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.