Investigation of the physics of transduction in nafion / Ionic liquid composite membranes

Abstract

Ionomeric polymer actuators based on Nafion membranes exhibit large bending motion (1%) under the application of small voltages (1-5 V). Actuation in these materials is believed to arise from the field-induced motion of mobile charges when a voltage is applied. In order for this charge motion to occur, the material must be swollen with a diluent, typically water. However, dehydration of the water limits the lifetime of these actuators in non-aqueous environments. Recently, highly stable ionic liquids have been demonstrated as viable diluents for these actuators. In the current paper, the physics of transduction in these ionic liquid-swollen Nafion membranes are investigated. Small-angle X-ray scattering reveals that the structure and properties of the ionic liquid have a strong influence on the morphology of the composites. Infrared spectroscopy is used to probe the ion associations within the films and shows that the ionic liquids are able to effectively mobilize the counterions of the Nafion membrane. Nuclear magnetic resonance spectroscopy is also used to investigate the composites and reveals that the mobility of the counterions increases as the content of ionic liquid within the membrane is increased. The results of these characterizations are compared to an experimental investigation of transduction in Nafion / ionic liquid composites to form an interpretation of the mechanisms of actuation. This comparison reveals that the counterions of the Nafion membrane are the primary charge carriers and that it is the motion of these mobile charges that gives rise to the actuation behavior of the films.