Imidazolium cation is a well-known moiety of ionic liquids, but also important in solid-state ion conductors, such as plastic crystals and ionic copolymers. Interesting phase behaviors are found from symmetric dicationic imidazolium salts with an ethylene bridge and linear alkyl side chains; multiple solid-soild phase transitions are found on DSC. Some of the salts can be called as a true plastic crystals from the low entropy of fusion values, ΔSf &lt; 20 J mol-1 K-1. Along the phase transitions, the discontinuities and changes in ion conductivity slopes near the phase transition temperatures. Solid-state NMR study of the deuterated imidazolium salts shows the fraction changes of the moving cations and anions in solid-state as temperature varies; this result corresponds to the phase and conductivity changes. Linear imidazolium-based poly(ethylene glycol) (PEG) copolyesters also shows interesting crystallization behaviors, which are dependent on the different polymer structures about PEG molecular weight, spacer length and counter-anion. The copolymers with relatively larger and more polarizable Tf2N- counter-anions has higher conductivity than that with the smaller PF6- counter-anions, the differences are more dominant at lower temperature. The abrupt change in the conductivity is also observed for the copolyesters with longer PEG-2000 segments at the observed crystallization temperatures. Interestingly, the imidazolium polyester with a higher ion density shows lower ionic conductivity than the imidazolium-PEG copolyesters within less ionic units per mass, because of the higher activation energy about the ion pair dissociation without an assistant by PEG units. More detail structures, phase changes and ion transport behaviors will be discussed for the various dicationic imidazolium plastic crystals and the imidazolium-PEG copolyesters.

Ion conductions and phase bahaviors of imidazolium-based ionic plastic crystals and semicrystalline copolymers

2019 Fall National Meeting

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