Encapsulation of inorganic particles via miniemulsion polymerization. II. Preparation and characterization of styrene miniemulsion droplets containing TiO₂ particles

The encapsulation of titanium dioxide (TiO₂) particles via styrene miniemulsion polymerization requires two successive dispersion steps. First, the TiO₂ particles must be successfully dispersed in the monomer phase. Second, this phase must be dispersed in an aqueous surfactant solution to form stable submicron droplets. The oil/water interface between the droplets and the aqueous phase can be affected not only by the surfactant used but also the components present in the oil phase. Interfacial tensions between the phases were measured. The presence of insufficient or excess stabilizer both reduced the interfacial tensions. This was attributed to migration of the partially covered hydrophilic TiO₂ particles or the excess OLOA 370 stabilizer (polybutene–succinimide pentamine) to the oil/water interface. The free surfactant in the aqueous phase and the average droplet size of the miniemulsions were characterized as a function of the process variables. The presence of TiO₂ particles within the droplets limited the ability of the process to reduce the droplet size although miniemulsion droplets in the size range of 150–200 nm were still obtained for subsequent polymerizations. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4431–4440, 2000