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

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Abstract

New bis(ω-hydroxyalkyl)imidazolium and 1,2-bis[N-(ω-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-C11-sebacate-C6 (4e), mono-imidazolium-C11-sebacate-C11 (4c), bis(imidazolium)ethane-C6-sebacate-C6 (5a), and bis(imidazolium)ethane-C11-sebacate-C11 (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 × 10−6 to 3 × 10−5 S cm−1) are higher than those of the corresponding bis-imidazolium polyesters (4 × 10−9 to 8 × 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-C11-sebacate-C11 (5c), displays almost 400-fold higher room temperature ionic conductivity (1.6 × 10−6 S cm−1) than the 1,2-bis(imidazolium)ethane-C6-sebacate-C6 analog (5a, 4.3 × 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, Tg) amorphous phase and good mechanical properties of the crystalline phase.

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