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

Abstract

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