Unraveling the biaxial deformation mechanism of isotactic polypropylene film via in-situ synchrotron radiation X-ray scattering

Abstract

The structure evolution of polymers during biaxial stretching presents a complex, critical challenge in both polymer industry processing and polymer physics. Despite its significance, biaxial deformation of polymers at high temperature has been less studied due to experimental difficulties, leaving underlying mechanism poorly understood. Here we employ a self-developed in-situ biaxial stretching device combined with synchrotron radiation small and wide-angle X-ray scattering (SAXS and WAXS) to track the real-time structural evolution of isotactic polypropylene (iPP) films under sequential (SEQ) and simultaneous (SIM) biaxial stretching. Our results reveal distinct deformation pathways. In the first step of SEQ stretching, the initial linear deformation is governed by lamellar separation, interlamellar shear, and lamellar rotation, followed by coarse slip-induced yielding. Subsequent melting-recrystallization equilibration maintains constant crystallinity while forming fibrous crystals. During the second SEQ stage, stretch-induced melting dominates initially, and the chain reorganization drives the formation of new crystals. In contrast, SIM stretching primarily involves crystal melting. Lamellae undergo separation, shear, and slip, followed by progressive melting, first in thinner lamellae, then within lamellar clusters. Notably, recrystallization is suppressed in SIM due to the absence of in-plane orientation, underscoring chain alignment, rather than entropy reduction, as the primary driving force for recrystallization.