Zwitterionic Dielectric Elastomer with Ultrahigh Permittivity for Actuators

Dielectric elastomer actuators (DEAs) have shown great promise in soft robotics applications given their merits of large deformation, fast response and high energy density. However, the voltage required to drive DEAs is usually up to several kilovolts because of low intrinsic relative permittivity of current dielectric elastomers (DEs). To obtain DEs with high dielectric constant for low-voltage operation, high permittivity or conductive fillers are introduced. However, the existence of fillers will usually lead to lower breakdown field and high stiffness. Meanwhile, the dielectric properties of the composite are still dominated by the DE matrix according to the percolation theory. Therefore, approaches to achieve DEs with large dielectric constant and high breakdown field are highly desirable.

Zwitterion is a kind of small molecules connecting a cation and an anion by covalent bonds. This unique structure renders zwitterion with very large molecular dipole from increased polarizability, as well as enormous dielectric constant which is highest (e.g., 200-270) among reported soft materials. Also, the alkyl chain connecting cation and anion within zwitterion can eliminate long-range migration of ions under electric field, making it suitable for high-performance dielectrics. Herein, we introduce the zwitterionic in designing of DE materials, which copolymerizes zwitterionic moiety into polyurethane elastomer. This strategy endows the resultant zwitterionic polyurethane (ZPU) to have not only large dielectric constant (36.7, 1 kHz) but also high breakdown field (78.5 V/μm), resulting in an extraordinary maximum electrostatic energy density over all reported DEs. The DEAs based on the ZPU also exhibited outstanding actuation performance and high specific energy density at low electric field as compared to commercial DEs. Moreover, the DEAs can perform the actuation task, coupled with self-proprioceptive capacitive sensing, without electrostatic and electrical interference from high driving voltage, respectively.