Multi-block versus random quaternary ammonium Poly(arylene ether sulfone) membrane dependent water and alcohol mass uptake, directionally-dependent swelling and ion conductivity, and alcohol transport

Abstract

Quaternary ammonium poly(arylene ether sulfone) copolymer random and multi-block ionomers were synthesized using multiple condensation reactions. Hydrophilic and hydrophobic block-length relationships were investigated with respect to directionally-dependent water swelling, hydroxyl conductivity, percolation-dependent properties, and alcohol transport. 1H nuclear magnetic resonance was used to evaluate brominated multi-block poly(arylene ether sulfone) composition, degree of functionalization, and ion-exchange capacity (IEC). These highly aminated ionomers did not display any counterion condensation associated with their conductivity as predicted by Oosawa-Manning Counterion Condensation Theory. Water-swollen quaternary ammonium poly(arylene ether sulfone) copolymer random and multi-block ionomers; and Nafion 117 membrane conductivity was modeled using Percolation Theory. In general, multi-block ionomer in-plane hydroxide conductivity was greater than its randomly functionalized counterpart at a similar IEC. Multi-block ionomer films with the largest hydrophilic block length (24k) exhibited a hydroxide conductivity of 62.1 mS/cm. An equivalent random copolymer's conductivity was 1.25 times lower. Multi-block and random copolymer's in-plane conductivity was 23.5% lower than its through-plane conductivity at an IEC greater than 2.02 meq/g. This decrease was strongly coupled with its swelling ratio and water uptake (WU). Increased swelling and water dilutes the available charge carriers responsible for through-plane conductivity. A percolation threshold at an IEC of 1.75 was observed for the random copolymer. Its maximum WU was 223 wt%. The multi-block ionomer's WU ranged between 172 wt% and 78.6 wt% at a similar IEC. Swelling suppression at high IEC was attributed to the sequential hydrophilic-hydrophobic block architecture. Multi-block ionomers with smaller hydrophilic block lengths appeared to inhibit alcohol transport more effectively than larger ones at the same IEC. Increasing block length led to greater water and alcohol uptake that was related to ion-domain size, alcohol polarity, and ionomer solubility parameter. Smaller hydrophilic block size was more effective at decreasing alcohol permeation at a similar ion-group concentration. This was attributed to reduced ion-domain cluster size not achievable with larger hydrophilic multiblock ionomers. These results suggest that block length has an important role in controlling ionomer swelling, and ion and alcohol transport, which is critical to fuel cells, and solid-state electrochemical devices.