Solution-intercalation/mixing

Figure 1.15 Schematic representation of formation of polymer-based CMP systems viz. (a) conventional bulk CMP (tactoid), (b) intercalated NCs, and(c) exfoliated NCs, via physical mixing (dry/solution phase mixing or melt compounding) and in-situ polymerization routes.

Solution intercalation is based on a suitable solvent system in which the polymer is soluble and the silicate layers are easily swellable. Prior to the nanocomposite preparation biodegradable polymer is allowed to dissolve in a suitable solvent. The same solvent is used to disperse nanoclays through optimized cycle of mechanical stirring and sonication process. Futher, both solutions are mixed through a coupled process of mechanical stirring and sonication. The cycle time and order depends upon the nature of the matrices and fillers and also the composition. A similar mechanical stirring and sonication coupled process has been used by Miyagawa et at... 

In the melt mixing method, nanoclays are incorporated into the polymer in the molten state. This technique has considerable advantages over either the in situ intercalative polymerization or polymer solution intercalation techniques. Firstly, this method is environmentally benign due to the absence of organic solvents. Secondly, melt processing is compatible with current industrial processes, such as extrusion and injection moulding. The melt intercalation method allows the use of biopolymers that were not suitable for in situ polymerization. This has been the most widely used method in the literature for obtaining PLA/clay nanocomposites. " ... 

Common solvent technique is based on a solvent system in which the polymer or pre-polymer is soluble and the silicate layers are swellable. The layered silicate is first swollen in a solvent, such as water, chloroform, or toluene. When the polymer and layered silicate solutions are mixed, the polymer chains intercalate and displace the solvent within the interlayer of the silicate. Upon solvent removal, the intercalated structure remains, resulting in the formation of PCN. Polyimide based nanocomposites are made by using a common solvent like dimethyl acetamide (DMAc) [22],... 

Solution intercalation or solution mixing is based on the solubility of biobased epoxy matrices and an appropriate solvent. Commonly used solvents include acetone, chloroform, methanol, and ethanol as listed in Table 6.4. Compatibility between the solvents, nanoclays, and polymer matrices depends on factors such as the polarity of the solvent, evaporation temperature, volatility of the solvent, and degree of dispersion with high amounts of nanoclays (Haq et al., 2009b Donescu et al., 2013). 

The conditions for dispersing clay nanolayers into both cis-1,4-polyisoprene (IR) and epoxidized natural rubber (ENR) have been reported. The incorporation of the clays into these elastomers was achieved by mixing the components in a standard internal mixer/mixing mill (melt compounding) or by mixing their dispersions produced in toluene or methyl ethyl ketone solvents (solution Intercalation). X-ray diffraction studies indicated the intercalation of IR and ENR into the silicate interlayers, followed by exfoliation (delamination) of the silicate layers. The reinforcing effect strongly depended on the extent of dispersion of the silicate layers. 

Even though this technique has been mostly used with water-soluble polymers, such as PEO, polyvinyl ether (PVE), polyvinylpyrrolidone (PVP), and poly(acrylic acid) (PAA) [134-141], intercalation from nonaqueous solutions has also been reported [142-145]. For example, high-density polyethylene (HDPE)-based nanocomposites have been prepared by dissolving HDPE in a mixture of xylene and benzonitrile with dispersed organomodified layered silicates (OMLSs). The nanocomposite was then recovered by precipitation from tetrahydrofuran (THE) [143], Polystyrene (PS)/OMLS-exfoliated nanocomposites have also been prepared by the solution intercalation technique, by mixing pure PS and organophilic clay with adsorbed cetyl pyrid-ium chloride [146]. Similarly, several studies have focused on the preparation of polylactide (PLA)-layered silicate nanocomposites using intercalation from solution. 

The process of separating the nanoclay platelets is referred to as the intercalation process. Without this separation, the nanoclay would not be capable of allowing the polymer to penetrate the platelet layers. There are two techniques for intercalating the matrix polymer molecule between clay platelets melt intercalation and solution intercalation. In case of melt intercalation, a layered silicate is mixed with the polymer matrix material in the molten state. In case of solution intercalation, the modified nanoclay is swelled in monomer, allowing it to enter the clay gallery. Subsequently, the monomer is polymerized with the result that polymer is formed inside the clay gallery. In either case, once mixed the clay platelet material swells in the polymer matrix and forms a very strong interaction with the polymer chains to produce a composite matrix with enhanced performance. 

Polyimide-clay nanocomposites constitute another example of the synthesis of nanocomposite from polymer solution [70-76]. Polyimide-clay nanocomposite films were produced via polymerization of 4,4 -diaminodiphenyl ether and pyromellitic dianhydride in dimethylacetamide (DMAC) solvent, followed by mixing of the poly(amic acid) solution with organoclay dispersed in DMAC. Synthetic mica and MMT produced primarily exfoliated nanocomposites, while saponite and hectorite led to only monolayer intercalation in the clay galleries [71]. Dramatic improvements in barrier properties, thermal stability, and modulus were observed for these nanocomposites. Polyimide-clay nanocomposites containing only a small fraction of clay exhibited a several-fold reduction in the... 

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