Charge transport and water molecular motion in variable molecular weight nafion membranes: High pressure electrical conductivity and NMR

Abstract

Measurements of the electrical conductivity and deuteron NMR spin-lattice relaxation times ( T 1) in three different molecular weights of acid form NAFION conditioned at various levels of relative humidity have been carried out. Complex impedance studies were made along the plane of the film at frequencies from 10-10 8 Hz at room temperature and pressures up to 0.3 GPa. The high pressure electrical data were only obtained for water contents less than 8 wt%. The NMR measurements were also made at room temperature and pressures up to 0.25 GPa. The NMR data are primarily for water contents greater than 6 wt%. The calculated activation volume exhibits a large decrease (from 16 to 3 cm 3/mol) as the water content is increased from 2.4–8 wt%. In addition, the activation volumes are larger and the electrical conductivity is smaller for the higher molecular weight material. These results represent further evidence that the transport mechanism in low water content materials is dominated by segmental motions of the polymer chain and that proton transport and water molecular rotation are correlated. The activation volumes extracted from the NMR data show only a small further decrease as the water content is increased from 6–22 wt%. Possible explanations for the high water content NMR pressure results are given.