Studies of the mesophase development in polymeric fibers during deformation by synchrotron SAXS/WAXD

On-line structural and morphological studies on Kevlar 49 and isotactic polypropylene (iPP) fibers during deformation were carried out using synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). A novel image analysis method was used to extract quantitative fractions of the crystal phase, the amorphous phase and a “mesomorphic” (intermediate) phase from two-dimensional (2D) WAXD patterns. Results showed that about 20 wt% in the Kevlar 49 fiber had an intermediate mesophase morphology. The transitions between crystal phase, amorphous phase and mesophase were not obvious during deformation of Kevlar 49 fiber at room temperatures. 2D SAXS patterns indicated that the superstructure of the Kevlar 49 fiber was fibril in nature. 2D WAXD results of iPP fibers showed that the α-form crystals were quite defective in the initial state and were converted to the well-known mesomorphic form by drawing at room temperatures. The mesophase in Kevlar 49 fibers was then compared with that in iPP fibers. The shape of one-dimensional equatorial peak of the mesophase in the iPP fiber was similar to that in the Kevlar fiber, indicating that the mesophase in both iPP and Kevlar fibers could be similar in some aspects of molecular arrangement. Corresponding 2D SAXS patterns showed that there was no obvious long period in the mesophase of the drawn iPP fiber. We speculate that the constituents of the mesomorphic fraction extracted in the drawn iPP fibers may consist of partially oriented bundles of helical chains with random helical hands as well as oriented chains with no helical structures (consisting of stereo/tacticity defects). The latter is similar to the mesophase of rigid chains in Kevlar fibers, consisting of only oriented chains with no helical structures.