Influence of the stereo-defect distribution on the crystallization behavior of Ziegler-Natta isotactic polypropylene

Abstract

The relationship between the stereo-defect distribution and the crystallization behavior of Ziegler-Natta isotactic polypropylene (ZN-iPP) is an important issue, which has not been clearly studied up to now. In this work, the crystallization behavior of a series of iPP samples with similar average isotacticity but different stereo-defect distribution, polymerized with the same Ziegler-Natta catalyst system, was studied by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) observation. The results of isothermal crystallization kinetics indicated that, as the distribution of stereo-defects becomes more uniform, the overall crystallization rate decreases gradually. Meanwhile, the results of self-nucleation isothermal crystallization kinetics showed that, the crystal growth rate decreases gradually and the energy barrier of crystal growth increases. Moreover, the POM observation illustrated that not only the crystal growth rate, but also the nucleation rate decrease gradually as the stereo-defect distribution becomes more uniform. The results above indicated that for iPP polymerized with the same Ziegler-Natta catalyst system, stereo-defect distribution plays an important role in determining the nucleation kinetics, crystal growth kinetics and the overall crystallization kinetics of the resin.