Strain Rate Dependence of Amorphous Phase Instability in Semicrystalline Polymers: Insights from the Scale of Lamellar Stacks

Given the structural hierarchy in semicrystalline polymers, there is a compelling need to elucidate the mechanisms behind the instability of the interlamellar amorphous phase at the scale of lamellar stacks, which constitute fundamental building units with a biphasic nature. We specifically chose a hard-elastic isotactic polypropylene film composed of highly oriented lamellar stacks as a model sample. By utilizing synchrotron-based in situ wide-, small-, and ultrasmall-angle X-ray scattering techniques (WAXS/SAXS/USAXS), along with postmortem scanning electron microscopy (SEM) analysis, we studied the structural instabilities of lamellar stacks across a wide range of strain rates (from 0.001 to 0.5 s–1). Owing to the inherently dynamical asymmetry of the amorphous phase, we propose an insight into its instability characterized by stress-induced microphase separation based on the stress–concentration coupling model, where the extreme outcome aligns with the classical viewpoint, the formation of a fibrillar bridge/void system. With an increase in the Weissenberg number, a greater number of stress transmitters within the amorphous phase tend to be retained, thereby impeding the advancement of stress-induced microphase separation but promoting the crystalline phase instability. Furthermore, during the transition from a slow to a rapid stretching process, the amorphous phase instability undergoes a shift from a growth-dominated to a nucleation-dominated mode. This kinetic transition results in a more uniform dispersion of lamellar clusters that encompass unstable amorphous layers.