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 °C, i.e., close to the nominal melting point of the material, 163 °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 (∼7%) of the volume. Moreover, the lateral growth rate of the kebabs is an order of magnitude larger than expected from quiescent spherulitical growth.