By Lorenz Gubler, Günther G. Scherer (auth.), Günther G. Scherer (eds.)

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These considerations evolved later into a new structural model of Nafion-type membranes, which gives a continuous transition between dry membrane state and highly diluted polymer solution [40]. Gebel’s group utilized SAXS, SANS and USAXS (Ultra Small-Angle X-ray Scattering) techniques to study characteristic dimensions of hydrated Nafion membranes from 1 nm to 1000 nm. The USAXS upturn in scattering curves was related to large-scale inhomogeneities in electron density (∼100 nm). The so-called matrix-knee corresponds to the supralamellar distance in the crystalline domains.

Pronounced effects of pore size on activation energies of proton transfer and pore conductances were found in good agreement with experimental conductivity data [50]. The refined Poisson–Boltzmann theory in [84], based on a more realistic electrostatic model of surface charges, showed activation energies that were almost independent of pore sizes. 36 eV in “dry” PEMs. Seemingly, the transition from bulk to surface mechanism of proton transport upon pore deswelling is not simply due to a rescaling of electrostatic interactions but it involves more dramatic reorganizations of the interfacial conformations which should invoke full ab initio studies.

As corroborated above the effect of arrangement and fluctuations of charged surface groups on proton transport in membranes is the more pronounced the smaller the water content in the membrane is. Squeezing the pore leaves no room for bulk-like transfer. Upon deswelling of pores, sidechain separations are likely to decrease [33]. The proton concentration, surface charge density and corresponding electrostatic interactions will thus increase, and explicit sidechain-sidechain correlations will become more pronounced.

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