Phase behavior of polymer/liquid crystal blends

Abstract

The thermodynamic phase behavior of blends of a low molecular-weight liquid crystal (LC), 4-cyano-4'-n-heptylbiphenyl (7CB), and each of two amorphous polymers, poly(methyl methacrylate) (PMMA) and polystyrene (PS), was studied as a function of temperature or LC concentration. Phase diagrams for both the binary PMMA/7CB and PS/7CB blends were constructed from DSC and optical microscopic observations showing that 7CB was partially miscible in both PMMA and PS. The study revealed an important observation that there existed a polymer/isotropic LC two-phase state at a given composition besides the polymer/nematic LC two-phase state and the polymer/solid crystalline LC two-phase state. The polymer/isotropic LC two-phase morphology was clearly observed by the phase-contrast optical microscope in the temperature range between the homogeneous single-phase state and the polymer/nematic LC two-phase state, while DSC or cross-polarized light microscopy failed to detect the existence of the polymer/isotropic LC two-phase state. This was also confirmed by a light-scattering experiment. At a low ratio of LC in both the PMMA/7CB and PS/7CB blends, direct transformation between the homogeneous single-phase state and the polymer/nematic LC two-phase state was observed without going through the polymer/isotropic LC two-phase state.