Structural changes during deformation of Kevlar fibers via on-line synchrotron SAXS/WAXD techniques

On-line studies of structure and morphology changes in Kevlar 49 fibers dur ing stretching were carried out using synchrotron simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. A unique two-dimensional (2D) image analysis method was used to extract qu antitative information of crystal, amorphous and mesomorphic fractions from 2D WAXD patterns. Results showed that about 20% of the fraction (mass) in the Kevlar 49 fiber was mesophase, 50% was crystalline and 30% was amorphou s. There were transitions between crystal, amorphous and mesomorphic fracti ons during deformation. The crystal orientation was obtained in terms of th e Herman's orientation function f(2) from the azimuthal scan of the (200) c rystal reflection. The crystal orientation was found to be quite high in th e Kevlar 49 fiber and increased with the stretch ratio. The fibril length a nd misorientation were also obtained from 2D SAXS patterns by using the Rul and method. Results showed that the fibril length decreased with the stretc h ratio until 2.0% and then increased. The misorientation decreased with in creasing stretch ratio. (C) 2000 Elsevier Science Ltd. All rights reserved.