ARTICLES High Pressure NMR Study of Water Self-Diffusion in NAFION-117 Membrane J. R. P. Jayakody, ² P. E. Stallworth, ‡ E. S. Mananga, ² J. Farrington-Zapata, ² and S. G. Greenbaum* ,² Physics Department, Hunter College of the City UniVersity of New York, New York, New York 10021, and Physics Department, City College of the City UniVersity of New York, New York, New York 10031 ReceiVed: NoVember 26, 2003; In Final Form: February 9, 2004 Measurements of the self-diffusion coefficient of water in NAFION-117 as a function of pressure have been carried out. The natural field gradient of a 7.3 T superconducting magnet was used for the diffusion measurement. The measurements were carried out at 288 K and variable pressure up to 0.25 GPa. The high- pressure diffusion data were obtained for four different water contents between 6.6 and 22 wt %. The calculated activation volume decreased from 13.2 to 2.7 cm 3 /mol as the water content increased from 6.6 to 22 wt %. These results agree with previously published activation volumes extracted from electric conductivity and NMR T 1 measurements. It is suggested that segmental motions within the polymer play a significant role in the transport mechanism for low water content membranes, and that the transport mechanism for high water content membranes is similar to that in liquid bulk water. Introduction There are a number of powerful NMR spectroscopic tech- niques that yield information for both structural and dynamic properties of materials. Additionally, diffusion and flow can be measured with good precision using NMR methods. When NMR is applied to the study of electrolytes, the gathered self-diffusion coefficients can be of great importance in characterizing ionic conduction and molecular motion. In this study, self-diffusion coefficients are measured and then used to evaluate mass transport in NAFION, a model material useful for hydrogen fuel cell applications. In general, the study of ion and molecular transport in polymers can be greatly assisted by employing pressure as the thermodynamic variable. Although many investigations of NMR self-diffusion and electrical conductivity have been carried out in proton conducting membranes, 1-11 and some electrical conductivity and NMR spin-lattice relaxation time (T 1 ) studies as a function of applied pressure have been published, 12-17 to our knowledge, no work has been reported on the effect of high pressure on self-diffusion in a membrane. The NMR signal is dominated by water in the kinds of membranes used in fuel cells; nevertheless, it has been shown that water mobility is strongly correlated with electrical conductivity in these materials. 2,7 Molecular motions and ionic diffusion are associated with volume fluctuations that can be probed directly by employing pressure as the thermodynamic variable. It is of interest to compare these new results (pressure dependent diffusion measurements) with previous work reported for variable pressure electric conductivity and NMR T 1 mea- surements for NAFION membranes at different water contents. Activation volumes for four different water concentrations in NAFION-117 are reported in the present work. Self-diffusion measurements are customarily made by the pulsed gradient spin-echo method, 20 but in this work the static gradient associated with the fringe field of the NMR magnet was used. Two advantages of the static gradient are its typically much larger magnitude than obtainable with NMR probe gradient coils, and the relative ease of incorporating a pressure cell into the experiment. Experimental Details All studies were carried out on NAFION-117 supplied by E. I. du Pont de Nemours. Samples were made from 13 rectangular strips of film comprising a stack with dimensions 0.6 cm × 0.5 cm × 1.8 mm. Samples were first dried in a vacuum oven at 50 °C for several days and then saturated in distilled water for a few hours. After this preparation, the sample weight increased due to water uptake such that 100% × (weight of absorbed water)/(dry weight of NAFION) ) 22%. Variation of the water uptake in samples was achieved by exposing the saturated sample (22 wt %) to the atmosphere for a limited time until the required water percentage was reached. For NMR measure- ments, the samples were hermetically sealed in thin polyethylene bags, which were shown to produce a negligible proton NMR background signal compared to the signal from the NAFION. This was necessary to isolate the sample from the pressure transmitting fluid (hydrogen-free FLUORINERT electronic fluid, FC-77 manufactured by 3M Co.). The naturally existing field gradient of a conventional 7.3 T superconducting magnet was used for the measurements. The central field and gradient strength were varied continuously, within the limits of the magnet, by moving the NMR probe head within the bore of the magnet. The position of the NMR coil (which contains the sample) determines both the resonant frequency and the magnetic field gradient. A home-built ² Hunter College of the City University of New York. ‡ City College of the City University of New York. 4260 J. Phys. Chem. B 2004, 108, 4260-4262 10.1021/jp037621+ CCC: $27.50 © 2004 American Chemical Society Published on Web 03/17/2004