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 th e nematic melt phase of a thermotropic Liquid crystalline polymer of t he Vectra type, a 70/30 random copolymer of p-hydroxybenzoic acid (HBA ) and 2-hydroxy-6-naphthoic acid (HNA). The periodic simulation box, w hich contained 12 chains of 10 monomers each, was orthorhombic in shap e, and its a, b, and c dimensions could independently adjust under con stant pressure. The X-ray scattering pattern calculated for the melt i s in good agreement with experimental results. Analysis showed the cha ins to be highly disordered conformationally in that the dispositions of successive ester group directions and aromatic ring staggers were f ound to be distributed over all four available positions, at ca. +/- 6 0 degrees and +/-120 degrees. Translational register of chains, a feat ure believed to be present experimentally in the 3D ordered or ''cryst alline'' lower temperature state, is found here to be present also in the nematic melt state. Thus, conformational disorder appears to be th e principal distinction between the melt and the lower temperature ord ered state. The melt remains in the nematic state on cooling to ambien t temperature in the simulations.