On the emulsion polymerization of styrene in the presence of a nonionic emulsifier

The batch emulsion polymerization kinetics of styrene (St) initiated by a w ater-soluble peroxodisulfate in the presence of a nonionic emulsifier was i nvestigated. The polymerization rate versus the conversion curves showed tw o nonstationary rate intervals, two rate maxima, and Smith-Ewart Interval 2 (nondistinct). The rate of polymerization and number of nucleated polymer particles were proportional to the 1.4th and 2.4th powers, respectively, of the emulsifier concentration. Deviation from the micellar nucleation model was attributed to the low water solubility of the emulsifier, the low leve l of the micellar emulsifier, and the mixed modes of particle nucleation. I n emulsion polymerizations with a low emulsifier concentration, the number of radicals per particle and particle size increased with increasing conver sion, and the increase was more pronounced at a low conversion. By contrast , in emulsion polymerizations with a high emulsifier concentration, the num ber of radicals per particle decreased with increasing conversion. This is discussed in terms of the mixed models of particle nucleation, the gel effe ct, and the pseudobulk kinetics. The formation of monodisperse latex partic les was attributed to coagulative nucleation and droplet nucleation for the polymerizations with low and high emulsifier concentrations, respectively. The effects of the continuous release of the emulsifier from nonmicellar a ggregates and monomer droplets, the close-packing structure of the droplet surface, and the hydrophobic nature of the emulsifier on the emulsion polym erization of St are discussed. (C) 1999 John Wiley & Sons, Inc.