Effects of Trace Water on Self-Assembly of Sulfonated Block Copolymers During Solution Processing

Solution-cast films of sulfonated block copolymers are investigated for applications in water purification, gas separations, and fuel cells. It is well known that solvent composition influences the structures that form in solutions of sulfonated block copolymers and in the corresponding films. The solutions often contain trace water due to the hygroscopic nature of sulfonated polymers, and the effect of trace water on structure formation has not been previously explored. Water will drive dissociation of sulfonic acid, which in turn might accelerate the rearrangement of polymer chains by disrupting hydrogen bonds between acid groups. In this work, we examine the effects of trace water on the solution-state structure of a sulfonated pentablock copolymer (SPC) and the ultimate film morphology using small-angle X-ray scattering. The solvent used was a mixture of toluene and n-propanol with varying molar ratios of water to sulfonic acid (lambda). In solutions with lambda = 0, the SPC adopts a disordered structure. The addition of trace water (lambda = 3.2) to SPC solutions drives a rapid self-assembly into an ordered lamellar structure. However, the presence of trace water in the solution has little effect on processing the SPC into films as water will rapidly escape from the drying film in a dry environment. For film processing, ambient humidity is more important than trace water in the solution as moisture in air provides a constant supply of water to the film as the solvent evaporates. While there are many variables that control the morphology of solution-cast SPC films, these studies demonstrate that trace amounts of water play a critical role in kinetics and thermodynamics of solution-state self-assembly and the evolution of nanoscale structures during film drying.