Probing the flow-induced shish-kebab structure in entangled polyethylene melts by synchrotron X-ray scattering

ABSTRACT

In-situ rheo small-angle X-ray scattering (SAXS) and rheo wide-angle X-ray diffraction (WAXD) techniques were used to investigate the flow-induced crystalline structure in entangled melts of ultrahigh-molecular-weight polyethylene (UHMWPE)/low-molecular-weight polyethylene (LMWPE) blends (0, 2 and 5 wt% of UHMWPE). Immediately after a step shear at 415  K, SAXS and WAXD results confirmed that only the shish structure was formed in the melts without kebabs. The topological deformation of entangled UHMWPE chains in the blend was responsible for the formation of shish. The missing kebab growth in the presence of shish indicated that secondary nucleation of coiled chains on the shish surface was frustrated at high temperatures close to the equilibrium melting temperature (418.5  K). When the temperature was quenched to 407  K, both blends (but not pure LMWPE) clearly exhibited oriented kebab growth. An Avrami analysis was applied to investigate the nucleation and growth of kebabs. Results indicated that kebabs were probably grown under athermal nucleation and diffusion-controlled conditions. In addition, the crystallization rate under predetermined nucleation was strongly governed by the concentration of shish.