A study on the appearance of extended chain fibrils of ultrahigh molecular weight polyethylene at melt/solid interface

摘要

Pressing of ultra high molecular weight polyethylene (UHMWPE) melt on a flat substrate of HOPG, Si/SiO _2, or mica resulted into molecular alignment at the polymer melt/solid interface. From the resultant morphology, as visualized from AFM, it appears that the extended chain crystals are formed at the interface. The chain alignment in the extended chain crystals could be preserved even after annealing well above the equilibrium melting temperature (20 h at 160°C). It is thus concluded that the atomically flat surface provides efficient constraint that refrains chain relaxation of the extended chains to random coil state. It is shown that at the melt/hydrophilic mica interface the extended chain fibrils are formed easier from the polymer obtained from the single-site catalyst compared to the heterogeneous catalyst. The extended chain fibrils at the interface promote the oriented lamella growth. Isothermal crystallization at low supercooling (at 133°C) results in the formation of lamella thicker than 200 nm. The lamellae exhibit a characteristic tapered shape indicating the thickening growth mechanism via chain sliding diffusion.