Polymer-layered silicate nanocomposites by melt processing
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Polymer-layered silicate nanocomposites formed from the organically modified clay mineral montmorillonite and related materials have attracted a great deal of technological and scientific interest in the past decade. These composites offer the promise of greatly improved mechanical, thermal, and barrier properties over those of the matrix polymer owing to the nanoscale reinforcement and constraints of the polymer caused by dispersing the one nanometer thick, high aspect ratio aluminosilicate (clay) layers. The central scientific issue is how to achieve a high level of dispersion, and ultimately full exfoliation of the clay platelets within the polymer matrix since this is necessary to realize the large filler aspect ratios. Although several factors play a role in organoclay exfoliation, it seems to be largely dependent upon a complex array of interactions between the polymer matrix and the organoclay. Recently, there has been a strong commercial drive for producing such nanocomposites from low cost polymers like polyolefins. Unfortunately, polyolefins are highly inefficient at exfoliating the organoclays by themselves, since there is no favorable interaction with the polar aluminosilicate surface of the clay. Hence, the principal goal of this research work was to explore the various routes to improve polyolefin-organoclay interactions, and thus, organoclay exfoliation in these systems. Three mutually exclusive strategies were employed to achieve this objective. First, the polyolefin matrix was made more polar by several techniques viz., surface treating the polyolefin particles, grafting of maleic anhydride on the polyolefin backbone, copolymerizing with polar monomers like methacrylic acid, and incorporating ionic groups (ionomers). These modifications resulted in significant improvements in organoclay exfoliation. Second, the organoclay structure was engineered to improve polyolefin-organoclay compatibility. It was determined that surfactants whose structure lead to more shielding of the silicate surface or increased alkyl material within the organoclay galleries result in improved levels of exfoliation. Finally, the melt processing conditions were fine tuned to generate optimum amounts of shear, and reduce thermal degradation of the surfactant during the preparation of nanocomposites. Once sufficient levels of organoclay exfoliation were attained, these materials were tested for barrier film applications.