Effects of dispersion and aggregation of silica in the reinforcement of poly(methyl acrylate) elastomers

Composites were prepared from methyl acrylate monomer and silica particles to which 3-(trimethoxysilyl)propyl methacrylate groups had been grafted. The silica particles were bonded onto poly(methyl acrylate) through participation of their methacrylate groups in the polymerization of the methyl acrylate to form an elastomeric matrix. If the polymerization was done promptly after the ingredients were mixed, the silica particles were randomly dispersed. On the other hand, if sufficient time was permitted, the particles spontaneously formed regular arrays within the monomer which, on polymerization, yields the elastomer. Finally, if the particle dispersion was first dried and then blended into the monomer prior to polymerization, an aggregated arrangement was obtained. Although the elastomers having regularly ordered silica dispersions exhibited novel optical properties, they showed little difference in mechanical properties relative to those with randomly dispered silica. The elastomers having aggregated silica, however, had significantly improved mechanical properties, relative to the other two types. This is consistent with frequent observations in the rubber industry that halting compounding short of complete filler dispersion can give the best balance of mechanical properties.