Molecular Dynamics Simulation of the Nematic Melt of a p-Hydroxybenzoic Acid/2-Hydroxy-6-naphthoic Acid Liquid Crystalline Copolyester

A molecular dynamics simulation has been carried out on a model for the nematic melt phase of a thermotropic liquid crystalline polymer of the Vectra type, a 70/30 random copolymer of p -hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). The periodic simulation box, which contained 12 chains of 10 monomers each, was orthorhombic in shape, and its a, b, and c dimensions could independently adjust under constant pressure. The X-ray scattering pattern calculated for the melt is in good agreement with experimental results. Analysis showed the chains to be highly disordered conformationally in that the dispositions of successive ester group directions and aromatic ring staggers were found to be distributed over all four available positions, at ca. ± 60° and ±120°. Translational register of chains, a feature believed to be present experimentally in the 3D ordered or “crystalline” lower temperature state, is found here to be present also in the nematic melt state. Thus, conformational disorder appears to be the principal distinction between the melt and the lower temperature ordered state. The melt remains in the nematic state on cooling to ambient temperature in the simulations.