Shear induced crystallization of bimodal and unimodal high density polyethylene

Abstract

In-situ step-shear experiments by synchrotron X-ray techniques were performed to study shear-induced crystallization of bimodal (BM1) and unimodal (UM1) high-density polyethylene (HDPE). The BM1 sample consisted of a bimodal molecular weight (MW) and composition distribution with the low MW components having less short chain branches (SCBs), or hexene co-monomer, and the high MW components having more SCBs. The average molecular weight of BM1 was higher than that of UM1, where both BM1 and UM1 covered the same molecular weight range. The UM1 sample contained a unimodal but broad molecular weight distribution (MWD) with a constant SCB value between those of the low and high MW components in BM1. Discrete primary and secondary lamellae were found in BM1 and UM1, whereas UM1 exhibited faster secondary lamellar growth. Shear-induced crystallization resulted in higher lamellar orientation in UM1, but higher crystallinity in BM1. The large fraction of medium high MW components in UM1 was found to be effective in crystallization under deformation, leading to stronger shear-induced crystalline orientation.