Influence of lamellar structure on double yield behavior and pore size distribution in β nucleated polypropylene stretched membranes

Abstract

In this article, five β nucleated polypropylene (β-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 β-iPP precursors, which were composed of parallel-stacked lamellae without fully developed spherulite, had similar crystallinity and extremely high contents of β-crystals. Thus, the influence of the lamellar structure on the stress–strain behavior of β-iPP precursors and the performance of such films upon stretching can be investigated. We first discovered a peculiar double yield phenomenon in β-iPP under tensile loading. Based on the detailed characterization of the morphological evolution during stretching, we found that a narrower β-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 β-iPP.