Factors Affecting Electromechanical Properties of Ionic Polymer Actuators Based on Ionic Liquid-containing Sulfonated Block Copolymers

Abstract

We investigate the electromechanical properties of ionic polymer actuators based on sulfonated block copolymers and imidazolium ionic liquids (ILs) to unveil key factors affecting the actuation performance. First, the extent of electromechanical deformation of the actuators was proven to be largely affected by the type of anion in the ILs, as understood by the anion-dependent ionic conductivity, charging time, and Young's modulus of IL-containing polymers. In particular, upon switching the anion from hexafluorophosphate to bis(trifluoromethane)sulfonimide, more than a 2-fold enhancement in the bending strain was observed, which has not been previously reported. Second, we show that the bending strain and durability of the actuators are tunable in a straightforward manner by controlling the block architecture and molecular weight of the polymer, where the use of triblock copolymers was found to be beneficial in enhancing the actuation performance. The high mechanical strength of IL-containing triblock copolymers at large IL loadings was responsible for advancing the actuation performance because of bridge/loop formations in the hydrophobic domains of the triblock copolymers, which is essentially not affected by the incorporation of ILs. Lastly, our actuators demonstrated up to a 10 times increase in the bending strain, compared to actuators based on conventional poly(vinylidene fluoride-co-hexafluoropropene) with the same type of IL. The key to success stemmed from the self-assembled structure of sulfonated block copolymers having continuous ionic phases with highly interactive -SO3H surfaces, which facilitated fast ion diffusion with a reduced tendency to form ion clusters. A molecular-level understanding of actuation mechanisms underlying the improved actuation performance was acquired by combining in situ spectroscopy and in situ scattering experiments under voltage stimuli.