Pressure-Volume-Ternperature Relation of Liquid Crystals and the Effect of Molecular Rigidity on the Nematic-Isotropic Phase Transition

Abstract

Pressure-Volume-Temperature relations and the slope of the log-log plot of the transition temperature vs. molar volume at the transition points (-d InTc/d InVc) were measured in the vicinity of the isotropic to nematic phase transition point for 4'-mono-4-propyl cyanophenyl cyclohexane (3PCH). The value of -d InTc/d In Vc was found to be 8.29 for 3PCH, which is larger than that obtained for other nematogenic substances, but is smaller than the extrapolated value for the alkylcyanobiphenyl homologous series. The results are well explained by the hypothesis that the cyclohexyl ring behaves more softly than the benzene ring under pressure in the isotropic to nematic phase transition. This suggests that the effective potential is not only intermolecular, but also involves an intramolecular potential.