S-PEEK

Depending on the number of ether and keto groups in the constitutional repeating units one distinguishes between poly(ether ketone)s (PEK, x = y = 1), poly(ether ether ketone)s (PEEK, x = 2, y = 1), poly(ether ketone ketone)s (PEKK, X = 1, y = 2), and poly(ether ether ketone ketone)s (PEEKK, x = y = 2). 

It was through such research that ICI s PEEK (polyether ether ketone), one of the first high-performance aromatic polymers, was put on sale, as well as Du Pont s aramide fibers Nomex and Kevlar, more resistant than steel in like volume. 

Monomers for poly(ether ether ketone)s (PEEK)s and poly(ether ketone)s (PEK)s are shown in Table 6.1. Varieties of PEEK are shown in Figure 6.2. PEEK-WC is poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene). In addition to PEEK, the carbonyl group is partly modified with phthalide units. It is an amorphous PEEK. 

C. Zhao, X. Li, Z. Wang, Z. Dou, S. Zhong, and H. Na. Synthesis of the block sulfonated poly(ether ether ketone)s (S-PEEKs) materials for proton exchange membrane. J. Membr. ScL, 28(Xl-2) 643-650, September 2006. 

Among the proton-conducting membranes, Nafion or Nafion-like sulfonated perfluorinated polymers should also be mentioned. These materials are used for polymer electrolyte membrane (PEM) FCs, and in addition to being chemically very stable, they exhibit high proton conductivity at temperatures lower than 100°C. It is believed that permeability and thermal stability may be increased if tailor-made lamellar nanoparticles are added to a proton-conducting polymer. The sulfonated poly(ether ether ketone) (S-PEEK) type of polymers is also widely reported as an alternative to fluorinated polymers such as Nafion or Hyflon [51]. 

FIGURE 21.17 Schematic representation of the microstructures of (a) Nafion and (b) an s-PEEK illustrating the less pronounced hydro-phobic/hydrophilic separation of the latter compared to the forma-. (Reprinted from J. Membr. Sci., On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells, 185, 2001, 29, Kreuer, K.D. et al. With permission from Elsevier Kreuer, K.D., J. Membr. ScL, 185,29,2001.)... 

Furthermore, in 2001, Ballard entered an alliance with Victrex to produce two new membrane alternatives. One membrane is based on sulfonated poly(arylether) ketone (a variant of s-PEEK) supplied by Victrex, which may be better suited to PEM fuel cell fabrication applications. In March 2002, U.S. Patent 6359019 was issued to Ballard Power for a graft-polymeric membrane in which one or more trifluoro-vinylaromatic monomers are radiation graft polymerized to a preformed polymeric base. The structures of BAM membranes have been studied by way of small-angle neutron scattering or SANS. ° The study of the ionomer peak position suggests the existence of relatively small ionic domains compared to Nafion, despite large water content. Phase separation in the polymer matrix is possibly crucial for the manbrane s mechanical and transport properties. 

The chemical structures of sulfonated poly(4-phenoxybenzoyl-l,4-phenyl-ene) (S-PPBP) (1), poly(p-xylylene) (S-PPX) (2), poly(phenylene sulfide) (S-PPS) (3), poly(phenylene oxide) (S-PPO) (4), poly(ether ether ketone) (S-PEEK) (5), poly(ether ether sulfone) (S-PEES) (6), arylsulfonated poly(ben-zimidazole) (S-PBl) (7) sulfonated polyphenylquinoxiiline (S-PPQ) (8) and sulfonated phenoxy polyperyleneimide (PSPPI) (9) are shown below. ACPs are sulfonated using common sulfonating agents [82-85]. In particular, PEEK can be sulfonated in concentrated sulfuric acid [50], chlorosulfonic acid [86], SO3 (either pure or as a mixture) [53,65,86,87], a mixture of methanesulfonic acid with concentrated sulfuric acid [88] and acetyl sulfate [89,90]. 

The solubility of polymers changes while the degree of sulfonation increases. For instance, S-PEEK containing 30 mol % sulfonic add groups dissolves in DMF, DMSO and N-methyl-2-pyrrolidone (N-MP) at 70% sulfonation the polymer is soluble in methanol, and at 100% - in water. Non-sulfonated PPBP is soluble in conventional chlorinated solvents (e.g., chloroform and dichloromethane), whereas S-PPBP with 30% sulfonation is insoluble in these solvents. However, the polymer can be dissolved in DMF, DMSO and N-MP. At the sulfonation levels above 65%, S-PPBP swells in methanol and water. 

The dependence of the degradation temperature, (Tj), of S-PPBP and S-PEEK on the degree of sulfonation is presented in Fig. 12. 

As the degree of sulfonation increased, the degradation temperatures decreased from 500 down to 300 °C for S-PEEK, and from 500 down to 250 °C for S-PPBP. The results of elemental analysis of residues indicate a dramatic (nearly ten-fold) decrease in sulfur content of the polymers after heating at temperature above 400 °C. These data confirm that thermal stabilities of polymers are sufficient for fuel cell appHcation even at high sulfonation levels [7,35]. 

Fig. 12 Degradation temperature of S-PEEK (1) and S-PPBP(2) as a function of sulfona-tion level [7,35]...

Absorbtion of water vapour by polymer films prepared from S-PEEK and S-PPBP was studied by placing films into the atmosphere with different humidities and subsequent measuring of the equihbriiun water content. The results obtained were foimd to be close to those reported in similar studies for Nation membranes [ 10]. The dependence of water uptake for S-PEEK and S-PPBP films on relative hiunidity at room temperature is shown in Fig. 13. 

Fig. 13 Water uptake of S-PPBP (1-3) and S-PEEK (4) at room temperature as a function of relative humidity [7]. Concentration of SO3H groups in the polymer (mol%) 30 (1), 65 (2), 80 (3) and 65 (4)...

The content of water in S-PPBP at 65 mol % sulfonation is higher than for S-PEEK with the same sulfonation level. At a relative humidity of 100% and room temperature, the content of water in S-PPBP and S-PEEK is 8.7 and 2.5 molecules per sulfonic acid group, respectively. 

Picnometric measurements showed that the densities of the polymers with a sulfonation level of 65 mol % were 1.338 (S-PEEK) and 1.373 gcm (S-PPBP). According to the results obtained by scanning electron microscopy, both polymers exhibited very close characteristics of their surface and fracture surface. 

The difference in water uptake between S-PEEK and S-PPBP can be attributed to flexibihty of the phenoxybenzoyl group in the side chain of S-PPBP, which favours water permeation into the polymer and water absorption by the terminal sulfonic acid group. Water uptake of S-PPBP is comparable to that of Nation membranes. 

Experiments [7] on water absorption by S-PEEK and S-PPBP films showed that proton conductivities of the films containing equihbrium amounts of absorbed water depend on the relative hiunidity. Fig. 15 represents the dependency of the proton conductivities of S-PEEK and S-PPBP with different sulfonation levels as a function of relative humidity. 

It becomes clear that proton conductivities of the films increase with the relative humidity and water uptake and can become as high as 10 Scm (for S-PEEK). 

The proton conductivities for S-PEEK and S-PPBP with equal degrees of sulfonation (65mol%) at a 100% relative hiunidity can be compared using the graphs shown in Fig. 16. 

It is obvious that the proton conductivities and water uptake for S-PPBP are much higher than those for S-PEEK. Moreover, the proton conductivity for S-PEEK dramatically decreases at temperature above 100 °C, whereas that of S-PPBP appears to be much less temperature dependent. 

Fig. 15 Proton conductivity of S-PEEK (1) and S-PPBP (2-5) with different sulfonation levels as a function of relative humidity at room temperature [7]. Sulfonation level (mol%) 65 (1), 30 (2), 65 (3), 80 (4) and 85 (5)...

Fig. 16 Temperature dependences of proton conductivity as S-PPBP (1) and S-PEEK (2) with the same degrees of sulfonation (65 mol %) at a relative humidity of 120% [7]...

Figure 17 represents temperature dependences of the proton conductivity of S-PEEK with a sulfonation degree of 85 mol % at different relative humidity values. 

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