Zwitterionic Liquids and Elastomers with High Permittivity for Soft Dielectric Actuators

Current interest in soft robotics, sensors, and shape-morphing materials have driven demand for mechanically compliant devices. While a broad palette of soft elastomers has emerged for these applications, dielectric materials remain limited to compositions with low elastic modulus <i>Y</i> (<i>e.g.</i>, poly(dimethylsiloxane), <i>Y</i> ~ 10-1000 kPa) and permittivity (<i>ε_r</i> = 2-10). A common strategy to increase permittivity is the incorporation of high-permittivity fillers, but this affords a concomitant increase in their stiffness. We posit that zwitterions, which are composed of charge pairs tethered by a covalent linker, offer a path to creating soft, high-permittivity dielectric liquids and elastomers. Unfortunately, most zwitterions synthesized to date possess high melting points, <i>T_m</i> &gt; 200 °C, preventing field-mediated alignment under ambient conditions. Here, we report a new class of zwitterions that are stable liquids at room temperature and exhibit static dielectric constants <i>ε_r</i> as high as 420. Importantly, they also exhibit dramatically lower viscosity (~ 500x) compared to previously reported supercooled zwitterions. When incorporated as pendent groups on a flexible polymer backbone, they afford polymer melts; these can be crosslinked to form soft dielectric elastomers. These are the first examples, to our knowledge, of polyzwitterion melts and elastomers, making them ideal candidates for dielectric elastomer applications.