Tuning quaternary ammonium ionomer composition and processing to produce tough films

Abstract

Multi-block and random quaternary ammonium poly (arylene ether sulfone) copolymer ionomers were synthesized using sequential reactions. Ionomer film processing involved two separate methods: heterogeneous-conversion and solution-casting a pseudo-solution. Film hydroxyl conductivity, water swelling, and tensile strength were dependent upon the hydrophilic and hydrophobic block-length. 1H nuclear magnetic resonance was used to evaluate brominated multi-block poly (arylene ether sulfone) composition, degree of functionalization (DF), and ion-exchange capacity (IEC). In general, multi-block ionomer hydroxyl conductivity was greater than its randomly functionalized counterpart at a similar IEC. Multi-block ionomer films with largest hydrophilic block length exhibited a hydroxide conductivity of 49.8 mS/cm. However, the equivalent random copolymer's conductivity was 1.02 times lower with a water uptake of 223 wt%, which were 190% higher than its multi-block counterpart. This was attributed to ion-clustering improvements, which is not present in a random ionomer. Equivalent copolymer ionomers had a percolation threshold associated with excessive swelling when its IEC exceeded 2.02 meq/g. In contrast to the random ionomer, the maximum swelling observed for the multi-block copolymers was 33.6% at an IEC of 2.86 meq/g. This swelling suppression at high IEC was attributed to the sequential hydrophilic-hydrophobic block architecture. Moreover, the solution-cast multi-block ionomer was found to possess the highest toughness of 1185 × 104 J/m3, which was 237% greater than its heterogeneous counterpart. These results suggest that block length and ionomer processing play a critical role in controlling swelling, improving mechanical strength, and enhancing ion transport.