Free volume and percolation in S-SEBS and fluorocarbon proton conducting membranes

Electrical conductivity results at a variety of pressures, temperatures and water contents are evaluated for sulfonated styrene/ethylene–butylene/styrene (S-SEBS) triblock polymer, Nafion 117, and Dow 800 proton conducting membranes. In addition, room temperature and atmospheric pressure diffusion coefficients determined from conductivity and 1H pulsed gradient spin-echo nuclear magnetic resonance (NMR) studies are considered. While the S-SEBS and fluorocarbons exhibit a percolation threshold at 10 and 4 wt.%, respectively, all materials exhibit this phenomenon at a volume water fraction of C≈0.05. Above the threshold the conductivity exhibits a power law behavior. When the volume of the hydrophobic portion of the membrane is subtracted the threshold occurs at the adjusted volume fraction of CA≈0.12 which approaches that expected for continuum percolation. The activation volume results are shown to be consistent with free volume considerations.