Surface-Induced Ordering Depresses Through-Film Ionic Conductivity in Lamellar Block Copolymer Electrolytes

Lamellar block copolymers based on polymeric ionic liquids (PILs) show promise as electrolytes in electrochemical devices but often display structural anisotropy that depresses the through-film ionic conductivity. This work hypothesizes that structural anisotropy is a consequence of surface-induced ordering, where preferential adsorption of one block at the electrode drives a short-range stacking of the lamellae. This point was examined with lamellar diblock copolymers of polystyrene (PS) and an acrylate-based PIL. The bulk PS−PIL structure was comprised of randomly oriented lamellar grains. However, in thin PS−PIL films (100−400 nm), the lamellae were stacked normal to the plane of the film and surface relief structures were formed when the as-prepared film thickness was incommensurate with the natural lamellar periodicity. Both of these attributes are well-known consequences of preferential wetting at surfaces. The ionic conductivity of thick PS−PIL films (50−100 μm) was approximately 20x higher in the in-plane direction than in the through-plane direction, consistent with a mixed structure comprised of randomly oriented lamellae throughout the interior of the film and highly oriented lamellae at the electrode surface.