Effects of trace water on the self-assembly kinetics and nanoscale structure of sulfonated block copolymers during solution processing*

Solution-cast films of sulfonated block copolymers (SBC) are investigated for applications in water purification, gas separations, and fuel cells. The influence of organic solvent composition on the self-assembled morphology is well known. Trace amounts of water might also play a role in the self-assembly process, as water disrupts the hydrogen bond interactions between the sulfonic acid groups, but this point has not been examined in prior studies. In this work, we examine (1) the structure of an SBC in an organic solvent mixture with varying molar ratios of water to sulfonic acid (λ); and (2) the morphology of the corresponding solvent-cast films. In solutions with λ = 0, the SBC adopts a disordered structure. The addition of trace water (λ = 3.2) to SBC solutions drives a rapid self-assembly into an ordered lamellar structure. When processed into films, trace water in the solvent has little effect on the SBC morphology, but the ambient humidity is a critical process parameter: under dry conditions, the films have a poorly developed structure, while humidity facilitates the assembly of well-defined lamellae. These outcomes demonstrate that trace water can strongly impact the self-assembly kinetics in block copolymer electrolytes.