Molecular Aspects of the Formation of Shish-Kebab in Isotactic Polypropylene

To elucidate the mechanism of formation of shish-kebab, flow-induced crystallization of isotactic polypropylene is investigated using model slit-flow experiments in combination with in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The results, consistent with nucleation and growth theory, show that a brief but intense pulse of shear is sufficient to trigger structure formation, even at temperatures as high as 165 degrees C, i.e., close to the nominal melting point of the material, 163 degrees C. Working at such a high temperature allows for a clear separation of the nucleation step, taking place during flow, and the growth step, taking place after flow ceases. A small degree of crystallinity stabilizes the fibrillar crystallization precursors, formed in the early stages, and prevents them from dissolving by allowing the molecules involved to retain a stretched conformation. The stretched molecular configuration is essential for further crystallization in this high temperature range. A kinetic analysis indicates that crystallization within the fibrillar shish is based on the unidirectional propagation of a growth front, whereas in a later stage when kebab crystallizes, a bidimensional growth front is observed and the space is rapidly filled until impingement occurs. The lateral dimensions of shish-kebab (as obtained from SAXS analysis) indicate that shish occupy only a small fraction (similar to 7%) of the volume. Moreover, the lateral growth rate of the kebabs is an order of magnitude larger than expected from quiescent spherulitical growth.