Phase transitions of nematic main-chain polyesters under high pressure

Abstract

The nematic-isotropic (NI) and crystalnematic (KN) transition temperatures were measured from atmospheric pressure to 1 kbar for a homologous series of thermotropic polyesters based on regularly alternating 2,2'-dimethylazoxybenzene mesogen and alkanedicarboxylic acid spacer. The values of (dP/dT) for the NI transitions show an odd-even alternation with n, consistent with the previously reported alternation of T_NI and (ΔH)_NI. The characteristic temperature T^∗ calculated for n = odd is approximately 300 K, similar to the values for noncybotactic nematics. On the other hand, for n =even, T^∗ could not be reliably calculated, perhaps due to the existence of cybotactic ordering. At the KN transition there is no such clear alternation in (dP/dT). The specific volume changes (ΔV)_KN at melting are very small and comparable in magnitude with (ΔV)_NI. They are much smaller than the corresponding values for low molar mass liquid crystals and conventional polymers. This suggests that the crystals of these polyesters may have a high degree of imperfection.