Pore formation mechanism of beta nucleated polypropylene stretched membranes

In this article, two beta nucleated polypropylene (beta-iPP) precursor films were prepared at crystallization temperatures (T-c) of 110 degrees C and 135 degrees C. The differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM) results reveal that the two samples, which were composed of bundle-like lamellae, had similar crystallinity and particularly high contents of the beta-crystal. However, they behaved differently when stretched at 25 degrees C and 90 degrees C. Based on the detailed characterization of morphological evolutions during stretching by 2D-WAXD and SEM measurements, a mechanism of pore formation in stretching beta-iPP is presented. We found that the phase transformation of beta-smectic and beta-alpha are not the primary origin of pore formation in stretching b-iPP, instead, the micropores are directly initiated from the substantially weak interfaces between beta-lamellae. Since the bundle-like beta-lamella without fully developed spherulites is an asymmetric three dimensional structure, the lamellae at different angles to the tensile axis would lead to various deformational modes during stretching. Therefore, it is the polydispersity of b-lamellae in beta-iPP that causes the poor pore size distribution in stretching beta-iPP membrane.