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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