Dynamic Anion-Adaptive Poly(ionic liquid) Films via Surface-Initiated Ring-Opening Metathesis Polymerization

We report the surface-initiated ring-opening metathesis polymerization (SI-ROMP) of ionic liquid-tethered monomers to grow poly­(ionic liquid) (PIL) films on gold and silicon substrates. The kinetics of film growth are rapid, with profilometric thicknesses approaching 600 nm within 15 min of polymerization in a 0.1 M monomer solution and substantial film growth observed at monomer concentrations as low as 0.02 M. The ionic liquid (IL) monomer consists of the cation 3-[(bicyclo[2.2.1]­hept-5-en-2-yl)­methyl]-1,2-dimethylimidazol-3-ium (N1-dMIm+) with the hexafluorophosphate (PF6–) anion. SI-ROMP of the IL monomer results in a surface-tethered poly­(3-[(bicyclo[2.2.1]­hept-5-en-2-yl)­methyl]-1,2-dimethylimidazol-3-ium hexafluorophosphate) (p­[N1-dMIm]­[PF6]) film whose PF6– anion can be easily interchanged to tune the film properties. The p­[N1-dMIm]­[PF6] films were shown to be adaptive to their anionic environment with the extent of anion exchange characterized by reflectance-absorption infrared spectroscopy and utlraviolet-visible spectroscopy. Anionic dyes incorporated into the p­[N1-dMIm] films via anion exchange resulted in a reversible color change in the films. The surface and bulk interaction of the p­[N1-dMIm] films with water was analyzed by contact angle goniometry and quartz crystal microbalance with dissipation (QCM-D). The p­[N1-dMIm] film with the perchlorate (ClO4–) ion exhibited the lowest advancing contact angle with water of 35 ± 3° compared to the p­[N1-dMIm] film with the bistriflate (−NTf2) ion which exhibited the highest water contact advancing angle of 65 ± 3°. The p­[N1-dMIm]­[ClO4] film demonstrated an 8% increase in bulk water content, as determined by QCM-D, compared to the p­[N1-dMIm]­[PF6] film. The rate of ion transfer through the film was highly dependent on the anion as characterized by electrochemical impedance spectroscopy (EIS) in different electrolytes. In particular, resistances to ion transfer ranged from 7.90 ± 0.11 KΩ·cm2 for films containing the PF6– anion to ≤ (4.34 ± 0.04) × 10–3 KΩ·cm2 for films containing the perchlorate ClO4– anion.