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.