Simultaneous enhancement in processability and mechanical properties of polyethylenes via tuning the molecular weight distribution from unimodal to bimodal shape

Abstract

Semicrystalline polymers with bimodal molecular weight distribution (MWD) have captured broad interest in academia and industry. Current efforts to understand the effect of bimodal MWD shape compared with unimodal shape are mixed with factors such as molecular weight, chain branching or phase separation. The collective effect is difficult to decouple in order to elucidate the independent contribution of MWD shape on the properties of polymers. In this work, a chain transfer polymerization method has been utilized to prepare well-defined linear unimodal and bimodal polyethylenes (PEs), to serve as a model system for the study of the impact of the MWD shape on the crystallization, rheology and mechanical properties of semicrystalline polymers. It has been demonstrated that PEs with bimodal shape display simultaneously enhanced crystallinity, processability, Young's modulus and tensile strength while ductility is unaffected. Compared to unimodal PEs with comparable molecular weights, bimodal PEs show about 40% lower processing viscosity, while exhibiting up to 30% greater tensile strength. This represents the first systematic comparative investigation of MWD shape-property relationship at the same molecular weight over a wide molecular weight range. This work indicates that semicrystalline polymers with bimodal MWD shape have the merit to overcome the trade-off between processing and performance without altering average molecular weight, chain structure or chemical composition.