A MONTE-CARLO SIMULATION OF THE HIGH-TEMPERATURE PHASES OF POLY(P-HYDROXYBENZOIC ACID)

The two high-temperature structures (identified as phase III and phase IV) of poly(p-hydroxybenzoic acid) (PHBA) are investigated through a molecular modeling methodology employing a Monte Carlo sampling of con figurational phase space. Realistic chains of PHBA are represented by the explicit atom representation of the dimer repeat unit of the unit cell. States are sampled from the NVT ensemble using a scheme consisti ng of (1) valence angle and torsional angle variations and (2) rigid b ody rotations of the chain about the chain axis. Simulated X-ray diffr action data for the two phases are compared with experimental data fro m the literature. Intermolecular orientational probability density dis tributions indicate that phase III exhibits a wide range of intermolec ular phenylene orientations, but with a retention of a herringbone-typ e packing in the a-b plane. The phenylene rings; of phase ni experienc e a greater intermolecular orientational freedom over phase III, but w ithout free rotation. Intramolecular orientational probability density distributions indicate a preference for staggering the successive phe nylene rings along the chain, with phase IV exhibiting a higher probab ility of the coplanar arrangement over that of phase III. The packing environments do not impose any severe restrictions on the torsion angl es; the torsional angle populations of phases III and IV exhibited onl y minor variations relative to those exhibited by the individual chain s free of interchain interactions.