Cross-Linking Density Effect of Fluorinated Aromatic Polyethers on Transport Properties

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 1H NMR, 19F 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 cm2/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 × 103−587 × 103 S·s/cm3, and they were thus also much higher than the selectivity of Nafion 212 (56 × 103 S·s/cm3).