Synthesis and Characterization of Highly Fluorinated Cross-linked Aromatic Polyethers for Polymer Electrolytes

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 −8 cm 2 /s) and higher selectivities (130 × 10 3 −492 × 10 3 s Ω −1 cm −3 ) than those of Nafion 117 (167 × 10 −8 cm 2 /s and 55 × 10 3 s Ω −1 cm −3 ).