Markedly improving mechanical properties for isotactic polypropylene with large-size spherulites by pressure-induced flow processing

Mechanical property of semicrystalline polymer with large-size spherulites is rather poor, and needs to be improved by adding nucleating agent to make spherulites smaller. However, the nucleating agent causes many problems limiting its applications. In this paper, we report a method to simultaneously improve the toughness and tensile strength of isotactic polypropylene (iPP) with large-size spherulites by deformation of spherulites and orientation of lamellaes through pressure-induced flow processing (PIF-processing) at solid state. The initial spherulite sizes of iPP samples with the same crystallinity and crystal form were controlled by adding nucleating agent (NA, Talc, nano-scaled), and then the samples were subjected to PIF-processing. The resulting mechanical performance of the bulk materials, including impact and tensile strength, was simultaneously enhanced. In particular, the impact strength increases upto 8 folds higher than those obtained by conventional processing for the same bulk materials, attributing to the synergistic effect of both the orientation and the large-size spherulites. The measurements of SEM, XRD and SAXS revealed the spherulites deformation and lamellae orientation during PIF-processing, as the basis of discussing the mechanism.