Theoretical studies of models of thermotropic liquid crystalline polymers

摘要

The molecule 4-hydroxyphenylcueterephthalato (HPT) has been investigated using several computational tccliniquiss ran-ging Irom molecular mechanics to high quality, ab inilio techniques, This molecule represents a model for the repeut unit of a group of interesting liquid crystalline polymers: poly(sub.sliluted-p-phenylenetereplUlu)lates), Ontr results indicate that the tcrephthalic part of the molecule is planar and rigid. The molecule is quite flexible, however, with respect lo internal rotation around the C(phenyl)-C) ester bond. The preferred conformation is twisted, and conformations with dihedral angles of 90° 30° have almost the same energies. The barrier to internal rotation around the ester bonds is estimated lo be .1.5—2.0 kca mol~(-1) These eonlbrmalional features are conserved For longer polymer chains, The I IRS TO-.10 results are in excellent agreement with the results obtained with the 6-31(3 " basis set, While the 3-210 basis set appear to predict an incorrect molecular conformation. Molcculur mechanics results are in qualitative agreement with results obtained with accurate ab initio techniques.