Effects of sodium benzoate on distributions of lamellar structures of sheared isotactic polypropylene: A study using small angle synchrotron X-ray scattering

Small-angle X-ray scattering (SAXS) studies were carried out to investigate effects of sodium benzoate (SB) on the shear-induced distributions of lamellar structures through the depth direction of injection-moulded isotactic polypropylene (iPP) plates, using the micro beam available from a synchrotron to gain good spatial resolution. It was found that in the presence of SB, the shish-kebab structure emerges over a broader range to include regions in which a random arrangement of crystalline lamellae is found without SB. The oriented crystalline lamellae are even found in positions that are very close to the core center. The SB-induced shish-kebab structure under low levels of shear flow is attributed to lower free energy barriers for the coil-to-stretch transition and shorter times to start the crystallization. The effect of SB on the lamellar dimensions is significant only in the core region, and only when the concentration of SB is sufficiently high. A down-U distribution of the long spacing is found, which is mainly ascribed to the variation of thickness of crystalline lamellae with the depth. The results show that a higher level of lamellar orientation is not necessarily accompanied by a higher degree of linear crystallinity.