Influence of Hydrogen Bonding Effects on Methanol and Water Diffusivities in Acid-Base Polymer Blend Membranes of Sulfonated Poly(ether ether ketone) and Base Tethered Polysulfone

Atomistic molecular dynamics simulations were used to study the water and methanol diffusivities in acid-base polymer blend membranes consisting of sulfonated poly(ether ether ketone) (SPEEK) and polysulfone tethered with different bases (2-amino-benzimidazole, S-amino-benzotriazole, and 1H-perimidine). Consistent with experimental trends, methanol and water diffusivities in all the SPEEK-based systems were found to be lower than those in Nafion. When the base group attached to the polysulfone was varied, the methanol diffusivities were found to exhibit the same trends as observed in the experimentally measured crossover current densities. Such trends were however observed only when we explicitly accounted for hydrogen bonding interactions between the hydrogen attached to the nitrogen of the base and the oxygen of the sulfonate of SPEEK. Furthermore, in almost all cases, methanol diffusivities were found to be highly correlated with the pore sizes of the membranes, which, in the case of blends, were found to be influenced by the strength of parasitic hydrogen bonding interactions between the sulfone oxygen of polysulfone and H(N-base). The influence of pore sizes on the methanol diffusivity behavior was rationalized by using both the coordination behavior and the residence time distributions of methanol in various regions of pores. Together, our results unravel the physicochemical origins of methanol diffusivities in acid-base blend membranes and highlight the crucial role played by the hydrogen bonding interactions in influencing methanol transport in acid-base polymer blend membranes.