X-RAY STUDIES ON THE STRUCTURE OF POLYACRYLONITRILE FIBERS

Using an X-ray fiber diffractometer, 2D intensity distributions of homopolymer polyacrylonitrile (PAN) fibers are measured in reciprocal space in the plane containing the fiber axis. The equatorial intensity distributions show four relatively sharp reflections and three diffuse maxima. The sharp reflections correspond to a pseudohexagonal 2D lattice of intermolecular packing. The diffuse maxima are due to the Laue scattering of a rotational disorder in this lattice. The off-equatorial intensity distribution is mainly determined by intramolecular scattering. A comparison with calculated 2D intensity distributions for various conformations of short PAN chain segments reveals that the predominant conformation is planar zigzag. 2D correlation functions obtained by Fourier transformation of the intensity distribution corroborate this interpretation. To reconcile the results of the WAXS studies with tacticity measurements using C-13 NMR, the continuity of the zigzag conformation is considered to be interrupted by defects (kinks) related to sterically unfavorable sequences of CN groups. No evidence is found for the existence of a two-phase structure with ordered and disordered domains.