TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The hydrophilic segment has structure of formula (2).m=3-1500.Preferred Properties: The ion exchange capacity of the aromatic polyether sulfone block copolymer is 0.8-2 mEq/g. The weight fraction (Fa) of hydrophilic segment in the aromatic polyether sulfone block copolymer is 0.2-0.8. The weight fraction (Fa) is calculated from a formula: Fa=Wa/Wa+Wb, where Wa is weight of hydrophilic segment and Wb is weight of hydrophobic segment. The polyelectrolyte film has methanol permeability coefficient (P) of less than 1x10-6 cm2/second at 30degreesC and less than 1.5x10-6 cm2/second at 60degreesC, in 10 wt.% methanol aqueous solution and water. The methanol permeability coefficient (P) is calculated from a formula: P=transmission amountxthickness/areaxtimex1/concentration gradient. EXAMPLE - Bis(4-chlorophenyl) sulfone (in g) (21.3), 2,7-dihydroxy naphthalene (12) and potassium carbonate (14) were mixed. Dimethyl sulfoxide (120 ml) and toluene (50 ml) were added to the resulting mixture. The resulting mixture was heated and stirred under nitrogen stream. The reaction solution was maintained at 175degreesC, and generated water was removed with toluene. The resulting solution was stirred at 175degreesC for 16 hours, to obtain a polymer solution. A solution obtained by dissolving Sumica excel 7600P (43.3) in dimethyl sulfoxide (170 ml), was added to the polymer solution, and stirred at 170degreesC for 1.5 hours. The reaction solution was added into large amount of water, and white solid was precipitated. The precipitated product was filter-separated. The obtained solid was washed with methanol and water, to obtain block polymer having reduced viscosity of 0.68. The obtained block polymer (20) was dissolved in 98% sulfuric acid (200 ml), and stirred at room temperature for 24 hours, such that the sulfonic acid group was introduced into the block polymer. The reaction solution was added into large quantity of water, and white solid was precipitated. The precipitated product was filter-separated. The obtained solid was washed 5 times with water, to obtain a block copolymer having ion exchange capacity of 1.3 mmol/g. The obtained block copolymer was dissolved in N,N-dimethyl acetamide, such that the weight fraction of hydrophilic segment of the block copolymer became 20 wt.%. The obtained block copolymer was flow-spread on a glass plate, and hot-air drying was carried out at 130degreesC for 1 hour and 200degreesC for 0.5 hour, to obtain a polyelectrolyte film with thickness of 50 microns. The obtained polyelectrolyte film had proton conductivity of 3.3x10-2 S/cm, and methanol permeability coefficient of 0.2x10-6 cm2/second at 30degreesC and 0.7x10-6 cm2/second at 60degreesC.