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 miniemul sion polymerization requires two successive dispersion steps. First, the Ti O2 particles must be successfully dispersed in the monomer phase. Second, t his phase must be dispersed in an aqueous surfactant solution to form stabl e 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 phase s were measured. The presence of insufficient or excess stabilizer both red uced the interfacial tensions. This was attributed to migration of the part ially covered hydrophilic TiO2 particles or the excess OLOA 370 stabilizer (polybutene-succinimide pentamine) to the oil/water interface. The free sur factant in the aqueous phase and the average droplet size of the miniemulsi ons were characterized as a function of the process variables. The presence of TiO2 particles within the droplets limited the ability of the process t o reduce the droplet size although miniemulsion droplets in the size range of 150-200 nm were still obtained for subsequent polymerizations. (C) 2000 John Wiley & Sons, Inc.