Schematic diagram of pore formation mechanism of β-PP during stretching at 25 °C and 90 °C.

In this article, five b nucleated polypropylene (b-iPP) precursor films were prepared through crystallization within the temperature window of 110-135 C, and microporous membranes were fabricated by a successive biaxial stretching. Differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM) showed that all the five b-iPP precursors, which were composed of parallel-stacked lamellae without fully developed spherulite, had similar crystallinity and extremely high contents of b-crystals. Thus, the influence of the lamellar structure on the stress-strain behavior of b-iPP precursors and the performance of such films upon stretching can be investigated. We first discovered a peculiar double yield phenomenon in b-iPP under tensile loading. Based on the detailed characterization of the morphological evolution during stretching, we found that a narrower b-lamellar thickness distribution would result in the delayed occurrence of a second yield point under tensile loading and a more uniform pore size distribution in the microporous membranes prepared from b-iPP.

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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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Two isotactic polypropylene (iPP) samples (PP-A and PP-B) were obtained by utilizing two different heterogeneous Ziegler-Natta catalysts in a given polymerization system. The molecular structure and conformational behavior of the samples were studied. The results of determination of xylene soluble material (XS) and 13 C NMR showed that the average isotacticity of the samples were nearly same. However, the results of high-resolution high-temperature 13 C NMR (HRHT 13 C NMR) and successive self-nucleation and annealing (SSA) fractionation revealed that the amount of high isotacticity of PP-B was lower than that of PP-A, and the amount of relative medium and low isotacticity of PP-B was higher than PP-A, indicating that the stereodefect distribution of PP-B was more uniform than PP-A. The calculation of average meso sequence length from SSA was found to be in good agreement with that calculated from the results of HRHT 13 C NMR. Moreover, Fourier transformation infrared was utilized to study the influence of stereodefect distribution on the conformational behavior of iPP. The result suggested that the molecular conformation of the PP-B was more disordered than PP-A, the regularity of molecular structure for PP-B was lower than that of PP-A. The related action mechanism and the influences of which on crystallization behavior were discussed.

Polyamide thin film composite (TFC) reverse osmosis (RO) membranes were prepared in this study.

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