Deformation and pore formation mechanism of β nucleated polypropylene with different supermolecular structures

In this article, four β nucleated polypropylene (β-iPP) precursor films were prepared through adding four conventional β nucleating agents (β-NA) and crystallizing at 126 °C. The differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM) results show that four cast films had similar crystallinity, high contents of β-crystal but different supermolecular structures. Then a sequential biaxial stretch method was performed to fabricate porous membranes. Based on the detailed characterization of morphological evolutions during stretching by 2D-WAXD and SEM measurements, two models were presented to explain the influence of supermolecular structures on the deformation and pore formation mechanism of β-iPP. The "bundle-like" β-crystal and β-hedrites whose β-lamellae distributed randomly in the cast films would lead to the wide pore size distribution in the membranes. On the other contrary, the "flower-like" β-crystals and β-spherulites which were completely three dimensional symmetrical would lead to homogeneous deformation during stretching and result in the narrow pore size distribution in the membranes.