Modeling the nanophase structural dynamics of phenylated sulfonated poly ether ether ketone ketone (Ph-SPEEKK) membranes as a function of hydration

Solvated phenylated sulfonated poly ether ether ketone ketone (Ph-SPEEKK) membranes in the presence of hydronium ions were modeled by classical molecular dynamics simulations. The characterization of the nanophase structure and dynamics of such membranes was carried out as a function of the water content λ, where λ is the number of water molecules per sulfonate group, for λ values of 3.5, 6, 11, 25, and 40. Analysis of pair correlation functions supports the experimental observation of membrane swelling upon hydration as well the increase in water and hydronium ion diffusion with increasing λ. Whereas the average number of hydrogen bonds between hydronium ions and sulfonate groups is dramatically affected by the hydration level, the average lifetime of the hydrogen bonds remains essentially constant. The membrane is found to be relatively rigid, and its overall flexibility shows little dependence on water content. Compared with Nafion, water and ion diffusion coefficients are considerably smaller at lower hydration levels and room temperature. However, at higher λ values of 25 and 40, these coefficients are comparable to those in Nafion at a λ value of 16. This study also shows that water diffusion in Ph-SPEEKK membranes at low hydration levels can be significantly improved by raising the temperature with important implications for proton conductivity.