**4. Techniques of characterization**

**Figure 13.** Effect of shearing on the dispersion of the nanoparticles during melt blending.

reduction in the permeability of gases [74], easy operation, and general technique for all types of nanofillers and to both thermoset and thermoplastic polymers [75]. The main limitations are aggregation and environmental constraints [73, 76]. This technique will likely be limited

In the melt blending method, the nanofillers are directly dispersed into the molten polymer. During mixing in the melt state, the strain that the polymer applies on the particles depends on its molecular weight and weight distribution. High levels of shear stress reduce the size of the agglomerates [61]. The mechanism for the action of shear flow during the dispersion and distribution of nanoparticles is shown in **Figure 13**. Initially, large agglomerates break down and form smaller ones dispersed through the polymer matrix. The transfer of strain from the polymer to these new agglomerates leads to stronger shearing, which breaks them into individual particles; this step depends fundamentally on time and on the chemical affinity

Both single and twin-screw extruders are usually applied for melt blending [79], although it must be noted that in some cases high temperatures can have unfavorable effects on the modified surface of the nanofiller and an optimization must be employed [80]. Intermeshing co-rotating twin-screw extruders are quite popular for this purpose. This method has some drawbacks that involve parameters that are not easy to control, such as the interaction between the polymer and the nanoparticles and the processing conditions (temperature and residence time) [81]. Therefore, in some cases, it can be difficult to obtain well-dispersed nanoparticles. An example of a medium dispersive screw profile for a twin-screw extruder is shown in **Figure 14**. It was designed with transport and kneading block elements and one turbine element at the

between the polymer and the surface of the nanoparticles [59, 78].

to polymers that are soluble in water [77].

*3.2.2. Melt blending*

114 Nanocomposites - Recent Evolutions

end of the melting zone [82].

The knowledge and use of techniques of characterization is determinative to understand the basic physical and chemical properties of polymer nanocomposites. For several applications, it facilitates the study of emerging materials by giving information on intrinsic properties [95]. Various techniques have been used extensively in polymer nanocomposite research.

#### **4.1. Structural and morphological characterization**

The commonly used techniques are wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [10, 96, 97]. The SEM provides images of surface features associated with a sample. The atomic force microscope (AFM) uses a sharp tip to scan across the sample. Raman spectroscopy has proved a useful probe of carbon-based material properties [95, 98].

Due to the easiness and availability, WAXD is most commonly used to probe the nanocomposite structure [99, 100] and to study the kinetics of the polymer melt intercalation, when using layered silicates [100]. In these systems, a fully exfoliated system is characterized by the absence of intensity peaks in WAXD pattern [101]. Therefore, a WAXD pattern concerning the mechanism of nanocomposite formation and their structure are tentative issues for making any conclusion. On the other hand, TEM allows a qualitative understanding of the internal structure, spatial distribution of the various phases and views of the defective structure through direct visualization. Thus, TEM complements WAXD data [102]. **Figure 15** illustrates some results obtained from both analyses.

matrix [104]. In addition, understanding rheological properties of nanocomposites is crucial for application development and understanding polymer processability. The nanocomposites usually demonstrate a change of pattern in dynamic mechanical spectrum, as a function of

**Figure 16.** Schematic representation of the rheological response to the increase in the number of particles per unit

**Figure 17.** Steady shear viscosities as a function of shear rate at different montmorillonite concentrations in PA12 and a

scheme of a percolated network in a nanocomposite [103, 105].

, G" ~ ω<sup>1</sup>

], then to a pattern with double crossover frequencies, and

Polymer Nanocomposites with Different Types of Nanofiller

http://dx.doi.org/10.5772/intechopen.81329

] to a

117

the degree of exfoliation/dispersion, from typical polymer response [G' ~ ω<sup>2</sup>

, G" ~ ω<sup>1</sup>

terminal response [G' ~ ω<sup>1</sup>

volume [104].

SAXS is used to observe structures on the order of 10 Å or larger, in the range of 0–5°. The TEM, AFM and SEM are also required to characterize the dispersion and distribution of nanoparticles. WAXD has found relatively limited success in CNT research [95]. In the Raman spectra of graphite and SWNTs, there are many features that can be identified with specific phonon modes that contribute to each feature. The Raman spectra of both materials can provide much information about the exceptional 1D properties of carbon materials, such as their phonon structure and their electronic structure, as well as information about sample imperfections. Since mechanical, elastic and thermal properties are also strongly influenced by phonons, Raman spectra provide general information about the structure and properties of SWNTs [98].

#### **4.2. Thermal, mechanical, rheological and other techniques of characterization**

For further characterization of polymer nanocomposites, the commonly used techniques are Fourier-transform infrared (FTIR), rheometry [82], differential scanning calorimeter (DSC), thermogravimetric (TGA), thermomechanical (TMA) and dynamic modulus analysis (DMA) [96].

Because viscoelastic measurements are highly sensitive to the nano- and mesoscale structure of polymers, when combined with WAXD, TEM, DSC, TGA and DMA, they will provide fundamental understanding of the state and mechanism of dispersion of the nanoparticles in the

**Figure 15.** (a) WAXD diffractograms and (b) TEM micrograph of a 2.5 wt% MMT-PA12 nanocomposite [103].

matrix [104]. In addition, understanding rheological properties of nanocomposites is crucial for application development and understanding polymer processability. The nanocomposites usually demonstrate a change of pattern in dynamic mechanical spectrum, as a function of the degree of exfoliation/dispersion, from typical polymer response [G' ~ ω<sup>2</sup> , G" ~ ω<sup>1</sup> ] to a terminal response [G' ~ ω<sup>1</sup> , G" ~ ω<sup>1</sup> ], then to a pattern with double crossover frequencies, and

**Figure 16.** Schematic representation of the rheological response to the increase in the number of particles per unit volume [104].

**Figure 17.** Steady shear viscosities as a function of shear rate at different montmorillonite concentrations in PA12 and a scheme of a percolated network in a nanocomposite [103, 105].

**Figure 15.** (a) WAXD diffractograms and (b) TEM micrograph of a 2.5 wt% MMT-PA12 nanocomposite [103].

Due to the easiness and availability, WAXD is most commonly used to probe the nanocomposite structure [99, 100] and to study the kinetics of the polymer melt intercalation, when using layered silicates [100]. In these systems, a fully exfoliated system is characterized by the absence of intensity peaks in WAXD pattern [101]. Therefore, a WAXD pattern concerning the mechanism of nanocomposite formation and their structure are tentative issues for making any conclusion. On the other hand, TEM allows a qualitative understanding of the internal structure, spatial distribution of the various phases and views of the defective structure through direct visualization. Thus, TEM complements WAXD data [102]. **Figure 15** illustrates

SAXS is used to observe structures on the order of 10 Å or larger, in the range of 0–5°. The TEM, AFM and SEM are also required to characterize the dispersion and distribution of nanoparticles. WAXD has found relatively limited success in CNT research [95]. In the Raman spectra of graphite and SWNTs, there are many features that can be identified with specific phonon modes that contribute to each feature. The Raman spectra of both materials can provide much information about the exceptional 1D properties of carbon materials, such as their phonon structure and their electronic structure, as well as information about sample imperfections. Since mechanical, elastic and thermal properties are also strongly influenced by phonons, Raman spectra provide general information about the structure and properties

**4.2. Thermal, mechanical, rheological and other techniques of characterization**

For further characterization of polymer nanocomposites, the commonly used techniques are Fourier-transform infrared (FTIR), rheometry [82], differential scanning calorimeter (DSC), thermogravimetric (TGA), thermomechanical (TMA) and dynamic modulus analysis (DMA) [96].

Because viscoelastic measurements are highly sensitive to the nano- and mesoscale structure of polymers, when combined with WAXD, TEM, DSC, TGA and DMA, they will provide fundamental understanding of the state and mechanism of dispersion of the nanoparticles in the

some results obtained from both analyses.

116 Nanocomposites - Recent Evolutions

of SWNTs [98].

finally to a solid-like response with G' > G" in all frequency ranges, as seen in **Figure 16**. The number of particles per unit volume is a key factor determining the characteristic response of nanocomposites [82, 104].

when 7 wt% of MMT was added, an exfoliated structure was obtained due to the predominant linking reactions between the residual monomer and the polar organic surfactant. Solutions of these nanocomposites in formic acid were prepared, and the 3 and 5 wt% nanocomposites were successfully electrospun; however, electrospinning of the 7 wt% nanocomposite was not possible. WAXD, SEM and TEM results showed that the 3 and 5 wt% nanofibers with average diameter between 80 and 250 nm had exfoliated structures. These results indicate that the high elongational forces developed during the electrospinning process changed the initial intercalated/exfoliated structure of the nanocomposites to

Polymer Nanocomposites with Different Types of Nanofiller

http://dx.doi.org/10.5772/intechopen.81329

119

The use of an aqueous dispersion of polyethylene copolymer with a relatively high content of acrylic acid as a compatibilizer and as an alternative medium to obtain polyethylene NFC nanocomposites was a matter of recent study [113]. The NFC content was varied from 1 to 90 wt%, and the appearance, optical, thermal, mechanical and rheological properties, as well the morphology of the films, were evaluated. The PE/NFC films were transparent up to 20 wt% of NFC indicating a good dispersion of NFC, with PE-rich and NFC-rich regions observed by SEM. Improved mechanical properties were achieved with an increase in the Young's modulus.

Water suspensions of NFC with xylan, xyloglucan and pectin were studied for foaming and structural properties as a new means for food structuring [114]. They were analyzed by rheometry, microscopy and optical coherence tomography (OCT). A combination of xylan with TEMPO-oxidized NFC produced a mixture with well-dispersed air bubbles, while the addition of pectin improved the elastic modulus, hardness and toughness of the structures. Shear flow caused NFC to form plate-like flocs in the suspension that accumulated near bubble interfaces. This tendency could be affected by adding laccase to the dispersion. Xyloglucan

Polymer nanocomposites offer excellent opportunities to explore new functionalities beyond those of conventional materials. The field of nanocomposites has been one of the most promising and emerging research areas. They find special attention due to the unique properties such as light weight, ease of production and flexibility. A defining feature of polymer nanocomposites is that the small size of the fillers leads to an enormous increase in interfacial area as compared to traditional composites. The interfacial area creates a significant volume fraction of interfacial polymer with properties different from the bulk polymer even at low loadings of the nanofiller. Interfacial structure is known to be different from bulk structure, and in polymers with nanoparticles possessing high surface areas, most parts of the polymers are present near the interfaces, in spite of the small weight fraction of the filler. This is one of the reasons why the nature of reinforcement is different in nanocomposites. The crucial parameters which determine the effects of fillers on the properties of composites are filler size,

The rheological behavior indicated good melt processability [113].

shape and aspect ratio and filler-matrix interactions.

interacted strongly with TEMPO-oxidized NFC (high storage modulus) [114].

an exfoliated one [112].

**6. Conclusions**

The presence of nanofillers caused these nanocomposites to have solid-like behaviors and slower relaxation. This behavior can be explained in terms of the development of a graftingpercolated nanoparticle network structure [105]. Its formation is a consequence of physical interactions between dispersed nanoparticles, polymeric chains and surfactants, which promote a considerable resistance to flow [103]. This behavior is shown in **Figure 17**.

The next section will briefly discuss the main properties and characteristics of some polymer nanocomposites, which have been used in relevant applications.
