Complex impedance studies of S-SEBS block polymer proton-conducting membranes

Water uptake, swelling, H-1 pulsed gradient spin-echo nuclear magnetic resonance (NMR) and variable temperature and pressure complex impedance/electrical conductivity studies have been carried out on sulfonated styrene/ethylene-butylene/styrene (S-SEBS) triblock polymer proton conducting membranes. At the highest water contents, the activation volume calculated from the effect of pressure on the electrical conductivity is negative. Previously reported results for Nafion 117 show the same behavior. In addition, above about 10 wt% water, the diffusion coefficients, D from NMR and D-sigma calculated from conductivity data, are similar for S-SEBS. The same result is obtained for Nafion 117. The conclusion is that proton transport at high water content is by molecular diffusion for both materials. For low water contents, however, the materials are significantly different. For low water content S-SEBS, D and D-sigma are different while they are the same for Nafion 117. In addition, the variation of the conductivity with temperature for S-SEBS is Arrhenius while that for Nafion 117 is not. Finally, the variation of the electrical conductivity with pressure gives rise to activation volumes on the order of 14 cm(3)/mol for S-SEBS while those for Nafion 117 are about four times larger. These results indicate that proton transport in low water content S-SEBS occurs via a thermally activated process (ion motion via energy barriers) that is consistent with the more rigid side chains in that material. (C) 2001 Elsevier Science Ltd. All rights reserved.