Order and molecular motion in wholly aromatic copolyesters

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 degrees 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 C-1-C-4 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.