Softening and heating effects in ionic polymer transducers

Abstract

This study reports the softening and heating of ionic polymer transducers during actuation. This is the first account of such effects that will impact the understanding of the actuation mechanisms and the physical modeling of these actuators. The ionic polymer transducer samples are characterized in the extensional mode under a variety of mechanical boundary conditions, as a function of the electrode architecture and the cation species. For instance, the electrode thickness is varied from 10 to 40 µm while three extensional actuators with lithium, cesium, and tetraethylammonium mobile cations are characterized. The actuators are characterized under the following boundary conditions: free displacement, spring loaded with a varied amount of prestress, and constant applied pressure. The softening behavior is observed when the prestressed actuator contracts rather than expands as expected in the extensional mode. While the heating effect is observed when transducers characterized with a high-frequency alternating current, excitation generated a logarithmic like response similar to that of a step input. The shape of this response is correlated with the measured temperature of the actuator. However, when actuated with a low-frequency (<0.1 Hz) alternating current signal, the ionic polymer transducer responded with a sine displacement containing a strong first harmonic. Furthermore, experimental results demonstrate a strong correlation between electrode architecture and the peak strain response. Extensional strains on the order of 1.35% are observed with air stable ionic liquid-based transducers. Ionic polymer transducers with cesium (Cs) cation outperformed all other tested actuators with other mobile species.