Confined Interfacial Structure of Poly(ionic liquid) Thin Films

Abstract

Thin films of poly(ionic liquids) (PILs) provide a versatile platform to explore how confinement and interfacial interactions affect structural organization at the nanoscale. Here, we investigate the interfacial ordering of poly(1-vinyl-3-alkylimidazolium)s (PCnVImTFSI) thin films under confinement with different side-chain lengths (n). A combined use of specular X-ray reflectivity (XRR) and grazing incidence wide-angle X-ray scattering (GIWAXS) provides three-dimensional structural information in the vicinity of the interface. We show that confinement induces lamellar arrangements and preferential orientation of imidazolium groups. The interplanar spacing, quantitatively probed by a Fourier transform analysis, appears to depend on the overall thickness of the film. Such a confinement effect has in-plane repercussions and, most notably, an increase in the distance between neighboring chains. A molecular picture encompassing both observed effects is proposed, relying on a preferential orientation of imidazolium groups close to the interface as the material is increasingly confined. Such structural changes are expected to directly impact ion mobility and charge transport, thereby linking interfacial organization to the functional properties of PILs. This structure–property relationship highlights how confinement-induced ordering can influence performance in electrochemical devices, including batteries, fuel cells, and sensors.