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New series of ethynyl-terminated sulfonated-fluorinated poly(arylene ether)s (ESF-6Fs) were synthesized via nucleophilic aromatic substitution reaction (SNAr) from potassium 2,5-dihydroxybenzenesulfonate (SHQ), 4,4′-(hexafluoroisopropylidene)diphenol (6FBPA), and decafluorobiphenyl (DFBP), followed by the reaction with 3-ethynylphenol. The cross-linked ESF-6F membranes exhibited high glass-transition temperatures in the range of 258−276 °C and good thermal degradation temperatures in the range of 308−328 °C. To optimize the proton conductivity and water uptake of the cross-linked ESF-6F membranes, various volume-related parameters (length-scaled parameters) were introduced. Using those parameters, the proton conductivity and water uptake of the cross-linked ESF-6F membranes were compared with sulfonated poly(arylene ether sulfone) random copolymer (BPSH), Nafion membrane, and another cross-linked membrane. In addition, the proton conductivity of optimized ESF-6F membrane was observed as a function of relative humidity (RH) at 80 °C and compared with other membranes. Excellent proton conductivity was observed at the percent conducting volume (PCV) (0.39, ESF90-6F) corresponding to the maximum volume-based ion exchange capacity in the hydrate state (IECv(wet)) and the atomic force microscopy (AFM) image showed good ionic channels at that point. Also, for the direct methanol fuel cell (DMFC) application, the cross-linked ESF-6F membranes exhibited lower methanol crossover (11 × 10−8−103 × 10−8cm2/s) and higher selectivities (130 × 103−492 × 103s Ω−1cm−3) than those of Nafion 117 (167 × 10−8cm2/s and 55 × 103s Ω−1cm−3). [ABSTRACT FROM AUTHOR]

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 × 10−8cm2/s, as compared with 154 × 10−8cm2/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 cross-linked membrane (5−20 nm) are much smaller than those of noncross-linked membrane (20−50 nm). [ABSTRACT FROM AUTHOR]