Structural and morphological studies of isotactic polypropylene fibers during heat/draw deformation by in-situ synchrotron SAXS/WAXD

On-line studies of structural and morphological changes during the heating and drawing process of isotactic polypropylene (iPP) fiber were carried out using synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. A unique image analysis method was used to deconvolute the two-dimensional (2D) WAXD patterns into quantitative fracti ons of crystal, mesomorphic, and amorphous phases. Results showed that the alpha -form crystals were quite defective in the initial iPP fibers and wer e converted into the mesomorphic modification by drawing at room temperatur e. Corresponding 2D SAXS patterns showed that there was no obvious long per iod (i.e., no lamellar structure) in the mesophase of the iPP fiber. We pos tulate that the constituents of the mesophase in iPP fibers include oriente d bundles of helical chains with random helical hands and perhaps oriented chains with no helical structures; both have only partial packing ordering. The formation of the mesophase is through the destruction of the lamellar crystalline phase probably by pulling chains out from crystals. The alpha - form crystals were not converted into the mesophase by drawing at high temp eratures. At higher temperatures, the alpha -form crystals became perfect a nd the crystallinity increased when the fiber was drawn. However, the draw ratio showed an inverse effect. The increase in draw ratio had a minimal ef fect on the crystallinity, but the transformation from the amorphous phase to the mesophase became dominant.