Magnetic resonance imaging study of a soft actuator element during operation

Abstract

The physical structure and the actuation mechanism of a Nafion-based soft actuator – a Pt-containing ionic polymer–metal composite (IPMC) – were investigated using in situ magnetic resonance imaging (MRI) during application of four different electrical regimes. Importantly, the raw MRI data were used to generate spatial maps of both proton density (PD) and proton spin–spin relaxation time (T₂) across the sample. These were successfully employed to study changes in the distribution and chemical environment of water molecules absorbed within the operating actuator device. The IPMC sample was mapped in this way during the application of a small d.c. potential across its thickness. Three main phenomena were observed in the results: initial rapid increase in T₂ at both electrodes, without an observed change in PD; slower formation of a region of high T₂ and high PD at the IPMC cathode; and contraction of the polymer along the anode and its expansion along the cathode, giving rise to bending actuation. Reversing the polarity of the applied potential resulted in the reversal of the direction of the bending deformation of the IPMC sample and of the distribution of PD and T₂ within it. These phenomena were explained in terms of the unusual structure of Nafion and its interaction with host ions and the electric field. Up to 20% of the total water content of the IPMC was found to be involved in long-range electro-diffusion.