A study of molecular motion in a liquid-crystalline copolyester by broad-line nuclear magnetic resonance

Broad-line nuclear magnetic resonance (NMR) measurements have been carried out for the proton resonance in an oriented liquid-crystalline copolyster prepared from 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). The Second moments were determined as a function of the angle γ between the specimen orientation direction and the static magnetic field. The low-temperature NMR results are consistent with a rigid structure of random copolymer chains arranged on a hexagonal lattice. As the temperature is raised the NMR signal changes and can be modeled very satisfactorily by considering that there is free rotation of the benzene ring residues about the 1-4 substitution direction. At more elevated temperatures the observed NMR spectra are consistent with rotation of both HBA and HNA groups.