Shear-enhanced crystallization in isotactic polypropylene Part 2. Analysis of 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 history was selected from the range that induces a highly oriented skin core morphology (Kumaraswamy G, Issaian AM, Komfield JA. Macromolecules 1999;32:7537.). At the chosen crystallization temperature (T-cryst = 141 degrees C, characteristic 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 rapidly during the first 100 s (similar to 10(-2)t(Q)) after cessation of shear, and very slowly after that. Further, for the first 1200 s, the orientation 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 stages of crystallization arose mainly from crystallites in the oriented skin. The skin consisted of densely nucleated thread-like line structures from which a-phase crystalline lamellae radiate. The spacing between the row nuclei in the skin increases as a function of distance from the wall of the shear 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 in the skin. (C) 2000 Elsevier Science Ltd. All rights reserved.