CHARGE-TRANSPORT AND WATER MOLECULAR-MOTION IN VARIABLE MOLECULAR-WEIGHT NAFION MEMBRANES - HIGH-PRESSURE ELECTRICAL-CONDUCTIVITY AND NMR

Measurements of the electrical conductivity and deuteron NMR spin-latt ice relaxation times (T-1) in three different molecular weights of aci d 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 pr essures up to 0.3 GPa. The high pressure electrical data were only obt ained for water contents less than 8 wt%. The NMR measurements were al so made at room temperature and pressures up to 0.25GPa. The NMR data are primarily for water contents greater than 6wt%. The calculated act ivation volume exhibits a large decrease (from 16 to 3 cm(3)/mol) as t he water content is increased from 2.4-8 wt%. In addition, the activat ion 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 i s 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 explanation s for the high water content NMR pressure results are given.