TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The highly sulfonated polysulfone copolymer has 60-80 mol% of disulfonated pendant groups to obtain 0.17-0.30 S/cm of proton conductivity. Preferred Process: The blending step is carried out by blending sulfonated poly(arylene ether sulfone)copolymer and nonsulfonated poly(ether sulfone)copolymer having 1:1 weight based blend ratio in N,N-dimethylacetamide. The process comprises suppressing phase separation at early stage of spinodal decomposition carried out by freeze-drying, or maintaining phase separation until late stage of spinodal decomposition. The removal of solvent is accelerated or delayed by using solvent with low or high boiling point, increasing or lowering viscosity of solution, or lowering or increasing drying temperature, respectively. Preferred Component: The polymer blend membrane has co-continuous or two-layer morphology. The two-layer morphology is prevented from being delaminated by interfacial adhesion increased by in-situ formation of two-layer structure, and comprises first and second layers having highly sulfonated and nonsulfonated polysulfone, and faces cathode and anode of fuel cell, respectively. A difference in specific gravity is greater than or equal to 0.01. EXAMPLE - Polymer blend membrane having co-continuous morphology was prepared by blending sulfonated poly(arylene ether sulfone) copolymer and nonsulfonated poly(ether sulfone) copolymer having 1:1 weight based blend ratio in N,N-dimethylacetamide. Initial casting concentration was 20-15 wt.% (Blend 1-2) and cast solution was freeze-dried at - 75 degrees C for 140 hours under vacuum condition and temperature was raised to 100 degrees C to remove residual solvent completely. The co-continuous domain has less than 1 mu m size. Well developed hydrophilic channels facilitated proton movement and hydrophobic network restricted methanol crossover. Consequently, fuel leakage was effectively limited and membrane selectivity was maximized, and excellent selectivity was maintained even at high temperature.