MULTIPLE PHASE-EQUILIBRIA IN POLYDISPERSE POLYMER LIQUID-CRYSTAL BLENDS

A thermodynamic analysis of phase equilibria in polydisperse polymer-l iquid crystal blends is presented. Three different systems were analyz ed: (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 blende d with a mixture of small mesogenic molecules usually called E7. In th e latter case the analysis was performed in both pregel and postgel st ages. Macroscopic phase separation taking into account the fractionati on of the polydisperse polymer among different phases, was simulated w hen cooling from an initially homogeneous state. Predicted isotropic-i sotropic and isotropic-nematic transitions showed a good agreement wit h experimental results. The relative volume fraction and compositions of isotropic and nematic phases were predicted. A temperature range wa s 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 pha se. Implications of this behavior on morphologies developed in polymer -dispersed liquid crystals (PDLC) are discussed.