**3. Examples of testing polymers by FTIR-ATR**

Tests were carried out by means of a single-beam FTIR-Nicolet 6700 spectrometer from Thermo Scientific, equipped with a diamond crystal (refractive index n = 2.4). IR spectra were recorded as changes in absorption as a function of wave number ranging from 600 cm-1 to 4000 cm-1. A DTGS KBR detector was used. The following measurement technical conditions were used: measurement recording accuracy - 4 cm-1, mirror travel rate - 0.31 cm-1/s, aperture - 50, minimal scans number – 32.

#### **3.1. Assessment of the modification effects on the surface of polypropylene (PP) films and nonwovens**

88 Advanced Aspects of Spectroscopy

diagnostic tool in medicine.

There the system generates an FTIR-ATR absorption spectrum characteristic of the given sample. The FTIR-ATR absorption spectrum slightly differs from that obtained by the transmission method. The differences concern the intensity and frequency of absorption peaks characteristic of chemical groups in view of the phenomenon of reflection, e.g. Goos-Hänchen's displacement. Thus it is necessary to take corrective action that can be realized automatically. The penetration depth of IR beam can be controlled within some range by selecting an appropriate prism (selection of the refractive index) and the incident angle of beam. The commonly used prisms are made of diamond, germanium, silicon and ZnSe, whose refractive indices are equal to 2.4, 4.0, 3.4 and 2.4, respectively, and the beam penetration depths: 2.03 m, 0.67 m, 0.84 m and 2.03 m, respectively, at = 1000 cm–1 (Material Thermo Scientific Smart ITR). During testing sub-micrometric coating, the beam penetrates a higher depth than the coating depth and also passes to the substrate, on which the coating is deposited. The absorption spectrum then constitutes a superposition of the spectrum of coating material and substrate. In such cases, qualitative analysis is carried out,

The basic requirement for ATR technique is to place a sample in direct contact with the prism as only such conditions allow the IR evanescent wave to penetrate the sample surface layer. Moreover, there should be a considerable difference between the refractive indices of

The drawback of ATR technique is a relatively low sensitivity and susceptibility to the effect of environmental conditions, which makes it necessary to calibrate the IR spectrum. Modern spectrometers have an option of automatic computer-aided spectrum correction. ATR technique has numerous advantages. FTIR-ATR shows the features of a routine method for testing the chemical and physical surface structure of materials such as polymers, films and membranes provided that these well adhere to the crystal. Tests with a modern instrumentation are characterized by a high reproducibility (better than 0.1%) (Urbanczyk, 1988). FTIR-ATR makes it possible to record spectra within a wider frequency range of IR radiation than transmission spectroscopy owing to the lack of limitations caused by the absorption of cuvette windows. An important advantage of this technique is the possibility of recording spectra *in situ* and *in vivo*, e.g. in testing biological objects and using it as a

In this work, the FTIR-ATR technique was used to analyze the surfaces of modified

Tests were carried out by means of a single-beam FTIR-Nicolet 6700 spectrometer from Thermo Scientific, equipped with a diamond crystal (refractive index n = 2.4). IR spectra were recorded as changes in absorption as a function of wave number ranging from 600 cm-1 to 4000 cm-1. A DTGS KBR detector was used. The following measurement technical conditions were used: measurement recording accuracy - 4 cm-1, mirror travel rate - 0.31 cm-

which takes into account the absorption spectrum of substrate.

prism and sample to get the phenomenon of internal reflection occurred.

polymers and to test the polymeric layers deposited on substrates.

**3. Examples of testing polymers by FTIR-ATR** 

1/s, aperture - 50, minimal scans number – 32.

Polypropylene products are commonly used in commodity production due to their special chemical properties (resistance to organic and inorganic solvent, hydrophobic properties) and physical characteristics (lightness, mechanical strength, electro- and thermal insulating capabilities). In the methods of making products such as composites with the use of PP films or fibers as reinforcing components, a serious drawback of these materials is their low free surface energy, which results in weak molecular interactions between the composite components. The free energy of PP material surface can be increased by creating new functional chemical groups in the material surface layer. This task has been fulfilled by exploring different approaches such as chemical, electrochemical, physical and plasma methods . The effectiveness of the methods used was assessed by means of the FTIR-ATR technique.

The moleculare structure of polypropylene is the same in the use of films and nonwovens products:

In one unit of PP molecule chain are tree atoms of carbon, in the form of different groups: - CH2- ; >CH- ; and –CH3. Each of them is correlated in IR spectra with the suitable absorption peak by definite wavenumber values (Figure 2). The proper characteristic, concerning this correlation is presented in Table 1.

**Figure 2.** FTIR-ATR spectra of commercial PP films : non-oriented, non-crystalline PP Cast (Sample 1); bidirectionally oriented, crystalline PP AG (Sample 2) and PP nonwoven (Sample 3).

IR spectra of PP for film and nonwoven can differ between themselves only in defiles (the differences in shape and intensity of peaks), because in ATR technique, the contact of the samples with measure crystal, can be different for different structure of samples (film – continuous structure, nonwoven – porous structure). The explanation of this problems shown as an example at Figure 3.


\*) s - stretching vibration symmetrical and a -asymmetrical, s - deformation vibration symmetrical and a – asymmetrical, w - wagging vibration, r – rocking vibration

**Table 1.** IR absorption bands of Polypropylene (Urbanczyk, 1988; Rau, 1963)

**Figure 3.** SEM images of type surface structure : PP nonwoven fabrics (Sample A), PP Cast film (Sample B)

#### *3.1.1. Effects of PP film modification*

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shown as an example at Figure 3.

asymmetrical, w - wagging vibration, r – rocking vibration

(Sample B)

**Table 1.** IR absorption bands of Polypropylene (Urbanczyk, 1988; Rau, 1963)

**Figure 3.** SEM images of type surface structure : PP nonwoven fabrics (Sample A), PP Cast film

Sample A Sample B

IR spectra of PP for film and nonwoven can differ between themselves only in defiles (the differences in shape and intensity of peaks), because in ATR technique, the contact of the samples with measure crystal, can be different for different structure of samples (film – continuous structure, nonwoven – porous structure). The explanation of this problems

Wave number, cm-1 Absorbing group and type of vibration

2916 a (CH2) 2959 a (CH3) 2881 s (CH3) 2841 s (CH2) 1460 a (CH3) 1376 s (CH3) 1357, w (CH2- CH) 1328 w (CH2 - CH) 1302, 1224, 941 Carbon lattice pulsation 1170, 1153 w (CH3), (CH2), (CH) 975, 899, r (CH3), r (CH2), r (CH) 841, 810 r (CH2), r (CH), r (CH3) 765 w (CH2) \*) s - stretching vibration symmetrical and a -asymmetrical, s - deformation vibration symmetrical and a – Commercial PP, non-oriented, non-crystalline (PP Cast) and bidirectionally oriented, crystalline (PP-AG) films were modified in media of strong oxidants, such as: 3M nitric acid, 30% hydrogen peroxide, and a saturated solution of potassium dichromate in 70% sulfuric acid (K2Cr2O7+H2SO4). The electrochemical oxidation was carried out with the use of anolyte (AgNO3 solution in nitric acid) and catholyte (nitric acid solution). The physical modification of PP was performed by means of a Xenotest apparatus, irradiating PP film with UV radiation according to EN ISO 105-B02:2006 (Urbaniak-Domagala, 2011). Plasma modification processes were carried out with the use of RF glow discharge of special gases under decreased pressure (Urbaniak-Domagala, 2011). Figures 4, 5 show the FTIR-ATR spectrograms of the PP film surfaces after oxidation compared with unmodified PP films. In the FTIR-ATR spectrograms of the PP film surface layer, one can observe absorption bands that are consistent with those of isotactic PP obtained by the authors mentioned in Table 1.

**Figure 4.** FTIR-ATR spectra of commercial PP films before and after oxidation. 1- PP Cast untreated, 2 - PP AG untreated. Samples 3÷7 PP Cast modified: by using electrochemical method, current intensity: 100 mA/cm2, 30 min. (Sample 3), UV treatment (Xenotest) 170h (Sample 4), K2Cr2O7+H2SO4 solution at 70oC, 3 min. (Sample 5), 3M nitric acid at 20oC, 24 h (Sample 6), 30% hydrogen peroxide at 20oC, 1 h (Sample 7).

Moreover, the spectrograms of PP surface layer oxidized by chemical methods show a new absorption band within the wave number range of (1730 – 1680) cm-1 that corresponds to a carbonyl group formed in a oxidizing medium as a results of the nucleophilic substitution of PP, mainly at the tertiary carbon atom: - CH2 – C < R H – CH 2 – (the substitution susceptibility of the tertiary, secondary and primary carbon is 7000: 1100: 1, respectively) (Wiberg & Eisenthal, 1964). The absorption maximum of carbonyl group is slightly shifted depending on the type of oxidizing medium.

**Figure 5.** FTIR-ATR spectra of PP Cast films before and after plasma oxidation. Sample 1 - PP untreated, Sample 2. PP treated Ar plasma p=0.05Tr, power 300W, t=10 min., Sample 3 - PP treated Acetic Acid Vapour plasma: p=0.05Tr 300W, t=10 min., Sample4 - PP treated Water Vapour plasma: p=0.05Tr; 300W, 10min, Sample 5 - PP treated Air plasma: p=0.05Tr; 300W. t= 5min., Sample 6 - PP treated Air plasma: p=0.05Tr; 300W, t= 10min.

The absorption band of carbonyl group in the PP spectrum is broad, which can indicate the presence of carbonyl group in various products of oxidation, such as aldehydes and ketones (Carlsson & Wiles, 1969): 1700 cm-1absorption (>C = CH-CO-OH), 1710 cm-1absorption (-CO-OH), 1715 cm-1absorption (>C = O), 1718 cm-1 absorption (-CCH3 –CH2 –CO- CH2- CH3), 1726 cm-1absorption (-CCH3 –CH2 –CO- CH3).

The spectrogram of oriented and crystalline PP AG shows no absorption band of carbonyl group despite the fact that the determination of the contact angle of PP surface wetted with polar liquids indicated an increase in free energy (Urbaniak-Domagala, 2011). One may assume that the active center concentration is too low for the FTIR-ATR method. The spectrogram of PP oxidized by chemical methods also indicates changes in two bands at 940 cm-1 and 765 cm-1 (Figure 4).

The first one indicates the skeleton vibration of mer links with a relative phase shift of 2/3, being mainly characteristic of the crystalline phase (Rau, 1963). In the case of PP AG film, this band is intensive, while in PP Cast, it decreases and after oxidation is absent, which can indicate that the PP surface layer becomes amorphous due to the oxidation process. The absorption band at 765cm-1 is characteristic of non-crystalline PP, caused by the deformation vibration of methylene group (–CH2 –) (Kazicina.&Kupletska, 1976). This band is absent in the spectrogram of PP after oxidation, which can be due to the decrease in the number of methylene groups caused by the degradation of the polymer in its surface layer. This band is also absent in crystalline PP due to spherical limitations caused by a long-range order.

FTIR-ATR absorption spectra (Figure 5) present the chemical effects of plasma on PP film. The gases used in this process included: argon and air and vapors of acetic acid and water under optimal conditions of plasma treatment (time and power applied to the system).

The spectra of the plasma-treated PP film show a new absorption band within the range of wave numbers of (1640 ÷ 1660) cm-1 , which can indicate the formation of carbonyl group, >C=O, (valence vibration) as well as –C=C- groups (valence vibration) (Kazicina&Kupletska, 1976). The prolongation of plasma treatment and increase in power leads to the increase in the IR radiation intensity of the band of new functional groups. The position of IR absorption maximum slightly shifts depending on the plasma composition. These new active centers can be regarded as a result of PP surface oxidation with plasma particles. In the case of Ar plasma, the effect of surface functionalization can result from the so-called post-treatment process (Guruvenket et al, 2004). The results obtained indicate a particular activity of air plasma as oxidizing medium for polypropylene.

#### *3.1.2. Effects of PP nonwovens modification*

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treated Air plasma: p=0.05Tr; 300W, t= 10min.

1726 cm-1absorption (-CCH3 –CH2 –CO- CH3).

cm-1 and 765 cm-1 (Figure 4).

**Figure 5.** FTIR-ATR spectra of PP Cast films before and after plasma oxidation. Sample 1 - PP untreated, Sample 2. PP treated Ar plasma p=0.05Tr, power 300W, t=10 min., Sample 3 - PP treated Acetic Acid Vapour plasma: p=0.05Tr 300W, t=10 min., Sample4 - PP treated Water Vapour plasma: p=0.05Tr; 300W, 10min, Sample 5 - PP treated Air plasma: p=0.05Tr; 300W. t= 5min., Sample 6 - PP

The absorption band of carbonyl group in the PP spectrum is broad, which can indicate the presence of carbonyl group in various products of oxidation, such as aldehydes and ketones (Carlsson & Wiles, 1969): 1700 cm-1absorption (>C = CH-CO-OH), 1710 cm-1absorption (-CO-OH), 1715 cm-1absorption (>C = O), 1718 cm-1 absorption (-CCH3 –CH2 –CO- CH2- CH3),

The spectrogram of oriented and crystalline PP AG shows no absorption band of carbonyl group despite the fact that the determination of the contact angle of PP surface wetted with polar liquids indicated an increase in free energy (Urbaniak-Domagala, 2011). One may assume that the active center concentration is too low for the FTIR-ATR method. The spectrogram of PP oxidized by chemical methods also indicates changes in two bands at 940

The first one indicates the skeleton vibration of mer links with a relative phase shift of 2/3, being mainly characteristic of the crystalline phase (Rau, 1963). In the case of PP AG film, this band is intensive, while in PP Cast, it decreases and after oxidation is absent, which can indicate that the PP surface layer becomes amorphous due to the oxidation process. The absorption band at 765cm-1 is characteristic of non-crystalline PP, caused by the deformation vibration of methylene group (–CH2 –) (Kazicina.&Kupletska, 1976). This band is absent in the spectrogram of PP after oxidation, which can be due to the decrease in the number of methylene groups caused by the degradation of the polymer in its surface layer. This band is also absent in crystalline PP due to spherical limitations caused by a long-range order.

FTIR-ATR was also used to assess the effects of plasma-treated PP nonwovens. PP meltblown nonwovens (surface weight: 80 g/m2, average thickness: 1.5mm) made of PP fibers with an average thickness of 2.12 μm were modified by means of synthetic air plasma to form chemically active centers on the PP fiber surface.

**Figure 6.** ATR IR spectra of PP nonwoves untreated (Sample 1) and air plasma treated, pressure 0.1Tr : Sample 2 - power 50W, time 5min. Sample 3- power 100W, time 5min., Sample 4 - power 100W, time 10min

The FTIR-ATR spectrogram of the air plasma-treated PP nonwoven shows two broad bands at 1660 cm-1 and 3320 cm-1 that indicate the formation of carbonyl group >C=O and hydroxyl

group – OH (Kazicina.&Kupletska, 1976). One may assume that the air plasma oxidizes the fiber surface with the aid of reactive oxygen, peroxide and nitrogen groups that together with electrons react with the PP fiber surface causing not only the etching of surface layer but also its functionalization.
