Shear-Enhanced Crystallization in Isotactic Polypropylene. 3. Evidence for a Kinetic Pathway to Nucleation

Abstract

In situ rheo-optical techniques are combined with synchrotron wide-angle X-ray diffraction (WAXD) to investigate the oriented crystallization precursors that develop upon strongly shearing an isothermal melt of polydisperse isotactic polypropylene (iPP). The “short-term shearing” experimental protocol, developed by Janeschitz-Kriegl and co-workers, is used under conditions previously determined to induce oriented crystallite growth. Surprisingly, the time for these precursors to appear decreases with increasing temperature, tracking the melt dynamics of the polymer moleculesa behavior unanticipated by current models. Thus, nucleation proceeds via a “nonclassical” kinetic pathway that effectively eliminates the activation barrier for nucleation. To characterize the importance of chain length distribution for the formation of nucleation precursors, experiments were performed with model bidisperse systems containing a small percentage of high molecular weight iPP blended with lower molecular weight iPP of matched stereoregularity. Oriented crystallization was not observed for the individual components of the blend under the most extreme experimental conditions investigated, but it was readily observed for the bidisperse blend. This suggests that, under intense shear, nucleation of oriented crystallites is governed by the rheologically determined formation of a critical anisotropic configuration of polymer chains in the melt.