Effects of filler particle/elastomer distribution and interaction on composite mechanical properties

Some new characterization results are reported for composites prepared from methyl acrylate monomer and from reinforcing silica particles at various degrees of dispersion. In some cases, 3-(trimethoxysilyl)propyl methacrylate groups were grafted onto the silica (PMA) through participation in the methyl acrylate polymerization used to form an elastomeric PMA matrix. In some cases, the usual random dispersion of the silica particles was ''aged'' or converted into regular arrays within the monomer prior to its polymerization. As an alternative, placing chloropropyltrimethoxysilane groups on the particle surfaces was used to obtain random arrangements in which the strong bonding between the particles and elastomer was suppressed. In another approach, the particle dispersion was first dried and then blended into the monomer before its polymerization, thereby giving an aggregated arrangement. These various composites were characterized with regard to their mechanical properties in elongation (using techniques allowing a close appproach to elastic equilibrium), and with regard to chain orientation (using birefringence measurements and infrared spectroscopy). The elastomers having randomly dispersed and regularly dispersed silica dispersions were very similar in mechanical properties and chain orientation, but extensibility was significantly improved by decreasing the strength of the particle-elastomer bonding. Additional improvements in extensibility, and associated increases in toughness, were obtained when these same particles were aggregated.