Compromise between dominant polymerization mechanisms in preparation of polymer microspheres

Abstract

A system consisting of uniform-sized styrene monomer droplets dispersed in an aqueous phase was employed to study the compromise between dominant polymerization mechanisms in the preparation of polymer microspheres. Monomer droplets were prepared by a microporous membrane emulsification technique, with negligible size distribution. Two polymerization mechanisms were identified—polymerization in micro-sized monomer droplets and nano-sized secondary nuclei, even in the presence of a hydrophobic initiator. Factors affecting the compromise between these two dominant mechanisms were investigated. Increasing the amount of hydrophobic additive (hexadecane) and/or adding water-soluble inhibitor in the aqueous phase suppressed the polymerization in the secondary nuclei, while adding a relatively hydrophilic monomer in the styrene feed and/or increasing the amount of the initiator and the size of the monomer droplets promoted the polymerization in the secondary nuclei. Under the condition where a hydrophilic monomer was added, hollow microspheres were formed when polymerization in the secondary nuclei was dominant, while one-hole microspheres resulted when polymerization occurred only inside the larger monomer droplets, indicating that the compromise between dominant mechanisms is critical to the polymerization process.