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. At the chosen crystallization temperature (T_cryst = 141 °C, characteristic time for quiescent crystallization, t_Q to approximately 10⁴ s), crystalline WAXD peaks emerge during the brief interval of shear (σ_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 (approximately 10^-2t_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 α-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.