Electroactive ionic polymer actuators
Electroactive actuators that show reversible mechanical deformation in response to an electric field have received enormous interest in the era of biomimetic technology such as artificial muscles, robotics and microsensors. Recently, ionic polymer actuators comprising a layer of polymer electrolyte sandwiched in between electrodes have emerged as promising candidates in such applications, owing to their lightweight, flexibility, mechanical robustness and ease of fabrication at low cost. The most important requirements are large displacements in bending motion, fast response time, low operating voltage and durability in air.
We developed new ionic polymer actuators comprising phase-separated block copolymers and ionic liquids. [Figure 1] The use of sulfonated block copolymer possessing well-connected ionic channels resulted in fast response and the best performance under sub-1V condition, in comparison to the state-of-the-art PVdF-based actuators, upon building up efficient ion conduction pathway. In-situ SAXS experiments were carried out to underpin the actuation mechanism of the new actuator and the key to success stemmed from the evolution of the unique nanostructure of the electrolyte layer with dimensional gradients beneath the cathode during actuation, which promoted the bending motion of the actuators. (Kim, O. et al. Nature Communications 2013, 4, 2208.)
Furthermore, we investigated the electromechanical properties of ionic polymer actuators to unveil key factors affecting the actuation performance. [Figure 2] 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. 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. Lastly, our actuators demonstrated up to a 10 times increase in the bending strain, compared to actuators based on conventional PVdF-HFP 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. (Kim, O. et al. Macromolecules, 2014, 47 (13), 4357.)
Our results convince that the nanostructured block copolymer electrolytes allow the successful actuator operation with a small battery, which paves the way for advanced biomimetic technologies in the future. [Figure 3] Furthermore, the actuation performance may be tuned by varying the type of ionic liquids and by employing a range of block copolymers with different molecular weights and compositions. In addition, experiments on whether the morphology effects and development of new types of actuators for practical biomimetic devices are present in the actuators will be subjects of future studies.
