Effects of shear and thermal history on the microstructure of solution polymerized poly(methyl methacrylate)-clay composites

Poly(methyl methacrylate)-clay composites were synthesized through free-radical solution polymerization of methyl methacrylate in the presence of clay that was preintercalated with dodecylammonium or hexadecyltrimethylammonium ions. Neither the diffraction peak from the (001) plane nor the 001 reflections for the (001) plane were observed during x-ray characterization, showing that the clay particles were greatly expanded (i.e., exfoliated) in the reaction products, due to the presence of large quantities of polymer between the clay layers. Syntheses carried out with high clay loadings resulted in reductions in the weight and number-average molecular weights and increases in the polydispersities of the polymers, caused by chain transfer to active sites on the surface of the clay. It was found that heat and shear, during subsequent processing of the reaction products, resulted in reordering of the microstructure through narrowing of the clay galleries with the partial exclusion of the polymer. The final interlayer spacings were determined by the orientation and lengths of the preintercalated alkylammonium ions. It was concluded that the exfoliated structure created during polymerization was unstable under the processing conditions and, consequently, reordered into a more stable form.