Order and molecular motion in wholly aromatic copolyesters

Abstract

The order and molecular dynamics of wholly aromatic thermotropic copolyesters prepared from hydroxybenzoic acid (HBA), hydroquinone (HQ) and phenoxyterephthalic acid (Ph-O-TPA) have been studied by X-ray analysis and deuteron NMR spectroscopy of specimens containing either deuterated hydroquinone or deuterated phenoxy side-chain units. X-ray analysis shows the coexistence of nematic and three-dimensionally ordered regions that are characterized by registry of adjacent chains with interleaved side groups. The molecular motions in these nematic/three-dimensionally ordered polymers are compared to those of a highly crystalline copolyester prepared from deuterated TPA and HQ. In the fast motional limit the motion in the crystalline system is characterized by 180° flips about the TPA axis, whereas in the nematic/three-dimensionally ordered copolyester the same type of axially symmetric spectrum is observed for both the nematic and the three-dimensionally ordered regions. The motion of the HQ units is characterized by rotational diffusion about the C₁C₄ axis, augmented by fast rotational fluctuation of this axis about the main-chain direction. A model of free rotation about the axis of the phenyl group augmented by librations of restricted amplitude about the PhO-TPA bond for the ordered regions superimposed by slow axial motions about the main-chain axis in the nematic phase is consistent with both the NMR data and X-ray analysis.