Pore formation mechanism of oriented β polypropylene cast films during stretching and optimization of stretching methods: In-situ SAXS and WAXD studies

Abstract

In this article, the relationship between the initial microvoids or cavities variations and lamellar structures evolutions of oriented β-PP cast films along different tensile directions was mainly investigated via In-situ small-angle X-ray (SAXS) and wide-angle X-ray diffraction (WAXD) measurements. The pore formation mechanism, initial cavities were directly originated from the lamellar separation and its specific positions were at the weak boundary between β lamellae and adjacent amorphous phase, was proposed. Although the pore formation mechanism of both different stretching directions was consistent, the separation process of β lamellae was greatly different during stretching. Lamellae on β-transcrystals (β-TCs) were preferred to be separated along machine direction, while twisted and inter-crosslinked β lamellae were more likely to separate along transverse direction. Both stretching directions mainly affected the stage of microvoids growth that could be promoted along transverse direction. Moreover, for the sample of stretching along transverse direction, microporous membranes with higher porosity and more homogeneous micropores distribution could be obtained through subsequent biaxial stretching. Therefore, the suitable stretching methods were optimized for β-TCs to prepare PP microporous membranes with excellent performance.