Water–polymer interfacial area in Nafion: Comparison with structural models
(1971-1974) Polymer 52 #9 (2011)
Kong and  Schmidt-Rohr, Iowa State University, Iowa, compared published experimental polymer interfacial area per volume, S/V data of Nafion over a wide range of hydration levels with various structural models featuring stiff polymer backbones, in particular the parallel water-channel and the polymer ribbon models.  The S/V curve at intermediate hydration levels typical of fuel-cell conditions (ca. 20 vol% water) matches that of the parallel water-channel model with molecular corrugation.  At higher hydration levels, i.e. for membranes soaked in water or autoclaved at elevated pressures, the polymer-ribbon model matches the decreasing S/V ratio with increasing water content, while the polymer-bundle model predicts a higher surface area. However, the ribbon or bundle models cannot apply at low hydration (<3 water molecules per Nafion side group), since we show that the interfacial area in this regime must increase strongly with hydration, being determined by the available surface area of the water molecules. The pronounced asymmetry of the plot of S/V vs. water volume fraction is explained in terms of the difference in the diameters of the water molecules and the polymer aggregates.  (RDC 6/1/2011)

Surface-Induced Micelle Orientation in Nafion Films
(2893–2899) Macromolecules 44 #8 (2011)
Bass et al of Ben-Gurion University of the Negev, Israel, used grazing incidence small angle exray scattering to examine the surface structure of thin Nafion films spin-cast on hydrophobic silanized Si substrate and exposed to water and vapor.  Results showed that in vapor Nafion micelles at the surface tend to align parallel to the surface, while under water they are preferentially oriented normal to the surface, in agreement with previously proposed structural picture and with AFM and contact angle results.  However, spectra at near-critical incidence angles representing the bulk of the films indicate the micelles preferentially align normal to the surface, regardless of the external phase.  This observation is in contrast with previous studies that employed hydrophilic native Si substrates, in which the observed micelle orientation was parallel to the surface.  The proposed explanation assumes that lateral interactions with octadecyl tails and/or heterogeneities at the silanized Si surface, could be responsible for perpendicular alignment in the present case. The heterogeneities could allow preferential interactions of the substrate with both microphases of Nafion, rather than with one microphase, as in the case of bare Si. The observed effect of substrates on micelle orientation suggests an attractive possibility of enhancing transport within Nafion films in a desired direction.  (RDC 5/27/2011)