Contactless actuation of perfluorinated ionomer membranes in salt solution: an experimental investigation

Alain Boldini,Maurizio Porfiri,Maxwell Rosen,Youngsu Cha

doi:10.1038/s41598-019-48235-9

Abstract

A variety of modeling frameworks have been proposed for ionic polymer metal composites (IPMCs), but the physical underpinnings of their actuation remain elusive. A critical step toward the validation of existing theories and transition to engineering practice entails the design of new experimental paradigms that could support hypothesis-driven research. While several factors exacerbate the complexity of experimenting with IPMCs, the presence of the electrodes plays a major role by hindering the repeatability of the results and bringing a number of difficult-to-measure parameters into the picture. Here, we seek to address these experimental confounds by investigating contactless actuation of perfluorinated ionomer membranes in salt solution. In contrast to IPMCs that bend toward the anode in response to an applied voltage, ionomer membranes display a consistent deflection toward the cathode. Through hypothesis-driven experiments where the membrane width, solution concentration, and voltage applied across the electrodes are systematically varied, we elucidate electrochemistry and mechanics of contactless actuation. The applied voltage and solution concentration have a dominant role on the electrochemistry, while mechanics is mainly affected by the applied voltage and membrane width. Our results depict a complex scenario, which is expected to inform future theoretical inquiries about IPMC actuation.

Highlights

Ionic polymer metal composites (IPMCs)[1,2] are a class of electroactive materials that hold promise as actuators for biomedical engineering[3] and soft robotics[4,5]
Other authors have focused on physically-based theories of ionic polymer metal composites (IPMCs), encompassing models grounded in micromechanics[15], theory of mixtures[16], Poisson-Nernst-Planck systems[17,18,19], and theory of porous media[20]
Observed for the first time by Asaka and colleagues[23], this phenomenon consists of fast bending toward the anode, followed by slow relaxation toward the cathode, upon the application of a step voltage. This surprising effect is commonly associated with the so-called added mass, whereby solvent molecules in the ionomer are first dragged toward the anode by counterions migration and slowly diffuse back to drive the relaxation of the IPMC24. This explanation presents some inconsistencies when compared with experimental observations[16,25,26], which could be partially resolved by embracing our thermodynamically-consistent continuum model as we have demonstrated in previous efforts[27,28]

Summary

Introduction

Ionic polymer metal composites (IPMCs)[1,2] are a class of electroactive materials that hold promise as actuators for biomedical engineering[3] and soft robotics[4,5]. The main element of novelty of the model is the presence of Maxwell stress tensor[22], whose interaction with osmotic pressure is hypothesized to determine IPMC actuation Enticing to this interaction is the possibility to explain the phenomenon of back-relaxation from first physical principles. Observed for the first time by Asaka and colleagues[23], this phenomenon consists of fast bending toward the anode, followed by slow relaxation toward the cathode, upon the application of a step voltage This surprising effect is commonly associated with the so-called added mass, whereby solvent molecules in the ionomer are first dragged toward the anode by counterions migration and slowly diffuse back to drive the relaxation of the IPMC24. While we are able to resolve some of the qualitative discrepancies of the added mass explanation with respect to existing experiments, it is difficult to confidently validate any theory of IPMC actuation and propose which approach should be preferred when designing IPMC actuators

Objectives

Methods

Results

Conclusion