Exponential Water Uptake in Ionomer Membranes Results from Polymer Plasticization
A. Barnett, J. J. Karnes, J. Lu, D. R. Major Jr., J. S. Oakdale, K. N. Grew, J. P. McClure, V. Molinero, Macromolecules, 55, 6762–6774 (2022).
Water content is the most influential factor in the performance of ion-exchange membranes (IEMs), controlling their mechanical rigidity, ionic conductivity, and degradation rates. Membranes absorb water exponentially at high water activity (aw), making that region of the sorption isotherm the most influential on membrane properties. Plasticization of the polymer by water has been proposed to cause this exponential uptake at high aw. However, an integrated microscopic picture of the structure, thermodynamic, and mechanical properties of IEMs as a function of aw has remained elusive. Here, we use large-scale molecular simulations validated with experimental measurements to compute the sorption isotherms, Young’s modulus, polymer dynamics, and structure of IEMs. The simulations unveil that the exponential increase in water uptake (WU) coincides in all cases with the glass to rubber transition of the membrane, as measured through its Young’s modulus and segmental polymer dynamics. Functionalization of the polymer with alkyl groups further contributes to its plasticization, increasing the WU at a given aw and ion-exchange capacity (IEC). The alkyl chains act synergistically with water to plasticize the polymer matrix and allow water penetration in the membrane. The simulations reveal that the width of the water channels depends on the ratio λ of water to ions in the polymer but is independent of its IEC. We conclude that differences in the polymer matrix─not the water channels─are responsible for the distinct uptake response of IEMs to the thermodynamic driving force of water activity.
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