Characterization of the solvent-induced nonlinear response of ionic polymer actuators

Ionic polymer transducers exhibit coupling between the electrical, chemical, and mechanical domains, allowing their use as both sensors and actuators. Because of their compliance, light weight, and low voltage operation, ionic polymers have spawned an area of much research, although their fundamental mechanisms are still open for debate. While most of the existing models provide linear, dynamic approximations of the response, nonlinear characteristics have been observed experimentally. Some of these include the introduction of permanent strain in the step response and distortion in the forced response to harmonic excitations. Recent experimental results have shown that the solvent plays a significant role in the dynamic response of ionic polymer actuators. Given a single-frequency input voltage, the major difference from changing solvent materials was concluded to be a nonlinear distortion with varying influence, seen in both the actuation current and tip velocity measurements. These results compared the response of a water-based sample to a sample prepared with the ionic liquid EMI-Tf, where it was found that the voltage-to-current relationship was much more nonlinear in the water sample, while it was predominantly linear with the ionic liquid sample. This research looks to further explore this nonlinear distortion by incorporating a larger set of candidate solvent materials and investigating the impact of how changing properties affect the overall response. System identification techniques using the Volterra series are employed to aid in the characterization of the harmonic distortion. The knowledge gained in this study will provide useful information about the nature of the nonlinearity and some of the factors that affect its relative influence, which will assist physical model development.