Influence of Prereserved Shish Crystals on the Structural Evolution of Ultrahigh-Molecular Weight Polyethylene Films during the Hot Stretching Process

Prereserved shish crystals and their influence on the macroscopic properties and structural evolution of ultrahigh-molecular weight polyethylene (UHMWPE) films during the hot stretching process were studied by in situ small-angle X-ray scattering (SAXS)/ultrasmall-angle X-ray scattering (USAXS)/wide-angle X-ray diffraction (WAXD) measurements and ex situ differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) measurements. The SEM result that the fibrous crystals in UHMWPE powder induced the formation of shish-kebab crystals in solution confirmed that the fibrous crystals were shish crystals. The initial UHMWPE films containing prereserved shish crystals were obtained by compression molding of the nascent UHMWPE powder at 135 and 138 °C. With hot stretching, a USAXS/SAXS signal representing the prereserved shish crystals and the prereserved shish-induced shish-kebab crystals appeared, which was also confirmed by ex situ SEM. This means that the presence of prereserved shish crystals made the easier epitaxial crystallization of the fragmented ordinary lamellae form shish-kebab crystals, which accelerated the evolution of ordinary lamellar crystals to shish-kebab or shish crystals. Moreover, the prereserved shish crystals induced the formation of more and longer new shish crystals, resulting in a significant increase in the relative intensity and average length of the final shish crystals. The corresponding crystallinity and the degree of orientation also increased during hot stretching. Prereserved shish crystals, which induced the formation of more shish crystals, resulted in a significant improvement in the mechanical properties of UHMWPE films.