Influences of casting solvents on proton dynamics within sulfonated polyether ether ketones (S-PEEKs) studied using high-resolution solid-state NMR

Proton mobilities of S-PEEK membranes with variable degrees of sulfonation (DS), and of S-PEEK membranes cast from different solvents were studied using high-resolution solid-state 1H MAS NMR. In the hydrated S-PEEKs, a single resonance for water-associated sulfonic acid protons was observed for S-PEEK with DS = 58 and 95%, which indicates that the acid protons in either of these S-PEEK samples have similar environments, suggesting a similar distribution of water at each sulfonic acid site. However, there are multiple resonances for water-associated sulfonic acid protons in S-PEEK with DS = 72 and 86%. This suggests distinct acid proton environments resulting from an uneven distribution of water to sulfonic acid protons in these S-PEEK membranes, explaining the fact that these two fully hydrated S-PEEKs show similar conductivities although they have large differences in water content and DS. For S-PEEK membranes cast from different solvents, stronger polymer chain interactions were observed in S-PEEK from N,N-dimethylformamide (DMF) than in those from N,N-dimethylacetamide (DMAc). This could explain why the former exhibits lower proton conductivity although it has the same water content and DS as the latter. S-PEEKs cast from DMAc and dimethylsulfoxide (DMSO) have large differences in water content, but show very similar proton conductivity at high relative humidity. In all cases, solid-state 1H NMR is shown to be a very sensitive probe of local environments as a function of the casting solvent, degree of sulfonation, and relative humidity, as such spectra provide detailed information about the distribution of acid protons in the membranes.