Enhancing Interfacial Strength in Self‐Reinforced Polypropylene Composites: Annealing‐Induced Structural Reorganization of Polypropylene Film With Fibrous Crystal

Self‐reinforced polypropylene (PP) composites were fabricated using both annealed and unannealed PP films containing fibrous crystals, and their tensile and interfacial strengths were evaluated. The effects of annealing on the microstructure and mechanical properties of the PP films were investigated using wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), dynamic mechanical analysis (DMA), and other analytical techniques. The results indicate that annealing the PP films enhances the interfacial strength of the self‐reinforced PP composites and broadens the processing window. When annealed at higher temperatures, the less‐perfect fibrous crystals in the PP films can melt and subsequently recrystallize into crystals along stretching direction with a lower melting point, explaining the appearance of two melting peaks in the differential scanning calorimetry (DSC) curves of PP films annealed at elevated temperatures (e.g., 150°C). After hot pressing, the presence of recrystallized crystals with lower melting temperature contributes to a greater proportion of the matrix phase, which is the primary reason for the improved interfacial performance and the widened processing window observed in the self‐reinforced PP composites; while the presence of high‐melting‐point fibrous crystals, the increased processing stability of annealed PP films, and stronger interfacial bonding together provide tensile strength of self‐reinforced PP composites. Furthermore, a physical model describing the microstructural evolution of PP films containing fibrous crystals during annealing was established.