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College Park, Maryland      June 6 - 10 , 2004

MP20: New structural model for Nafion® membranes and Study of proton dynamics

O. DIAT (UMR 5819 SPrAM CEA Grenoble 17 av des martyrs, 38054 Grenoble cedex 9 France), L. Rubatat (UMR 5819 SPrAM CEA Grenoble 17 av des martyrs, 38054 Grenoble cedex 9 France; Department of Physics, Simon Fraser University, Burnaby, Canada)

A global understanding of the ionomer Nafion® membrane structure, a benchmark system, from the Ångström up to the micrometer in relation with the transport properties, is essential for the optimisation of H2/O2 fuel cell system.

We propose a new fibrillar structural model, based on polymeric chains aggregation in elongated objects, decorated with charges to explain i) the swelling process from the dry membrane to colloidal suspension, ii) the birefringence properties, iii) and the mechanical degradation in fuel cell operation. By using small angles scattering techniques (x-rays & neutrons) and microscopy (AFM & TEM), we have studied the polymer arrangement in bundles over a large range of scale and water content. Drawing experiments have permitted to confirm our model. The main characteristic of the material being to exhibit a hierarchical multi-scale structure, from nanometric aggregates to micronic domains of varying densities, the relaxation times associated to the proton dynamics in these systems are expected to range from the pico-second (local jump) to few tenths of seconds (macroscopic conductivity).

We have started a complete study of the proton dynamics with various complementary techniques, in order to couple these results to the knowledge that we have now on the structure. Quasi-elastic neutron scattering studies allowed to measure correlations at very short times (pico-second) and gradient field-pulsed NMR allows to measure macroscopic diffusion coefficients in the range of few milliseconds to few seconds. Intermediary region in between is directly accessible by the field cycling NMR relaxometry technique. Our paper is more especially dedicated to the interest of “neutron” tools in using contrast variation method for SANS and QENS.

A.L. Rollet et al, J. Phys. Chem B 106(12), (2002) 3033-3036.
L.R. Rubatat et al, Macromolecules 35(10), (2002) 4050-4055.
L. Rubatat et al under submission to Macromolecules;
P. van der Heijden et al submitted to Macromolecules.

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