Elongation induced β- to α-crystalline transformation and microvoid formation in isotactic polypropylene as revealed by time-resolved WAXS/SAXS

Abstract

Time-resolved synchrotron small- and wide-angle X-ray scattering (SAXS) and (WAXS) was carried out during the high-speed elongation of unoriented exclusively β-crystal-containing isotactic polypropylene, and the results were compared with those for α-crystal polypropylene. SAXS and WAXS indicated that there were three different stages in the deformation. In the first stage, microvoid formation started at the upper yield point, which is associated with lamellae perpendicular to the elongation. In the second stage, the meridional β-lamellar SAXS disappeared; the second stage results in a β–α polymorphous transformation that produces a highly oriented α-crystal parallel to the elongation direction. In this stage, equatorial SAXS remained intact mainly because of microvoid formation. In the third stage, we observed decreasing equatorial SAXS and re-emerged meridional SAXS. After the disappearance of the parallel lamellae, which indicates the depletion of the source for the β–α transformation, the remaining perpendicular lamellae became the transformation source. The perpendicular lamellae underwent inter- and intralamellar slipping and fragmentation and then unfolded to produce the oriented α-crystal. The β-crystal film contained many microvoids, which absorbed the stress. This means that the β- and α-crystals may not have been fully subjected to the applied stress, causing lower orientations for the α-crystals and the remaining β-crystals.