Shish and its relaxation dependence of re-crystallization of isotactic polypropylene from an oriented melt in the blends with high-density polyethylene

Abstract

Shish structure and its relaxation dependence of re-crystallization of isotactic polypropylene (iPP) from an oriented melt, caused by melting of shish kebab in original samples (indicated by 2D SAXS and 2D-wide-angle X-ray scattering experiments (2D WAXS) measurements), has been investigated by differential scanning calorimetry (DSC) and optical microscopy (OM). Shish was obtained by dynamic packing injection molding and its size was controlled by addition of high-density polyethylene (HDPE). An increase in shish size was observed with increasing of HDPE content, as indicated by an increase in the crystallization temperature for iPP during re-crystallization. This is understood as, on one hand, the overall decrease in viscosity by addition of HDPE, thus an increase in shear rate. Higher shear rate can promote larger orientation of molecules and continuous growth of shish structure. On the other hand, the relaxation mode of shish in the melt while re-crystallization is also dominated by its size. Shish with larger size has higher thermal stability and can endure more duration time in the melt. Even more, shish with a larger size cannot be transformed to random coil entirely even subjected to annealing at 200 °C for 60 min, and thus re-crystallization via self-seeding is always observed on primary nuclei originated from shish structure. A permanent ordered structure, most likely with chain helical conformation, is proposed for iPP with large shish size. However, shish with smaller size can only maintain for a short time and then relax into random coil completely, resulting in almost absence of self-seeding in re-crystallization. Re-crystallization of isotactic polypropylene was discussed based on: (1) self-seeding with respect to size of shish structure and (2) relaxation of shish with different size.