The influence of preparation method upon the structure and relaxation characteristics of poly(methyl methacrylate)/clay composites

Poly(methyl methacrylate) (PMMA)/clay composites were prepared using two methods: through solution polymerisation of methyl methacrylate in the presence of alkylammonium-modified clay and through the direct melt intercalation of PMMA. Both composites were fluxed in an internal mixer to give comparable melt processing histories. In the reaction product of the polymerisation method, the clay layers were separated to a distance greater than 80 Å; after melt processing, the layers were partially re-aggregated. The length and orientation of the alkylammonium ions principally controlled the interlayer spacings within these aggregates. A finer degree of dispersive mixing of the clay in the polymerisation system was observed. The presence of modified clay in the polymerisation system led to branching, reduction of average molecular weight, increase in polydispersity, and to a bimodal molecular weight distribution of the PMMA. The glass–rubber transitions were observed around 120°C in the polymerisation system in comparison with 105°C for the melt intercalation materials and the PMMA resin. Moreover, PMMA extracted from the polymerisation composites had similarly elevated glass transition temperatures. The chain branching in the polymerisation system was the principal reason for the differences in the thermomechanical behaviour of the two composites.