Molecular Weight Distribution Shape Dependence of the Crystallization Kinetics of Semicrystalline Polymers Based on Linear Unimodal and Bimodal Polyethylenes

The manipulation of the crystallization kinetics and crystallinity of polymers without altering their chemical composition, chain structure, or average molecular weight is challenging, while little attention is paid to the role of molecular weight distribution (MWD) shape. In this work, a series of well-defined linear unimodal and bimodal polyethylenes (PEs) with molecular weights ranging from 300 k to 1200 k g/mol were synthesized to serve as a model system to study the impact of bimodal MWD shape on the crystallization kinetics of semicrystalline polymers in comparison with unimodal MWD shape at the same weight-average molecular weight (Mw). It is shown that PEs with a bimodal shape exhibit a faster nucleation rate and crystallization rate with a smaller lamellar width at low isothermal temperatures. A higher crystallization enthalpy of bimodal PEs is shown in non-isothermal experiments. The mechanism behind this is elucidated, which suggests that MWD shapes mainly affect the small-scale nucleation process without altering the large-scale growth process. This first systematic comparative study on the crystallization kinetics of linear unimodal and bimodal PEs gives insight into tailoring the crystallization behavior of semicrystalline polymers from an MWD shape perspective.