The role of cations on contactless actuation of perfluorinated ionomer membranes in salt solution

Ionic polymer metal composites (IPMCs) are electroactive materials that comprise an ionomer core with metal electrodes. In the ionomer, anions are attached to the polymeric backbone, while cations can migrate within the membrane. The application of a potential difference across the electrodes causes macroscopic bending of the material. Despite the availability of many physically-based models to explain IPMC actuation, experimental validation of theoretical predictions remains a challenging task, due to the sensitivity of IPMC electrochemical properties to the electrodes' microstructure. Contactless actuation of perfluorinated ionomer membranes (with- out plated electrodes) offers a unique possibility to experiment with these materials, at a smaller variability. Previous experiments showed that, under the effect of an external electric field, ionomer membranes in salt solution deflect toward the cathode. However, the physical underpinnings of their actuation remain unclear. Previously, we have studied the role of the voltage level, salt concentration, and membrane width on membrane deflection. Here, we experimentally explore the effect of the size of the cations in the solution and membrane on the actuation of ionomer membranes. By increasing the size of cations in the solution, we observe a significant reduction of membrane deflection. Surprisingly, for combinations of large cations in both the solution and mem- brane, membranes deflect toward the anode. By increasing membrane cations' size, we find larger deflections for smaller cations in the solution and smaller deflections for larger cations in the solution. These results can help inform theoretical models to explain actuation of ionomer membranes.