Shear-enhanced crystallization in isotactic polypropylene Part 2. Analysisof the formation of the oriented "skin"

In-situ synchrotron wide angle X-ray diffraction (WAXD) was used to follow crystallization in a polydisperse isotactic polypropylene during and after a brief interval of shear under isothermal conditions. A specific flow hist ory was selected from the range that induces a highly oriented skin core mo rphology (Kumaraswamy G, Issaian AM, Komfield JA. Macromolecules 1999;32:75 37.). At the chosen crystallization temperature (T-cryst = 141 degrees C, c haracteristic time for quiescent crystallization, t(Q) similar to 10(4) s), crystalline WAXD peaks emerge during the brief interval of shear (sigma(w) = 0.06 MPa, t(s) = 12 s) showing a highly oriented fiber-like diffraction pattern; primary lamellae with c-axis orientation were present, along with their associated crosshatched daughter lamellae. The crystallinity grew rap idly during the first 100 s (similar to 10(-2)t(Q)) after cessation of shea r, and very slowly after that. Further, for the first 1200 s, the orientati on distribution did not change from that generated during the shear pulse. Ex-situ transmission electron microscopy showed a characteristic skin-core morphology with a thin skin region consisting of oriented crystallites near the walls of the shear device, adjacent to weakly anisotropic spherulites farther from the walls. This indicates that the in-situ WAXD at the early s tages of crystallization arose mainly from crystallites in the oriented ski n. The skin consisted of densely nucleated thread-like line structures from which a-phase crystalline lamellae radiate. The spacing between the row nu clei in the skin increases as a function of distance from the wall of the s hear device. Our data suggest that the lamellae grow from a central thread until they impinge at about 100 s to form the dense crystalline structure i n the skin. (C) 2000 Elsevier Science Ltd. All rights reserved.