A Comparison of the Thermodynamic Stability and Phase Separation Kinetics of Polymer Blends Containing Cyclic Chains of High Molecular Weight

Two blends of polystyrene (PS, 240K molecular weight) and poly(vinyl methyl ether) (PVME, 171K) were compared, where one blend contained exclusively linear chains and the second contained cyclic PS. The PS/PVME system has been well-studied in the past and is known to exhibit a lower critical solution temperature (LCST) and a critical point near 80% PVME/20% PS when the components exceed 10⁵ in molecular weight. The cloud point measurements reported here show that over a broad composition range, the cycle-containing blend phase separates at temperatures ~7 °C above the cloud point temperatures of the exclusively linear blend. The critical composition does not shift appreciably in the cycle-containing blend. Time-resolved light scattering studies were used to probe the phase separation kinetics near the critical composition following temperature jumps into the unstable region. Analysis of the intensity growth via Cahn-Hilliard-Cook theory for early stage spinodal decomposition yielded an apparent mutual diffusion coefficient. When this mutual diffusion coefficient was combined with an estimate for the second derivative of the free energy with respect to composition, it was possible to calculate the mobilities of the cyclic and linear polystyrenes in the PVME. While the diffusion coefficients in the two blends were nearly the same when the systems were compared at identical temperature increments above their respective spinodal temperatures, the PS cycles appeared to be less mobile than their linear analogs in linear PVME matrices. Our data cannot be compared directly with other diffusion and rheological studies of cycles in the literature, but our results appear consistent with some of the published trends.