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 cm2/s) and higher selectivities (130 × 103−492 × 103 s Ω−1 cm−3) than those of Nafion 117 (167 × 10−8 cm2/s and 55 × 103 s Ω−1 cm−3).