Electrically stimulated gradients in water and counterion concentrations within electroactive polymer actuators

Abstract

While ionic polymer metal composites (IPMCs) have been studied for more than 10 years, the specific actuation mechanism is still unclear. In this work, neutron imaging, applied potential atomic force microscopy (APAFM) and current sensing atomic force microscopy (CSAFM) methods are employed to fundamentally investigate the actuation mechanism of this electroactive polymer system. Direct neutron imaging allowed a mapping of the water–counterion concentration gradient profile (i.e., a non-flat optical density profile sloping from the cathode to the anode) across an IPMC cross-section. While the neutron imaging method was capable of visualizing inside an operating IPMC, APAFM–CSAFM characterized changes in the nanoscale morphology and local surface properties due to redistribution of water–counterions under electrical stimulation. In APAFM, the darker, more energy dissipative features disappeared as the applied bias was varied from 0 V to 3 V, indicating that the surface became dehydrated. Surface dehydration undoubtedly supports the concept of proton and water migration to the negatively charged substrate. Water–counterion redistribution was further evidenced by CSAFM. With a negatively charged substrate (a 2 V bias), 2.8 pA of the average current were detected over the perfluorosulfonate ionomer (PFSI) surface in contact with AFM tip, which suggests the depletion of positively charged cations on the surface. On the contrary, a positively charged substrate (a −2 V bias) led to the average current of −90 pA over the PFSI surface in contact with the AFM tip, which indicates the formation of a cation-rich fluid on the top surface of the PFSI membranes. The observed water–counterion redistribution upon electrical stimulation directly supports a hydraulic contribution to the overall mechanism of actuation in IPMCs.