Anomalous phase separation behavior in dynamically asymmetric LCST polymer blends

Abstract

The interplay of thermodynamic forces and self-generated stresses induced in different compositions, and at different quench depths on the phase behavior of dynamically asymmetric PS/PVME blends are studied. The thermodynamic phase diagram is obtained from dynamic temperature sweep experiments. Phase contrast optical microscopy and rheological measurements including linear viscoelastic behavior and the stress growth behavior are employed to investigate the time evolution of the different phase-separating morphologies. At an intermediate quench depth of 14 °C, in addition to thermodynamically controlled phase separation mechanisms (nucleation and growth, NG, and spinodal decomposition, SD), three different kinds of phase separation behavior are induced by transient gel formation due to self-induced stresses in the early stage of phase separation: (i) conventional viscoelastic phase separation (VPS), (ii) nucleation of aggregate-like PS-rich phase and subsequent formation of a percolated network by coalescence, and (iii) nucleation of an aggregate-like PS-rich phase while the dispersed-matrix morphology is preserved in the later stages of phase separation. While it is generally accepted that VPS occurs at deep quench depths, we observe the VPS mechanism at shallow quench depths which is attributed to dynamic heterogeneity in the phase-separated domains. A dynamic phase diagram which shows the effect of dynamic asymmetry on phase behavior is proposed.