Equivalent weight (EW) is the number of grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form. This is an average EW in the sense that the comonomer sequence distribution (that is usually unknown to the investigator and largely unreported) gives a distribution in m in this formula. EW can be ascertained by acid-base titration, by analysis of atomic sulfur, and by FT-IR spectroscopy. The relationship between EW and m is EW ) 100m + 446 so that, for example, the side chains are separated by around 14 CF2 units in a membrane of 1100 EW. Common at the time of this writing are Nafion 117 films. The designation “117” refers to a film having 1100 EW and a nominal thickness of 0.007 in., although 115 and 112 films have also been available. Early-reported studies involved 1200 EW samples as well as special experimental varieties, some being rather thin. The equivalent weight is related to the property more often seen in the field of conventional ion exchange resins, namely the ion exchange capacity (IEC), by the equation IEC ) 1000/EW. The mention of the molecular weight of high equivalent weight (EW > 1000 g‚mol-1) Nafion is almost absent in the literature, although the range 105-106 Da has been mentioned. As this polymer does not form true solutions, the common methods of light scattering and gel permeation chromatography cannot be used to determine molecular weight as well as the size and shape of isolated, truly dissolved molecules. Studies of the structure of this polymer in solvent (albeit not a true solution) will be mentioned in the scattering section of this review. It should be noted that Curtin et al. performed size exclusion chromatography determinations of the molecular weight distribution in Nafion aqueous dispersions after they were heated to high temperatures (230, 250, and 270 °C).1 Before heating, there was a high molecular weight shoulder on a bimodal distribution, due to molecular aggregates, but this shoulder disappeared upon heating, which indicated that the aggregates were disrupted. The peaks for the monomodal distribution for the heated samples were all located at molecular weights slightly higher than 105 g‚mol-1. Also, light scattering experiments revealed that the radius of gyration had a linear dependence on the molar mass of the aggregates, which suggests that the particles are in the form of rods or ribbons, or at least some elongated structure. Nafion ionomers are usually derived from the thermoplastic -SO2F precursor form that can be extruded into sheets of required thickness. Strong interactions between the ionic groups are an obstacle to melt processing. This precursor does not possess the clustered morphology that will be of great concern in this article but does possess Teflon-like crystallinity which persists when the sulfonyl fluoride form is converted to, for example, the K+ form by reacting it with KOH in water and DMSO. Thereafter, the -SO3H form is achieved by soaking the film in a sufficiently concentrated aqueous acid solution. Extrusion of the sulfonyl fluoride precursor can cause microstructural orientation in the machine direction, * Address correspondence to either author. Phone: 601-266-5595/ 4480. Fax: 601-266-5635. E-mail: Kenneth.Mauritz@usm.edu; RBMoore@usm.edu. 4535 Chem. Rev. 2004, 104, 4535−4585

State of Understanding of Nafion

Advanced functional polymer membranes

There is disclosed hemoglobin modified in that a poly(alkylene oxide) is bonded thereto by a bond between a terminal group of said poly(alkylene oxide) and an amino group of said hemoglobin, said bond having the structure: --CH.sub.2 --O--(CH.sub.2)--CONH--Hb wherein Hb represents said hemoglobin and n represents a positive integer, preferably 1 to 7. The modified hemoglobin is an effective oxygen carrier and can therefore be used as a blood substitute.

Hemoglobin combined with a poly(alkylene oxide)

Recent advances in perfluorinated ionomer membranes : structure, properties and applications

Pervaporation is one of the most active areas in membrane research, and the pervaporation process has been shown to be an indispensable component for chemical separations. In this paper, the recent development in pervaporation membranes and pervaporation processes is reviewed, and some outstanding questions involved in membrane pervaporation are discussed with emphasis on the following issues:  mass transport in the membrane, membrane material selection, concentration polarization in the boundary layer, pressure buildup in hollow fiber membranes, asymmetric and composite membranes, and the activation energy for permeation. We attempt to provide insight into this dynamic field and to highlight some of the outstanding problems yet to be solved or clarified.

Liquid Separation by Membrane Pervaporation: A Review

Industrial state-of-the-art of pervaporation and vapour permeation in the western countries

Sorption, diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis

Some experimental data are given on the infrared spectra between 3300 and 3500 cm−1 of dilute solutions in carbon tetrachloride of three types of model compounds: CH3−CONH-CH(R1)-CONH(R2), (I); CH3-CON(CH3)-CH(R1)-CONH(R2), (II) and CH3-CONH-CH(R1)-CON(R2)2, (III). In studying the N-H stretching bands, it was found that there are two types of intramolecular hydrogen bonds in these molecules; these result in two different cyclized conformations, C5 and C7, which contain respectively, five and seven atoms in the ring. By using model substances I, II, and III, in which the nitrogen atoms are unequally substituted, it is possible to identify the N-H stretching bands which are to be ascribed to the N-H oscillators included in the two different chelated conformations. It is found also that the stretching frequency of a free N-H oscillator depends upon the substituent on the nitrogen atom. Thus, it is possible to observe, with some of the model compounds I, four different absorption bands located at 3340, 3420, 3440, and 3460 cm−1. The first two are ascribed to the N-H oscillators included in the Hbonds which lock the C7 and C5 conformations; the last two correspond to free N-H which differ with the substituent on the nitrogen atom.

Etude, par spectroscopie infra-rouge, de la conformation de quelques composés peptidiques modèles

Sorption isotherms of various organic solvents and aqueous mixtures into dense polydimethylsiloxane (PDMS) membranes have been analysed and the results are compared with Flory–Huggins theory, already reported to hold for solvent sorption into elastomeric materials. The original Flory–Huggins equation, as well as more complex versions derived later by different authors (variable interaction parameter, elastic contribution, modified entropic contribution) have been screened; it appears that although excellent agreement is offered by the Flory–Huggins simplest expression for good PDMS solvents (e.g. hydrocarbons and halogenated hydrocarbons), none of the modified Flory–Huggins theories leads to satisfactory data interpretation for poor solvents (ketones, alcohols), unless at least three adjustable parameters are used for the enthalpic contribution term. Cluster formation at high solvent activity, considered on the basis of Zimm–Lundberg clustering analysis, is supposed to explain the observed deviations for poor solvents. A semi-empirical power law (Freundlich type) gives the best fit in the latter case.

Jean Neel

Papers

Industrial state-of-the-art of pervaporation and vapour permeation in the western countries

Sorption, diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis

Etude, par spectroscopie infra-rouge, de la conformation de quelques composés peptidiques modèles

Sorption of organic solvents into dense silicone membranes. Part 1.—Validity and limitations of Flory–Huggins and related theories

The engaged species induced clustering (ENSIC) model: a unified mechanistic approach of sorption phenomena in polymers