The influence of short chain branch on formation of shear-induced crystals in bimodal polyethylene at low shear temperatures

Abstract

In-situ synchrotron X-ray scattering and diffraction techniques were employed to explore the effect of short chain branch on shear-induced crystallization of bimodal polyethylene, and the shear experiment was conducted at a low shear rate under the temperatures lower than melting point. It has shown that both shish-kebabs and conventional lamellar crystals can form and the crystallinity increases sharply during the shear process. In addition, there are more shish-kebabs but less lamellar crystals generated and the crystallinity reduces markedly with the increase of content of short chain branch. While in the subsequent isothermal crystallization at 121 °C, all samples display a slightly enlarged crystallinity and an approximately unvaried lower crystal orientation. The results illustrate that although the introducing of short chain branch in high molecular weight fraction depresses the crystallizablity of bimodal polyethylene, but it can facilitate the formation of shish-kebabs at low shear temperatures, and the structure formation follows the stretched network mechanism. Combined with our previous papers that the structure formation followed the CST mechanism at the shear temperature higher than melting point, the modulation of shish crystals formation owing to different mechanisms of the CST and the stretched network by changing shear temperature was achieved in the bimodal PE samples.