Transport fluorocarbons

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. ... 

It was also mentioned by McLean et al. that ions and polar solvent molecules must necessarily diffuse across thin amorphous fluorocarbon or interfacial regions between swollen polar domains. However, all of this does not require the need for channels with diameters of 10 A that are coated with SOs groups for long-range transport. In any case, a simple consideration of the steric volume of SOs groups in relation to the size of these channels leads to the conclusion that more than one group would have difficulty fitting into this very small volume. Related... 

This chapter presents the state of the art of the use of highly fluorinated liquids in ophthalmology and perspectives of future applications in the eye. In different medical disciplines, the characteristics of these fluids are directly used, like in the case of ocular endotamponades in ophthalmology, of gas carriers in liquid ventilation, or of preservation and transport media in transplantation medicine [1-3]. For these applications, the highly fluorinated liquids are used in a purified form or as mixtures. The intended effect is created by the physicochemical characteristics themselves. The extraordinary behaviour of the fluorocarbon liquids (FCLs) requires specialised biocompatibility testing, adjusted to this class of components. 

Conversely, the role of perfluorocarbons for oxygen transport and in vivo delivery is investigated. In addition to possible use as temporary blood substitute, these fluorocarbon molecules can be applied as respiratory gas carriers, for instance as lung surfactant replacement compositions for neonates and possibly for the treatment of acute respiratory distress syndrome for adults. Another... 

Early fundamental studies of gas transport in polymers were almost entirely confined to hydrocarbon materials above their glass transition temperatures. The essentially nonpolar structures of the elastomers led to a number of reasonably successful attempts to correlate gas transport parameters with various physical characteristics of the gases and the polymers. These have been summarized and discussed in a number of papers In addition to studies with hydrocarbon elastomers a few studies of other amorphous polymers above their glass transition temperatures have dealt with polyvinyl acetate silicones and fluorocarbon polymers Recent studies have also dealt with poly(methyl aciylate) poly-(vinyl methyl ether) and poly(vinyl methyl ketone) With these more... 

Johnson OL, Washington G, Davis SS. Thermal stability of fluorocarbon emulsions that transport oxygen. Int J Pharm 1990 59 131-135. 

The global presence of radioactive fallout from the nuclear bomb tests of the 1950s and 1960s clearly illustrated the capacity of the atmosphere to distribute chemicals around Earth. Global transport has been demonstrated for a large number of chemicals that do not rapidly degrade or settle out of the atmosphere. Examples are methane (CH4), nitrous oxide (N20), and chloro-fluorocarbons (CFCs), all of which have half-lives in the atmosphere ranging from years to decades. 

Surface and transport properties of solvents are very important for solvents. Surface tension of a solvent shows how easy or difficult it would be to wet the surface on which the solvent is being applied. Low surface tension implies better wetting ability and vice versa. Water and other polar organic solvents have very high surface tension, whereas silicones, fluorocarbons, and aliphatic hydrocarbons have low surface tension. Solvents with low surface tension are easier to leak through threaded joints compared to those with high surface tension. 

One of these, which has recently become increasingly important in electrochemical applications, and which is one of the materials discussed extensively in this volume, is the Nafion ionomer family. These materials were developed by the duPont company, and consist of hydrophobic fluorocarbon backbone chains, with hydrophilic per-fluorinated ether side chains terminated by sulfonic acid groups or corresponding alkali salts. The Nafions possess many exceptional properties which are not encountered in other ionomer systems, particularly the high water permeability (26,27), permselectivity with regard to ion transport (28-30), durability in strong alkali (26), thermal stability (26,31), and others. 

A model for ionic clustering in "Nafion" (registered trademark of E. I. du Pont de Nemours and Co.) perfluorinated membranes is proposed. This "cluster-network" model suggests that the solvent and ion exchange sites phase separate from the fluorocarbon matrix into inverted micellar structures which are connected by short narrow channels. This model is used to describe ion transport and hydroxyl rejection in "Nafion" membrane products. We also demonstrate that transport processes occurring in "Nafion" are well described by percolation theory. 

The solvent and ion exchange sites in "Nafion" perfluori-nated membranes phase separate from the fluorocarbon matrix to form clusters (1-5). This ionic clustering will not only affect the mechanical properties of the polymer (JL), but should also have a direct effect on the transport properties across these membranes (2). In addition the exchange sites in the resin are strongly acidic and the polymer is extremely hydrophilic. Combined with the polymer s exceptional thermal and chemical stability, these properties make "Nafion" membranes particularly suitable for a variety of applications. These include applications as membrane separators in several electrochemical processes (6-9), as a superacid catalyst in organic syntheses (10-12), and as a membrane electrode (13). 

Figure 8. A two-dimensional illustration for the concept of percolation. The shaded and crossed areas correspond, respectively, to sites that were previously occupied and sites that have just been occupied. Those marked L in (b) are empty sites that must be occupied before the onset of ion transport. The percentage of occupancy of the grid are 18,31, 45, and 53% for Cases a to d, respectively. In this context, the empty and occupied sites would represent the fluorocarbon backbone and the electrolyte phase, respectively.

Typically, chemically modified surface layers involve thicknesses ranging from less than 1 micrometer(H) up to 20H-, so the overall mechanical properties of the treated objects are hardly affected by the process. Even if one limits the discussion to materials containing C-H bonds, practically all engineering plastics are covered except pure fluorocarbons and some silicones. Clearly, various gases can react with the carbon-hydrogen bonds on the surface of a plastic article and can reduce the diffusion coefficient of penetrants in the material. The choice of sulfonation as the preferred treatment, therefore, is not based solely on the ability to modify transport properties. 

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