Elucidating the role of the ionic liquid in the actuation behavior of thermo-responsive ionogels

The characterization of thermo-responsive ionogels with electrochemical impedance spectroscopy using gold interdigitated electrodes is described. The ionogels are synthesized using poly(N-isopropylacrylamide) as thermo-responsive gel and polymerized in the presence of two ionic liquids: ethyl-3-methylimidazolium ethyl sulfate or trihexyltetradecyl-phosphonium dicyanamide. The changes on the charge-transfer resistance show a clear dependence on the porosity and on the chemical structure of the ionogel. Moreover, the charge-transfer resistance parameter can be used to track in real time the photopolymerization and the hydration process of the ionogels. After exposing them to several drying/rehydration cycles the switching performance is fully understood. The results show that the ionogel with 1-ethyl-3-methylimidazolium ethyl sulfate requires less time to absorb and release water and is excellent for fast response applications. On the contrary, the one with the trihexyltetradecyl-phosphonium dicyanamide ionic liquid presents a more robust actuator behavior and a huge potential to be included in long-term applications. Finally, the observed microstructures have been correlated with the measured charge-transfer resistance providing a better understanding of the actuator behavior of these smart materials.