Probing Artificial Muscles with Neutrons
Ionic polymer metal composites (IPMCs) are a class of materials that exhibit actuation due to electrical stimulation with similarities to biological muscles, and are often referred to as artificial muscles. Although IPMCs have been studied for more than 10 years, the exact actuation mechanism is still not well understood. IPMCs are based on a solid polymer electrolyte, which is plated on both surfaces by dense layers of metal nanoparticles (e.g., platinum or gold) to serve as conductive electrodes, which resembles the membrane electrode assemblies used in fuel cells.Under the stimulus of a relatively low electric field (2 V – 3 V), IPMCs are capable of undergoing significant mechanical bending motion; an example of this is shown in figure 1.
One proposed mechanism for IPMC actuation that has commonly been discussed in the literature is an electrophoresis like counterion and water redistribution within the nanostructured ionomer membrane. Since the ionic polymer is negatively charged with protons or alkali-metal cations as the counterions, these mobile cations in the hydrated membrane readily redistribute in response to an applied electric field to create cation-rich and cation-poor boundary layers near the electrode/membrane interfacial regions. As cations migrate toward the cathode, they drag along water molecules. This hydraulic action results in swelling near the cathode and contraction near the anode that gives rise to the bending force that curves the IPMC towards the anode.
The results of the neutron imaging are shown in figure 2. Here the IPMC was oriented to allow neutrons to travel parallel to the anode and cathode surfaces. The resulting neutron transmission allows the mapping of water distributed through the plane of the IPMC from anode to cathode. In the figure, the image of the unstimulated state is compared to the state after an electrical stimulus of 3 V was applied for 30 s. The resulting change in neutron attenuation showed a rapid redistribution of water/conterions in the IPMC when the electrical stimulus was applied. When the polarization of the applied electric field is reversed the corresponding water/counterion distribution is also rapidly reversed, in direct support of the hydraulic model for the deformation of the IPMC due to the applied electric potential. Additional measurements were made using D2O swollen IPMCs neutralized to contain sodium and tetramethylammonium counterions were also studied to separate the cation motion from the hydraulic motion. These results showed that both the cations and water migrate under electrical stimulation.
Neutron Interactions and Dosimetry Group
|David L. Jacobson
|Daniel S. Hussey
Robert B. Moore
Virginia Polytechnic Institute and State University