Aromatic Ionomers

Interestingly, ionic cluster formation was observed in the case of another aromatic ionomer, the Na salt of sulfonated polyether-ketone47,67. 

B. Bae, T.Yoda, K. Miyatake, H. Uchida, M.Watanabe, Proton-conductive aromatic ionomers containing highly sulfonated blocks for high-temperature-operable fuel cells. Angewandte Chemie International Edition 2010,49(2), 317-320. 

K. Xu, H. Oh, M.A. Hickner, Q. Wang, Highly conductive aromatic ionomers with perfluoro-sulfonic acid side chains for elevated temperature fuel cells. Macromolecules 44 (12) (2011) 4605 609. 

Guiver et al. of National Research Council, Canada developed comb-shaped poly(arylene ether) electrolytes containing 2-A sulfonic acid groups on aromatic side chains (d) [76]. Their membranes showed relatively high proton conductivity and well-developed and continuous ionic domains. However, trade-off relationship between water uptake and proton conductivity of their membranes was not better than that of Nafion. In order to pronounce the hydrophilic/hydropho-bic differences, another series of comb-shaped aromatic ionomers with highly fluorinated main chains and flexible poly(a-methyl styrene sulfonic acid) side chains were developed [77]. The membranes seemed to have better properties than their previous version, however, chemical instability of the side chains needed to be improved. 

Yoshimiu-a and Iwasaki of Sumitomo Chemical Co. have synthesized aromatic ionomers containing pendant perfluorosulfonic acid groups (Fig. 7.29) [79]. Poly (arylene ether sulfone) was brominated and then perfluorosulfonated via Ullmann coupling reaction in the presence of copper catalyst. The lEC was controllable up to 1.58 meq/g. The obtained ionomer membranes behaved very differently from the typical sulfonated aromatic ionomer membranes. Characteristic hydrophobic/hydro-philic separation (ca. 3-4 nm) was observed in the smaU-angle X-ray scattering (SAXS) analyses of the hydrated samples. The ionic clusters were slightly smaller... 

The group of GKSS Research Center Germany extensively researched the effect of a variety of inorganic nanoparticles on the properties of aromatic ionomer membranes [86-93]. Composite membranes were prepared from silicates and sulfonated poly(ether ketone)s or sulfonated poly(ether ether ketone)s. For DMFC applications, the composite membranes showed promising properties with lower methanol and water permeability and comparable (or higher) proton conductivity compared to the parent polymer membranes. The flux of water and methanol decreased with the increase in content of silicates. 

The syntheses of three new aromatic ionomers with equivalent weights of ca. 800, 600 and 400 have been carried out. Thus, 3,3(oxydi P phenylene)bis[2,4,5-triphenylcyclo-pentadienone] and 3,3 -(oxydi-p-phenylene)bis[2,5-diethoxy-carbonyl-4-phenylcyclopentadienone] were copolymerized with 4,4 -diethynyldiphenyl ether in molar ratios of 70 30 100, 60 40 100 and 40 60 100 to afford the corresponding alkoxy-carbonyl-substituted polyphenylene oxides, which were converted to the ionomers by treatment with potassium hydroxide. 

Excellent candidates for membrane materials in alkaline electrolyzer applications are the aromatic ionomers (5). 

Stress relaxation experiments performed in the dry state on the aromatic ionomer films were complicated by the hydrophilic nature of the materials. The curves obtained to date are characteristic of the water plasticized system rather than the dry material. More reliably, experiments were... 

Both the Nation and aromatic ionomer systems show an affinity for water. However, they differ significantly in one respect. An important feature of the aromatic ionomers, particularly in view of their possible use in aqueous environments, is their excellent surface wettability. 

Investigation of further sulfonated and phosphonated aromatic ionomers as acidic macromolecular cross-linkers (and possibly as proton-conducting blend component) for intermediate-T fuel cell membranes... 

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

Many research studies have focused on improving the nanophase-separated structures between the hydrophilic and hydrophobic units to increase the proton conductivity of the aromatic ionomers under low RH. In this section, recent approaches to improve the membrane properties, especially the proton conductivity, which is usually the first characteristic considered when evaluating membranes for fuel cells, and morphology will be discussed as follows (1) multiblock SPES copolymers, (2) locally and densely SPES, (3) SPES with high lEC values and high free volume, (4) SPES with pendant perfluoroalkyl sulfonic acids, (5) cross-linked SPES, and (6) thermally annealed SPESs. 

Despite its higher lEC values, the proton conductivity of the membranes was lower than that of the Nafion membrane. The authors concluded that acidity was a crucial factor to determine the proton conducting properties. However, other structural factors, such as the hydrophobicity and flexibility of the main chain, would have to be optimized for further improving the proton conductivity of the aromatic ionomer membranes [51]. On the other hand, Ueda and coworkers reported a novel PES containing binaphthyl units with pendant perfluoroalkyl sulfonic acids (BNSH-PSA) for PEM [52]. The BNSH-PSA (1EC=1.91 meq./g) was prepared by the aromatic nucleophilic substitution reaction of l,T-binaphthyl-4,4 -diol and 4,4 -dichlorodiphe-nylsulfone, followed by bromination with bromine, and then the Ullmann coupling reaction with PSA-K (Scheme 4.19). 

Miyatake, K., Shimura, T., Mikamiac, T., and Watanabe, M. 2009. Aromatic ionomers with superacid groups. Chem. Commun. 42 6403-6405. 

Knauth, P., Di Vona, M.L., Sulfonated aromatic ionomers Analysis of proton conductivity and proton mobility, Solid State Ionics, 2012, 225, 255-259. 

Water absorbing capacity of the AEMs depends on the number of ion exchange groups (lEC value) and the nature of the polymer matrix of the membrane. In the majority of situations, the former factor is more important larger lEC value corresponds with the larger water uptakes and vice versa. For the AEMs derived from different types of aromatic ionomers, they exhibit rather different level of water uptakes. Therefore, to better understand the difference of the water absorbing ability... 

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