Thixotropic interparticle interaction between silica and nonionic polymer particles prepared by static dispersion polymerization

Static dispersion polymerization of styrene, methyl methacrylate, ethyl methacrylate, butyl methacrylate and benzyl methacrylate, initiated by a lipophilic initiator in an aqueous dispersion with unmodified silica particles was investigated. Polymerization of styrene gave a mixed dispersion of silica and monodisperse polystyrene (PS) particles, which was extremely stable compared with the aqueous dispersion of silica particles. The enhanced dispersion stability is due to a thixotropic effect based on weak attractive interparticle interactions between silica and PS particles in aqueous dispersion. An aggregate in which silica particles are surrounded by the smaller PS particles is formed and inhibits flocculation of silica particles, thus giving enhanced dispersion stability. The results of similar static polymerization reactions with methacrylate analogs indicate that hydrogen bonds between benzene ring and hydroxy groups on the silica surface, and not electrostatic interaction derived from initiator residues, are the origin of weak interaction for thixotropic aggregation.