Dielectric Relaxation in Phase-Segregated Mixtures of Polystyrene and Liquid Crystal 5CB

Abstract

Phase behavior and dynamics in blends of a liquid crystal 4-cyano-4‘-n-pentylbiphenyl (5CB) and polystyrene(PS) were investigated by using dielectric spectroscopy. The blends exhibit phase separation of a UCST type. Below the phase-separation temperature, the blends separate into the isotropic PS-rich and 5CB-rich phases. With decreasing temperature the 5CB-rich phase transforms into the nematic liquid crystalline state and subsequently into the regular crystalline states. The nematic phase supercools about 30 K below the nematic-to-crystal transition point TNC. Optical micrographs of blends indicate that spherical 5CB-rich domains are dispersed in the matrix of PS-rich phase even at the 5CB content of 80%. A strong dielectric relaxation due to rotation of the 5CB molecules in the isotropic PS-rich phase has been termed α. The α process exhibits complex temperature dependence due to the changes in the composition and morphology with temperature. Although the 5CB-rich phase crystallizes mostly at low temperatures, some 5CB-rich droplets of a small size remain without crystallization and exhibit dielectric relaxations termed β and γ. The β and γ processes are assigned to rotation of the 5CB molecules around the short and long axes of the rodlike 5CB molecule, respectively. Besides those dipolar relaxation processes, a relaxation termed α‘ is observed at PS/5CB compositions of 5/5 and 4/6 and assigned to the Maxwell−Wagner effect.