Time-Resolved Mapping of Water Diffusion Coefficients in a Working Soft Actuator Device

Diffusion-weighted imaging was employed to spatially map the distribution of the diffusion coefficient of water, D, in bare, water-soaked, Li⁺-exchanged, cast Nafion and in an ionic polymer−metal composite (IPMC) soft actuator element, prepared from this bare Nafion by impregnation with Pt electrodes. D was evaluated in two orthogonal directions: along one of the long dimensions of the sample (D_x) and through its thickness (D_z). D-maps of the IPMC element were obtained both in the absence of an applied potential and in situ during the application of a 3 V dc potential across the thickness of the sample. In the bare Nafion, D-maps showed uniform values of both D_x and D_z of about 6 × 10⁻¹⁰ m² s⁻¹. In the IPMC two effects were observed: (i) D at the electroded surfaces of the IPMC was higher than at the center of the sample; (ii) this difference was much greater in D_z than in D_x. Both effects were explained by the influence of the impregnated Pt electrodes on polymer structure. The D-maps in the electrochemical measurements showed high values of D (up to 8 × 10⁻¹⁰ m² s⁻¹) at the cathode and low values (from 1 × 10⁻¹⁰ m² s⁻¹) at the anode. This was explained in terms of the effect on the Nafion nanostructure of the forced electro-migration of Li(H₂O)_x⁺ species toward the cathode.