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

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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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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.

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The lithium‐ion batteries (LIBs) have been widely used in the world since the first introduction in 1991. The microporous polyolefin separator is the key component to determine the electrical properties and safety of LIBs. In China, the LIBs separators were completely imported and expensive before 2008. We have realized the industrialization of LIBs separators by either dry or wet process successfully. Nowadays, China has become the biggest manufacturing country of LIBs separators in the world and the price is very much reduced. Among the separator processing techniques, the dry process based on biaxial stretching β nucleated polypropylene (β‐iPP) was originated from China and has the lowest manufacturing cost. However, the separator produced by this method has non‐uniform pore size distribution which limits its wider application. In this article, based on the better understanding of original crystal morphology on the pore formation during stretching, we present our recent works to improve the performance of dry process separator through the preparation of β‐spherulites, casting technique optimization, improved annealing treatment and multi‐stages longitudinal stretching. As a result, we produced the dry process separator with much more uniform pore size distribution than that made from the traditional dry process.