Tracking Early Cavitation in iPP: Insights from SEM During Uniaxial Stretching

Semicrystalline polymers exhibit complex structural evolution during uniaxial stretching, including the formation of cavities, which significantly affect their material properties. However, in-depth studies are still lacking in understanding the early stage of cavitation due to limitations in existing techniques. This study presents a detailed investigation of the early-stage cavitation of isotactic polypropylene (iPP) during uniaxial stretching using SEM-based in situ observations, which provides real-time, high-resolution visualization of the morphological changes. The results reveal that cavitation initiates following the linear elastic region due to the fracture of lamellae mainly aligned parallel to the stretching direction. Initial nano-sized cavities form approximately perpendicular to the stretching direction. As strain increases, these cavities evolve into elliptical craze-like structures with fibril bridges connecting adjacent cavities before yielding, and the size distribution of craze-like structures broadens. Complementary small-angle X-ray scattering (SAXS) measurements capture the formation of the cavities and the craze-like structures at larger strains than SEM measurements.