Anal. Chem. 1996, 68, 3793-3796
Density and Solubility of Nafion: Recast, Annealed, and Commercial Films Lois Anne Zook and Johna Leddy*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
The density of Nafion membranes wetted in water and acetonitrile and the solubility of Nafion in a 50:50 ethanol/water mixture are reported. Commercially available membranes, solution recast membranes, and recast membranes thermally processed for varying lengths of time were examined. Unheated recast membranes were found to be ∼20% less dense in water than both commercial membranes and recast membranes heated at 140 °C for 40, 50, and 60 min. For heating times of g40 min, the density of recast membranes in water was invariant and equal to the density of commercial films. Similarly, in acetonitrile, commercial membranes and films heated for 1 h had the same density; unheated recast films were ∼25% less dense. Nafion density in acetonitrile was ∼15% less than that in water. Acid pretreatment reduced Nafion film density by ∼15%. Unheated recast films were 22-100% soluble in a sonicated ethanol/water mixture. Commercial membranes and recast films heated for as little as 10 min were insoluble under the same conditions. Electrode surfaces are modified to enhance specificity and reactivity. Modification often entails either binding species such as enzymes and catalysts to the electrode surface or enhancing specificity by excluding solution species by, for example, electrostatic repulsion and size exclusion. The polymer Nafion (E. I. du Pont de Nemours and Company, Inc.) often serves as a matrix for electrode modification, as it is readily cast from solution, and the film provides strong anion exclusion and cation selectivity. The properties of Nafion are linked to its microstructure, and the microstructure is modified by heating. The microstructure of Nafion consists of three regions: the hydrophobic fluorocarbon backbone, hydrophilic ionic clusters of sulfonic acid groups, and an interfacial region.1 The superselectivity of Nafion and its chemical and thermal stability are ascribed to the structure. Studies in the literature focus on three types of Nafion films: commercial membranes, films recast from solution at room temperature,2 and recast films heat-processed either in high-boiling solvents3 or by annealing.4,5 Recast films are more soluble and permeable than commercial membranes and heatprocessed films.6,7 Commercial and annealed films undergo (1) Yeo, S. C.; Eisenberg, A. J. Appl. Polym. Sci. 1977, 21, 875-98. (2) Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Anal. Chem. 1982, 54, 163941. (3) Moore, R. B., III; Martin, C. R. Macromolecules 1988, 21, 1334-9. (4) Michael, A. C.; Wightman, R. M. Anal. Chem. 1989, 61, 2193-200. (5) Fan, Z.; Harrison, D. J. Anal. Chem. 1992, 64, 1304-11. (6) Striebel, K. A.; Scherer, G. G.; Haas, O. J. Electroanal. Chem. 1991, 304, 289-96. (7) Moussy, F.; Jakeway, S.; Harrison, D. J.; Rajotte, R. V. Anal. Chem. 1994, 66, 3882-8. S0003-2700(96)00604-X CCC: $12.00
© 1996 American Chemical Society
morphological changes when heated above the glass transition temperature (Tg ) 109 °C for protonated Nafion).1,8 Cold cast films have a micellar configuration with the sulfonate exchange sites on the outside of the micelle; heat reorganizes the structure to an inverted micelle with the sulfonates on the interior of the micelle. The permeability of Nafion is important in sensors, electrode modifications, and separators in electrochemical cells. The correct interpretation of permeability depends on the thickness of the wetted film. Wetted film thickness is most readily determined from the mass of Nafion and its wetted density. Densities for several types of dry Nafion films9 and the hydrated sodium form of the commercial films10 have been reported. However, we have found no reports of densities for either recast or heat-processed films wetted with solvent. The majority of wetted film thicknesses in the literature are calculated using 1.58 g/cm3. This value was reported by Mauritz and co-workers10 for commercial Nafion 1100 in 1 M sodium ion, where the density was reported as a line without data points (or statistics) for sodium ion concentrations between 1 and 14 M, over which range the density increased linearly by approximately 0.1 g/cm3. Because electrodes are modified with recast and annealed films, and electrochemical measurements are carried out at electrolyte concentrations
