Multiple Phase Equilibria in Polydisperse Polymer/Liquid Crystal Blends

Abstract

A thermodynamic analysis of phase equilibria in polydisperse polymer−liquid crystal blends is presented. Three different systems were analyzed: (a) a high molar mass polydisperse polystyrene (PS) blended with 4-cyano-4‘-n-heptylbiphenyl (7CB), (b) a low molar mass polydisperse PS blended with 7CB, and (c) an epoxy-based thermosetting polymer blended with a mixture of small mesogenic molecules usually called E7. In the latter case the analysis was performed in both pregel and postgel stages. Macroscopic phase separation taking into account the fractionation of the polydisperse polymer among different phases, was simulated when cooling from an initially homogeneous state. Predicted isotropic−isotropic and isotropic−nematic transitions showed a good agreement with experimental results. The relative volume fraction and compositions of isotropic and nematic phases were predicted. A temperature range was found where three macroscopic phases, two isotropic and one nematic, coexisted at equilibrium. This was the result of a liquid−liquid (or gel−liquid) phase separation preceding the appearance of a nematic phase. Implications of this behavior on morphologies developed in polymer-dispersed liquid crystals (PDLC) are discussed.