Electro-Optical Studies on Polymer Dispersed Liquid Crystal Composite Films. II. Composite of PVB/E44 and PMMABA/E44

Abstract

Aggregation states and electro-optical effects based on light scattering have been investigated for PVB/E44 and PMMABA/E44 composite films. The composite films were prepared by solvent-evaporation technique. Scanning electron microscopy observations showed that E44-phase is continuously embedded in a sponge-like PVB and PMMABA matrix polymers. Electro-optical responses of the composite films under the conditions of an externally applied AC electric field (0-240 Vp-p; 50, 100, 500 and 1 KHz) temperature (28°C) and film thickness (10±2μm) were determined using He-Ne (wavelength 632.8 nm) laser as a light source. The composite films (of both PVB/E44 and PMMABA/E44) with positive dielectric anisotropy exhibited characteristic reversible light scattering-light transmission switching, strongly dependent on the liquid crystal (LC) channel size and shape in the composite films. The electro-optical response of LC molecules in both of the composite systems is influenced by the miscibility of the LC molecules at LC-polymer interface. Thus the distribution of relaxation time for the interfacial polarization corresponds to the miscibility state at the interface between matrix polymer and LC. The miscibility between two phases at interface was investigated by employing Fourier-Transform Infrared Spectroscopy/Attenuated Total Reflectance (FT-IR/ATR) spectrophotometry and differential scanning calorimetry (DSC). The results obtained indicate that under the conditions imposed the output can be controlled and the response time is on the order of several milliseconds or less. A memory effect is observed in PVB/E44 composite films. The transmittance of the memory state is preserved for a longer time after an applied voltage is removed, which can be erased to the scattering OFF state by heating the film to the E44-clearing temperature. In the case of PMMABA/E44 composites, which exhibit electro-optical switching hysteresis, a possible mechanism that the applied electric field-induced phase-mixing of PMMABA and E44 would be a main cause of hysteresis is discussed.