Probing the Origin of Shish-Kebab Formation in Model Polyethylene Blend under Shear by In-situ Rheo-SAXS and Rheo-WAXD

The formation of shish-kebab structure in polyethylene melt under intense shear flow was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) techniques. Model polyethylene blend was prepared by solution blending of low molecular weight polyethylene (Mw = 50,000, MWD = 2) matrix with 2 wt% of ultra high molecular weight polyethylene (Mw = 5 - 6 million). In this model blend, the low molecular weight PE matrix, which was non-crystallizing at the experimental condition (120^oC), behaved as a ¡°solvent¡+/-. A clear shish-kebab structure was observed in this blend after shear (shear rate = 60 1/s, shear duration = 5s). The total crystallinity of the blend after was around 2 %, indicating that the shish-kebab structure mainly came from the UHMWPE component. SAXS results indicated that shish formed first and subsequently kebab structure developed, indicating the existence of two populations of chain segments (stretched and coiled) in UHMWPE induced by shear. We argue that the stretched segments form the entity of shish while the coiled segments crystallize into kebabs. The evolution of shish length and the kebab spacing calculated from SAXS results will also be discussed.