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Influence of Network Structural Modifications on the Dynamic Relaxation Characteristics of Crosslinked Poly(ethylene oxide) Copolymer Membranes

Douglass S. Kalika1, Jeffrey J. Richards1, Victor A. Kusuma2, and Benny D. Freeman2. (1) Chemical and Materials Engineering, University of Kentucky, 177 Anderson Hall, Lexington, KY 40506-0046, (2) Chemical Engineering, The University of Texas at Austin, 10100 Burnet Rd., Bldg 133, Center for Energy and Environmental Resources, Austin, TX 78758

The structure and dynamic relaxation characteristics of a series of crosslinked poly(ethylene oxide) copolymer networks based on the UV photopolymerization of bisphenol A ethoxylate diacrylate [BPA-EDA] crosslinker have been investigated. Rubbery amorphous networks with high ethylene oxide content have recently been identified as promising membrane materials for the selective removal of carbon dioxide from light gas mixtures. By strategic control of crosslink density and the introduction of selected pendants along the network backbone it is possible to modify both the solubility and free volume characteristics of the membranes and thereby tailor them for specific gas separations.

Polymer networks were synthesized by UV photopolymerization of commercially-available BPA-EDA crosslinker. Crosslink density within the networks was controlled in two ways: (i) variation of crosslinker molecular weight, with the diacrylate crosslinkers encompassing 4, 8 and 30 total ethylene oxide segments, respectively; and (ii) copolymerization of selected monofunctional ethylene oxide acrylates into the network backbone. Copolymerization results in the insertion of fixed-length pendants into the network structure and a corresponding reduction in crosslink density. Variation in the size and end group associated with these pendants provides additional variables by which to tune the chemical composition, architecture and free volume of the resulting membrane networks.

In this study, dynamic mechanical analysis and broadband dielectric spectroscopy have been used to investigate the sub-glass and glass-rubber viscoelastic relaxation characteristics of the networks in order to elucidate the underlying motional origins of each relaxation and their correlation with the structural and morphological features of the materials. These complementary techniques provide detailed information with respect to relaxation intensity and breadth, and the influence of crosslink constraint on the local and segmental motions inherent to the materials. Based on these studies, a series of design rules have been established to guide the formulation of the networks for optimum gas transport performance. The gas transport characteristics of the BPA-EDA networks are compared to other families of crosslinked PEO membranes with similar properties, and the overall performance of these materials are interpreted within the context of the selectivity-permeability “upper bound” concept.