thiol groups etc.) but also to determine at any moment the actual degree of conversion and hence the residual unreacted groups content. This analytical method has proved extremely valuable for measuring the polymerization rates and quantum yields of reactions that develop in the millisecond time scale.

The polymerization rate, Rp, being the rate of monomer conversion, can be determined by measuring the decrease of the infrared absorption of the reactive group:

$$R\_p = \frac{-d\left[M\right]}{dt} = \left(\frac{A\_1 - A\_2}{A\_0}\right) \cdot \left(\frac{\left[M\_0\right]}{t\_1 - t\_2}\right) \tag{1}$$

where A1 and A2 are IR absorption values of the reactant A after exposure to UV-light during time t1 and t2, respectively; A0 and [M0] are the absorption and the molar initial concentration values of the monomer, A, before irradiation.

The first method, which provides quantitative and reliable information on the extent of cure, is a time-consuming technique. In addition, there is certain error included in the measurement owing to the post-polymerization reaction, which occurs during the lapse of

Among the real-time (RT) techniques RT-FTIR spectroscopy is one of the most powerful analytical methods for monitoring UV-initiated curing processes, which proceed rather rapidly. RT-FTIR has several advantages over other real-time methods such as photo-DSC. The most important limitation of photo-DSC is its long response time, which makes it inadequate for the monitoring of fast polymerization reactions. In addition, using photo-DSC requires the knowledge on the theoretical enthalpy of reaction for the conversion of functional groups to be calculated using the heat release measured. Finally, the sample thickness is much higher than that in most practical applications (coatings, printing, etc),

In contrast, RT-FTIR spectroscopy allows a rapid and quantitative measurement of the conversion of specific reactive functional groups under variable conditions as light intensity, photoinitiator concentration, coating thickness, etc., which are closer matched to those in technical coating and printing processes. In the past, RT-FTIR spectroscopy has been successfully used to study the kinetics of photopolymerization reactions in dependence on the irradiation conditions and other experimental parameters, the reactivity of monomers

One of the unique advantages of the IR technique is to permit an instant and precise evaluation of the amount of un-reacted groups (residual bonds or monomer groups), which remain trapped in the glassy polymer network. Its value is highly dependent on the monomer functionality as well as on the glass transition temperature, Tg, of the network. It should be emphasized that RT-FTIR spectroscopy has been proven to be very valuable for the precise determination both of the rate of polymerization and the amount of residual un-

Real Time (RT-) FTIR spectroscopy permits not only to follow quantitatively the polymerization by monitoring the disappearance of the IR absorption characteristic of the polymerizable reactive groups (acrylates, methacrylates, epoxy rings, vinyl ether double bonds, thiol groups etc.) but also to determine at any moment the actual degree of conversion and hence the residual unreacted groups content. This analytical method has proved extremely valuable for measuring the polymerization rates and quantum yields of

The polymerization rate, Rp, being the rate of monomer conversion, can be determined by

<sup>0</sup> 1 2

*dt A t t* 

*d M A A M*

where A1 and A2 are IR absorption values of the reactant A after exposure to UV-light during time t1 and t2, respectively; A0 and [M0] are the absorption and the molar initial

0 12

(1)

measuring the decrease of the infrared absorption of the reactive group:

*p*

*R*

concentration values of the monomer, A, before irradiation.

moreover, the thickness of the layer in the sample pan is poorly controlled.

and oligomers and the efficiency of newly developed photoinitiator systems.

reacted groups where the knowledge of the initial group content is not required.

**2. Real time FTIR spectroscopy** 

reactions that develop in the millisecond time scale.

time between the end of exposure and the measurements

In the RT-FTIR technique, the sample is simultaneously exposed to the polymerizing UVirradiation beam and to the analyzing IR beam (Fig. 1), which monitors the resulting drop of absorbance. In all the reported data the UV light was a medium pressure mercury lamp, with a broad UV spectra emission.

As an example the RT-FTIR spectra for a methacrylic resin at different irradiation time are shown in Figure 2 [Amerio et al. 2008]. It is evident that the band at 1630 cm-1 (C=C), attributed to the methacrylic functional group, decreases during irradiation of the sample with the UV beam. The decrease in intensity of the band at 1630 cm-1 is accompanied by an increase and shift to higher wavenumbers of the C=O oscillation mode related to the change in mobility of the C=O bonds with the gradual opening of the C=C bonds.

Fig. 1. Set up for RT-FTIR monitoring of UV irradiation curing

Fig. 2. RT-FTIR spectroscopy during UV curing of a methacrylated system [Amerio et al. 2008]

Infrared Spectroscopy as a Tool to Monitor Radiation Curing 329

Overcoming this unwanted reaction has turned into a major challenge. Different methods have been considered: A clear, illustrative example is reported in Figure 4 in which the effect of air and inert atmosphere (CO2 and N2) was investigated [Studer et al., 2003]. In the presence of air, the polymerization starts after an induction period of 0.2 s, at a speed four times lower than that in an inert atmosphere (Fig. 4). After 0.35 s, once 15% of the acrylate double bonds have polymerized, the reaction begins already to slow down because of the continuous diffusion of air into the sample. To prevent the diffusion of oxygen into the sample, a UV-transparent polypropylene (PP) film was placed on top of the liquid coating (laminated sample). The induction period is then slightly shorter than in air (oxygen is still dissolved in the film) and the polymerization proceeds three times faster than in air to reach

Fig. 4. Conversion curves as a function of irradiation time. Profiles recorded by RT-FTIR spectroscopy for a PUA (Polyurethane) resin exposed to UV light under different atmospheres and with a PP-film as oxygen barrier ("laminate"). [Studer et al., 2003]

The efficiency of the radical photo-initiator can be described by two quantum yields: The *quantum yields of initiation*, which represents the number of starting polymer chains per photons absorbed, and the *quantum yields of polymerization*, which is the number of monomer units polymerized per photons absorbed. Therefore, it is clear that if the monomers are absorbing in the same UV range as the photoinitiator the competition will limit the photons absorbed by the latter. This can lead to a decrease of quantum yields and therefore to a lower degree of conversion as well as polymerization rate. In such cases special care needs to be taken when selecting the proper photoinitiator. This issue becomes particularly important when dealing with formulations containing fillers which absorb light in the same

an 85% conversion by the end of the UV-irradiation (35 s).

**2.3 Photoinitiator** 

spectral range as the photoinitiator.

The reactivity and the kinetics of a UV curable system is affected by numerous parameters i.e. nature and amount of photoinitiator and functionalized monomers and oligomers, thickness, light intensity, concentration of O2, the presence of fillers or additives etc. RT-FTIR has been extensively used for the rapid and quantitative assessment of the effect of such parameters on the UV curing kinetics. Further down characteristic examples of the use of this powerful analytical technique are listed.

#### **2.1 Irradiation time**

The reaction kinetics curve follows a characteristic sigmoid, S-shaped profile (Fig. 3) due to two major factors: (i) the initial induction period is resulting from the well-known inhibition effect of O2 on the radical-induced polymerization, which disappears completely for experiments carried out in vacuum or in N2; (ii) the progressive slowing down observed at degrees of conversion above 30-40% is the direct consequence of the network formation and the subsequent gelification, which reduces the segmental mobility of the growing polymer chains and of the un-reacted double bonds [Fouassier & Rabek, 1993].

Fig. 3. Conversion curve and Rp curve as a function of irradiation time for a radical induced polymerization [Fouassier & Rabek, 1993].

#### **2.2 Presence of O2**

The free radicals formed by the photolysis of the initiator are rapidly scavenged by O2 molecules to yield peroxyl radicals. These species are not reactive towards the acrylate double bonds and can therefore not initiate or participate in any polymerization reaction. An additional amount of photoinitiator (and of UV energy) is therefore needed to consume/compensate the oxygen dissolved in the resin, as well as the atmospheric O2 diffusing into the sample during the UV exposure, in order to obtain coatings with the desired mechanical properties and tack-free surfaces.

The reactivity and the kinetics of a UV curable system is affected by numerous parameters i.e. nature and amount of photoinitiator and functionalized monomers and oligomers, thickness, light intensity, concentration of O2, the presence of fillers or additives etc. RT-FTIR has been extensively used for the rapid and quantitative assessment of the effect of such parameters on the UV curing kinetics. Further down characteristic examples of the use

The reaction kinetics curve follows a characteristic sigmoid, S-shaped profile (Fig. 3) due to two major factors: (i) the initial induction period is resulting from the well-known inhibition effect of O2 on the radical-induced polymerization, which disappears completely for experiments carried out in vacuum or in N2; (ii) the progressive slowing down observed at degrees of conversion above 30-40% is the direct consequence of the network formation and the subsequent gelification, which reduces the segmental mobility of the growing polymer

R Convertion (%) <sup>p</sup>

chains and of the un-reacted double bonds [Fouassier & Rabek, 1993].

I II III

0.0 0.5 1.0

Exposure Time (seconds)

Fig. 3. Conversion curve and Rp curve as a function of irradiation time for a radical induced

The free radicals formed by the photolysis of the initiator are rapidly scavenged by O2 molecules to yield peroxyl radicals. These species are not reactive towards the acrylate double bonds and can therefore not initiate or participate in any polymerization reaction. An additional amount of photoinitiator (and of UV energy) is therefore needed to consume/compensate the oxygen dissolved in the resin, as well as the atmospheric O2 diffusing into the sample during the UV exposure, in order to obtain coatings with the

0

25

50

75

100

of this powerful analytical technique are listed.

**2.1 Irradiation time** 

0

**2.2 Presence of O2** 

polymerization [Fouassier & Rabek, 1993].

desired mechanical properties and tack-free surfaces.

5

10

 (mol l-1 s-1 ) Overcoming this unwanted reaction has turned into a major challenge. Different methods have been considered: A clear, illustrative example is reported in Figure 4 in which the effect of air and inert atmosphere (CO2 and N2) was investigated [Studer et al., 2003]. In the presence of air, the polymerization starts after an induction period of 0.2 s, at a speed four times lower than that in an inert atmosphere (Fig. 4). After 0.35 s, once 15% of the acrylate double bonds have polymerized, the reaction begins already to slow down because of the continuous diffusion of air into the sample. To prevent the diffusion of oxygen into the sample, a UV-transparent polypropylene (PP) film was placed on top of the liquid coating (laminated sample). The induction period is then slightly shorter than in air (oxygen is still dissolved in the film) and the polymerization proceeds three times faster than in air to reach an 85% conversion by the end of the UV-irradiation (35 s).

Fig. 4. Conversion curves as a function of irradiation time. Profiles recorded by RT-FTIR spectroscopy for a PUA (Polyurethane) resin exposed to UV light under different atmospheres and with a PP-film as oxygen barrier ("laminate"). [Studer et al., 2003]

#### **2.3 Photoinitiator**

The efficiency of the radical photo-initiator can be described by two quantum yields: The *quantum yields of initiation*, which represents the number of starting polymer chains per photons absorbed, and the *quantum yields of polymerization*, which is the number of monomer units polymerized per photons absorbed. Therefore, it is clear that if the monomers are absorbing in the same UV range as the photoinitiator the competition will limit the photons absorbed by the latter. This can lead to a decrease of quantum yields and therefore to a lower degree of conversion as well as polymerization rate. In such cases special care needs to be taken when selecting the proper photoinitiator. This issue becomes particularly important when dealing with formulations containing fillers which absorb light in the same spectral range as the photoinitiator.

Infrared Spectroscopy as a Tool to Monitor Radiation Curing 331

Fig. 6. Photopolymerization profiles (left) and variation of the rate of photopolymerization, Rp, (right) recorded by RT-FTIR spectroscopy for: (1) ethyldiethylenglycol monoacrylate, (EDGA) (2) EDGA + TPGDA, (3) tripropylenglycol diacrylate (TPGDA), (4) Oxazolidone

Fig. 7. Variation of the rate of photopolymerization, Rp, recorded by RT-FTIR spectroscopy for: (1) ethyldiethylenglycol monoacrylate, (EDGA) (2) EDGA + TPGDA, (3) tripropylenglycol diacrylate (TPGDA), (4) Oxazolidone monoacrylate. [Decker & Moussa, 1988]

monoacrylate. [Decker & Moussa, 1988]

In Figure 5 the UV-Vis spectra of a radical photoinitiator and the BaTiO3 (BT) filler that was used in an acrylic formulation are presented. It is evident that there is a competitive absorption between the photo-initiator and the ceramic powder in the UV region between 200 and 400 nm. That leads to a linear decrease of acrylic double bond during UV irradiation by increasing the filler content in the photocurable formulation [Lombardi et al., 2011].

Fig. 5. UV-VIS spectra of the 2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR® 1173, Ciba®) photo-initiator and the BaTiO3 (BT) filler. [Lombardi et al., 2011]

#### **2.4 Monomers**

From the slope of the RT-FTIR kinetic curves it is easy to evaluate the rate of polymerization (Rp) which can be calculated at any moment of the reaction. It is therefore possible to plot the Rp values as a function of the conversion rate. The overall polymerization quantum yield, p, can be calculated from the ratio of polymerization, Rp, over the absorbed light intensity.

In a pioneer study from Christian Decker the photopolymerization of polyurethanediacrylates was investigated by using the RT-FTIR technique [Decker & Moussa, 1988]. The RT-FTIR conversion curves as a function of irradiation time were examined for different systems (Fig. 6).

The polymerization rate, Rp, values as a function of the percentage of conversion are presented in figure 7. Rp reaches its maximum value (= 8 mole 1-1s-1 for the most reactive system) at a degree of conversion of 25% for the 3 systems investigated. This peak corresponds to the phase at which the O2 inhibition has been overcome and gelification has not yet slowed down the polymerization rate. Higher Rp values up to 103 mole l-1 s-1, were obtained with such multiacrylic monomers by merely increasing the intensity of the UV source [Decker & Bendaika, 1984].

In Figure 5 the UV-Vis spectra of a radical photoinitiator and the BaTiO3 (BT) filler that was used in an acrylic formulation are presented. It is evident that there is a competitive absorption between the photo-initiator and the ceramic powder in the UV region between 200 and 400 nm. That leads to a linear decrease of acrylic double bond during UV irradiation by increasing the filler content in the photocurable formulation [Lombardi et

Fig. 5. UV-VIS spectra of the 2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR®

From the slope of the RT-FTIR kinetic curves it is easy to evaluate the rate of polymerization (Rp) which can be calculated at any moment of the reaction. It is therefore possible to plot the Rp values as a function of the conversion rate. The overall polymerization quantum yield, p, can be calculated from the ratio of polymerization, Rp, over the absorbed light

In a pioneer study from Christian Decker the photopolymerization of polyurethanediacrylates was investigated by using the RT-FTIR technique [Decker & Moussa, 1988]. The RT-FTIR conversion curves as a function of irradiation time were examined for different

The polymerization rate, Rp, values as a function of the percentage of conversion are presented in figure 7. Rp reaches its maximum value (= 8 mole 1-1s-1 for the most reactive system) at a degree of conversion of 25% for the 3 systems investigated. This peak corresponds to the phase at which the O2 inhibition has been overcome and gelification has not yet slowed down the polymerization rate. Higher Rp values up to 103 mole l-1 s-1, were obtained with such multiacrylic monomers by merely increasing the intensity of the UV

1173, Ciba®) photo-initiator and the BaTiO3 (BT) filler. [Lombardi et al., 2011]

al., 2011].

**2.4 Monomers** 

intensity.

systems (Fig. 6).

source [Decker & Bendaika, 1984].

Fig. 6. Photopolymerization profiles (left) and variation of the rate of photopolymerization, Rp, (right) recorded by RT-FTIR spectroscopy for: (1) ethyldiethylenglycol monoacrylate, (EDGA) (2) EDGA + TPGDA, (3) tripropylenglycol diacrylate (TPGDA), (4) Oxazolidone monoacrylate. [Decker & Moussa, 1988]

Fig. 7. Variation of the rate of photopolymerization, Rp, recorded by RT-FTIR spectroscopy for: (1) ethyldiethylenglycol monoacrylate, (EDGA) (2) EDGA + TPGDA, (3) tripropylenglycol diacrylate (TPGDA), (4) Oxazolidone monoacrylate. [Decker & Moussa, 1988]

Infrared Spectroscopy as a Tool to Monitor Radiation Curing 333

The high reactivity of the epoxy groups results to a quite high initial rate of polymerization (slope of the curve). The epoxy groups' conversion levels off, after 2 minutes of irradiation, to a value of about 60%. This is due to the formation of a glassy polymer network, which hinders the mobility of the reactive species so that a large number of un-reacted epoxy groups remained trapped. Introduction of CNT leads to a slight decrease if the epoxy group photocuring rate compared to the neat epoxy system. In addition, the final conversion (after

**0 20 40 60 80 100 120**

Fig. 9. RT-FTIR conversion curves as a function of irradiation time for dicycloaliphatic epoxy

In a clear formulation, UV radiation is absorbed mainly by the photoinitiator, so that the curing depth is directly controlled by its concentration. For each specific application, the best compromise must be found between curing speed and depth. Ideally, the initiation

By selecting an initiation wavelength where the monomer does not absorb, one can significantly increase the maximum photoinitiation rate and the rate of spatial propagation of the polymerization front. It is advantageous to use photobleaching initiators whose light absorption is higher than the one of the initiator products, thereby allowing more light to

Simulation results have confirmed that, at any given time, the initiation rate profile resembles a wave front, and the breadth of this front is determined by factors such as:

**Time (s)**

resin and in the presence of increasing amount of CNT. [Sangermano et al., 2008]

wavelength should be selected so that the initiator is the only absorbing specie.

**0**

**10**

**20**

**30**

**Conversion (%)**

**40**

**50**

**60**

**70**

120 seconds of irradiation) decreases from about 60% to 50%.

 **Epoxy w. 0.05wt% CNTs Epoxy w. 0.10wt% CNTs**

 **Pure Epoxy**

**2.7 Thickness and photoinitiator concentration** 

pass through the system.


#### **2.5 Additives: Photosensitizers**

In a recent paper [Beyazit et al., 2011] long wavelength free radical photopolymerization of (meth)acrylic monomers (TMPTA) is described. The polymerization was carried out using a conjugated thiophene derivative (3,2-diphenyldithieno[3,2-b-2,3-d]thiophene, DDT) as photosensitizer and a diphenyliodonium hexafluorophosphate (Ph2I+PF6) as photoinitiator. The progress of conversion versus time as well as the final conversion level (Fig. 8) shows that the presence of DDT affects significantly the polymerisation of TMPTA. In general the extension of spectral sensitivity by adding a photosensitizer consists of an energy transfer processes as schematized in the following:

Photosensitizer (T1) + Photoinitiator (S0) Photoinitiator (T1) + Photosensitizer (S0)

The photosensitizer is excited by light and it is able to transfer the energy, through orbital overlap, to the photoinitiator that is indirectly activated. In the cited work, the photosensitizer DDT that is photoexited by a UV source in the range of 350–450 nm is rapidly quenched by the onium salt, which is transparent in that range.

Fig. 8. Real-time FT-IR conversion curves as a function of irradiation time for TMPTA w/wo a photosensitizer, DDT. [Beyazit et al., 2011]

#### **2.6 Fillers**

Recently [Sangermano et al., 2008] reported for the first time the preparation of antistatic epoxy coatings via cationic UV curing of an epoxy resin in the presence of a very low content of carbon nanotubes (CNT). After dispersing the CNT into the epoxy resin, in the range between 0.025-0.1 wt.-%, the formulations were cured by means of UV light in the presence of a sulfonium salt as cationic photoinitiator.

The effect of the presence of CNT on the photopolymerization process was investigated by means of real-time FT-IR. The conversion curves as a function of irradiation time for the epoxy resin with and without CNT are presented in Figure 9.

In a recent paper [Beyazit et al., 2011] long wavelength free radical photopolymerization of (meth)acrylic monomers (TMPTA) is described. The polymerization was carried out using a conjugated thiophene derivative (3,2-diphenyldithieno[3,2-b-2,3-d]thiophene, DDT) as photosensitizer and a diphenyliodonium hexafluorophosphate (Ph2I+PF6) as photoinitiator. The progress of conversion versus time as well as the final conversion level (Fig. 8) shows that the presence of DDT affects significantly the polymerisation of TMPTA. In general the extension of spectral sensitivity by adding a photosensitizer consists of an energy transfer

Photosensitizer (T1) + Photoinitiator (S0) Photoinitiator (T1) + Photosensitizer (S0) The photosensitizer is excited by light and it is able to transfer the energy, through orbital overlap, to the photoinitiator that is indirectly activated. In the cited work, the photosensitizer DDT that is photoexited by a UV source in the range of 350–450 nm is

Fig. 8. Real-time FT-IR conversion curves as a function of irradiation time for TMPTA w/wo

Recently [Sangermano et al., 2008] reported for the first time the preparation of antistatic epoxy coatings via cationic UV curing of an epoxy resin in the presence of a very low content of carbon nanotubes (CNT). After dispersing the CNT into the epoxy resin, in the range between 0.025-0.1 wt.-%, the formulations were cured by means of UV light in the

The effect of the presence of CNT on the photopolymerization process was investigated by means of real-time FT-IR. The conversion curves as a function of irradiation time for the

rapidly quenched by the onium salt, which is transparent in that range.

**2.5 Additives: Photosensitizers** 

processes as schematized in the following:

a photosensitizer, DDT. [Beyazit et al., 2011]

presence of a sulfonium salt as cationic photoinitiator.

epoxy resin with and without CNT are presented in Figure 9.

**2.6 Fillers** 

The high reactivity of the epoxy groups results to a quite high initial rate of polymerization (slope of the curve). The epoxy groups' conversion levels off, after 2 minutes of irradiation, to a value of about 60%. This is due to the formation of a glassy polymer network, which hinders the mobility of the reactive species so that a large number of un-reacted epoxy groups remained trapped. Introduction of CNT leads to a slight decrease if the epoxy group photocuring rate compared to the neat epoxy system. In addition, the final conversion (after 120 seconds of irradiation) decreases from about 60% to 50%.

Fig. 9. RT-FTIR conversion curves as a function of irradiation time for dicycloaliphatic epoxy resin and in the presence of increasing amount of CNT. [Sangermano et al., 2008]

### **2.7 Thickness and photoinitiator concentration**

In a clear formulation, UV radiation is absorbed mainly by the photoinitiator, so that the curing depth is directly controlled by its concentration. For each specific application, the best compromise must be found between curing speed and depth. Ideally, the initiation wavelength should be selected so that the initiator is the only absorbing specie.

By selecting an initiation wavelength where the monomer does not absorb, one can significantly increase the maximum photoinitiation rate and the rate of spatial propagation of the polymerization front. It is advantageous to use photobleaching initiators whose light absorption is higher than the one of the initiator products, thereby allowing more light to pass through the system.

Simulation results have confirmed that, at any given time, the initiation rate profile resembles a wave front, and the breadth of this front is determined by factors such as:


Infrared Spectroscopy as a Tool to Monitor Radiation Curing 335

Fig. 11. Rate of polymerization of 50/50 BisGMA/TEGDMA (25°C) as a function of doublebond conversion for various light intensities of (a) 2.9 mW/cm2, (b) 1.5 mW/cm2, and (c) 0.4

Real-Time FTIR spectroscopy has been used extensively for the monitoring of UV-induced polymerization reactions. As shown in the present chapter starting from the pioneer work of Prof. Decker in the late 80s RT-FTIR has been proven to be a powerful technique, which allows following such reactions quantitatively, even if they occur in fraction of a second. Using this technique it is possible to calculate the conversion as a function of irradiation time during the progress of a reaction. From this data the polymerization rate can be calculated at any time, obtaining the true rate of photopolymerization and the amount of the residual reactive groups in the cured polymers. The efficiency of new photoinitiators as well as the reactivity of the different monomers can be studied in detail. Furthermore, the effect of experimental parameters on the photopolymerization rate and on the final conversion can

The RT-FTIR spectroscopy has a number of advantages over other existing methods:

Real-Time monitoring, providing a species analysis of the quasi-instant liquid-solid

The great sensitivity of IR spectroscopy allows very small changes in the monomer

 The kinetics of cure reactions can be studied over a very broad range of light intensity. In spite of a number of advantages the RT-FTIR spectroscopy has also several limitations,

It is not possible to investigate samples containing black pigments and high

mW/cm2. [Lovelh et al., 1999]

be investigated in depth.

such as:

phase change in a fraction of a second.

The sample thickness is limited to the range 1-100 m.

The coatings support must be transparent to IR radiation.

concentration of other coloured pigments.

concentration to be detected.

**3. Conclusion** 

Fouassier & Rabek, 1993, show that there is an optimum initiator concentration for the efficient photopolymerization of thick samples (Fig. 10). As the initiator concentration is increased, the initiation rate at the surface is increased, but the rate of propagation of the front through the sample is decreased.

Fig. 10. RT-FTIR conversion curves as a function of sample thickness and concentration of the initiator [Fouassier & Rabek, 1993].

Low initiator concentration and/or photoinitiator of low extinction coefficient is required for the photocuring of thin films:


#### **2.8 Light Intensity**

Lovelh et al., 1999 investigated the photo-copolymerization of 50/50 BisGMA/TEGDMA at various light intensities (0.4, 1.5, and 2.9 mW/cm2). The effect of light intensity on the polymerization rate of the comonomer mixture at 25°C is depicted in Figure 11. The maximum rate of polymerization and the final conversion are both significantly affected by the differences in light intensity. Increasing the intensity of the UV light from 0.4 to 2.9 mW/cm2 leads to an increase of the maximum rate to more than double. This is a clear investigation/result evidencing the effect of light intensity to photocuring reaction.

Fig. 11. Rate of polymerization of 50/50 BisGMA/TEGDMA (25°C) as a function of doublebond conversion for various light intensities of (a) 2.9 mW/cm2, (b) 1.5 mW/cm2, and (c) 0.4 mW/cm2. [Lovelh et al., 1999]

## **3. Conclusion**

334 Infrared Spectroscopy – Materials Science, Engineering and Technology

Fouassier & Rabek, 1993, show that there is an optimum initiator concentration for the efficient photopolymerization of thick samples (Fig. 10). As the initiator concentration is increased, the initiation rate at the surface is increased, but the rate of propagation of the

Fig. 10. RT-FTIR conversion curves as a function of sample thickness and concentration of

Low initiator concentration and/or photoinitiator of low extinction coefficient is required



Lovelh et al., 1999 investigated the photo-copolymerization of 50/50 BisGMA/TEGDMA at various light intensities (0.4, 1.5, and 2.9 mW/cm2). The effect of light intensity on the polymerization rate of the comonomer mixture at 25°C is depicted in Figure 11. The maximum rate of polymerization and the final conversion are both significantly affected by the differences in light intensity. Increasing the intensity of the UV light from 0.4 to 2.9 mW/cm2 leads to an increase of the maximum rate to more than double. This is a clear

investigation/result evidencing the effect of light intensity to photocuring reaction.

front through the sample is decreased.

the initiator [Fouassier & Rabek, 1993].

for the photocuring of thin films:

higher rates of bleaching.

the sample decreases.

**2.8 Light Intensity** 

Real-Time FTIR spectroscopy has been used extensively for the monitoring of UV-induced polymerization reactions. As shown in the present chapter starting from the pioneer work of Prof. Decker in the late 80s RT-FTIR has been proven to be a powerful technique, which allows following such reactions quantitatively, even if they occur in fraction of a second. Using this technique it is possible to calculate the conversion as a function of irradiation time during the progress of a reaction. From this data the polymerization rate can be calculated at any time, obtaining the true rate of photopolymerization and the amount of the residual reactive groups in the cured polymers. The efficiency of new photoinitiators as well as the reactivity of the different monomers can be studied in detail. Furthermore, the effect of experimental parameters on the photopolymerization rate and on the final conversion can be investigated in depth.

The RT-FTIR spectroscopy has a number of advantages over other existing methods:


In spite of a number of advantages the RT-FTIR spectroscopy has also several limitations, such as:


**Section 3** 

**Materials Technology** 

#### **4. References**

