**3. Processing of polymer nanocomposites**

Polymer nanocomposites can be produced by three methods: in situ polymerization, solution and melt blending. An appropriate method is selected according to the type of polymeric matrix, nanofiller and desired properties for the final products [61].

#### **3.1. In situ polymerization**

During the in situ polymerization, the nanofiller must be properly dispersed in the monomer solution before the polymerization process starts, ensuring the polymer will be formed between the nanoparticles. Polymerization can be started using several techniques (heat, use of an appropriate initiator, etc.) [62]. Using this technique, a polymer grafted nanoparticle and high loading of nanofillers without aggregation can be carried out [63]. Organic modifiers may be used to help the dispersion of the nanoparticles and take part in the polymerization [64]. It can be an alternative way for the production of nanocomposites using polymers that are non-soluble or thermally unstable [65]. In some cases, this technique can be applied in solvent-free form [66]. It is also a technique which can result to higher performance products [67]. Mini-emulsion polymerization is based on the creation of monomer droplets that are dispersed in a solution in a nanoscale [68]. The procedure for the production of polymer nanocomposites by this technique is shown in **Figure 11**.

**Figure 11.** Schematic illustration for the in situ polymerization method.

Some of the benefits are controllable particle morphology [69], good interfacial adhesion of the nanofillers [70] and high transparency [71, 72]. When using this method, it is possible to [61] apply higher contents of nanofillers without agglomeration, have better performance of the final products, expand to the solvent-free form, have covalent bond among the nanoparticle functional groups and polymer chains and use both thermoset and thermoplastic polymers. One main limitation is the ease of agglomeration [63, 65].

#### **3.2. Blending**

**3. Processing of polymer nanocomposites**

**3.1. In situ polymerization**

**Figure 9.** LaNi<sup>5</sup>

112 Nanocomposites - Recent Evolutions

matrix, nanofiller and desired properties for the final products [61].

**Figure 10.** Scheme of production of compatibilized nanocomposite of PVDF/SWCNT [60].

/ABS after a mechanical-dry particle coating process in a tumbling mill [57].

Polymer nanocomposites can be produced by three methods: in situ polymerization, solution and melt blending. An appropriate method is selected according to the type of polymeric

During the in situ polymerization, the nanofiller must be properly dispersed in the monomer solution before the polymerization process starts, ensuring the polymer will be formed between the nanoparticles. Polymerization can be started using several techniques (heat, use of an appropriate initiator, etc.) [62]. Using this technique, a polymer grafted nanoparticle and high loading of nanofillers without aggregation can be carried out [63]. Organic modifiers may be used to help the dispersion of the nanoparticles and take part in the polymerization [64]. It can be an alternative way for the production of nanocomposites using polymers that are non-soluble or thermally unstable [65]. In some cases, this technique can be applied in solvent-free form [66]. It is also a technique which can result to higher performance products [67]. Mini-emulsion polymerization is based on the creation of monomer droplets that are dispersed in a solution in a nanoscale [68]. The procedure for the production of polymer nanocomposites by this technique is shown in **Figure 11**.

This method is widely used for the production of polymer nanocomposites due to its simplicity. However, reaching a proper dispersion of the nanofiller in the polymer matrix can be more difficult when compared to other methods [61, 62].

#### *3.2.1. Solution blending*

Solution blending is actually a system including the polymer and nanofiller, which are easily dispersed in an appropriate solvent [62]. Ultrasonic irradiation, magnetic stirring or even shear mixing can be used to disperse the nanofiller within the polymer [63]. In this method, when the solvent evaporates, the nanoparticle remains dispersed into the polymer chains, as shown in **Figure 12**. The produced nanocomposite can also be obtained as a thin film [61].

There are some problems for the solution blending from the economic and environmental point of view. A proper decision must be taken to choose a correct method according to the situation and the desired product [73]. Some of the benefits of using solution blending are [61]

**Figure 12.** Schematic illustration for the solution blending method.

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 to polymers that are soluble in water [77].

#### *3.2.2. Melt blending*

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

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 end of the melting zone [82].

Melt blending has been used for the production of polymer nanocomposites with different types of matrices: polypropylene [83–85], poly(methyl methacrylate) [86], poly(lactic acid)

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The melt blending is well matched with several industrial operations, such as extrusion and injection molding, and consequently, it can be commercialized [61]. Some of the benefits of this technique are good dispersion of the nanoparticles [92], enhancement of the heat stability [93], improvement of mechanical properties [83–86] and low cost-effectiveness and ecofriendly (do not use solvent). A considerable limitation is the use of high temperatures, which

By and large, each technique has some advantages over the others and can be selected as the

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.

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 spectros-

copy has proved a useful probe of carbon-based material properties [95, 98].

[75], poly(vinyl chloride) [87], polycarbonate [88, 89], polyamide 6 [4, 90, 91], etc.

can damage the modified surface of the nanofillers [94].

**Figure 14.** Schematic illustration of a screw profile of a twin-screw extruder [82].

**4. Techniques of characterization**

**4.1. Structural and morphological characterization**

best method according to the conditions and applying materials [61].

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

**Figure 14.** Schematic illustration of a screw profile of a twin-screw extruder [82].

Melt blending has been used for the production of polymer nanocomposites with different types of matrices: polypropylene [83–85], poly(methyl methacrylate) [86], poly(lactic acid) [75], poly(vinyl chloride) [87], polycarbonate [88, 89], polyamide 6 [4, 90, 91], etc.

The melt blending is well matched with several industrial operations, such as extrusion and injection molding, and consequently, it can be commercialized [61]. Some of the benefits of this technique are good dispersion of the nanoparticles [92], enhancement of the heat stability [93], improvement of mechanical properties [83–86] and low cost-effectiveness and ecofriendly (do not use solvent). A considerable limitation is the use of high temperatures, which can damage the modified surface of the nanofillers [94].

By and large, each technique has some advantages over the others and can be selected as the best method according to the conditions and applying materials [61].
