Molecular Dynamics Studies of the Phase Transitions of Homopolymers of p-Hydroxybenzoic Acid

Abstract

Studies of liquid crystals composed of the homopolymer of p-hydroxybenzoic acid are presented based upon force-field (FF) and molecular dynamics (MD) methods. Structural data has been extracted from MD simulations to characterize the nature of the high-temperature phase transitions which have been experimentally identified for these polymers. Discontinuities in the isobaric heat capacity calculated as a function of temperature are taken to be signatures for the various phase transitions that HBA undergoes. Typically, the structural transitions are predicted by the simulations at temperatures agreeing within 10 to 15 °C of the transitions observed in differential scanning calorimetry measurements. Variations in crystallographic lattice constants determined from X-ray diffraction patterns are reproduced well by the simulations. The transition from an orthorhombic unit cell in a low-temperature phase (below 340 °C) to a pseudohexagonal unit cell at higher temperatures is experimentally observed for HBA homopolymers and has also been observed in the course of these simulations. In general, the simulations and methods applied are suitable for a description of the occurrence of phase transitions within these novel liquid crystal materials, are useful for characterization of the packing and relative orientations between chains, as well as providing a detailed description of the reorganization of the phenyl rings and ester linkages along the chain axes.