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 TNI and (b.DeltaH)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 (b.DeltaV)KN at melting are very small and comparable in magnitude with (b.DeltaV)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.