Shear-induced crystallization of isotactic polypropylene with different molecular weight distributions: in situ small- and wide-angle X-ray scattering studies

In situ synchrotron wide-angle X-ray scattering (WAXS) and small-angle X-ra y scattering (SAXS) were used to monitor the structural and morphological d evelopments during crystallization of a series of isotactic polypropylene ( iPP) blends with different molecular weight distributions (MWD). The experi ments were carried out in the undercooled melt at 150 degreesC subjected to a high shear strain (1428%) at a fixed shear rate (57 s(-1)). The final WA XS patterns showed arcing of the main Bragg reflections, indicating the pre sence of oriented crystallites. These oriented crystallites, in the form of lamellae, were also revealed by two strong meridional reflections in the S AXS patterns. The crystallization kinetics exhibited a strong dependence on the molecular weight distribution, which were due to the production of pri mary nuclei induced by orientation. A method to deconvolute the total integ rated scattered intensity (SAXS and WAXS) into the isotropic and anisotropi c contributions from the crystallized polymer was used to analyze the data. The fraction of oriented morphology determined by SAXS and that of oriente d crystallinity determined by WAXS showed good agreement with each other. H owever, it has been demonstrated that the oriented fraction from SAXS is mo re suitable for determining the 'critical orientation molecular weight' (M* ) value. Only polymer chains above M* in the distribution can become orient ed at a given shear rate. It is observed that, regardless of the MWD, a sim ilar value for M* is obtained under our experimental conditions. (C) 2001 E lsevier Science Ltd. All rights reserved.