THEORETICAL-STUDIES OF MODELS OF THERMOTROPIC LIQUID-CRYSTALLINE POLYMERS

The molecule 4-hydroxyphenyleneterephthalate (HPT) has been investigat ed using several computational techniques ranging from molecular mecha nics to high quality ab initio techniques. This molecule represents a model for the repeat unit of a group of interesting liquid crystalline polymers: poly(substituted-p-phenyleneterephthalates). Our results in dicate that the terephthalic part of the molecule is planar and rigid. The molecule is quite flexible, however, with respect to internal rot ation around the C(phenyl)-O ester bond. The preferred conformation is twisted, and conformations with dihedral angles of 90 +/- 30 degrees have almost the same energies. The barrier to internal rotation around the ester bonds is estimated to be 1.5-2.0 kcal mol(-1). These confor mational features are conserved for longer polymer chains. The HF/STO- 3G results are in excellent agreement with the results obtained with t he 6-31G* basis set, while the 3-21G basis set appear to predict an i ncorrect molecular conformation. Molecular mechanics results are in qu alitative agreement with results obtained with accurate ab initio tech niques.