The gas transport and physical properties of sulfonated poly(arylene ether sulfone) was studied as a function of sulfonated and unsulfonated block length (5k:5k, 10k:10k, and 15k:15k). Viscoelastic properties were evaluated using Dynamic Mechanical Analysis (DMA) to observe polymer relaxations and domain compatibility. A decrease in glass transition temperature T-g was observed with increasing block length. 5k:5k had a single T-g (241 degrees C), while micro-phase separation between 10k:10k and 15k:15k domains create two T-g's that are slightly merged (191 degrees C, 236 degrees C and 182 degrees C, 233 degrees C). Swelling measurements revealed that film dimensional changes were greater in the plane normal to the film than parallel with increasing block size. He, H-2, CO2, and O-2 permeability decreased with increasing sulfonated block length with no interchain spacing dependence. The apparent activation energy for permeation E-p increased with gas kinetic diameter size and had a maximum value for 15k:15k. A trade-off relationship between sulfonated and unsulfonated polymer block length is linked to phase separation, water swelling, and gas permeability. (C) 2012 Elsevier B.V. All rights reserved.