Highly fluorinated ion-exchange polymers, such as the sulfonyl type disclosed in U.S. Pat. No. 3,282,875, are used in membrane form as separators in electrochemical cells. The polymers are also useful as acid catalysts. These applications first used melt-fabricated, i.e. melt-processed, shapes such as films and pellets. Because the ion-exchange polymers are difficult to melt-process in the ionic form, fabrication is carried out on polymer in a melt-processible precursor form, and the fabricated article is then hydrolyzed to convert the polymer to the ionic (also referred to as the ion-exchange) form. Later, methods were discovered for making liquid compositions of highly fluorinated ion-exchange polymers: U.S. Pat. Nos. 4,433,082 and 6,150,426. From such liquid compositions, ion-exchange membranes can be made by film-casting techniques. Catalyst can be made by coating liquid compositions on inert substrates. Liquid compositions have also found use in making electrodes for fuel cells.
Dissolution of the above polymers in their ionic forms requires solvents and temperatures sufficient to overcome the forces that hold the polymer together in the solid state. These forces include the polar attractions of the ionic groups for one another. Polar solvents such as water and alcohol can solvate the ionic groups of the polymer, weakening their interaction, and promoting dissolution. Heat further weakens intermolecular attractions.
Liquid compositions are typically made by putting highly fluorinated ion-exchange polymer that is in the sulfonic acid form, in alcohol, in water, or in aqueous alcohol, and heating the combination to achieve the dissolution of the polymer. The temperatures necessary are generally 220° C. or higher. These temperatures are above the boiling point of the solvent at atmospheric pressure and therefore the dissolution is conducted in an autoclave. Higher temperatures are necessary with water. Alcohol is a better solvent, and lower temperatures can be used. However, the formation of side products such as ether and olefin through the reaction of the alcohol with the strongly acid polymer contributes to the development of pressure during dissolution. Water-alcohol, i.e. aqueous alcohol, is effective at lower temperatures than are necessary with water alone, and also does not develop pressures so high as occur with alcohol alone. Nevertheless, though reduced in quantity, side products still form, adding to reaction pressure, requiring separation from the liquid composition and disposal, and resulting in loss of solvent. New solvents are needed that are effective at lower temperatures and that produce less side product.