Sulfonated Aromatic Polymers

To date, much effort has been undertaken to develop new alternatives. For example, sulfonated aromatic polymers, i.e., polymers with the sulfonic acid groups directly attached to the main chain or carrying short pendant side chains with terminal sulfonic acid units, attract increasing interest because of their chemical and thermal stability, and the ease of the sulfonation procedure. Some of the proposed polymers are sulfonated polysulfone (SPSU) [134] sulfonated poly(phenylene oxide) (SPPO) [135] sulfonated poly-(ether ether ketone) (SPEEK) [136] poly(phenylquinoxaline) (PPQ) [137] and poly(benzeneimidazole) (PBI) [138],... 

Fenton test (3% H2O2 solution containing 2 ppm FeS04) results for sul-fonated aromatic polymers have also been reported. The stability of sulfonated aromatic polymers depends on backbone structure, sulfonation degree, and testing temperature. For polyimides, the time before the polymer is totally dissolved ranges from several minutes to 9 hours at or as... 

The most common way to modify aromatic polymers for application as a PEM is to employ electrophilic aromatic sulfonation. Aromatic polymers are easily sulfonated using concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide (or complexs thereof). Postmodification reactions are usually restricted due to their lack of precise control over the degree and location of functionalization, the possibility of side reactions, or degradation of the polymer backbone. Regardless, this area of PEM synthesis has received much attention and may be the source of emerging products such as sulfonated Victrex poly (ether ether ketone). 

Poly(arylene sulfone)s and poly(arylene ketone)s are important engineering thermoplastics, and display high Tg-values, high thermal stabilities, good mechanical properties, and an exceptional resistance to both oxidation and acid-catalyzed hydrolysis. It is only during the past decade that the sulfonated aromatic polymers have been considered to be well-suited as PEM candidates for fuel cells [61-64]. 

One alternative approach to obtain sulfonated aromatic polymers is the direct synthesis via aromatic nucleophilic substitution reaction from the sulfonated-functionalized monomers. This direct synthesis process has been proven to be more advantageous than post-sulfonation, namely ... 

The concentration and positions of the sulfonate groups within the directly-synthesized sulfonated aromatic polymers can be easily controlled. 

Sulfonation of aromatic polymers has been explored as a method to produce hydrophilic polymers with water permeability and salt rejection characteristics. These have been of interest because of their potentially high degree of chlorine resistance. The use of sulfonated aromatic polymers for reverse osmosis membranes began in the late 1960 s with the work of Plummer, Kimura and LaConti of General Electric Company.82 Polyphenylene oxide [poly(2,6-di-... 

In sulfonated aromatic polymer membranes, therefore, Donnan ion exclusion appears to be an important contributor to performance. In contact with dilute feedwaters, these strongly anionic membranes exclude feedwater anions from entering the membranes by charge repulsion effects, thus inhibiting salt passage. In more concentrated salt solutions, the charge repulsion effects are shielded by the high ionic activity of the feedwater. 

Sulfonated aromatic polymers have been widely studied as alternatives to Nafion due to potentially attractive mechanical properties, thermal and chemical stability, and commercial availability of the base aromatic polymers. Aromatic polymers studied in fuel cell apphcations include sulfonated poly(p-phenylene)s, sulfonated polysulfones, sulfonated poly(ether ether ke-tone)s (SPEEKs), sulfonated polyimides (SPIs), sulfonated polyphosphazenes, and sulfonated polybenzimidazoles. Representative chemical structures of sulfonated aromatic polymers are shown in Scheme 3. Aromatic polymers are readily sulfonated using concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide. Post-sulfonation reactions suffer from a lack of control over the degree and location of functionalization, and the... 

Scheme 3 Chemical structures of representative sulfonated aromatic polymers...

Recent efforts in the synthesis of sulfonated aromatic polymers are directed to the polymerization of sulfonated monomers (such as (b), (d), (g), (j), (k), and (1) shown in Scheme 3) [14,15,53,54,96-102] or coupling reactions of sulfonated compoimds with fimctional groups attached to a polymer backbone [ 103,104]. In post-sulfonation, attachment of the sulfonic acid group is restricted to the activated position ortho to the aromatic ether bond, as indicated in Scheme 4a, while in direct polymerization of sulfonated monomers, the sulfonic acid groups are attached to the deactivated site on the ring (Scheme 4b). An enhancement of stabUity toward desulfonation and a modestly higher acidity are expected for the structure shown in Scheme 4b. Recently, polymerization of sulfonated monomers was also used to obtain sulfonated polysulfone (m) via oxidation of a sulfonated polysulfide-polysulfone copolymer [105]. 

Compared to fluoropolymers, aliphatic and aromatic polymers are less hydrophobic, and the sulfonic acid functional group is less acidic and less polar. Nafion is a superacid, with a piCa -6 as estimated using pJCa database 4.0 [130]. Ma et al. [131] determined the pKa of Nafion 117 and the sulfonated aromatic polymer, BPSH (Scheme 3b), to be -3.09 and -2.04, respectively, while the simplest aromatic sulfonic acid, benzene sulfonic acid, has a pJCa of 0.70 [132]. Aromatic polymers are more rigid than Nafion and possess shorter ionic side chains, and therefore are expected to exhibit a lesser degree of separation between hydrophilic and hydrophobic domains. SAXS measurements on rigid aromatic polymers, such as SPIs based on naphthalenic dianhydride (Scheme 3k, x = 5, xly = 30/70) indicate the ab-... 

Compared to Nafion , a stronger confinement of water in the narrow channels of the sulfonated aromatic polymers leads to a significantly lower dielectric constant of the waters of hydration (20 compared to 64 in fully hydrated Nafion [185,186]). Of particular relevance to macroscopic models are the diffusion coefficients of water. As the amount of water sorbed by the membrane increases and molecular-scale effects are reduced, the properties approach those of bulk water on the molecular scale. Figme 26 shows the trend in proton mobility. Da, and water self-diffusion, Dh20. for Nafion and the sulfonated polyetherketone membrane [134]. 

Sulfonated Aromatic Polymers for Fuel Cell Membranes... 

Degradation of PBI-PS was studied by elemental analysis and FT-IR spectroscopy. It was foimd that the intensities of SO stretching vibrations decreased after heating the PBI-PS samples above 400 °C for 1 h. These results are similar to those reported by Giesehnan and Reynolds [166] who found that the degradation of PBI-PS occurs due to desulfonation. Hence, alkylsulfonated PBI is more thermally stable than sulfonated aromatic polymer electrolytes characterised by a degradation temperature between... 

The results obtained in the above mentioned studies suggest that the alkyl sulfonated aromatic polymer electrolyte exhibit sufficient thermal stabilities for fuel cell applications at 80 C (a typical operating temperature for per-... 

Anhydrous sulfonated aromatic polymers are highly brittle. Recently [ 197], new materials with high mechanical strength were reported. They were prepared using a polymer blending technique by combining PBI and sulfonated polymers (S-PEEK or ort/zo-sulfonated polysulfone) (Fig. 28). 

Sulfonated aromatic polymers have been widely investigated [32—42] for fuel ceU use due to their enhanced physical and chemical robustness, acid and water... 

Therefore, in a comparative study, different nonfluorinated and partially fluorinated sulfonated aromatic polymers were synthesized and characterized in terms of chemical and thermal stabilities. It was found that SO3H groups introduced in the electron-deficient sections of the aromatic polymer main chains were much more stable against splitting-off than SO3H groups pendent to electron-rich sections of the polymer backbones [29]. One of the most stable polymers of this series was a polymer prepared by polycondensation of decafluorobiphenyl and bisphenol (AF), followed by sulfonation with... 

Among these, acidity of ionic groups and membrane morphol-ogy appear to be crucial, and they are inter-related. Kreuer et al. [23] reported that typical sulfonated aromatic polymers are unable to form defined hydrophilic domains, as the rigid aromatic backbone prevents the formation of continuous conducting channels. Thus, various strategies have been pursued to obtain efficient ionic networks for enhancing the proton conductivity. 

Manufacture of sulfonated aromatic polymers and their use in the production of membranes 285... 

Manufacture of Sulfonated Aromatic Polymers and their Use in the Production of Membranes... 

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