Polymer-clay composites degree

Clays have long been used as fillers in polymer systems because of low cost and the improved mechanical properties of the resulting polymer composites. If all other parameters are equal, the efficiency of a filler to improve the physical and mechanical properties of a polymer system is sensitive to its degree of dispersion in the polymer matrix (Krishnamoorti et ah, 1996). In the early 1990s, Toyota researchers (Okada et ah, 1990) discovered that treatment of montmorillonite (MMT) with amino acids allowed dispersion of the individual 1 nm thick silicate layers of the clay scale in polyamide on a molecular. Their hybrid material showed major improvements in physical and mechanical properties even at very low clay content (1.6 vol %). Since then, many researchers have performed investigations in the new field of polymer nano-composites. This has lead to further developments in the range of materials and synthesizing methods available. 

Layered nanoparticles, like the aggregates of silicates talc and mica, form close proximity sheets of polymer—clay hybrids due to the immiscibility of clay in polymer. The degree of dispersion in these composites is normally referred to as the following ... 

In situ emulsion polymerization in aqueous solution is an effective and successful method for the synthesis of polymer-clay nanocomposites. The structure of the nanocomposites formed is usually characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD offers a convenient method to determine the position, shape and intensity of the nanofillers. The XRD patterns shows a basal (001) reflection when the spacing between the clay layers is small. However, X-rays cannot detect the (001) reflection when the layers are exfoliated in the composite. An increase in spacing between layers is correlated with an increase in the degree of exfoliation. In contrast, TEM observation gives a direct view of the internal structure, spatial distribution of the various phases and defect structures. By combining these methods, the structure of polymer nanocomposites can be determined. 

Various finishes can be achieved — gloss, satin (or egg-shell) or matt. This is accomplished by the addition of particles of size 1-5 pm of, for example, silica, china clay or the white pigment, TiCK The degree of mattness depends on various factors, such as particle size, surface treatment of the particles, rate of film formation, and the polymer composition, e.g., urethane/acrylate compared with epoxy/acrylate. The former requires smaller particles larger particles cannot be used as they create a rough surface. 

In conventional polymer composites, the interlayer spacing will remain the same, similar to clay, and there will therefore be no shift of the diffraction peak. But if there is intercalation, the peak position will shift to a lower angle than in the original nanomaterial. The absence of a characteristic diffraction peak is usually the first indication of exfoliation of the nanomaterial, although it is not conclusive. However, several other factors such as a decrease in the degree of coherent layer stacking, very low concentration of nanomaterial and strong absorption by heavy atoms/ions (e.g. bromide) may also affect the accuracy of this analysis. 

Direct comparison of the results found by Qu et al. for the LLDPE/clay nanocomposites [40] with those found for the HDPE composite by Jeon et al. [35] cannot be made because the differences between the two systems and procedures are too many. However, besides the solvent, which had much higher polarity in Ref. [35], one might consider the different degree of crystallinity of the two polymer matrixes to be another important factor. 

Incorporation of clay or layered silicates to polymer matrix provides three different stmctures conventional composites, intercalated and exfoliated nanocomposites and these are shown in Fig. 1. It is interdependent on the clay concentration the degree of clay layer separation and distribution in the composites [14]. In conventional composites the particles exist as aggregates with no insertion of polymer matrix. 

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