Pulsed Field Gradient NMR Investigation of Molecular Mobility of Trimethoxymethane in Nafion Membranes
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Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
drops as EW is increased, there is a trade-off associated with increased EW membranes-increased resistive loss in the cell versus decreased fuel permeation rates. In this investigation, we studied the mobility of water, methanol and TMM in NAFION membranes. We also assessed the effect of changing the equivalent weight of the proton exchange membrane on molecular crossover for both methanol and TMM. We carried out permeation and self diffusion measurements, by gas chromatography and pulsed field gradient (PFG) NMR techniques respectively, in NAFION membranes equilibrated in various solutions of water/TMM and water/methanol. The PFG NMR method is a powerful technique which yields directly the self-diffusion coefficient, D*, of a particular nucleus (e.g. 'H) associated with a diffusing molecule. 2 In certain favorable cases, as in the present investigation, it is possible to distinguish between several species (e.g. water and methanol) containing the same NMR nucleus, and thus determine their D* values separately. Inspection of the NMR results in light of known electrochemical crossover measurements led us to the chromatographic studies. EXPERIMENTAL NAFION membranes were obtained from E.I. DuPont de Nemours, Inc. in two ew forms, EW = 1100, which is commercially available as NAFION-1 17, and EW = 1500. The membranes were pre-treated in aqueous solutions of H 20 2 , and then H 2SO 4, and washed with deionized water, as described previously. 2,3 The PFG NMR measurements were performed on a Bruker AMX-400 NMR spectrometer equipped with a microimaging probe with gradient coils which can sustain a maximum gradient strength of 4.0 mT/cm. Gradients were calibrated with a water standard and temperatures were controlled to ± 1K with a Bruker VT 1000 temperature controller. For the NMR measurements, slivers of membrane of dimensions 8 mm X 15 mm were immersed for several days in aqueous solutions of TMM (obtained from Aldrich), with the TMM concentration varying from 0.5 to 14 M. The membranes were then removed from solution and quickly blotted dry to remove surface liquid, loaded into 10 mm OD glass NMR tubes and sealed with parafilm. The self diffusion coefficients, D* were determined by the pulsed gradient, stimulated spin echo sequence (PGSSE), utilizing the proton NMR signals originating from both water and methyl protons. The determination of methanol permeability in polymer electrolyte membranes was accomplished by measuring the change in concentration as a function of time, by gas chromatographic methods, of two vessels separated by membrane samples. The procedure involved preparing a solution of the permeate and placing it in one vessel, typically 3.OM solutions, while the other vessel (B) contained de-ionized water. Samples were taken from the vessel B at specified times and analyzed with a Varian 3400 gas chromatograph equipped with a Carbowax column. An internal standard, such as a higher molecular weight alcohol, was added to the samples to det
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