(251t) Tuning the Ionic Conductivity, Dielectric Constant, and Mechanical Properties in Protic Polymerized Ionic Liquid Homopolymers and Random Copolymers

Proton conducting membranes are of interest for a number of energy applications including use in fuel cells and artificial photosynthesis systems. We have synthesized a new class of protic polymerized ionic liquids (PILs) based on imidazolium cations which exhibit high conductivities in the solid state. In contrast to previous imidazolium based PILs, the ionic liquid moiety is attached via a carbon on the imidazole thus leaving the two nitrogens available to act as a proton donor/acceptor. The conductivities of these protic PILs, measured by dielectric spectroscopy, are orders of magnitude higher than the analogous non-protic PILs at a given distance above the glass transition temperature (Tg) as a result of a strong contribution from proton motion. The specific carbon through which the imidazolium is attached to a polymer backbone is also shown to influence the conductivity where poly(5-ethylamino imidazolium acrylamide) has a substantially higher conductivity than poly(2-ethylamino imidazolium acrylamide) when both contain the same bis(trifluoromethane sulfonimide) (TFSI) counteranion. This is attributed to a more extensive H-bonding network in the former polymer.

Routes to simultaneously improve the conductivity and dielectric properties of PILs have been undertaken by incorporating non-ionic repeat units into the polymer backbone. A variety of comonomers that are either polar or non-polar and either bulky or flexible have been incorporated to tune the dielectric constant, mechanical properties, and conductivity of PILs. We have found that up to ~ 20 wt% of a non-conductive species can be added into a random copolymer without substantially altering the Tg-normalized conductivity. This leads to improvements in processability and solubility of such materials and the ability to tune the overall material properties by the judicious choice of comonomer.