Combined effects of matrix molecular weight and crystallinity on the impact toughness of PP/EPR blends: The role of chain entanglement

Abstract

In recent years, rubber toughened polypropylene (PP) blends have been extensively investigated with special regard to the importance of PP matrix properties (e.g., crystal morphology and structure) in tailoring the toughening efficiency. Unfortunately, although high melt flowability is indispensable to the processing of polymers into complex thin-wall products, it remains a huge challenge to achieve a marvelous balance between stiffness and toughness in the blends with relatively low matrix molecular weight. Herein, taking PP/EPR (ethylene-propylene copolymer) blends as an example, the combined effects of matrix molecular weight and crystallinity on the mechanical properties of PP/EPR blends have been examined in both experiment and theory. The experimental results show that decreasing molecular weight leads to significant deterioration in notched impact toughness, tensile strength and Young's modulus. Impressively, the toughness loss caused by the decrease of molecular weight can be well-compensated by a slight reduction in the matrix crystallinity, without evidently sacrificing the strength and modulus, demonstrating an improved stiffness-toughness balance. Theoretical analysis indicates that the impact toughness of PP/EPR blends is heavily dependent on the chain entanglement density (ve) of PP matrix, and a linear relationship between critical interparticle distance (IDc) and the ve1/3 has been verified from both experiment and theory. Moreover, it is interesting to find that decreasing matrix crystallinity can increase the ve of low-molecular-weight PP matrix, which enables the effective toughening at lower concentration of EPR and thus gives rise to less loss in the strength and modulus. We believe this work not only gives a new insight into the role of chain entanglement in the toughening but also provides a promising guidance for the design of high-performance PP with high melt flowability.