form. At even higher KOH concentration, the electrolyte uptake of m-PBI starts to decrease. The observed electrolyte uptake disparity between STz25 and STz30 and m-PBI can be attributed to the difference in acidity between the two azoles. While the pKa of the benzimidazole units of m-PBI is around 12.8, tetrazoles are considerably more acidic with a pKa value close to that of acetic acid. The relatively strong acidity implies that the ionic potassium tetrazolide form of both STz25 and STz30 predominates under all conditions considered in this work as the pH of the solutions are far higher than the pKa of the tetrazole pendants, as also supported by the DFT results show in Figure . The high degrees of ionization make the polymers highly polar in alkaline environment, which promotes extensive electrolyte uptake. As the KOH concentration of the surrounding solution increased, the electrolyte uptake decreased. This seems consistent with the observed electrolyte uptake behavior of sulfonated ionomer membranes, where increasing ionic strength of the surrounding solution resulted in a decreased volume fraction of electrolyte within the membrane matrix. A similar trend has also been observed for quaternary ammonium functionalized m-PBI. The linear swelling followed the same trend as the electrolyte uptake behavior, i.e., decreasing swelling with increasing KOH concentration and decreasing electrolyte uptake.