Proton Transport Near the Polymer-Water Interface

Proton Transport Near the Polymer-Water Interface 

The confinement of water in nanometer-size pores between the walls of the polymer material affects the structure of water. The observed freezing-point suppression [49,50] and reduced dielectric constant of water [115-117], are macroscopic manifestations of this effect. The interfacial area between polymer and water provides a complex environment for proton transport. The complications for the theoretical description are caused by the flexibiHty of the sidechains, their random distributions and their partial penetration into the bulk of water-filled pores. The charged polymer sidechains contribute elastic ( entropic ) and electrostatic terms to the free energy [54,55]. Distribution and mobilities of protons depend strongly on the resulting sidechain-water interactions [54,56,118]. 

As corroborated above the effect of arrangement and fluctuations of charged surface groups on proton transport in membranes is the more pronoimced the smaller the water content in the membrane is. Squeezing the pore leaves no room for bulk-Uke transfer. Upon deswelling of pores, sidechain separations are likely to decrease [33]. The proton concentration, smface charge density and corresponding electrostatic interactions will thus increase, and explicit sidechain-sidechain correlations will become more pronoimced. The process will lead to with an array of micelles loosely connected by ultrathin aqueous necks, as described in [31]. 

Other experimental data for membranes with varying lEC suggest that mean sidechain separations in the dry state should not drop below 7 A, since otherwise random ionomers could not form a stable phase-separated 

Structure upon water uptake. At allowed separations, direct proton transfer between stiffly fixed SOs groups is still not possible. This situation could change, however, when random configurations of sidechains, sidechain flexibility, and fine structure and smearing of charges of sulfonate groups are taken into consideration [84]. 

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