Self-assembly of cationic rod-like poly(2,5-pyridine) by acidic bis(trifluoromethane)sulfonimide in the hydrated state: A highly-ordered self-assembled protonic conductor

Abstract

We show that acid–base complexation of rod-like poly(2,5-pyridine) (PPY) by bis(trifluoromethane)sulfonimide (TFSI) leads to highly-ordered lamellar self-assemblies in the hydrated films and shows relatively high room temperature conductivity of ca. 10−4 S/cm. Thin films with different nominal degrees of complexation were studied using X-ray diffraction, Fourier transform infrared spectroscopy, contact angle measurements, conductivity measurements, and polarised optical microscopy. We propose that the self-assembly is promoted by the amphiphilicity of TFSI and the interplay between the hydrophilic and hydrophobic sites within the complexes. The hydrophilic sites allow confinement of water molecules within the hydrated self-assemblies for low loading of TFSI to promote proton conductivity. For high loading of TFSI in the hydrated state, another coincident self-assembled structure is additionally observed, which we suggest to form due to phase separated water/TFSI domains, as resembling lamellar water/surfactant liquid crystalline phases. The new type of self-assembled acid–base material combining rod-like polymeric cations and ionic liquid anions suggests new routes for ionic and protonic transport and functional materials.