Comparative study of bending characteristics of ionic polymer actuators containing ionic liquids for modeling actuation

摘要

Ionic polymer metal composites (IPMCs) that can operate in air have recently been developed by incorporating an ionic liquid in ionic polymers. To understand transduction in these composites, it is important to determine the role of the ionic liquid in the ionic polymer (Nafion®), to identify the counter cation, and to investigate the interaction of IPMCs with water vapor in the air. We used Fourier-transform infrared spectroscopy to analyze three Nafion® membranes, which were soaked in mixtures of water and an ionic liquid (1-ethyl-3-methyl-imidazolium tetrafluoroborate (EMIBF_4), 1-buthyl-3-methyl-imidazolium tetrafluoroborate (BMIBF_4), and 1-buthyl-3-methyl-imidazolium hexafluorophosphate (BMIPF_6)). The results demonstrate that only cations (EMI~+ and BMI~+) in the ionic liquids are taken into the Nafion® membranes as counter ions and that the water content of the membranes in air is less than ～4% that of Nafion® swollen with water. Based on the experimental results, a transduction model is proposed for an IPMC with an ionic liquid. In this model, bending is caused by local swelling due to the volume effect of the bulky counter cations. This model can explain 30-50% of the experimentally observed bending curvature.