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, <i>D</i>, 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. <i>D</i> was evaluated in two orthogonal directions: along one of the long dimensions of the sample (<i>D</i>_<i>x</i>) and through its thickness (<i>D</i>_<i>z</i>). <i>D</i>-maps of the IPMC element were obtained both in the absence of an applied potential and <i>in situ</i> during the application of a 3 V dc potential across the thickness of the sample. In the bare Nafion, <i>D</i>-maps showed uniform values of both <i>D</i>_<i>x</i> and <i>D</i>_<i>z</i> of about 6 × 10⁻¹⁰ m² s⁻¹. In the IPMC two effects were observed: (i) <i>D</i> at the electroded surfaces of the IPMC was higher than at the center of the sample; (ii) this difference was much greater in <i>D</i>_<i>z</i> than in <i>D</i>_<i>x</i>. Both effects were explained by the influence of the impregnated Pt electrodes on polymer structure. The <i>D</i>-maps in the electrochemical measurements showed high values of <i>D</i> (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)_<i>x</i>⁺ species toward the cathode.