Analyzing long-period structural evolution of biaxially stretched ultra-high molecular weight polyethylene films

Ultra-high molecular weight polyethylene (UHMWPE) films are widely used in high-performance applications due to their excellent mechanical properties. However, understanding the structural evolution, particularly the long-period structure under tensile fields, remains a challenge in both practical use and processing. Here, we investigate the long-period structural evolution of biaxially stretched UHMWPE films under tensile fields using time-resolved small-angle X-ray scattering. Our results reveal distinct changes in the long-period structure during the stretching process. Initially, the isotropic crystalline regions of UHMWPE align along the stretching direction, transitioning from a diffuse scattering pattern to an ellipsoidal one. As stretching progresses, fibrillar crystals form, dominating the scattering pattern with sharp, oriented features. In the later stages, fragmentation of the fibrillar structure leads to smaller crystalline regions and a butterfly-shaped scattering pattern due to rearranged lamellar structures. Based on these findings, we propose a new model that suggests a reverse transformation from fibrillar crystals to lamellar crystals, contrasting with the traditional “shish-kebab” model. The reduced crystallinity, as shown by differential scanning calorimetry data, further supports this structural transformation. Understanding the structure and macroscopic property relationship of polymers is important for processing and applications. Here, time-resolved small-angle X-ray scattering shows a unique four-stage evolution pattern of biaxially stretched ultra-high molecular weight polyethylene films under stress.