Ionomers with sulfonic acid

FIG U RE 2.2 Chemical structures of perfluorinated ionomers with sulfonic acid (la = Nafion, Flemion lb = Aciplex 2a = Dow, Hyflon Ion 2b = 3M 2c = Asahi Kasei 3 = Asahi Glass) and bis[(perfluoro)alkyl sulfonyl] groups (4). (Reprinted with permission from Peckham, T. J., Yang, Y, and Holdcroft, S. et al., Proton Exchange Membrane Fuel Cells Materials, Properties and Performance, Wilkinson, D. P. et al., Eds., Figure 3.16, 138, 2010, CRC Press, Boca Raton. Copyright (2010) CRC Press.)... 

We will briefly introduce some important ionomers (see Fig. 1), but for a thorough treatment of ionomer chemistry, see, e.g., Refs. . The simplest classification of ionomers is semicrystalline vs. amorphous ionomers. The prototypical semicrystalline ionomer is EMAA (Surlyn, DuPont) neutralized with various cations. Also from DuPont, Nafion is a perfluorinated polyethylene with sulfonic acid or sulfonate groups on short side chains. Other commercial ionomers like Aciplex (Asahi Chemical Company), Flemion (Asahi Glass Company), and Neosepta (Tokuyama) are structurally similar to Nafion. For a recent review on Nafion see Mauritz and Moore. ... 

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. 

Perfluorinated ionomers functionalized with sulfonic acid groups (i.e., Nafion and Dow s PFSA) have been extensively studied and are used in a variety of applications [9]. The hydrophobic fluorocarbon backbone... 

Chang Y, Brunello GF, Fuller J, et al. Aromatic ionomers with highly acidic sulfonate... 

As a result, the overall performance and cost efficiency of the fuel cell system can be expected to be significantly improved. However, at high temperature, desuhbnation, which is a loss of the sulfonic acid unit through hydrolysis, may become a critical issue [29]. This is currently motivating the search for ionomers with alternative acidic moieties, such as phosphonic acid [30,31], which have higher chemical and thermal stabihties than sulfonic acid moieties [32- 34]. 

Perfluoroalkane ionomers consist of a perfluorinated backbone and pendant chains terminated with an anionic group, for example with sulfonic acid groups (Nafion H) [181-185] ... 

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. 

Weiss et al. [75] have synthesized Na and Zn salt of sulfonated styrene(ethylene-co-butylene)-styrene triblock ionomer. The starting material is a hydrogenated triblock copolymer of styrene and butadiene with a rubber mid-block and PS end-blocks. After hydrogenation, the mid-block is converted to a random copolymer of ethylene and butylene. Ethyl sulfonate is used to sulfonate the block copolymer in 1,2-dichloroethane solution at 50°C using the procedure developed by Makowski et al. [76]. The sulfonic acid form of the functionalized polymer is recovered by steam stripping. The neutralization reaction is carried out in toluene-methanol solution using the appropriate metal hydroxide or acetate. 

N.F. Hu and J.F. Rusling, Electrochemistry and catalysis with myoglobin in hydrated poly(ester sulfonic acid) ionomer films. Langmuir 13, 4119-4125 (1997). 

Ionomers are polymers which contain up to about 10% mole percent of ionic group [83-85]. Ionomers show interesting properties because of the presence of different interactions which include hydrogen bonding, formation of charge transfer complexes and ion-ion interactions. Polymers with carboxylic or sulfonic acid groups in its backbone on neutralisation with zinc, sodium or other metal salts form ionic aggregates. 

IR spectroscopy can be used to characterise the formation of ionomers by studying the environment of the anions [85, 86]. Risen and co-workers [87, 88] used far-IR spectra (150 to 400 cm"1) to demonstrate the sensitivity of low frequency vibrations to the anions and cations and the degree of cluster formation in ionomers. For example, styrene sulfonic acid ionomers with Na+ cation shows absorption bands at 220 cm 1, whereas the Cs+ cation shows bands at 100 cm 1. 

Nation ionomers are produced by copolymerization of a perfluorinated vinyl ether comonomer with tetrafluoroethylene resulting in the chemical structure shown in Figure 8.25 [162,166], This polymer and other related polymers consist of perfluorinated, hydrophobic, backbones that give chemical stability to the material. The material also contains sulfonated, hydrophilic, side groups that make hydration possible in the acidic regions, and also allow the transport of protons at low temperatures, since the higher limit of temperature is determined by the humidification of the membrane, since water is a sine qua non for conduction [166], The material exhibits a proton conductivity of 0.1 S/cm at 80°C [162], The membrane performance is then based on the hydrophilic character of the sulfonic acid groups, which allow proton transport when hydrated while the hydrophobic... 

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