Swelling of individual nanodomains in hydrated block copolymer electrolyte membranes

In this work, we examine the swelling of nanostructured block copolymer electrolytes immersed in liquid water. A series of sulfonated polystyrene-b-polyethylene-b-polystyrene (S-SES) membranes having the same nominal chemical composition but two different morphologies are prepared by systematic changes in processing. We start with a membrane comprising a mixture of homopolymer polystyrene (hPS) and a polystyrene-b-polyethylene-b-polystyrene (SES) copolymer. hPS is subsequently selectively removed from the membrane and the polystyrene domains are sulfonated to give S-SES membranes. The morphology of the membranes is controlled by controlling ϕ_v, the volume fraction of hPS in the blended membrane. The morphology of the membranes was studied by small angle X-ray scattering (SAXS), cryogenic scanning transmission electron microscopy (cryo-STEM), and cryogenic electron tomography. The overall domain swelling measured by SAXS decreases slightly at ϕ_v = 0.29; a crossover from lamellar to bicontinuous morphology is obtained at the same value of ϕ_v. The bicontinuous morphologies absorb more water than the lamellar morphologies. By contrast, the nanodomain swelling of the bicontinuous membrane (120%) is slightly less than that of the lamellar membrane (150%). Quantitative analysis of the STEM images and electron tomography was used to determine the swelling on the hydrophilic and hydrophobic domains due to exposure to water. The hydrophilic sulfonated polystyrene-rich domain spacing increases while the hydrophobic polyethylene domain spacing decreases when the membranes are hydrated. The extent of increase and decrease is not a strong function of ϕ_v.