Membranes fluorocarbon

Other hand, oxyfluorination is a technique offering unique possibiUties for the functionalization of fluorocarbons and for the preparation of functional fluorocarbon membranes (39). 

The hydrocarbon membrane can also operate at higher temperatures, of up to 95°C, which allows the use of smaller radiators to dissipate heat. It also lasts 50% longer, while generating up to 15% more power and operating at lower humidity levels. Fluorocarbon membranes can cost about 300 per square meter, the PolyFuel materials cost about half of this. While hydrocarbon membranes may have to prove themselves to many, Honda s FCX fuel cell cars use them. 

Photocyclisation - A wide variety of ring-forming reactions has again been reported. Irradiation of azepine derivative (16) results in 4-n-electrocyclisation to a mixture of the corresponding exo and endo cyclobutenes.6-Ti-Electrocyclisa-tion has been employed in a scaled-up synthesis (>300g) of 6-aza-l,10-phenan-throic anhydride (18) from the stilbazole (17). f-Azobenzene, incorporated into water-swollen acid-form Nafion fluorocarbon membranes, exists as the proto-nated form (19) and exhibits ambient temperature fluorescence, previously... 

Kobos et al. (1988) described the adsorption of urease on a fluorocarbon membrane for the construction of a urea sensor. The spontaneous adsorption was enhanced by a factor of 7 by perfluoroalkylation of the amino groups of the enzyme. The enzyme membrane was attached to an ammonia gas-sensing electrode. The urea sensor thus prepared exhibited a sensitivity of 50 mV per decade of urea concentration and a response time of 3 min. Only a small amount of enzyme could be adsorbed on the limited membrane surface, so the sensor was stable for only 7 days. 

Fluorocarbon membranes can cost about 300 per square meter, the PolyFuel materials should cost about half of this. While hydrocarbon membranes have yet to prove themselves to many, Honda s latest FCX fuel cell cars use them. 

Development of Alternative Membranes. Nafion membranes show considerable promise with respect to their performance characteristics, low resistivity and long-term stability. However, the present cost (about 30/ft ) of Nafion membranes is rather expensive for the SPE cell to be cost-effective for industrial and utility applications. TJje ultimate goal is to reduce the membrane cost to about 3/ft. Alternative membranes have been evaluated for use in the SPE cell. Only fluorocarbon membranes have been considered in this application (10), since the cell environment is corrosive, and cell tempperature is high (from 80°C to 150°C). <, 3, 3 - trifluorostyrene-sulfonic acid... 

Frankenfeld. Li, and Asher" 49 4 have described an apparatus quite similar to dial shown in Fig. 19.4-6B for the extracorporeal oxygenation of blood. Bubbles of gaseous Oj. encapsulated in a fluorocarbon membrane, rise ihrongh oxygen-depleted blood. As they do so oxygen diffuses from the membrane into the blood while CD2 differes in the opposite direction and is swept ont. This process was reviewed earlier. 1" The key to its potential success is the use of fluorocarbons in the membrane phase.11,45 4 Fluorocarbons are uniquely compatible with human blood and circumvent the damage to blood cells which is encountered with conventional devices. 

D.E. Maloney, Fluorocarbon Membranes for Chlor-Alkali Industry. In M.O. Coulter (ed.) Modem Chlor-Alkali Technology, Ellis Horwood Ltd, Chichester, Vol. 1 (1980), p. 173. 

Ohta et al. (1991) looked at using a partially hydrophilic dense fluorocarbon composite membrane and tested it for seawater desalination. The authors used the term MD for both porous and dense membranes. DCMD configuration was used and the obtained fluxes (<6kg/m h) were of similar magnitude to those obtained with porous hydro-phobic membranes. The effects of the DCMD operation parameters were similar to those observed using a single porous hydrophobic layer. It was also found that the permeability and thermal efficiency when using fluorocarbon membrane were superior than those of... 

The ion-pair extraction and colorimetric determination of cationics described in Chapter 12 has been automated using flow injection analysis. Orange n is a suitable anionic dye. A fluorocarbon membrane permits the separation of the organic from the aqueous phase for color measurement. Addition of methanol to the chloroform/water system aids in the efficiency of the extraction, so that nearly identical molar responses are obtained for a variety of cationic surfactants. As in the manual method, use of a neutral pH allows determination of only quatemaiy surfactants, while an acid pH permits determination of both quats and fatty amines (85,86). A flow injection method based on tetrabro-mophenolphthalein ethyl ester eliminates interference from amines by measuring the absorbance at 45 C, at which temperature the ion associates with amines are colorless (87). The amine interference may also be eliminated by conducting the determination at high pH (88). 

To be ionicaUy conducting, the fluorocarbon ionomer must be wet under equilibrium conditions, it will contain about 20 percent water. The operating temperature of the fuel cell must be less than 373 K (212°F), therefore, to prevent the membrane from drying out. 

A variety of ionomers have been described in the research literature, including copolymers of a) styrene with acrylic acid, b) ethyl acrylate with methacrylic acid, and (c) ethylene with methacrylic acid. A relatively recent development has been that of fluorinated sulfonate ionomers known as Nafions, a trade name of the Du Pont company. These ionomers have the general structure illustrated (10.1) and are used commercially as membranes. These ionomers are made by copolymerisation of the hydrocarbon or fluorocarbon monomers with minor amounts of the appropriate acid or ester. Copolymerisation is followed by either neutralisation or hydrolysis with a base, a process that may be carried out either in solution or in the melt. 

Diffusion of cations in a Nation membrane can formally be treated as in other polymers swollen with an electrolyte solution (Eq. (2.6.21). Particularly illustrative here is the percolation theory, since the conductive sites can easily be identified with the electrolyte clusters, dispersed in the non-conductive environment of hydrophobic fluorocarbon chains (cf. Eq. (2.6.20)). The experimental diffusion coefficients of cations in a Nation membrane are typically 2-4 orders of magnitude lower than in aqueous solution. 

In contrast to past environmental problems associated with fluorocarbon refrigerants, the exceptional properties of fluorine in polymers have great environmental value. Some fluoropolymers are enabling green technologies such as hydrogen fuel cells for automobiles and oxygen-selective membranes for cleaner diesel combustion. 

Most automotive fuel cells use a thin, fluorocarbon-based polymer to separate the electrodes. This is the proton exchange membrane (PEM) that gives this type of fuel cell its name. The polymer provides the electrolyte for charge transport as well as the physical barrier to block the mixing... 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

In a subsequent communication, Elliott and coworkers found that uniaxially oriented membranes swollen with ethanol/water mixtures could relax back to an almost isotropic state. In contrast, morphological relaxation was not observed for membranes swollen in water alone. While this relaxation behavior was attributed to the plasticization effect of ethanol on the fluorocarbon matrix of Nafion, no evidence of interaction between ethanol and the fluorocarbon backbone is presented. In light of the previous thermal relaxation studies of Moore and co-workers, an alternative explanation for this solvent induced relaxation may be that ethanol is more effective than water in weakening the electrostatic interactions and mobilizing the side chain elements. Clearly, a more detailed analysis of this phenomenon involving a dynamic mechanical and/ or spectroscopic analysis is needed to gain a detailed molecular level understanding of this relaxation process. 

Figure 6. Evolution of the membrane structure as a function of water content, 1 (moles of water per mole of sulfonic acid sites). The pictures are cross-sectional representations of the membrane where the gray area is the fluorocarbon matrix, the black is the polymer side chain, the light gray is the liquid water, and the dotted line is a collapsed channel. (Reproduced with permission from ref 89. Copyright 2003 The Electrochemical Society, Inc.)...

Most newer diaphragm materials are membranes made of highly alkali-resistant fluorocarbon polymers that incorporate cation-exchanging functional groups such as sulfonate.8 One such material, Nafion (Du Pont trade-name), is... 

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