Dynamic crystallization and melting behavior of β-nucleated isotactic polypropylene polymerized with different Ziegler-Natta catalysts

Abstract

Large amount of work has be published on the dynamic crystallization and melting behavior of β-nucleated polypropylene (β-PP). However, the relationship between molecular structure and dynamic crystallization behavior of β-PP is still not clear. In this study, the dynamic crystallization and melting behavior of two β-nucleated isotactic polypropylene (β-iPP) with nearly same average isotacticity but different stereo-defect distribution, were studied by differential scanning calorimetry (DSC), wide angel X-ray diffraction (WAXD) and temperature modulated DSC (TMDSC). The results indicated that stereo-defect distribution of iPP can significantly influence the dependence of the β-crystal content and thermal stability on the cooling rate. NPP-A with less uniform stereo-defect distribution favors the crystallization at higher temperature region and the formation of β-crystal with high thermal stability in all cooling rates concerned, moreover, the β-crystal content is influenced by cooling rate; for NPP-B with more uniform distribution of stereo-defect, the crystallization temperature and the regular insertion of molecular chains can be reduced in a larger extent. NPP-B is more suitable for the formation of high proportion of β-crystal in both low and high cooling rates, meanwhile, the thermal stability of crystal is sensitive to the cooling rate. This work provides a new insight into the design of β-iPP in dynamic crystallization.