Morphology of polypropylene/polystyrene composite particles prepared by seeded emulsion polymerization: influence of azo initiator intrinsic charge

We previously reported an interesting multilayer structure in polypropylene (PP)/polystyrene (PS) composite particles prepared by seeded emulsion polymerization. Herein, we investigate the influence of initiator type on PP/PS composite particle morphology. Three kinds of water-soluble azo initiators, each of a different charge, were used in seeded emulsion polymerization. In the case of the cationic initiator VA-044, the obtained PP/PS composite particles showed multilayer structures consisting of PP and PS phases. When the nonionic initiator VA-086 was used, the composite particles had a void structure, with the PS phase encapsulated by a PP shell, but the total amount of PS incorporated was lower than in the VA-044 system. On the other hand, most of the small amount of PS was not inside the PP shells but rather attached to the outer surfaces of PP particles in the case of the anionic initiator V-501. These differences were due to electrostatic interaction between the initiator and PP particles of negative zeta potential. Prior to polymerization, the cationic initiator VA-044 adsorbed to the negatively charged PP particles, thereby inducing the polymerization of styrene at the surface and within the PP particles. This behavior resulted in the formation of composite particles containing large amounts of PS. Moreover, the formation mechanism of the voids in PP/PS composite particles in the case of VA-086 was found to depend on the osmotic pressure and polymerization temperature rather than on the intrinsic charge of the initiator.