Carboxylate membranes perfluorinated

Perfluorinated carboxylate membranes were introduced about seven years ago. These membranes can be synthesized by a variety of methods or by various chemical conversions from the Nafion polymer.Composite membranes which contain both sulfonate and carboxylate functional groups have also been produced (see Section IV.l for more details). These carboxylate membranes have been widely employed in the advanced membrane chlor-alkali cells. This major chemical technology is in the process of being revolutionized by the use of these materials, a remarkable accomplishment for such a small group of polymers. ... 

Potential applications of perfluorinated carboxylate membranes have been focused to date on the chlor-alkali process. It has been pointed out previously that these polymers in acid form are not desirable for electrochemical applications because of rather high resistance. ... 

Ion clusters are commonly observed in the ionized forms of the perfluorinated membranes. The size of the clusters appears to be larger for sulfonate than for carboxylate membranes." " The size increases in the order Na, and Cs" and decreases with increasing number of functional groups per chain and with increasing temperature.As in the case of ethylene ionomers, the perfluorinated carboxylic acid membranes do not form ion clusters, at least in the dry state." The electrostatic interaction may be too weak to form ionic clusters. These observations are expected according to the Eisenberg theory (see Section II.2). 

The self-diffusion coefficients of sodium and cesium ions have been measured for perfluorinated sulfonate (Nafion ) and carboxylate membranes of similar structure. The exchange-site con-... 

Figure 17. Sodium ion transport number vs. caustic catholyte solution for a perfluorinated carboxylate membrane ( ) anolyte is 5 M NaCl and (O) anolyte and catholyte are identical concentrations of NaOH. (Ref. 149 reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

I, 1980, 76, 2558-2574 L.Y. Levy, A. Jenard and H.D. Hurwitz, Hydration and ion-exchange process in carboxylic membranes. Part 1. Infrared spectroscopic investigation of the acid membranes, J. Chem. Soc. Trans. 1, 1982, 78, 29-36 M. Falk, Infrared spectra of perfluorosulfonated polymer and water in perfluorosulfonate polymer, Perfluorinated Ionomer Membranes, ed. A. Eisenberg, H.L. Yeager, ACS Symposium Series, American Chemical Society, Washington DC, 1982, p. 139 C. Heitner-Wirguin and D. Hall, An infrared study of an anion exchange membrane,... 

Figure 10. Arrhenius plots of sodium ion diffusion in aromatic carboxylate polymer films open symbols, 5 M NaOH, closed symbols 11 M NaOH solution. O, - 400EW Q,l- 600EW A,i-perfluorinated carboxylate membrane.

Recently, a new type of diaphragm has impacted the industry for a few new plants. The new membranes are perfluorinated polymers with occasional sulfonate and/or carboxylate groups. They have the general structure outlined here. 

Ionomers are used to prepare membranes for a variety of applications including dialysis, reverse osmosis, and in electrolytic cells for the chlor-alkali industry. This latter application needs materials that show good chemical resistance and ionomers based on perfluorinated backbones with minor amounts of sulfonic or carboxylic acids are ideal. They also show good ion-exchange properties. 

M. Inaba, T. Kuroe, Z. Ogumi, Z. Takehara, K. Katakura, S. Ichikawa, and Y. Yamamoto. Oxygen permeation through perfluorinated carboxylate ion-exchange membranes. Electrochimica Acta 38, 1727-1731 1993. 

Nafion-perfluorinated membranes are manufactured from copolymers of tetrafluoroethylene and perfluorinated monomers and contain sulfonic groups on one side and carboxylic acid groups on the other. The membranes, originally designed for use in... 

These demands are fulfilled by membranes with a perfluorinated polyethene main chain with side-chains with sulfonic acid and/or carboxylic acid groups as produced by DuPont and Asahi Glass. 

Ltd.) and the perfluorinated membranes produced by the Asahi Chemical Industry Company (18). Recently, carboxylate perfluorinated materials have been available from DuPont Company. 

Figure 7. Depolarized small-angle light scattering patterns (under cross-polarIzatIon condition) from perfluorinate carboxylic and sulfuric acid membranes showing the effect of swelling In water and ethanol. Reproduced from Ref. 30. Copyright 1982 American Chemical Society.

The effect of NaOH concentration on the ion transport and rheological properties of the Nafion ion exchange membranes may be attributable to some variation in the ionic domain structure in the presence of NaOH. Therefore, it is extremely Important to understand the ionic domain structure under these conditions. The anomalous behavior of Na" " ion transport as a function of NaOH concentration is seen more frequently in bilayer Nafion membranes in which one layer is treated with diamine and also in perfluorinated carboxylic ion exchange membranes. Several mechanisms have been proposed to explain their ion transport results including water absorption, transport of hydroxide ion tunneling, ion pairing mechanisms, etc. (54-56). As the ion transport properties are beyond the scope of this review, no detailed discussion will be presented. 

Hopfinger and Mauritz and Hopfinger also presented a general formalism to describe the structural organization of Nafion membranes under different physicochemical conditions. It was assumed that ionic clustering does not exist in the dry polymer. This assumption is applicable to the perfluorinated carboxylic acid polymer" but not the perfluorosulfonate polymers." They consider the balance in energy between the elastic deformation of the matrix and the various molecular interactions that exist in the polymer. 

Three types of perfluorinated chlor-alkali membranes are noteworthy. The first of these, the homogeneous carboxylate films. 

The perfluorinated carboxylic acid membrane exhibits a higher resistance than Nafion membranes in SPE water electrolyzers. This is primarily due to small membrane swelling and slight dissociation of the carboxylic acid group in water or acid electrolyte with a pH <... 

A good measure of past and continuing interest in ionomer membranes issued from the development of perfluorinated ionomers, the first-announced being Nafion(44). These materials are characterized by remarkable chemical resistance, thermal stability and mechanical strength, and they have a very strong acid strength, even in the carboxylic acid form. The functionalities that have been considered include carboxylate, sulfonate, and sulfonamide, the latter resulting from the reactions of amines with the sulfonyl fluoride precursor. 

Since the conductivity of electrolytes and the cross section and thickness of the membrane are known, a can be determined from the voltage drops across the three pairs of probe electrodes 1-2, 3-4 and 5-6. The sodium current efficiency (CE) can also be determined by titrating the amount of caustic soda generated over a given period of time. The confinement chambers around the working electrodes are used to eliminate free bubbles near the membrane. Our normalized transport data for sulfonate, carboxylate and sulfonamide ionomers are plotted In Figure 5 the universal percolative nature of perfluorinated ionomers can be clearly eeij. The prefactor sulfonate ionomers. The exponent t is 1.5 0.1 in reasonable agreement with theory and the thresholds are between 8 to 10 vol. %, which are consistent with the bimodal distribution in cluster size postulated by the cluster-network model (5.18). This theory has also been applied recently to delineate sodium selectivity of perfluorinated ionomers (20). 

These performance goals have now largely been attained by continued improvements through several generations of materials. Currently, commercial perfluorinated ionomer materials for this application consist of membranes with carboxylate or mixed carboxylate-sulfonate functionality the latter membranes often have layered structures with the carboxylate layer exposed to the caustic catholyte solution. Fabric reinforcement is used in some instances to improve strength. 

Perfluorinated ionomers such as Nafion are of significant commercial importance as cation exchange membranes in brine electrolysis cells ( 1). Outstanding chemical and thermal stability make this class of polymers uniquely suited for use in such harsh oxidizing environments. The Nafion polymer consists of a perfluorinated backbone and perfluoroalkylether sidechains which are terminated with sulfonic acid and/or carboxylic acid functionality. 

Structure of Sulfonated and Carboxylated Perfluorinated Ionomer Membranes... 

Table II. Properties of Carboxylic-Acid Form of Perfluorinated Ionomer Membranes as a Function of Equivalent Weight.

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