Effects of the molecular weight of polymeric costabilizers on the Ostwald ripening behavior and the polymerization kinetics of styrene miniemulsions

The effects of the molecular weight of polystyrene (PS) and polymethyl methacrylate (PMMA) costabilizers on the Ostwald ripening behavior at 25 °C of styrene (ST) miniemulsions and the polymerization of these miniemulsions were investigated. The effectiveness of PS and PMMA costabilizers in retarding the diffusional degradation of ST miniemulsions decreases with increasing the molecular weight of PS or PMMA costabilizers. The Ostwald ripening rate data were used to determine the critical chain length of polymeric costabilizers to induce chain entanglements. The resultant critical chain length to induce chain entanglements is 36,387 ± 1475 g mol−1 and 7668 ± 521 g mol−1 for PS and PMMA costabilizer, respectively, which are comparable to those reported in the literature. The polymerization of the ST miniemulsions stabilized by PMMA costabilizers with different molecular weights at 70 °C was then carried out. The polymerization rate decreases with increasing the polymeric costabilizer molecular weight. This was attributed to the reduced number of latex particles (i.e., reaction loci) with the polymeric costabilizer molecular weight. The miniemulsion polymerization kinetics is primarily controlled by the particle nucleation process (the competition between the monomer droplet nucleation and homogeneous nucleation), which is closely related to the effectiveness of these costabilizers in retarding the Ostwald ripening process. Furthermore, the ideal one-to-one copy of homogenized monomer droplets into latex particles cannot be achieved during polymerization.