Water-soluble polyurethane resins as emulsifiers in emulsion polymerization of styrene: Nucleation and particle growth

Polystyrene/polyurethane hybrid latex particles were synthesized by employing water-soluble (or dispersible) polyurethane resins which had been prepared from isophorone diisocyanate (IPDI), poly(1,2-propylene glycols) (PPG-750 and PPG-2000), and 2,2-bis(hydroxymethyl)propionic acid (DMPA). Two kinds of polyurethane resin were used as emulsifiers and their effect on emulsion kinetics was investigated by measuring the rate of polymerization, the particle size distribution, the number of particles, and the average number of radicals per particle. Effects of the resin concentration on the rate of polymerization and the number of particles were found have a somewhat similar kinetic behavior with exponents in the range of 0.25-0.30 and 0.60-0.70, respectively. The average particle sizes, however, were very small (ca. 40-60 nm) and the particle size distribution showed a broad distribution. The particle nucleation period continued even over 0.7-0.8 fractional conversion of styrene. Moreover, the initial latex particle size was very large in comparison with the final particle size. Characteristic behavior of the particle nucleation and growth was mainly attributed to superior solubilizing ability and high internal viscosity of the polyurethane resin aggregates.