Co-Poly(ionic liquid) Films via Anion Exchange for the Continuous Tunability of Ion Transport and Wettability

This manuscript details a novel and simple approach to achieve surface-tethered co-poly(ionic liquid) (coPIL) films through the exchange of the resident anion of a poly(ionic liquid) (PIL) film with two or more anions. Initially, surface-tethered PIL films were prepared by the surface-initiated ring-opening metathesis polymerization of the ionic liquid monomer 3-[(bicyclo[2.2.1]hept-5-en-2-yl)methyl]-1,2-dimethylimidazol-3-ium hexafluorophosphate ([N₁-dMIm][PF₆]) whose PF₆ ^- anion was easily interchanged with aqueous solutions containing a binary mixture of the PF₆ ^- anion, along with perchlorate (ClO₄ ^-) or bis(fluorosulfonyl)imide (FSI^-) anions. The binary mole fraction of each anion in the film was determined from the infrared spectra of the coPIL films. The thermodynamically driven anion selectivity for exchange from the liquid phase into the coPIL films was determined to follow the order ClO₄ ^- < PF₆ ^- < FSI^-. The aqueous wettability of p[N₁-dMIm] coPIL films containing both the PF₆ ^- and ClO₄ ^- anions (p[N₁-dMIm][PF₆][ClO₄]) was quantified by contact angle goniometry with the observation that the surface showed an enrichment in the ClO₄ ^- anion compared to the average binary anion mole fraction of ClO₄ ^- in the film (y _ClO ). The rate of ion transport through the p[N₁-dMIm][PF₆][ClO₄] coPIL films, quantified by electrochemical impedance spectroscopy, linearly depends on the binary anion mole fraction of ClO₄ ^- in solution (x _ClO ), enabling continuous tunability by over three orders of magnitude for ion conductivity in the coPIL films.