MOLECULAR-DYNAMICS SIMULATIONS OF LIQUID-CRYSTALLINE ORDER FOR TETRAMERS OF P-HYDROXYBENZOIC ACID

Molecular dynamics simulations on condensed systems of dimers and tetr amers of p-hydroxybenzoic acid (HBA) are reported. A nematic liquid cr ystalline phase was observed in the simulations of a system of tetrame rs, while an isotropic phase was found for dimers. The simulations wer e started from a configuration with limited order. This configuration was obtained by compressing a low-density initial structure in which t he distribution of the angle between the long molecular axis and the d irector was in accordance with the results from mean field theory. The order parameter found for the nematic phase after relaxation is unexp ectedly high, a fact which is discussed extensively and compared to st andard theories for the formation of liquid crystals. The molecular se lf-diffusion and the distribution of the torsional angles of the ester group were determined. In the liquid crystalline environment, a copla nar arrangement of the phenyl rings is slightly more favorable than in isolated molecules, but in general the torsional angle distributions of the condensed phase are similar to the results obtained in vacuum s imulations. This demonstrates that the internal motions, which do not influence the shape of this rodlike molecule without spacer groups, ar e not influenced by the liquid crystalline environment.