Encapsulation of inorganic particles by dispersion polymerization in polar media 2. Effect of silica size and concentration on the morphology of silica-polystyrene composite particles

Following a previous work (Bourgeat-Lami, E., and Lang, J., J, Colloid Interface Sci. 197, 293 (1998)), encapsulation of silica beads has been achieved by dispersion polymerization of styrene in an aqueous ethanol medium using poly(hi-vinyl pyrrolidone) as stabilizer. Silica beads, prepared according to the Stober method, were coated prior to polymerization by grafting 3-(trimethoxysilyl)propyl methacrylate onto the surface. A great number of silica beads per composite particle were previously found using beads that had diameters between 49 and 120 nm. In the present work, larger silica beads with diameters between 191 and 629 nm are investigated. We demonstrate by transmission electron microscopy that, consequently, only a small number of silica beads are contained in the composite particles. By counting the composite particles containing precisely zero, one, two, three, four, and more than four silica beads, it clearly appears that the encapsulation of only one silica bead can be obtained simply by increasing the size of the beads. Under our experimental conditions, the optimal bead diameter for achieving composite particles containing only one silica bead turns out to be around 450 nm. We show that increasing the silica bead size above this value results in an increased number of composite particles without silica beads. In contrast, the number of composite particles with two, three, four, or more than four silica beads increases with decreasing silica bead size. In addition to the above variations in composition of the composite particles, changes in particle shapes were also observed as a function of the size of the silica beads and the styrene concentration in the polymerization medium. Hypotheses concerning these variations are presented. (C) 1999 Academic Press.