Investigation on the structure and crystallization behavior of controlled-rheology polypropylene with different stereo-defect distribution

Abstract

Understanding concerning the influence of peroxide degradation on the stereo-defect distribution and related crystallization kinetics of controlled-rheology polypropylene are of great importance. In this study, two iPP samples (PP-A, PP-B) with similar molecular weights and average isotacticities, but different stereo-defect distributions and their degradation productions after the addition of dicumyl peroxide (DCP) were prepared. Their melt flow indexes (MFI), stereo-defect distributions, crystallization behaviors and kinetics were studied using MFI measurement, differential scanning calorimetry, wide-angle X-ray diffraction, non-isothermal crystallization kinetics and successive self-nucleation and annealing fractionation. The results showed that as the content of DCP increased, the MFI, the crystallization temperature and crystallization rate of PP increased gradually, the crystallization peak width became narrower, and the crystallite size decreased. Meanwhile, the addition of DCP greatly restrained the formation of thick lamellae and narrowed its stereo-defect distribution [meso-sequence length (MSL) distribution]. On the other hand, results showed that PP resins with different stereo-defect distribution have similar MFI variations after degradation. PP-A (whose stereo-defect distribution was less uniform than PP-B) and its degradation products still possess higher crystallization rate, stronger crystallizability, smaller crystallite size and higher fractions of long MSL, compared with their counterparts of PP-B.