Liquid-crystalline transitions in homologous p-phenylenes and their mixtures. Part 1.—Experimental results

Abstract

Temperatures of the nematic → isotropic (T_NI) and smectic-A → nematic (T_SN) transitions in p-quaterphenyl, p-quinquephenyl and p-sexiphenyl have been determined from differential scanning calorimetry measurements carried out on the pure compounds and their binary mixtures. Both transitions are located below the melting point (T_f) of pure p-quaterphenyl; for p-quinquephenyl T_SN lies a few degrees below the crystal–nematic (T_KN) transition. The transition temperature T_NI= 565 °C found for p-sexiphenyl, although slightly higher than the value reported by Lewis and Barr, is confirmatory of their observations; it is much lower than was predicted by Flory and Ronca. The low density of p-sexiphenyl has been identified as a major factor responsible for this disparity between the previous theoretical calculations and experimental results. The densities of the fluids have been determined and thermal expansivities have been estimated. At T_NI the densities of p-quinquephenyl and p-sexiphenyl, 0.905 and 0.81 g cm^–3, respectively, are quite low. Biphasic equilibria in the binary mixtures have been observed as functions of composition and the complete phase diagrams have been delineated.