Effect of the Distribution of Short-Chain Branches on Crystallization Kinetics and Mechanical Properties of High-Density Polyethylene

The effects of the placement of short-chain branches (SCB) on crystallization kinetics, morphology and mechanical properties of high-density polyethylene (HDPE) are examined using bimodal blends of short (S ∼40 kg/mol) and long (L ∼400 kg/mol and 550 kg/mol) polyethylenes with SCB (1-hexene comonomer) incorporated in either the high or the low molecular weight component. A pair of blends that has nearly matched molecular weight distribution and average SCB content shows that placement of branches preferentially on the longer molecules results in slower crystallization kinetics at high crystallization temperatures relative to the material with branches on the short chains. Blends with SCB on the high molecular weight components have superior ultimate mechanical properties, and their resistance to slow crack growth is tremendously enhanced. These improved mechanical properties are attributed to an increase in the amount of tie chains that form when branches are placed on the long chains. A conceptual model based on the interplay of inherent crystallization kinetics of each species and their competition at the growth front in binary blends qualitatively explains the effects of SCB distribution on crystallization kinetics, lamellar thickness and inferred formation of tie chains.