Fluorinated polymer membranes

Surface modification of the polymeric membranes via molecular design is one of the most versatile means to improve the surface properties without affecting bulk properties. Surface modification of fluoropolymer membranes, especially for fully fluorinated polymer membranes, such as PTFE membranes, has been of particular interest, due to their physical and chemical inertness. Surface modification of fluoropolymer membranes can be classified into two categories surface coating and surface grafting. 

Beside this, corrosive attack to the metallic bipolar plate is further supported by the common PEFC operating temperatures of 80-100°C. Elevated temperatures in particular increase the chemical attack on the polymer membrane. Especially if the membrane tends to dry out, the increasing formation of peroxides results in inaeasing attack on the membrane polymer (Cleghom and Kolde 2007 Liu et al. 2001 Endoh et al. 2004). In the case of fluorinated polymers, membrane degradation results in continuous release of fluoride ions, which also support corrosive attack on the stainless steel (Ningshen and Kamachi Mudali 2002). 

This article focuses on the commercial, ethylene-based ionomers and includes information on industrial uses and manufacture. The fluorinated polymers used as membranes are frequently included in ionomer reviews. Owing to the high concentration of polar groups, these polymers are generally not melt processible and are specially designed for specific membrane uses (see Fluorine compounds, organic—perfluoroalkane sulfonic acids Membrane technology). 

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

As was noted above, functional fluoropolymers produced by copolymerization of fluoroolefins with functional PFAVE have several unique properties, with the main disadvantage of these materials being the extremely high cost of functional monomers and the resulting high cost of the functional polymers produced from them. The fact that they are so expensive limits their wider industrial application in other fields such as catalysis and membrane separation, except for chlorine-alkali electrolysis and fuel cells, where the only suitable materials are fully fluorinated polymers because of the extreme conditions associated with those processes. 

Direct fluorination of polymer or polymer membrane surfaces creates a thin layer of partially fluorinated material on the polymer surface. This procedure dramatically changes the permeation rate of gas molecules through polymers. Several publications in collaboration with Professor D. R. Paul62-66 have investigated the gas permeabilities of surface fluorination of low-density polyethylene, polysulfone, poly(4-methyl-1 -pentene), and poly(phenylene oxide) membranes. 

Surface fluorination changes the polymer surface drastically, the most commercially significant use of polymer surface direct fluorination is the creation of barriers against hydrocarbon permeation. The effectiveness of such barriers is enormous, with reductions in permeation rates of two orders of magnitude. Applications that exploit the enhanced barrier properties of surface-fluorinated polymers include (1) Polymer containers, e.g., gas tanks in cars and trucks, which are produced mostly from high-density polyethylene, where surface fluorination is used to decrease the permeation of fuel to the atmosphere and perfume bottles. (2) Polymeric membranes, to improve selectivity commercial production of surface-fluorinated membranes has already started.13... 

The PEMFC (Proton Exchange Membrane Fuel Cell) is a fuel cell with a protonconducting fluorinated polymer as electrolyte. Figure 14.12 gives a schematic drawing of the PEMFC. At the anode, hydrogen is oxidized to protons. At the cathode, oxygen from air is reduced to water. The PEMFC is in development for various applications. 

Clark oxygen electrode. [D. t. Sawyer, A. Sobkowiak, and J. L. Roberts, Jr., Electrochemistry for Chemists, 2nd eel. (New York Wiley. 1995).] A modern, commercial oxygen electrode is a three-electrode design with a Au cathode, a Ag anode, a Ag I AgBr reference electrode, and a 50-(im-thick fluorinated ethylene-propylene polymer membrane. Leland Clark, who invented the Clark oxygen electrode, also invented the glucose monitor and the heart-lung machine. 

The great value of the unique characteristics of fluorinated polymers in the development of modern industries has ensured an increasing technological interest since the discovery of the first fluoropolymer, poly(chlorotrifluoro-ethylene) in 1934. Hence, their fields of applications are numerous paints and coatings [10] (for metals [11], wood and leather [12], stone and optical fibers [13, 14]), textile finishings [15], novel elastomers [5, 6, 8], high performance resins, membranes [16, 17], functional materials (for photoresists and optical fibers), biomaterials [18], and thermostable polymers for aerospace. 

M. Langsam, Fluorinated Polymeric Membranes for Gas Separation Processes, US Patent 4,657,564 (April, 1987) M. Langsam and C.L. Savoca, Polytrialkylgermyl-propyne Polymers and Membranes, US Patent 4,759,776 (July, 1988). 

The most important use of /3-sultones is for the preparation of fluorinated polymers such as Nafion 64. These solid acid catalysts containing perfluorinated sulfonic acid groups have been known for many years and the presence of the electron-withdrawing F atoms increases the acid strength of the terminal sulfonic acid groups, which become comparable to that of pure sulfuric acid. Prior to the last decade, Nafion had been in use as a superacid, a fuel cell electrolyte and as a membrane-ion separator <1996CHEC-II(1B)1083>. 

In the diaphragm (and in the membrane) process, the anode and cathode compartments are separated by a permeable -> diaphragm. The latter generally consist of asbestos, reinforced with fibers of fluorinated polymers, or more recently, they consist of asbestos-free diaphragms, that are instead... 

Roziere, J. and Jones, D.J., Non-fluorinated polymer materials for proton exchange membrane fuel cells, Ann. Rev. Mater. Res., 33, 503, 2003. 

There are special applications where it is required that essentially no dissolution or reaction takes place between the membrane/module material and the process stream. One such application area is food and beverages. For these uses, not only the membrane material but also the housing and gasket materials need to pass certain tests for sanitary reasons (e.g., FDA approval). Stainless steel (especially the 316L type) is typically used as the casing material and fluorinated polymers, EPDM, silicon or other specialty... 

The maximum pore size used to separate phospholipid micelles, in which color pigments and other impurities are physically bound, can be in the range of 10,000-50,000 Da depending on the polymer type. Considerable swelling occurs with poly-sulfone membranes, which, in turn, affects the membrane chemistry drastically and reduces flow rates and in some cases totally closes the pores. Similar results have also been observed with polyamide and fluorinated polymers. 

Recent tests by Sun (32) have shown that polyimide membranes have higher rejection rates than those of polyamide membranes, but polyamide membranes have higher flux. The highest flux obtained with hexane miscella and polyamide membranes was 6.6 LMH. The phosphorus rejection rates of 98.1-99.3% were obtained with hexane miscella. The addition of surfactants increased the phosphorus rejection rate from 83.3-78.7% to 96.4% with IPA miscella. The added surfactants facilitated the formation of large phospholipid clusters. Koseoglu et al. (33) reported that membranes made of polyamide were least affected by hexane, but that a membrane made from a fluorinated polymer was deteriorated by hexane. 

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