Phase-Separation Behavior and Electro-Optical Properties for (Polymer/Liquid Crystal) Composite Films

Dominant factors that control the aggregation structure and electro-optical properties were presented for (polymer/liquid crystal) composite films prepared by a solvent cast method from homogeneous solutions. Phase-separation behavior of the composite films consisting of a three dimensional polymer network and low molecular weight liquid crystals (LCs) embedded in the network was studied by evaluating the development of optical heterogeneity in the casting solution during film formation and by observing the morphology of the polymer networks. It was confirmed from light scattering studies of the casting solution during solvent evaporation that the phase-separated structure of polymer and LC was formed through spinodal decomposition. The resultant size of the LC domain in the composite film was controlled by regulating the solvent evaporation rate. The morphology of the polymer network in the composite film was also strongly influenced by the solubility between the polymer and the casting solvent. The electro-optical properties based on field controlled light scattering of the composite film were strongly dependent on the size of the LC domain. Though the rise response time decreased, the decay response time increased with an increase in the size of LC domains in the composite film. When poly(diisopropyl fumarate), which is not compatible with the LC, was used as the matrix polymer, no electro-optical hysteresis was observed, while considerable electro-optical hysteresis was observed for the composite film consisting of poly(methyl methacrylate), which shows a certain compatibility with the LC.