SEC-MALS method for the determination of long-chain branching and long-chain branching distribution in polyethylene

Abstract

This paper systematically describes a LCB determination method that can quantify both LCB content and LCB distribution across the molecular weight distribution in polyethylene homopolymers as well as copolymers. Coupling size-exclusion chromatography with multi-angle light scattering (SEC-MALS), this method quantifies molecular weights (MW) and radii of gyration ( R g) simultaneously. The number of LCB per molecule and LCB frequency as a function of MW can be calculated by comparing R g of a branched polymer with that of a linear control at the same MW using the Zimm–Stockmayer approach. Because the presence of short-chain branching in copolymers results in changes in R g of the copolymers, their LCB contents cannot be obtained before the short-chain branching (SCB) effect is corrected. Using well-characterized linear PE copolymers as standards, an empirical method is successfully established in this paper to correct the SCB effect. Consequently, this method can be applied to determine LCB in PE copolymers as well. Some practical aspects, such as the selection of formalism for data processing, the LCB detection sensitivity and precision, and long-term reproducibility of this method are also discussed. Finally, examples are given to demonstrate how this method is applied to determine LCB and LCB distribution in practical PE homopolymers and copolymers.