Thermal Stability of Shear-Induced Shish-Kebab Precursor Structure from High Molecular Weight Polyethylene Chains

Abstract

In-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques were used to investigate repeated melting and re-formation of the flow-induced shish-kebab precursor structure in a once-sheared polyethylene (PE) bimodal blend at the confined quiescent state. The blend consisted of a noncrystallizing low molecular weight PE matrix (LMWPE, M̄ w = 53 000 g/mol, polydispersity = 2.2) and a small amount (2 wt %) of crystallizing high molecular weight PE (HMWPE, M̄ w = 1 500 000 g/mol, polydispersity = 1.1) under the chosen experimental temperature. After a step shear (shear rate = 125 s -1, shear duration = 20 s, temperature = 126.5 °C), combined SAXS and WAXD results confirmed that the shish-kebab structure was developed mainly from HMWPE chains, following a diffusion-controlled-like process. Although shish formed first followed by microkebabs and then macrokebabs, shish and microkebabs were melted simultaneously as an integrated entity after the macrokebab melting. Upon cooling, the shish-kebab structure could re-form rather quickly from unrelaxed stretched chain segments, but the corresponding fraction decreased with the increase in temperature. Results indicated that the shish-kebab re-formation is directly related to the relaxation behavior of stretched chain segments confined in a topologically deformed entanglement network. Under the chosen experimental conditions, the deformed HMWPE entanglement network could withstand temperature until 154 °C for 3 min before totally relaxed into the isotropic state.