Tailoring surface and bulk morphologies to control transport properties of sulfonated block copolymer films

Abstract

We examine the effects of solvent composition and solution-casting protocol on the structure of a lamellar sulfonated pentablock copolymer (SPC) membrane. Films are prepared by a layering process, wherein the film is built up through sequential bar-coating steps. Process variables include bar gap height, which controls drying time and layer thickness, and solvent selectivity toward each block in the SPC, which controls solution-state structure and biases the domain orientations at the surface. We find that irrespective of solvent selectivity, one-layer films have poorly ordered structures in the bulk with minimal restructuring near the surface. However, when the process involves sequential deposition of two or more layers, then the bulk structure of dry films can be well-predicted by the solution-state structure, and domain orientations at the surface are templated by polymer-solvent interactions. Consequently, processing conditions strongly influence through-plane transport properties of the SPC films. When the casting process generates disordered lamellae in bulk with a perpendicular lamellar orientation at the surface, both water and protons can move across the film. However, when the process generates highly ordered lamellae in bulk and parallel lamellar orientations at the surface, transport of both water and protons is inhibited by limited pathways. Significantly, the methodology outlined in this work can be extended to other ionic block copolymer platforms, allowing researchers to tailor bulk and surface morphologies for optimization of critical transport properties.