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

The influence of bi-component β-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 β-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 β-phase; meanwhile, its increase of crystallization peak temperature Tc is obviously more prominent. Further analysis of melting behavior showed that the β-phase content greatly depends on the cooling rate applied, the stereo-defect distribution of iPP and the end temperature of cooling (Tend). When Tend = 50 °C, for iPP with less uniform stereo-defect distribution (PCPP-A), the slow cooling rate favors the formation of high β-content, while for PCPP-B, the fast cooling rate favors the formation of high β-content, which should be attributed to the combination effect of the ability for β-crystallization and the thermal stability of the β-phase. After elimination of the confusion of β–α recrystallization during heating (Tend = 100 °C), it turns out that for both PCPP-A and PCPP-B, higher cooling rate is more favorable for β-crystallization. PCPP-A is more favorable for the formation of β-phase with higher thermal stability compared with PCPP-B.