Ionic polymer actuator based on anion-conducting methylated ether-linked polybenzimidazole

A new type of ionic polymer actuator based on anion-conducting methylated poly[(1-(4,4′-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI–OO) membranes and a carbon composite electrode material was successfully developed. The anion-conducting membrane was prepared through N-methylation of PBI–OO with iodomethane. The ion exchange reaction of the methylated PBI–OO membrane to various anions largely affected the ion conductivity and the resulting actuator performance. The methylated PBI–OO membranes possessed unique small ion clusters with the size of 1.71–2.04 nm and exhibited high tensile moduli and strengths in the hydrated states (tensile modulus: 553 MPa, tensile strength: 32 MPa in the iodide form) due to the π–π stacking interaction among the benzimidazole rings in the polymer backbone. The actuators based on PBI–OO bent toward the negatively charged electrode direction reverse to the bending direction of conventional ionic polymer actuators with cation-conducting membranes. They also revealed exceptional charge-specific blocking forces (at least 17 times larger than those of Nafion-based actuators) because of a combinatorial effect of the larger modulus of PBI–OO-based actuators and the smaller size of ion clusters inside the PBI–OO layer, and the electrolysis of water restrained using a carbon electrode as well. Moreover, the actuators showed no back relaxation within the examined time period even under a high dc voltage beyond the electrolysis voltage of water. The distinct characteristics of the PBI–OO-based actuators are expected to be used as a unique anion-conducting polymer actuator.