Hydration, Prediction of the pK_a, and Infrared Spectroscopic Study of Sulfonated Polybenzophenone (SPK) Block-Copolymer Hydrocarbon Membranes and Comparisons with Nafion

We used long-range-corrected density functional theory to investigate the hydration, pKa values, and harmonic vibrational spectroscopy of sulfonated polybenzophenone (SPK) block-copolymer hydrocarbon membranes to ascertain the reasons why this gives comparable or higher proton conductivities against Nafion over a wide range of humidity. It was found that a minimum of three water molecules are required for proton dissociation in both membranes. From natural population analysis, it was noticed that the proton dissociation of SPK membranes is nearly comparable to Nafion at relatively low water content. Next, we explored the applicability of the appropriate treatment for pKa and proton's energy with a benchmark set (AKB) scheme to compute the pKa values for these membranes. These results indicate that the proton dissociative abilities of sulfonic acid groups of the SPK membrane are higher than those of Nafion. This could be one of the reasons for the SPK membrane to show higher proton conductivities at high relative humidity. Furthermore, the effect of hydration on the proton conductivity of membranes illustrates that asymmetric stretching of the SO3- mode was in agreement with Nafion ones but opposite trends were found in the case of symmetric stretching of the SO3- mode upon hydration.