Block copolymers based on polymeric ionic liquids (PILs) show promise as electrolytes in electrochemical devices.  With commodity block copolymers, it is well known that preferential wetting of one block at a surface can drive the layering of domains through the film thickness.  If such layers are present in PIL-based materials, then the ionic conductivity should be highly anisotropic. This hypothesis is investigated with a series of lamellar diblock copolymers of polystyrene (PS) and poly(1-(2-acryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide)) (PAEBIm-TFSI)), as well as a PAEBIm-TFSI homopolymer. Ultrathin films of the diblock copolymer (100-400 nm) exhibit island and hole formation, an established signature of domain layering through the film thickness, and grazing-incidence small angle X-ray scattering confirms the presence of well-defined layers.  The ionic conductivity of thick block copolymer films (50-100 mm) was observed to be roughly 20 times higher in the in-plane direction than in the through-plane direction, while no anisotropy was observed for the PAEBIm-TFSI homopolymer.  These data suggest that measurements of ion conductivity are sampling at least two types of structures: randomly-oriented lamellae in the bulk, and highly-oriented lamellae at the interface with the electrode. Both structures contribute to the in-plane ion conductivity, while the interface structure dominates (and suppresses) the through-film conductivity. As devices generally require ion transport along a particular direction, the interface-induced layering of domains could either help or inhibit ion transport, thereby enhancing or suppressing overall performance.

Anisotropic ionic conductivity due to wetting layer formation in block copolymers of polymerized ionic liquids

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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