Microgel formation in emulsion copolymerization. I. Polymerization without seed latex

Abstract

Microgel formation during emulsion copolymerization of methyl methacrylate and ethylene glycol dimethacrylate is investigated both experimentally and theoretically. It was found that the average crosslinking density is fairly high even from a very early stage of polymerization. The molecular weight distribution (MWD) development in emulsion crosslinking copolymerization is completely different from that in homogeneous polymerization. Because the maximum molecular weights allowed to exist is limited by the particle size, a comprehensive model for the MWD development in nonlinear emulsion polymerization must account for the size of polymerization locus properly. During the formation of microgels, a drastic change in the weight-average molecular weights, which is characteristic of gelation in homogeneous media, is not always required. In a typical microgel formation process where a large mole fraction of divinyl monomer is used, the average molecular weights may increase just linearly with conversion in which a sharp MWD shifts to higher molecular weights with the progress of polymerization. It is shown that the microgels formed in emulsion polymerization are characterized as intramolecularly crosslinked macromolecules that occupy a very large weight fraction in each polymer particle. © 1996 John Wiley & Sons, Inc.