Design and Characterization of an Architecturally Perfect Family of Linear Low-Density Polyethylenes (LLDPEs) by Multi-Detector Size Exclusion Chromatography
Sara V. Orski, Luke Kassekert, Wesley Farrell, Grace A. Kenlaw, Marc Hillmyer, Kathryn L. Beers
Separations of commercial polyethylenes, which often involve mixtures and copolymers of linear, short-chain, and long-chain branched chains, can be very challenging to optimize as species with similar hydrodynamic sizes or solubility often co-elute in various chromatographic methods. To better understand the effects of polymer structure on the dilute solution properties of polyolefins, a family of model linear low-density polyethylenes (LLDPEs) were synthesized by ring-opening metathesis polymerization (ROMP) of sterically hindered, alkyl substituted cyclooctenes, followed by hydrogenation. Within this series, the alkyl branch frequency was fixed, while systematically varying the branch length. These model materials were analyzed by ambient- and high-temperature size exclusion chromatography to determine their molar mass, intrinsic viscosity ([η]), and degree of short-chain branching across their respective molar mass distributions. Short-chain branching is fixed across the molar mass distribution, based on the synthetic strategy used, and measured values agree with theoretical values for longer alkyl branches, as evident by HT-SEC. Deviation from theoretical values is observed for ethyl branched LLDPEs when calibrated using either 1-octene or polyethylene/polypropylene standards. A systematic decrease of intrinsic viscosity is observed with increasing branch length across the entire molar mass distribution. This work demonstrates the applicability of these model materials to deconvolute structure-property relationships via chromatographic separation techniques and is a step towards determining if sequence control can minimize local polydispersity and generate improved standards for determining branching content in commercial polyolefins.
, Kassekert, L.
, Farrell, W.
, Kenlaw, G.
, Hillmyer, M.
and Beers, K.
Design and Characterization of an Architecturally ‘Perfect’ Family of Linear Low-Density Polyethylenes (LLDPEs) by Multi-Detector Size Exclusion Chromatography, Macromolecules, [online], https://doi.org/10.1021/acs.macromol.9b02623
(Accessed October 19, 2021)