Novel sulfonated poly(arylene ether) copolymers containing cross-linking moiety and fluorine atoms were prepared for proton exchange membranes of fuel cells. The copolymers were synthesized using potassium 2,5-dihydroxybenzenesulfonate (SHQ), 4,4′-(hexafluoroisopropylidene)diphenol (6F-BPA), decafluorobiphenyl (DFBP), and 1-ethynyl-2,4-difluorobenzeneand (CM) as a cross-linking moiety through polycondensation. The chemical structures of the cross-linkable copolymers were analyzed by 1 H NMR, 19 F NMR, and FTIR-ATR spectra. Cross-linked membranes are obtained by thermal curing of the cross-linkable copolymers. The thermal properties of the cross-linked membranes were characterized by TGA and DSC. Intrinsic viscosity and gel fraction were also evaluated in order to measure the molecular weight and cross-linking density, respectively. The cross-linked network structure greatly suppressed the water uptake and swelling of the cross-linked membrane. The proton conductivity and methanol permeability of the membranes decreased with increasing cross-linking moiety. The decreases in the proton conductivity and methanol permeability are mainly caused by the enhanced barrier properties of the membranes due to the introduction of CM. In other words, the membranes blocked the channels for the passage of water and methanol. The proposed membranes showed moderate proton conductivity and significantly lower methanol permeability when compared to Nafion 212. The SFPE90-CM20 membrane in particular showed low water uptake (37.5%), high proton conductivity (0.091 S/cm), and low methanol permeability (37 × 10 -8 cm 2 /s). The selectivities of all the cross-linked membranes, that is, the ratio of proton conductivity to methanol permeability, fell in the range of 122 × 10 3 -587 × 10 3 S • s/cm 3 , and they were thus also much higher than the selectivity of Nafion 212 (56 × 10 3 S • s/cm 3 ).

End-group cross-linkable sulfonated poly(arylene ether) polymer (E-SFQK) was synthesized via direct polymerization of potassium 2,5-dihydroxybenzenesulfonate (SHQ) and decafluorobiphenyl (DFBP), followed by a reaction with ethynylphenol (EP). The cross-linking reaction of the ethynyl end group of E-SFQK was performed at 250 °C. After cross-linking, proton conductivity, water uptake, and swelling ratio of cross-linked membrane decreased from 0.16 (noncross-linked membrane) to 0.13 S/cm, from 86% to 42%, and from 31% to 13%, respectively. The effect of cross-linking time on proton conductivity, water uptake, and swelling ratio were also investigated. Methanol permeability of cross-linked membrane was compared with Nafion 117 due to solubility of noncross-linked membranes in methanol. The cross-linked membrane performed better, with a methanol permeability of 88 × 10 -8 cm 2 /s, as compared with 154 × 10 -8 cm 2 /s for Nafion 117. The cross-linked membrane also exhibited improved chemical resistance and oxidative stability from solubility and Fenton's tests. In order to study morphological changes of cross-linked and noncross-linked membranes, hydrophilic domain sizes from an AFM phase image were evaluated. These results showed that the sizes of hydrophilic domains of the crosslinked membrane (5-20 nm) are much smaller than those of noncross-linked membrane (20-50 nm).

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Ethynyl-terminated fluorinated poly(arylene ether sulfide) (E-FPAESI) was synthesized via nucleophilic aromatic substitution from dihydroxy monomer and pentafluorophenyl sulfide, followed by the reaction with 3-ethynylphenol. Dihydroxy monomer was synthesized in four steps via bromination, Grignard, Suzuki cross-coupling, and demethylation techniques. The number-average molecular weights and polydispersities of the E-FPAESI were in the ranges of 7700−23 000 and 1.53−2.89, respectively. The glass transition temperatures of the polymers varied from 126 to 171 °C and upon curing in the range of 168−236 °C. E-FPAESI exhibited high thermal stability up to 451−483 °C. The refractive indices and birefringences of the spin-coated polymer films were in the ranges of 1.4943−1.5047 and 0.0004−0.0009 at 1550 nm wavelength, respectively. The optical loss for E-FPAESI was about 0.45 dB/cm at 1550 nm wavelength.