Unique crystal morphology and tensile properties of injection-molded bar of LLDPE by adding HDPE with different molecular weights

Abstract

In this research, a linear low density polyethylene (LLDPE) was first melt blended with a series of high density polyethylenes (HDPE) with different molecular weights at a fixed ratio of LLDPE/HDPE=90/10 (w/w). The prepared HDPE/LLDPE blends were then injection-molded into specimen bars through a dynamic packing injection molding (DPIM) technique, in which an oscillating shear field was imposed on the melt by two pistons that move reversibly with the same frequency during the packing stage. The crystal morphology, orientation and tensile properties were characterized by scanning electron microscopy, two-dimensional wide-angle X-ray scattering and Instron, respectively. Compared with conventional injection molding, DPIM caused an obvious increase in tensile strength in the injection-molded bars. Interestingly, LLDPE blended with low molecular weight HDPE (LMW-PE) was found to possess much higher tensile strength than that blended with high molecular weight HDPE (HMW-PE). Shish–kebab morphology was observed for all blends obtained, regardless of the molecular weight of the HDPE. However, thicker but shorter lamellae were observed for the LLDPE/LMW-PE blend, corresponding to a higher melting temperature; while thinner but longer lamellae were seen for the LLDPE/HMW-PE blend, corresponding to a lower melting temperature. Furthermore, the phase miscibility between HDPE and LLDPE was found to increase with increasing HDPE molecular weight, which would affect the sensitivity of molecular chains for response to external shear. The changed miscibility, together with the changed entanglement density in different HDPE was responsible for the change in tensile strength and unique crystal morphology of LLDPE induced by adding HDPE.