PERMEABILITY OF GLUCOSE AND OTHER NEUTRAL SPECIES THROUGH RECAST PERFLUOROSULFONATED IONOMER FILMS

The effect of casting solvent choice and thermal curing on the permeability and diffusion coefficients of substrates in the perfluorinated sulfonic acid ionomer, Nafion, has been examined for the neutral species glucose [pH 13], hydroquinone (QH2) [pH 7.4], and ascorbic acid [pH 1.3], as well as the cation [(trimethylammonio)methyl]methyl]ferrocene (FcTMA+) [pH 7]. Glucose and ascorbic acid exhibit effective diffusion coefficients, alpha-D(m) (alpha is the partition coefficient of substrate between solution and Nafion) of 3.4 and 22 X 10(-8) cm2/s, respectively, in Nafion films cast from alcohol solutions and cured at room temperature. Curing films cast from alcohol at 120-degrees-C reduces alpha-D(m) for QH2 from 5.4 to 2.6 X 10(-8) cm2/s, and from 1.8 to 1.1 X 10(-8) cm2/s for FcTMA+. There is no significant change in film thickness as a result of the curing procedure. The use of dimethylformamide as the coating, solvent gives alpha-D(m) values of 2.7 X 10(-8) cm2/s for QH2 when cured at room temperature and 0.7 X 10(-8) cm2/s when cured at 120-130-degrees-C. These results indicate the role both casting solvent and temperature play in affecting permeation characteristics. In addition, ellipsometry was shown to be an effective means of measuring the film thickness, by cross correlation to surface profilometry measurements on the same coatings.