The b-nucleation behavior of isotactic polypropylene (iPP) is a fascinating and important issue in polymer physics; however, little about this phenomenon or its physical nature has been clearly understood. In the present study, by tuning the heating temperature (fusion temperature, T f ), the amount of ordered structures in iPP melt was controlled. In this way, the influence of five types of representative bnucleating agents (b-NA) on the crystallization behavior of iPP with different melt structures (i.e. the amount of ordered structures) was comparatively studied by differential scanning calorimetry (DSC), polarized optical microscopy (PLOM), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and rheological measurement. A surprising synergetic effect was observed between b-NAs with a-/b-dual polymorphic selectivity (dual-selective b-NA) and the ordered structure of iPP, resulting in significant increases of the b-nucleation efficiency and the b-phase proportion of the sample. PLOM observation of the crystallization process confirmed that with the presence of ordered structures in iPP melt, a mass of dark, small crystal embryos derived from self-nuclei uniformly distributed in the melt and exhibited b-nucleation efficiency. This work provides the first evidence that for iPP nucleated with dualselective b-NA, the ordered structures play a determining role in the b-nucleation of iPP. Under the influence of the dual-selective b-NA, the ordered structures exhibited b-nucleation efficiency and therefore encouraged b-nucleation. A possible mechanism was proposed.

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In this study, the crystallization behavior of the b-isotactic polypropylene (b-iPP) samples nucleated by a rare earth based b-nucleating agent (b-NA) WBG-II and a metal salts compound b-NA NAB83 (denoted as WPP and NPP, respectively) under different cooling conditions were comparatively investigated. The thermal conditions such as the cooling rate, isothermal crystallization temperature, isothermal crystallization time, and the subsequent cooling to room temperature. The results of WAXD, SEM, and nonisothermal crystallization reveal that under the same processing conditions, the crystallite size of NPP is smaller, which arrange more compactly as compared with WPP. Meanwhile, NPP has shorter crystallization rate and higher b-nucleation selectivity, but WPP can crystallization at wider temperature range. The results of isothermal crystallization showed that NPP has higher selectivity and higher b-nucleation efficiency, which favors the formation of high proportion of b-phase at the isothermal crystallization temperature of 110-130 C with and without subsequent cooling; WPP has lower selectivity, which can only induce high content of b-phase under isothermal crystallization without subsequent cooling to 25 C. In tuning the crystallization behavior and the properties of b-PP, the joint influence of the efficiency and selectivity of the b-NA, and the thermal conditions should be taken into consideration.

Aiming at further investigating the combination effect of concentration of β-nucleating agent (β-NA) and stereo-defect distribution on the crystallization behavior of β-nucleated isotactic polypropylene (β-iPP), in this study, the crystallization behavior and polymorphic morphology of twoβ-iPP resins with nearly same average isotacticity (PP-A and PP-B) but different uniformities of stereo-defect distribution were investigated by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). The results of DSC and WAXD showed that the addition of TMB-5 increases the crystallization temperature and decreases the spherulite sizes of both PP-A and PP-B, and reduces their crystallization energy barriers as well; however, the polymorphic behaviors of PP-A and PP-B exhibit different dependence on the TMB-5 concentration. For PP-A with less uniform distribution of stereo-defects, β-phase can be observed only when the TMB-5 concentration is no less than 0.1 wt.%, while for PP-B with more uniform stereo-defect distribution, addition of 0.01 wt.% TMB-5 can induce the formation of β-phase. Moreover, the analysis of POM indicated that the crystalline morphologies of both PP-A and PP-B change greatly with the TMB-5 concentration, and the variation features of PP-A and PP-B are quite different from each other. PP-B with more uniform stereo-defect distribution was more favorable for the formation of large amount of β-phase in the presence of wide concentration range of TMB-5. The different polymorphic behaviors and their different dependences on the β-NA concentration were related to the different uniformities of stereo-defect distribution of the samples, since the distribution of stereo-defects could restrain the regular insertion of molecular chains during crystallization and thus determine the tendency the α-phase crystallization of the sample.