A novel side-chain poly(arylene ether ketone) functionalized with 1,4-butane-sultone (SQNPAEK-x, "x" refers to the number of the attaching sulfobutyl groups in one unit) were synthesized for proton exchange membrane fuel cell. SQNPAEK-x showed high proton conductivity up to 0.317 S cm(-1) at 80 degrees C and excellent dimensional stability due to the remaining hydroxyl groups. Morphological investigations revealed that SQNPAEK-x contained more uniform ionic clusters than the main-chain sulfonated poly(arylene ether ketone), which increased the proton conductivity. Other properties such as water uptake, mechanical properties were also investigated. It should be noted that SQNPAEK-x possesses the best overall performance with proton conductivity (0.204 S cm(-1) at 80 degrees C) higher that Nafion 117, while the swelling ratio was equivalent (13.2% at 80 degrees C). All the results indicate that the SQNPAEK-x membranes are promising candidates for proton exchange membrane fuel cells. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.