Locating sulfonic acid groups on various side chains to poly(arylene ether sulfone)s: Effects on the ionic clustering and properties of proton-exchange membranes

Poly(arylene ether sulfone)s carrying various aromatic mono-, di-, and trisulfonated side chains have been investigated with respect to their nanoscale structure and key membrane properties. Sulfobenzoyl, sulfonaphthoxybenzoyl, disulfonaphthoxybenzoyl, and trisulfopyrenoxybenzoyl side chains were attached to the poly(arylene ether sulfone) main chain by employing different combinations of metallation and nucleophilic aromatic substitution reactions. The nature of the sulfonated side chains was found to either promote or suppress the formation of ionic clusters, in relation to the ionic clustering occurring in corresponding polymers carrying sulfonic acid groups directly on the main chain. Analysis by small angle X-ray scattering (SAXS) of solvent cast membranes showed that the ionic clustering was promoted by placing the sulfonic acid groups on the relatively long naphthoxybenzoyl or pyrenoxybenzoyl side chains. This resulted in SAXS profiles that indicated larger characteristic separation lengths and narrower ionomer peaks, as compared with corresponding main chain sulfonated polymers. On the other hand, the ionic clustering was almost completely suppressed in membranes based on polymers functionalized with short 2-sulfobenzoyl side chains. Proton conductivity measurements at low or moderate water contents showed a trend of increasing conductivities with the length and the sulfonic acid functionality of the side chain. The structure of the side chain also influenced the thermal stability and glass transition temperature of the membranes.