Hollow particles are produced by the burying of sulfate end-groups inside particles prepared by emulsion polymerization of styrene with potassium persulfate as initiator in the absence/presence of a nonionic emulsifier

Abstract

We prepared submicrometer-sized polystyrene (PS) particles by emulsion polymerization with/without polyoxyethylene nonylphenyl ether nonionic emulsifier (Emulgen 911), and examined how water absorption was affected by polymer end-groups derived from hydrophilic (ionic) or hydrophobic (nonionic) initiator and/or incorporated nonionic emulsifier when they were inside these particles. PS particles having sulfate end-groups, which were prepared using a potassium persulfate initiator with/without Emulgen 911, absorbed a certain amount of water inside them, whereas those having isobutyronitrile end-groups derived from 2,2′-azobis(isobutyronitrile) initiator did not. Absorption of water inside the particles caused by the incorporated Emulgen 911 alone was minimal in the absence of the sulfate end-groups. Considering that sulfate end-groups existing at the particle surface do not contribute to water absorption into the particles, these results indicate that sulfate end-groups did not only exist at the surface of the PS particles but were also buried inside the PS particles during the emulsion polymerization. This offers a clear explanation of a longtime enigma in (emulsifier-free) emulsion polymerization. Both the ionic end-groups buried in the particles and the nonionic emulsifier incorporated inside cooperate to absorb water, thus resulting in the formation of hollow PS particles.