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

Abstract

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‐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 × 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‐C 11 ‐sebacate‐C 11 ( 5c ), displays almost 400‐fold higher room temperature ionic conductivity (1.6 × 10 −6 S cm −1 ) than the 1,2‐bis(imidazolium)ethane‐C 6 ‐sebacate‐C 6 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, T g ) amorphous phase and good mechanical properties of the crystalline phase.