Shear-Enhanced Crystallization in Isotactic Polypropylene. In-Situ Synchrotron SAXS and WAXD

Abstract

In-situ synchtrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) are used to follow the isothermal crystallization (lamellar thickness, crystallinity, orientation, and parent-to-daughter ratio) of a polydisperse isotactic polypropylene subjected to “short term shearing” as a function of imposed shear stress, shearing duration, and crystallization temperature. The X-ray data are interpreted in view of the real-space morphological information from ex-situ microscopy. Under “mild” shearing conditions (shear stress less than a critical value and shearing duration less than a critical time), needlelike nuclei are induced during shear but are so far apart that crystallites splay substantially as they grow to form somewhat distorted spherulites; the X-ray results show weakly oriented growth on a time scale that is rapid compared to quiescent crystallization and show that the orientation distribution broadens as crystallization progresses. Stronger shearing leads to the elaboration of these nuclei into threadlike structures that template the formation of highly oriented crystals with fiberlike orientation. The parent-to-daughter ratio is influenced by both temperature and flow. As expected, increasing the crystallization temperature leads to fewer daughter crystals relative to the parents. Shear also enhances the formation of parents relative to daughters: as parent crystals form with their chain axis along the flow direction, the epitaxial daughter crystals have their chain axis in an unfavorable direction, perpendicular to the flow.