Physical properties and supermolecular structure of perfluorinated ion-containing (nafion) polymers

Abstract

The supermolecular structure and viscoelastic and diffusion properties of a perfluorinated polymer containing sulfonic acid (Nafion) were investigated. The breakdown of time–temperature super-position for the dry salt and and acid in the presence of 0.5 H2O/SO3H as well as the results of small-angle x-ray scattering suggest that the ions in this material are clustered. Above 180°C, the reestablishment of the time–temperature superposition in the salt suggests that ions in the clusters become mobile. Dynamic mechanical studies were performed over a temperature range from −190°C to above the glass transition temperatures Tg of the materials. The Tg of the salts is found at ca. 220°C, while in the acid it occurs at 110°C. A β peak in the acid is found at ca. 20°C, while in the salts it occurs between 140°C and 160°C. The β peak shifts to a lower temperature with the addition of water in both the acid and the salts, while the α and γ peaks are unaffected. The latter is located at ca. −110°C at 1 Hz. Dielectric behavior has also been studied as a function of water content for the acid sample and the potassium salt at frequencies of 100 Hz to 10 kHz. Two relaxations with different activation energies were observed. The position of both peaks shifts to a lower temperature as the water content increases. Finally, the diffusion of water in Nafion in the acid form has been determined. The diffusion coefficient can be represented by the equation