Rheological study on PP/HDPE blends compatibilized by EPDM with droplet morphology using Lee and Park model

Abstract

Small-amplitude oscillatory shear (SAOS) was used to measure the dynamic viscosity and modulus of polypropylene (PP)/ethylene-propylene-diene monomer (EPDM) block copolymer/high-density polyethylene (HDPE) blends at 210, 230, and 250 °C. The scanning electron microscope images show that the two blends with weight ratios of 9.7/2.9/87.4 and 29.1/2.9/68 have a dispersed morphology. Due to the addition of EPDM, the relaxation time of PP/EPDM/HDPE ternary blends is longer than that of PP/HDPE binary blends. The storage modulus data cannot be fitted by the rheological models if we consider the compatibilized ternary blends as binary PP/HDPE blends. This is mainly because the ternary blend, especially the EPDM phase, has higher elasticity than neat PP and HDPE. A simplified morphology is proposed, with EPDM surrounding the droplets serving as the dispersed phase. Lee and Park have examined interactions between dispersions, including flow-induced coalescence and breakup due to interfacial tension. In SAOS, coalescence will not occur, and the droplet’s shape will change periodically. Therefore, shape relaxation cannot be overlooked. The Lee and Park model has only one adjusting parameter, d₁, which represents the degree of total relaxation. The balance between interfacial area Q and its anisotropy \({\text{q}}_{ij}\) is influenced by both the external flow and interfacial tension, which have opposite effects. The Lee and Park model with simplified morphology executed a better fitting than without the simplification using the initial values of Q and its anisotropy \({\text{q}}_{ij}\).