Ion transport properties of ionic liquid-based polyelectrolytes

Three imidazolium-based ionic liquid monomers with polymerizable vinyl groups and the resulting polyelectrolytes have been synthesized and characterized. Particular attention was paid to the purity of the materials. Furthermore, a synthetic route via a 2-methylidene-imidazoline was used, which should be of general interest for the synthesis of polyelectrolytes with different spacer lengths between backbone and side chains. Besides comprehensive monomer and polymer analytics, electrical impedance spectroscopy was carried out to obtain information about the ion conducting properties of the three systems under investigation: P1 poly(3-ethyl-1-vinylimidazolium) bis(trifluoromethanesulfonyl)imide (N(Tf)2), P2 poly(3-methyl-1-(4-vinylbenzyl)imidazolium)N(Tf)2 and P3 poly(1-butyl-3-methyl-2-(4-vinylphenethyl)imidazolium)N(Tf)2. The pure polymers, which are N(Tf)2 anion conductors, exhibit room-temperature conductivities in the range from 10− 11 S/cm to 10− 8 S/cm. The anion conduction mechanism is strongly influenced by the length of the spacer group between the polymer backbone and the imidazolium cations in the side chain. In polymers P1 and P2 with short spacer groups, intra- and inter-cation hopping of the N(Tf)2 anions can be distinguished below the glass transition temperature, while this is not possible in the case of polymer P3 with longer spacer groups. Furthermore, we have studied several mixtures of the best conducting polymer P2 with LiN(Tf)2, zwitterions and monomeric ionic liquid. While the zwitterions were capable of compensating for the conductivity drop due to Li salt addition, the addition of monomeric IL as plasticizer leads to a considerable conductivity enhancement without a significant loss of mechanical stability.