Thermal Stability of Shear-Induced Precursors of Shish-Kebab in a Model Polyethylene Blend by in-situ Rheo-SAXS and -WAXD

\textit{In-situ} rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques were used to investigate thermal stability of shear-induced shish-kebab structures in a model polyethylene (PE) blend. The blend consisted of a low molecular weight non-crystallizing PE matrix (LMWPE, M$_{w}$ = 50,000 g/mol, MWD = 2.2) and a small amount of high molecular weight crystallizing PE (HMWPE, M$_{w}$ = 1,500,000 g/mol, MWD = 1.1). After an imposed steady shear condition in a parallel plate shear stage, rate = 125 s$^{-1}$, duration = 20 s, Temperature = 126.5 $^{o}$C, both SAXS and WAXD results showed that a shish-kebab morphology developed in the PE blend. Subsequently, the shish-kebab crystals were subjected to several thermal cycles, consisting of melting and crystallization of the oriented structures. Although shish formed first, followed by kebabs; surprisingly, the shish-kebab structure was seen to melt as an integrated entity. This suggests that the shish-kebabs contained integrated stretched and coiled chains (or chain segments), even for near monodispersed chains. The HMWPE oriented structures were found to withstand a high temperature of 152 $^{0}$C (holding time = 3 min) and acted as nuclei for crystallization of unstretched LMWPE chains at lower temperatures.