(469a) Impact of Surface-Induced Order on Ion Conductivity in Block Copolymer Electrolytes

Lamellar block copolymers base on polymer ionic liquids (PILs) show promise as electrolytes in electrochemical devices but often exhibit structural anisotropy, resulting in depressed through-film ionic conductivity. Using lamellar diblock copolymers of polystyrene (PS) and poly(1-(2-acryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) (PIL), this work demonstrates that structural anisotropy in these materials is a consequence of short-range stacking of lamellae at the electrode-polymer interfaces induced by preferential adsorption of one block at the electrode. While transmission small-angle X-ray scattering (SAXS) of 50-100 μm films indicated randomly oriented lamellar grains, grazing-incidence small-angle X-ray scattering (GISAXS) of 100-400 nm thin films revealed lamellae stacked normal to the plane of the film. In addition, surface relief structures were observed in the thin films when the as-prepared film thickness was incommensurate with the natural lamellar periodicity, a well know attribute of preferential wetting at surfaces. The ionic conductivity of 100 μm PS-PIL films was found to be approximately 20x higher in the in-plane direction than in the through-plane direction, consistent with a mixed structure consisting of highly oriented lamellae at the electrode surfaces and randomly oriented lamellae throughout the film interior. Consideration of surface interaction on the ordering of domains is therefore an important component to fully optimizing the performance of block copolymer electrolytes.