Axial X-ray scattering on high-performance polymeric fibers

High‐temperature polymers were spum from liquid‐crystalline solutions into fibers of superior thermal stability and mechanical properties. Fibers of two extended‐chain polymers poly(p‐phenyleneterephthalamide), PPTA, and poly‐2,5‐benzoxazole, ABPBO, as well as a rod‐like polymer poly(p‐phenylenebenzobisoxazole), PBO, were examined by axial x‐ray scattering. Both wide‐angle scattering and small‐angle scattering were performed with CuKα radiation aiming along the fiber axis (c‐axis) for structural information on the a‐b lattice plane. In addition to previously reported lattice structure, the PPTA fibers (Kevlar® 29, 49, and 149) showed strong [004] and a [022] reflections suggesting that segments of the PPTA molecules were transverse to the fiber axis. This unique fiber structure is more prominent and the void content is less for the PPTA fibers with higher tensile modulus, (i.e., Kevlar® 149 > Kevlar® 49 > Kevlar® 29). Similar measurements on thermally annealed ABPBO and PBO fibers showed no [00l], [h0l], or [0kl] reflection indicative of a truly uniaxial molecular orientation. Evidence of microfibrillar order was discovered for the Kevlar® fibers and the ABPBO fiber. Results of conventional x‐ray scattering on these fibers were compared and reconciled. © 1994 John Wiley & Sons, Inc.