Rheology of Imidazolium-Based Ionic Liquids with Aromatic Functionality

A series of imidazolium-based ionic liquids with cyclic and aromatic groups and a bis­[(trifluoromethane)­sulfonyl]­amide anion were characterized using dynamic and steady shear rheology in a wide temperature range. The cations investigated include 1-(cyclohexylmethyl)-3-methylimidazolium ([CyhmC1im]+), 1-benzyl-3-methylimidazolium ([BnzC1im]+), 1,3-dibenzylimidazolium ([(Bnz)2im]+), and 1-(2-naphthylmethyl)-3-methylimidazolium ([NapmC1im]+). Rheological properties are reported from terminal flow to the glassy state. All ionic liquids show very similar flow behavior with a glassy modulus approaching 1 GPa. The temperature dependences of the shift factors used for time–temperature superposition and the viscosity both follow the same VFT or WLF relationship, and the dynamic fragility at T g ranges from 117 to 130 for the aromatic ionic liquids investigated, considerably more fragile, in the Angell sense, than the aliphatic analogs. Additionally, an anomalous aging effect in the dynamic viscosity response (i.e., a maximum observed at intermediate frequencies) was found using a strain-controlled rheometer but not using a stress-controlled rheometer.