Conductivity behavior and solid state NMR investigation of imidazolium-based polymeric ionic liquids

Novel proton-conducting anhydrous polymer electrolytes comprising imidazolium ionic liquid units in the polymeric structure have been synthesized. These polymeric ionic liquid electrolytes were doped with phosphoric acid to get Poly(AcIm-n)xH3PO4, where x is the number of moles of H3PO4 per polymer repeat unit and n is the number of alkyl spacers (–CH2). Thermogravimetric analysis (TGA) illustrates that these materials are thermally stable up to about 200 °C. DSC results show the softening effect of the length of the spacers as well as phosphoric acid. Mechanical studies displayed the cross-linking of the samples which were also confirmed by solid state 31P CPMAS NMR spectra. The proton conductivity of the samples increases with x and reaches 10−2 S cm−1 at 120 °C for both Poly(AcIm-2)2H3PO4 and Poly(AcIm-6)2H3PO4. The conductivity isotherms show that the proton transport in acid-doped polymeric ionic liquids obeys VTF behavior.