The study of room-temperature stretching of annealed polypropylene cast film with row-nucleated crystalline structure

Abstract

The room-temperature stretching process of polypropylene annealed film with row-nucleated crystalline structure was studied by in-situ small-angle X-ray scattering (SAXS) setup and off-line wide-angle X-ray scattering (WAXS), temperature-modulated differential scanning calorimetry (TMDSC) and stress–strain curves testing. The formation process of initial connecting bridges and pores was clarified. For the annealed film, except for the initial lamellae structure, the recrystallized part formed by the melting and crystallization of imperfect crystals during annealing, tie chains connecting the lamellae structure among the amorphous region, secondary crystals from the crystallization of tie chains during annealing and daughter crystals from the special cross-hatched crystalline structure of PP coexist. It was found that 10% stretching lead to the pronounced increase of long period and the appearance of a few initial connecting bridges. The stretching of daughter crystal and recrystallized part contributed to the formation of initial bridges. At stretching ratio of 30%, uniform distributed connecting bridges were observed and the stretched film showed maximum structure periodicity. At this stretching ratio, except for the stretching of daughter crystal, the stretching of tie chains and secondary crystals within the amorphous region lead to the formation of more connecting bridges. At higher stretching ratios into the strain-hardening region and beyond the second yield point, except for the stretching of the above mentioned crystalline and amorphous structure, the stretching of mother lamellae occurred and lamellae deformation was inevitable, resulting in lower structure periodicity. During room-temperature stretching, the pores were formed progressively and the stretching ratio of 30%, situated at the transition point from plastic plateau to strain-hardening region in the stress–strain curves, was appropriate for the initiation of pores.