Investigation of the crystallization behavior of isotactic polypropylene polymerized with different Ziegler-Natta catalysts

Detailed characterization of the crystallization behavior is important for obtaining better structure property correlations of the isotactic polypropylene (iPP), however, attributed to the complexity in ZN-iPP polymerization, the relationship between crystallization behavior and the stereo-defect distribution of iPP is still under debate. In this study, the crystallization kinetics of the primary nucleation, crystal growth and overall crystallization of two iPP samples (PP-A and PP-B) with nearly same average isotacticity but different stereo-defect distribution (the stereo-defect distribution of PP-B is more uniform than PP-A) were investigated. The results of isothermal crystallization kinetics showed that the overall crystallization rate of PP-A was much higher than that of PP-B; but the analysis of self-nucleation isothermal crystallization kinetics and the polarized optical microscopy (POM) observation indicated that the high overall crystallization rate of PP-A was attributed to the high primary nucleation rate of the resin. The stereo-defect distribution plays an important role in determining both the nucleation kinetics and crystal grow kinetics, and thus influence the overall crystallization kinetics. A more uniform distribution of stereo-defects restrains the crystallization rate of iPP, moreover, it has more influence on nucleation kinetics, comparing with the crystal growth. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013