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

Water uptake, swelling, H-1 pulsed gradient spin-echo nuclear magnetic reso nance (NMR) and variable temperature and pressure complex impedance/electri cal conductivity studies have been carried out on sulfonated styrene/ethyle ne-butylene/styrene (S-SEBS) triblock polymer proton conducting membranes. At the highest water contents, the activation volume calculated from the ef fect of pressure on the electrical conductivity is negative. Previously rep orted results for Nafion 117 show the same behavior. In addition, above abo ut 10 wt% water, the diffusion coefficients, D from NMR and D-sigma calcula ted from conductivity data, are similar for S-SEBS. The same result is obta ined for Nafion 117. The conclusion is that proton transport at high water content is by molecular diffusion for both materials. For low water content s, however, the materials are significantly different. For low water conten t S-SEBS, D and D-sigma are different while they are the same for Nafion 11 7. In addition, the variation of the conductivity with temperature for S-SE BS 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 abo ut four times larger. These results indicate that proton transport in low w ater content S-SEBS occurs via a thermally activated process (ion motion vi a energy barriers) that is consistent with the more rigid side chains in th at material. (C) 2001 Elsevier Science Ltd. All rights reserved.