Nafion membranes, morphology

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Schematic depiction of the structural evolution of polymer electrolyte membranes. The primary chemical structure of the Nafion-type ionomer on the left with hydrophobic backbone, side chains, and acid head groups evolves into polymeric aggregates with complex interfacial structure (middle). Randomly interconnected phases of these aggregates and water-filled voids between them form the heterogeneous membrane morphology at the macroscopic scale (right).

Figure 6.4 shows that long-range diff usivities of water in Nafion membranes measured by QENS, Di are equal to self-diff usivities determined by PFG-NMR, Dg, at A > 10. In well-hydrated membranes, the major geometric constraints for water mobility due to the phase-segregated, random network morphology of... 

Roche and co-workers used SAXS and small-angle neutron scattering (SANS) to study the morphology of 1200 EW Nafion membranes in the acid and neutralized forms with a range of water contents. ... 

Elliott and co-workers performed a detailed SAXS investigation of the morphology of Nafion membranes that were subjected to uniaxial and biaxial deformation. For as-received membranes, manufactured by Du Pont using an extrusion process, the cluster reflection was shown to exhibit a limited degree of arching in the direction perpendicular to the machine direction. Upon uniaxial extension, this arching was observed to increase in a manner consistent with previous studies. This arching was rationalized on... 

It has been recently reported " that Nafion membranes show an ohmic behavior in 5 M NaOH, while in 10 M NaOH solution the specific conductance of the membranes increases with increasing current density. It is suggested that the passage of high currents at a severely dehydrated membrane may produce morphological changes that alter the character of the ionic conduction paths in the polymer. Hsu et have observed that the membrane conductivity of Nafion in alkaline electrolyte exhibits ion percolation behavior and can be described by... 

Water, sodium ion, and hydroxide ion concentrations have been measured within the membrane phase as a function of bulk caustic solution concentration and temperature. These internal membrane concentrations are important because of their influence on the membrane polymer morphology, structural memory, plasticity and the resultant effects on its internal resistance, viscoelasticity and material transport. In addition, the self-diffusion coefficient of the sodium ions in various Nafion membranes has been measured as a function of temperature and external caustic concentration... 

There is now general consensus that a hydrated PFSA membrane forms a two-phase system consisting of a water-ion phase distributed throughout a partially crystallized perfluorinated matrix phase [25,26,27]. The crystallized portion of the membrane cross-links the polymer chains, preventing complete dissolution of the polymer at temperatures below the glass transition temperature of the polymer [25] ( 405 K for Nafion [26]). For a detailed review and discussion of membrane morphology, readers are referred to Weber and Newman [27], and to Kreuer et al. [16]... 

Morphology of Nafion Membranes Microscopic and Mesoscopic Modeling... 

Morphology of Nafion Membranes 463 known, e.g., the Percus-Yevick (PY) closure or the hypernetted chain (HNC)... 

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