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

Owing to the unique cross-hatched structure of alpha 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 alpha-iPP under tensile loading is still not well established. In this work, various iPP samples were prepared via crystallizing from 0 degrees C to 130 degrees 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 alpha-iPP precursor films change slightly, while the content of tangential lamellae in alpha-spherulite and the spherulite size increase significantly with the decreasing supercooling. On the other hand, the morphological evolution of alpha-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 alpha-spherulite exceeds 25 mu m. Furthermore, we disclose four typical deformation behaviors of alpha-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 alpha-iPP with different crystalline structures during stretching, which provides an effective guidance to produce diverse iPP films with desired structures and tunable functionalities.