Domain Alignment and Solvent Swelling Impact Ion Transport in a Multiblock Copolymer Ionomer

The addition of polar solvents is known to increase the ionic conductivity of ion-containing polymers toward relevant values. For nanostructured polymers, ionic conductivity can also be improved by aligning the nanostructures to eliminate discontinuities in conduction pathways associated with misorientations of the conducting domains, grain boundaries, and defects. Here, we report improvements in ion conductivity with both solvent swelling and alignment by studying thin films of a precise amphiphilic multiblock copolymer ionomer. Samples were characterized under saturated solvent vapor by grazing incidence X-ray scattering and broadband dielectric spectroscopy using interdigitated electrodes. The layered ionic assemblies remain parallel to the substrate upon swelling the polar sublayers with selective solvents (λ = 2–2.5 solvent molecules per sulfonate). Relative to isotropic and solvent-free bulk samples, the in-plane ionic conductivities increase significantly due to alignment and solvent swelling with propylene carbonate, γ-butyrolactone, dimethyl carbonate, or diglyme. Interestingly, the ionic conductivity of an isotropic bulk sample swollen with propylene carbonate (λ = 1) is ∼ 1.5 orders of magnitude greater than an aligned thin film swollen with propylene carbonate (λ = 2.4). This result suggests that grain boundaries and defects in bulk samples of this alternating multiblock copolymer are preferentially swollen with solvent, providing pathways for rapid ion transport, and indicates routes to further improve ionic conductivity in solvent-swollen nanostructured ionomers.