Computer Simulation of Rotator Phases in Liquid-Crystalline Polymers

Abstract

The high temperature behaviour of the main-chain liquid-crystal polymer poly(hydroxy-benzoic acid) (PHBA) is examined through the use of computer models. PHBA is known to undergo a first order phase transition, at 340°C, from two co-existing orthorhombic crystalline phases to a single pseudohexagonal rotator phase. Although several attempts have been made to establish the nature of this rotator phase, its structure is, as yet, poorly understood. Monte Carlo simulations have been performed on idealised rotator phase models, using energy terms and cell parameters derived from PHBA. Electron diffraction patterns have been simulated, allowing the phase behaviour to be followed. A loss of angular correlations between adjacent molecules is found, which coincides with the crystal-rotator phase transition. The simulated rotator phase structure is compared with previous models which were derived from diffraction data on the basis of symmetry arguments. A possible explanation is proposed for the continued existance of two orthorhombic phases on cooling from the rotator phase.