The influence of reserved shish crystals on the structural evolution of ultra-high molecular weight polyethylene films with molecular weights in the tens of millions during the thermal stretching process

Abstract

The molecular weight of polymers plays a critical role in determining the degree of chain entanglement, which in turn significantly constrains the structural evolution during thermal stretching. Therefore, understanding the influence of reserved shish crystals on the structural evolution of ultra-high molecular weight polyethylene (UHMWPE) films, particularly those with molecular weights in the range of tens of millions, is of paramount importance. In this study, UHMWPE films with molecular weights in the tens of millions were prepared by precisely controlling the molding temperature, allowing for the reserve of varying amounts of shish crystals. The structural evolution of these films during thermal stretching was systematically investigated using a combination of in-situ wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques, complemented by ex-situ differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) analyses. The results demonstrated that in this ultra-high molecular weight system, the reserved shish crystals exhibit greater lengths, leading to a significant reduction in the degree of chain entanglement. As the content of reserved shish crystals increased, the signal corresponding to tilted lamellae gradually diminished. Notably, in UHMWPE films with a high content of reserved shish crystals, no tilted lamellae signal was detected, and a higher proportion of shish-kebab crystals was generated by the end of the stretching process. These findings illustrate that the reserve of a greater number of shish crystals facilitates favorable structural evolution during the thermal stretching of UHMWPE films with extremely high molecular weights.