The Morphology of Polystyrene Latex Particles

Our understanding of the factors controlling the preparation of sty-rene-based latex products has recently undergone extensive revision. Brief-ly, we have found that, under a wide range of conditions, the growing latex particles possess a core-shell structure in which a monomer-rich shell encapsulates a polymer-rich core. This two-phase behavior is remarkable inasmuch as styrene and polystyrene are ordinarily (in macroscopic sys-tems) completely miscible in all proportions. The proposed core-shell mor-phology explains the extended constant-rate behavior observed in styrene emulsion polymerization and permits extension of the Smith-Ewart The-ory to conversions as high as 60%. In seed growth experiments, the added monomer actually encapsulates the original seed particles, up to their equi-librium saturation content, thereby indicating that the phenomenon of interest is controlled by thermodynamic factors. We believe that encapsu-lation is governed by the microscopic nature of the latex particles which has, in turn, a profound effect upon the conformational behavior of the long-chain polymer molecules as they interact with the particle-water in-terface. These findings have important fundamental implications and tech-nological applications in latex product design that are only beginning to be explored