Branched sulfonated poly(aryl ether ketone) copolymers (Br-SPAEKs) were synthesized based on 4,4'(hexafluoroisopropylidene) diphenol (6F-BPA, A(2) monomer), 4,4'-difluorobenzophenone (DFBP, unsulfonated B-2 monomer), 3,3'-disodiumsulfonyl-4,4'-difluorobenzophenone (SDFBP, sulfonated B-2 monomer) and 2,4',6-trifluoro-benzophenone (TFBP, branched BB2' 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 BrSPAEK 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 degrees 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. (C) 2013 Elsevier B.V. All rights reserved.