Competition of shearing and cavitation effects on the deformation behavior of isotactic polypropylene during stretching

Owing to the unique cross-hatched structure of α crystal, the deformation behavior of isotactic polypropylene (iPP) differs considerably with the traditional polymers. The influence of crystalline structures on the cavitation and shearing effects of α-iPP under tensile loading is still not well established. In this work, various iPP samples were prepared via crystallizing from 0 °C to 130 °C, after which the crystalline structures were characterized via differential scanning calorimetry, polarized optical microscopy and scanning electron microscopy. The lamellar thickness and crystallinity of α-iPP precursor films change slightly, while the content of tangential lamellae in α-spherulite and the spherulite size increase significantly with the decreasing supercooling. On the other hand, the morphological evolution of α-iPP during stretching was tracked by in-situ two-dimensional small angle X-ray scattering. It is found that cavitation effect become dominant instead of shearing effect when the size of α-spherulite exceeds 25 μm. Furthermore, we disclose four typical deformation behaviors of α-iPP resulting from the competition between intra-spherulitic deformation and inter-spherulitic deformation, namely the shearing without cavitation, shearing then localized cavitation, homogeneous cavitation with concomitant shearing, and cavitation with absence of shearing. Accordingly, we construct a route for the structural evolution of α-iPP with different crystalline structures during stretching, which provides an effective guidance to produce diverse iPP films with desired structures and tunable functionalities.