Network Structure of Polystyrene Microgels and Dispersion Mechanism in an Organic Solvent

Polystyrene microgels (PSMG) were prepared by soap-free emulsion co polymerization of styrene and divinylbenzene (DVB), and then dispersed in tetrahydrofuran (THF). The apparent diffusion constant D _app and the intrinsic viscosity [η] for PSMG were determined in THF. D _app was measured with dynamic light scattering technique, and the hydrodynamic radius R_h of PSMG swollen in THF was estimated using the Stokes-Einstein relation. The slope of plots of D _app vs. concentration of PSMG changed from positive to negative with decreasing DVB content. This result was interpreted by a core-shell type network structure of swollen PSMG. Formation of the network structure is also supported by the difference in reactivity ratio between styrene and DVB. The swelling ratio of PSMG, which was calculated from the ratio of R_h and the diameter of dried PSMG, showed maximum at intermediate DVB content, while [η] showed minimum around the similar DVB content. This behavior was ascribed to the aggregation of PSMG. The dispersion mechanism could be qualitatively explained by considering both steric and topological repulsive forces, to which the network structure of the core-shell type may contribute.