Imidazolium Polyesters: Structure-Property Relationships in Thermal Behavior, Ionic Conductivity, and Morphology

New bis(omega-hydroxyalkyl)imidazolium and 1,2-bis[N-(omega-hydroxyalkyl)imidazolium] ethane salts are synthesized and characterized; most of the salts are room temperature ionic liquids. These hydroxyl end-functionalized ionic liquids are polymerized with diacid chlorides, yielding polyesters containing imidazolium cations embedded in the main chain. By X-ray scattering, four polyesters are found to be semicrystalline at room temperature: mono-imidazolium-C-11-sebacate-C-6 (4e), mono-imidazolium-C-11-sebacate-C-11 (4c), bis(imidazolium) ethane-C-6-sebacate-C-6 (5a), and bis(imidazolium) ethane-C-11-sebacate-C-11 (5c), all with hexafluorophosphate counterions. The other imidazolium polyesters, including all those with bis(trifluoromethanesulfonyl) imide (TFSI-) counterions, are amorphous at room temperature. Room temperature ionic conductivities of the mono-imidazolium polyesters (4 x 10(-6) to 3 x 10(-5) S cm(-1)) are higher than those of the corresponding bis-imidazolium polyesters (4 x 10(-9) to 8 x 10(-6) S cm(-1)), even though the bis-imidazolium polyesters have higher ion concentrations. Counterions affect ionic conduction significantly; all polymers with TFSI- counterions have higher ionic conductivities than the hexafluorophosphate analogs. Interestingly, the hexafluorophosphate polyester, 1,2-bis(imidazolium) ethane-C-11-sebacate-C-11 (5c), displays almost 400-fold higher room temperature ionic conductivity (1.6 x 10-6 S cm-1) than the 1,2-bis(imidazolium) ethane-C-6-sebacate-C-6 analog (5a, 4.3 x 10(-9) S cm(-1)), attributable to the differences in the semicrystalline structure in 5c as compared to 5a. These results indicate that semicrystalline polymers may result in high ionic conductivity in a soft (low glass tranition temperature, T-g) amorphous phase and good mechanical properties of the crystalline phase.