Howard L. Yeager

Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes

Several properties of short side chain perfluorosulfonate ionomer membrane films, made by the Dow Chemical Company, have been measured as a function of equivalent weight of the polymer. Water and electrolyte sorption and the self-diffusion coefficients of Na+, Cs+, Cl−, and I− have been measured. along with activation energies of diffusion. Also, the near infrared combination band for OH has been used to develop insight into the nature of sorbed water in the ion clusters of these micro-phase separated materials. Results show that these polymers have properties which are similar to Nafion® perfluorinated ionomer membranes, with their diffusional properties being intermediate between those of the sulfonate and carboxylate polymers. Equivalent weight was found to influence the spreads in diffusion coefficients among ions, which increase with increasing equivalent weight. The sorbed water for these polymers show significantly reduced hydrogen bonding strength compared to bulk water, and this property is counter-ion dependent.

Structure and properties of perfluorinated ionomers

Perfluorinated ionomers have proven to be a scientifically interesting class of polymers with a wide range of applications, most notably as permselective membrane separators in chlor-alkali and fuel cells, Yeager and Eisenberg (1982). These membranes were first developed by Dupont in the mid-1960s under the Nafion® trade name. The general structure consists of a tetrafluoroethylene backbone and a vinyl ether side-chain terminating in an ion-exchange group.

Relationships between Polymer Structure and Ion Diffusion in Perfluorinated Ionomer Membranes

Perfluorinated ionomer membranes demonstrate quite unusual ionic diffusional properties compared to more conventional ion exchange polymers. These differences are significant in practical terms, due to the enhanced permselectivity which is seen for these materials. Also, the study of ionic diffusion allows insight to be gained into the relationship between their ion clustered morphology and this permselectivity. It is concluded that cations and anions diffuse largely by different pathways within the ion clustered regions of these polymers.

Factors Which Influence the Permselectivity of High Performance Chlor-Alkali Membranes

Membrane current efficiencies have been determined in a laboratory chlor-alkali cell for several test materials with similar polymer composition and different fabrication features. Experiments were conducted as a function of solution mixing and membrane electrode gap over a wide range of catholyte concentrations. The results show high inherent rejection of hydroxide ion transport by the polymer film, and that this rejection can be altered by the specifics of membrane design and cell geometry and operation. A proposed explanation of the results focuses on the nature of hydroxide ion transport in aqueous media, which involves quantum mechanical proton tunneling processes. The large significance of the membrane’s steady state water content in an operating cell as a factor in the determination of membrane performance is discussed.