High frequency dielectric studies of hydrated Nafion (R)

This study reports high frequency dielectric measurements (0.045-30 GHz) on Nafion(R) 117 at various states of hydration. A novel technique to measure the broad band frequency dependent real and imaginary parts of the relativ e permittivity is described. The basic experimental configuration and numer ical data analysis are reported along with a discussion of several difficul ties encountered and experimental validation of the method. The preliminary results show a strong dependence of the dielectric constant of the Nafion( R) 117 membrane with water content. The dielectric constant for all hydrate d membrane samples was observed to be constant over the initial part of the frequency span, ranging from a maximum of 20 in the samples with 13 waters per sulfonate to a minimum of four in very dry samples. The results reflec t the decreasing polar environment of the water at low water contents as we ll as the increasing extent of binding of the water at the fixed ionic site at low water content. A 'roll off' in the dielectric constant, the extent of which was dependent on water content, was observed also in the frequency spectrum. Several possible origins for this 'roll-off', including real eff ects of dipolar relaxations occurring in the hydrated polymers and experime ntal effects due to the similarity of the wavelength of radiation to the sa mple dimensions are discussed. Finally, the conductivity of the membranes a t various water contents and at frequencies below 5 GHz was extracted from the loss factor spectra. These values agreed well with previous conductivit y measurements (obtained at lower frequencies) suggesting that no relaxatio ns are observed in the intervening frequency range (roughly 5-50 MHz). (C) 1998 Elsevier Science S.A. All rights reserved.