Shear-induced crystallization of unimodal/bimodal polyethylene at high temperatures affected by C4 short-branching

Abstract

Generation of shish-kebab crystals of unimodal and bimodal polyethylenes (PEs) with different C4 short-branching sheared at high overmelt degree (the increment higher than the crystallization temperature) and a low shearing rate was investigated by using in-situ SAXS/WAXD techniques. Both PEs exhibiting SAXS streaks during the shearing indicate the shish crystals generated by shearing. The WAXD analysis indicates an orientation degree as high as 0.75–0.95 for both PEs. During the succeeding isothermal crystallization at 121 °C, kebab crystals and partially oriented stacked crystals in lamellae form, which elevate the crystallinity index but decrease the orientation degree. The shish crystal content of bimodal PE is higher than that of unimodal PE because the high molecular weight (MW) part facilitates the generation of shish crystals. The increase of the branching length is more favorable for the generation of shish crystals (comparing with C2). However, the high content of C4 short-chain branching makes shear-induced crystals less ordered. The generation mechanism of both PEs' shish crystals at high shearing temperatures can be ascribed to the coil-stretch transition (CST) mechanism.