UCST behavior observed for a binary polymer mixture of polystyrene/poly(vinyl methyl ether) (PS/PVME) with a PS rich asymmetric composition as a result of dynamic asymmetry & imbalanced local stress, viscoelastic phase separation, and pinning by vitrification

Abstract

By using small-angle neutron scattering (SANS), we observed the upper critical solution temperature (UCST) behavior for a binary polymer mixture of polystyrene/poly(vinyl methyl ether) (PS/PVME) with a PS rich asymmetric composition. It was found in a single mixed state and upon decreasing the temperature below the glass transition temperature of PS (T_g,PS), excess small-angle scattering appeared at a low q (q < 0.01 Å⁻¹) showing that demixing between PS and PVME had occurred, whereas the Lorentzian (Ornstein–Zernike type) scattering due to thermal concentration fluctuations covered a wide q-region of 0.01 < q < 0.1 Å⁻¹ and continued to decrease, where q is the wave number of scattered neutron. As the temperature decreases below T_g,PS, the mixture of PS/PVME (80/20) becomes dynamically asymmetric because the PS chains become less mobile due to vitrification and start to form a quasi-stationary gel network. Upon decreasing the temperature further, the polymer chains (PS and PVME) tend to cooperatively diffuse in order to achieve a new equilibrium state. However, the quasi-stationary gel network composed of PS prohibits the re-configuration of the chain distribution so that the local stress is imbalanced between PS and PVME. Consequently, viscoelastic phase separation due to stress–fluctuation coupling occurs giving rise to excess inhomogeneity (UCST behavior). For a further temperature decrease, a quasi-stationary gel network changes into a frozen network, which pins both the excess inhomogeneity and the thermal concentration fluctuations. This UCST behavior, which is distinguished from thermodynamics instability, is similar to the appearance of a static inhomogeneity according to the frozen blob model, established for chemical polymer gels with heterogeneous crosslink density (J. Bastide and L. Leibler, Macromolecules 1988, 21, 2649).