Role of Stretching-induced Crystallization on Mesoscale Morphology Transition of UHMWPE during Hot Stretching

In this work, a morphology transition mode is revealed in ultra-high molecular weight polyethylene (UHMWPE) when stretching at 120 °C: moving from the slightly deformed region to the necked region, the morphology transfers from small spherulites to a mixture of transcrystalline and enlarged spherulites, and finally to pure transcrystalline; meanwhile, the lamellae making up the transcrystalline or spherulite were fragmented into smaller ones; spatial scan by wide-angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) revealed that the crystallinity is increased from 25.3% to 30.1% and the crystal orientation was enhanced greatly, but the lamellae orientation was quite weak. The rise of enlarged spherulites or a mixture of transcrystalline and spherulites can also be found in UHMWPE stretched at 140 and 148 °C, whereas absent in UHMWPE stretched at 30 °C. In situ WAXS/SAXS measurements suggest that during stretching at 30 °C, the crystallinity is reduced drastically, and a few voids are formed as the size increases from 50 nm to 210 nm; during stretching at 120 °C, the crystallinity is reduced only slightly, and the kinking of lamellae occurs at large Hencky strain; during stretching at 140 and 148 °C, an increase in crystallinity with stretching strain can be found, and the lamellae are also kinked. Taking the microstructure and morphology transition into consideration, a mesoscale morphology transition mode is proposed, in the stretching-induced crystallization the fragmented lamellae can be rearranged into new supra-structures such as spherulite or transcrystalline during hot stretching.