Rheology and Morphology of Dynamically Asymmetric LCST Blends: Polystyrene/Poly(vinyl methyl ether)

We investigated the correlation between the time evolution of the different phase-separating morphologies and corresponding linear and transient rheological behaviors for the dynamically asymmetric PS/PVME blend in which there is a large difference between glass transitions of the pure components (about 125 degrees C). The phase diagram was obtained from dynamic temperature sweep experiments. Phase contrast optical microscopy was employed to investigate morphological evolution of PS/PVME blends at various regions of obtained phase diagram at a constant temperature of 105 degrees C. At this temperature depending on sample composition, the viscoelastic phase separation (VPS) was observed besides the usual phase separation mechanisms (nucleation and growth (NO) and spinodal decomposition (SD)), indicating the interplay between thermodynamics and viscoelastieity in phase-separation behavior of PS/PVME blends. The linear viscoelastic behavior for different phase separating mechanisms (NO, SD, and VPS) was measured to investigate the kinetics of phase separation. It was found that the linear viscoelastic behavior can be described by Palierne's emulsion model, if the self-generated stresses induced during the phase separation in the matrix phase are taken into account. Furthermore, the stress growth behavior of different phase-separating morphologies was investigated by transient start-up or shear flow.