Molecular mobility in liquid-crystalline phase of oriented wholly aromatic copolyester as revealed by proton magnetic resonance

Broad line proton magnetic resonance (PMR) has been used for the study of molecular motion in wholly aromatic thermotropic liquid-crystalline (LC) copolyester at high temperatures up to 340°C. Specimens were thin highly oriented fibers produced from a copolymer of hydroxybenzoic acid and hydroxynaphthoic acid (HBA/HNA). In the vicinity of the thermotropic transition temperature a fine structure of PMR spectra was revealed for as-spun fibers. Annealing leads to smoothing of this structure. Theoretical spectra were calculated and compared with the experimental ones. This made it possible to obtain information on large-scale segmental motion of macromolecules in the LC phase. Heat treatment leads to retardation of the chain cooperative mobility. Dynamic inhomogeneity of the fibers at high temperatures is discussed.