The role of water in transport of ionic liquids in polymeric artificial muscle actuators

Abstract

Ionic liquid (IL) based polymeric mechanical actuators show promise for providing an important and unprecedented combination of electromechanical and soft material properties. Measurement of ion transport in an ionomer/IL membrane is essential for understanding and designing these “artificial muscle” actuators. Furthermore, water forms a ubiquitous part of these hydrophilic membrane systems when used in open air, greatly affecting electrochemical and physical properties including the ion conductivity critical for actuation performance. Here we present the first study quantifying the delicate interplay between water and an IL absorbed in an ionomer membrane as used in ionic polymer actuators. We use ¹H and ¹⁹F NMR diffusometry to investigate cation and anion diffusion of 1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate (EMI-Tf), swollen into a perfluorosulfonate ionomer membrane. The EMI cation diffuses faster than the Tf anion in both the neat state and inside the membrane. We quantitatively evaluate the effects of temperature, IL uptake, and water content. A key factor is the co-existing water content, quantified by in situ¹H NMR, which dramatically accelerates the diffusive motion of the IL. When water content, χ_H2O, drops from 1 to 0.5 water molecules per EMI-Tf, IL diffusion coefficients decrease by 36–60%. Our experiments provide critical feedback for optimizing actuator performance via novel materials, device development, and control of operation conditions.