What Determines Yield Stress in Semicrystalline Polymers within Lamellar Stack Dimensions: Competition between the Yield Behaviors of Crystalline and Glassy Amorphous Phases

Through meticulous design involving prestretching and various annealing procedures, we prepared poly(ethylene terephthalate) (PET) films featuring a series-coupled lamellar composite model structure. Each film manifests distinct properties of the crystalline and glassy amorphous phases. Using synchrotron-based in situ small-angle X-ray scattering and wide-angle X-ray diffraction techniques, we investigated the yield mechanisms of these oriented PET samples. Our findings highlight a competition between the yield behaviors of the two phases. In PET samples exhibiting low crystallinity, thin and imperfect crystals, and limited chain mobility in the amorphous phase, it becomes evident that the yield of the crystalline phase significantly influences the macroscopic yield stress. For these samples, we observed a positive correlation between the yield stress and the cohesive strength of the crystalline structure (considering crystallinity, crystal thickness, and perfection), consistent with findings commonly reported in publications. However, an intriguing phenomenon emerged when PET samples underwent annealing at elevated temperatures, specifically at 230 °C for durations exceeding 10 min. In these treated samples, the yield of the amorphous phase emerged as the primary contributor to the macroscopic yield stress, exhibiting a surprising negative correlation between the cohesive strength of the crystalline structure and the macroscopic stress, which contradicts conventional wisdom.