Chain dynamics in the nematic melt of an aromatic liquid crystalline copolyester: A molecular dynamics simulation study

A molecular dynamics (MD) simulation study of chain dynamics and the relation to dipolar relaxation has been carried out on a 70/30 composition random copolymer of p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid. The dynamics is analyzed in terms of the relatively flexible torsion at the ester oxygen-aromatic carbon bond. The effective torsional potential in the bulk that results from sampled torsional angle populations is found to differ from that of free chains in that the barriers are higher in bulk and the torsion connecting phenyl and naphthyl units has a higher barrier than that between pairs of phenyl units. Activation energies for conformational transitions are higher than those in the effective potentials. These effects indicate sensitivity to packing not found in simpler systems like polyethylene, and that frictional effects are operative. The distribution of transitions over individual bonds is found to be quite heterogeneous over the temperature range studied and is a signature for a vitrifying system. Conformational jumps were not found to be highly correlated, although some preference for next-neighbor events was indicated. Dipolar time autocorrelation functions were constructed separately for phenyl and naphthyl dipoles. The naphthyl units were found to relax more slowly than the phenyls. Comparison of MD relaxation times with experimental loss maxima indicates a degree of correspondence that lends credence to the assignment of the experimental beta relaxation subglass process as associated with the naphthyl units and the gamma subglass process with the phenyl units. (C) 1999 American Institute of Physics. [S0021-9606(99)51319-6].