Non-isothermal crystallization behavior and melting behavior of Ziegler-Natta isotactic polypropylene with different stereo-defect distribution nucleated with bi-component beta-nucleation agent

摘要

The influence of bi-component beta-nucleating agent of pimelic acid/calcium stearate (Pa-Cast) on the non-isothermal crystallization behavior and polymorphic composition of two isotactic polypropylene (iPP) samples with nearly same average isotacticity but different uniformities of stereo-defect distribution were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and Scanning electronic microscopy (SEM). The results indicated that Pa-Cast has very high beta-nucleation efficiency, and greatly increases the crystallization rate of both samples. After the addition of Pa-Cast, the sample with more uniform distribution of stereo-defect and less fraction of high isotacticity (denoted as PCPP-B) is more favorable for formation of large amount of beta-phase; meanwhile, its increase of crystallization peak temperature T (c) is obviously more prominent. Further analysis of melting behavior showed that the beta-phase content greatly depends on the cooling rate applied, the stereo-defect distribution of iPP and the end temperature of cooling (T (end)). When T (end) = 50 A degrees C, for iPP with less uniform stereo-defect distribution (PCPP-A), the slow cooling rate favors the formation of high beta-content, while for PCPP-B, the fast cooling rate favors the formation of high beta-content, which should be attributed to the combination effect of the ability for beta-crystallization and the thermal stability of the beta-phase. After elimination of the confusion of beta-I +/- recrystallization during heating (T (end) = 100 A degrees C), it turns out that for both PCPP-A and PCPP-B, higher cooling rate is more favorable for beta-crystallization. PCPP-A is more favorable for the formation of beta-phase with higher thermal stability compared with PCPP-B.