A Comparison of Mechanical and Electrical Percolation during the Gelling of Nafion Solutions

Rheological and impedance measurements are performed on Nafion solutions at relatively high concentration. The viscosity scales with the Nafion volume fraction as ηr ∝ φ0.59, up to φ = 2.5%. At higher fractions the viscosity diverges. At φ = 2.5% the storage modulus exhibits scaling behavior with the frequency (G‘ ∝ ω0.81), and the solution behaves as a gel. The results for the viscosity seem to fit the Zimm, rather than the Rouse, model. For the sodium form of Nafion the conductivity behaves qualitatively the same as the viscosity, with σ ∝ φ0.89, and can be described by a percolation network of conductors and superconductors. Above φ = 2.5% a sharp increase is observed, and the conductivity scales with the frequency as σ ∝ ω0.75. For Nafion in the proton form at volume fractions higher than 2.5% no sharp increase is observed in the conductivity, and the conductivity is independent of frequency. This indicates that in the proton form no electrical percolation takes place. It is discussed how these results can be used to improve the properties of recast membranes.