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

The incorporation of polar solvents into ion-containing polymers is known to enhance the ionic conductivity towards application-relevant values. We previously investigated the incorporation of ethylene carbonate into bulk, isotropic samples of a multiblock copolymer consisting of alkyl blocks of precisely 12 carbons alternating with polar blocks containing lithium sulfonate groups (PESLi12). The ionic conductivity was enhanced by several orders of magnitude upon swelling. Another approach for improving the ionic conductivity of nanostructured polymers is alignment which eliminates grain boundaries, mixed orientations, and defects. This study investigates the effect of solvent swelling on aligned thin films of PESLi12 using in situ grazing incidence X-ray scattering and broadband dielectric spectroscopy with interdigitated electrodes. We demonstrate that the layered ionic assembles remain parallel to the substrate upon swelling with propylene carbonate, γ-butyrolactone, dimethyl carbonate, or diglyme. The in-plane ionic conductivity is significantly increased upon solvent incorporation, but a swollen, isotropic sample swollen with propylene carbonate displays 1.5 orders of magnitude greater ionic conductivity than the equivalent aligned thin film. These findings indicate that grain boundaries and defects in these alternating multiblock copolymers are preferentially swollen with solvent, providing pathways for rapid ion transport and suggesting routes for further improvement of ionic conductivity in solvent-swollen nanostructured polymer electrolytes.