Comonomer effects on co-permeation of methanol and acetate in cation exchange membranes

Co-permeation in hydrated, dense polymer membranes is crucial to many applications from energy conversion (i. e. photoelectrochemical CO2 reduction cells) to liquid separation (i.e. pervaporation), where such membranes are challenged with complex mixtures of species. For instance, a major challenge to the realization of efficient CO2 reduction cells is the design of ion exchange membranes with sufficient conductivity and minimal permeation of CO2 reduction products (e.g. methanol and acetate), such that understanding permeation and copermeation behavior of these solutes in ion exchange membranes is needed. Previously, the transport behavior of Nafion (R) 117 and crosslinked cation exchange membranes prepared with poly(ethylene glycol) diacrylate (PEGDA, crosslinker) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, sulfonated comonomer) to methanol and sodium acetate was investigated and distinct changes in permeabilities of these membranes to sodium acetate was observed in co-permeation with methanol. To further investigate this copermeation behavior, we modify the PEGDA-AMPS structure by varying the negatively-charged AMPS content with three different comonomers, acrylic acid (AA, n = 0), 2-hydroxylethyl methacrylate (HEMA, n = 1), and poly(ethylene glycol) methacrylate (PEGMA, n = 5), where n represents the number of ethylene oxide repeat units in each comonomer. While the observed permeability to sodium acetate in co-permeation with methanol was increased for membranes with comonomers with short pendant groups (PEGDA-AMPS/AA and PEGDA-AMPS/HEMA), it remained relatively consistent for PEGDA-AMPS/PEGMA membranes. While the underlying causes of this type of behavior remains unresolved, we propose a combination of assisted transport by methanol and disruption of electrostatic interactions by pendant ethylene oxide repeat units based on our experiments. Overall, such differences in transport behavior underscore the need for increased understanding of emergent copermeation behavior in hydrated, dense polymer membranes.