Interface-induced ordering in lamellar block copolymer electrolytes

Lamellar block polymers base on polymer ionic liquids (PILs) show promise as electrolytes in electrochemical devices but often exhibit structural anisotropy that depresses through-film ionic conductivity. We propose that one source of structural anisotropy in these systems is short-range stacking of lamellae adjacent to the electrodes due to preferential adsorption of one block at this surface. This point is demonstrated with lamellar diblock copolymers of polystyrene (PS) and a polymer ionic liquid (PIL). Grazing-incidence X-ray scattering (GISAXS) from thin films reveals lamellae stacked normal to the plane of the film, with no evidence of isotropic lamellar order, while transmission small-angle X-ray scattering (SAXS) in 50-100 μm films detects randomly oriented lamellar grains. The ionic conductivity of 100 μm PS-PIL films was found to be approximately 20x higher in the in-plane direction than the through-plane direction, consistent with a mixed structure consisting of highly oriented lamellae at the electrode surfaces and randomly oriented lamellae throughout the interior. To reduce the tendency to form highly-oriented interfacial structures, the electrodes can be coated with random copolymers that screen preferential interactions.