Effect of chain branching on the rheological properties of HDPE/LLDPE and HDPE/LDPE blends under shear and elongational flows and evaluation of die swell and flow instabilities

Abstract

Depending on the conditions of polymer synthesis, polyethylene can exhibit various molecular structures, such as long and short-chain branching, which influence its molecular weight and molecular weight distribution. The molecular structure of polyethylene in the blend significantly impacts its properties, making it essential to study these effects on the blend’s final properties. Based on the above, this work aims to investigate the effects of 1-hexene short chain branching (SCB) of linear low-density polyethylene (LLDPE) and long chain branching (LCB) of low-density polyethylene (LDPE) polymers on the rheological properties and processing of blends with high-density polyethylene (HDPE). LDPE showed higher complex viscosity at low frequencies and stronger shear thinning behavior than HDPE and LLDPE-H polymers. The strong shear thinning behavior of LDPE may be related to its higher molecular weight and presence of LCB. Capillary rheometry tests demonstrated a shear-thinning behavior for both the pure polymers and the blends, with the results for the blends being lower than those for the pure polymers, following the direct mixing rule. LDPE exhibited lower viscosity compared to HDPE, LLDPE-H, and all the blends, corroborating the observed results in complex viscosity at high deformation values. As for the flow instabilities, the occurrence of these instabilities at high shear rates was observed for all the polymers investigated in this study. Two methods were used to measure the swell of the extrudate (in-line and off-line), and both methods were efficient in describing the swell behavior as a function of shear rate. The elongational viscosity of the LDPE was found to exhibit “strain hardening” behavior, i.e., an increase in elongational viscosity with increasing strain and time.