Synthesis of branched sulfonated poly(aryl ether ketone) copolymers and their proton exchange membrane properties

Branched sulfonated poly(aryl ether ketone) copolymers (Br-SPAEKs) were synthesized based on 4,4′-(hexafluoroisopropylidene) diphenol (6F-BPA, A2 monomer), 4,4′-difluorobenzophenone (DFBP, unsulfonated B2 monomer), 3,3′-disodiumsulfonyl-4,4′-difluorobenzophenone (SDFBP, sulfonated B2 monomer) and 2,4′,6-trifluoro-benzophenone (TFBP, branched BB′2 monomer). Structures of Br-SPAEKs were confirmed by FT-IR and NMR. Compared to linear sulfonated poly(aryl ether ketone) copolymer (L-SPAEK) membrane with similar ion exchange capacity (IEC), these Br-SPAEK membranes showed a higher proton conductivity and a lower methanol permeability. This synergistic effect on proton conductivity and methanol permeability was mainly due to the characteristics of large free volume and similar to cross-linking architecture arising from the introduction of the branched monomers. Moreover, the Br-SPAEK membrane containing 20 mol% branched monomer possesses the best selectivity, and the value of selectivity is almost 3 times higher than that of L-SPAEK. Proton conductivities of Br-SPAEK membranes are higher than 10−2 S/cm and increase gradually with increasing temperature. At 80 °C, the proton conductivity of Br-20-SPAEK-40 (branched monomer of 20 mol% and sulfonated monomer of 40 mol%) membrane reaches 0.121 S cm−1, compare to that of Nafion 117 (0.139 S cm−1). Additionally, the methanol permeability of Br-20-SPAEK-40 is one order magnitude lower than that of Nafion 117.