Viscoelastic phase separation of polystyrene/poly(vinyl methyl ether) blends with different dynamic asymmetry

Abstract

Viscoelastic phase separation (VPS) can be induced in dynamically asymmetric polystyrene/poly(vinyl methyl ether) (PS/PVME) blend in which there is a large difference between glass transition temperature of the components (about 125 °C). In this study, the effect of molecular weight of the slower component, PS, on VPS mechanism is investigated. The correlation between the time evolution of the phase-separating morphologies and their linear and nonlinear rheological behavior is studied for blends with three different PSs. Rheological measurements and morphological observations are carried out at quench depth of 14 °C. While it was previously found that depending on composition of blend, three different types of VPS behavior can be observed due to self-induced stresses in the early stage of phase separation—(i) the transient gel induced VPS (TG-VPS), (ii) coalescence induced VPS (C-VPS), and (iii) aggregating nucleation and growth (ANG)—in this study, we show that different types of VPS can be observed for a single composition by changing the molecular weight of PS. In other words, dynamic asymmetry between PS-rich and PVME-rich phases decreases with a decrease of molecular weight of PS which results in weaker transient gel in the early stage of phase separation. Consequently, the phase separation mechanism changes for 10/90 blend from TG-VPS to C-VPS and, for 5/95 blend from C-VPS to ANG or NNG (normal nucleation and growth). Furthermore, the variation of fractal dimension by phase separation is studied for 10/90 blends containing PS with different molecular weights.