NOVELTY - A polymer electrolyte comprises at least one aromatic polyether copolymer with main chain pyridine groups and side chain carboxylic acid or carboxylic ester or toluene or methoxy phenyl or hydroxyl phenyl or propenyl or styrene groups and/or pyridine groups, which have the ability to be covalently crosslinked.    USE - As polymer electrolyte in layered membrane electrode assembly (claimed); in high temperature polymer electrolyte membrane fuel cells; and in proton exchange membrane fuel cells (PEMFC) used as power generators for automotive, stationary as well as portable power.    ADVANTAGE - The electrolyte has the ability to be covalently crosslinked. The fuel cell comprising the electrolyte exhibits high energy efficiency and low emission. The electrolyte exhibits improved mechanical properties and oxidative stability. The copolymers with different chemical structures exhibit excellent mechanical properties, high thermal and oxidative stability and high values of proton conductivity after doping with phosphoric acid. The polymeric material exhibits excellent mechanical properties, high oxidative and chemical stability, high doping level and proton conductivity and hence can be used as polymer electrolyte in high temperature polymer electrolyte membrane fuel cells. The produced membranes combine excellent mechanical properties with high thermal and oxidative stability. The membranes can be doped with phosphoric acid at high doping levels resulting in proton conductivities of 10-2 to 10-1 S/cm.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method (M1) for crosslinking aromatic polyethers selected from 5 copolymers as given in the specification e.g. Copolymers of formulae -(O-T1-T2-T1-O-T1-SO2-T1)x-(O-T3-T3-O-T1-SO2-T1)y-(O-T4-O-T1-SO2-T1)z- (I), -(O-T1-T2-T1-O-T1-SO2-T1)x'-(O-T3-T3-O-T1-SO2-T1)y'-(O-T5-O-T1-SO2-T1)z'- (II), -(O-T1-T2-T1-O-T1-R'-T1)x1-(O-T6-O-T1-R'-T1)y1-(O-T7-C(CH3)2-T7-O-T1-R'-T1)z1 (III) or -(O-T1-T2-T1-O-T1-SO2-T1)m-(O-T3-T3-O-T1-SO2-T1)n-(O-T8-SO2-T8-O-T1-SO2-T1)o-(O-T6-O-T1-SO2-T1)p (V), involving at least one of heat treating the aromatic polyether copolymers, or reacting the aromatic polyether copolymer with at least one crosslinking agent;    (2) a method (M2) for crosslinking aromatic polyethers i.e. Copolymers (I) o (II) involving reacting the aromatic polyether copolymers with at least ditetrazoles or poly(tetrazole)s or dihydrazides to form oxadiazole rings, or tetramines or poly(diamine)s to form imidazole rings;    (3) a method (M3) for crosslinking aromatic polyethers i.e. Copolymer (II) involving reacting the aromatic polyether copolymers with aromatic fluorides to form aromatic ether crosslinked bonds;    (4) a method (M4) for crosslinking aromatic polyethers i.e. Copolymer (III) involving thermally treating the aromatic polyether copolymers at about 80-150 degrees C in the presence of phosphoric acid, or reacting the aromatic polyether copolymers with bisazide at about 150-250 degrees C to form aziridines or secondary amines;    (5) a method (M5) for crosslinking aromatic polyethers i.e. Copolymer (II), (IV) or (V) involving thermally treating the aromatic polyether copolymers at about 200-300 degrees C, or at 80-150 degrees C in the presence of phosphoric acid; and    (6) a layered membrane electrode assembly comprising an anode electrode, a cathode electrode and the polymer electrolyte.    T1=1,4-phenylene;    T2=pyridin-3,6-diyl;    T3=3,5-dimethyl-1,4-phenylene;    T4=1,5-phenylene (substituted on 3-position by COOH);    T5=a group of formula (ia);    T6=a group of formula (ib);    T7=1,4-phenylene (substituted on 3-position by (E)-CH=CH-CH3);    x=5-95%;    y=0-95%;    z=5-95%;or    x+y+z=100%;    x'=50-80%;    y'=0-50%;    z'=5-50%;or    x'+y'+z'=100%;    R=-CH=CH2, CH3, COOH, COOCH3, OCH3 or OH;    x1=50-70%;    y1=0-30%;    z1=10-30%;or    x1+y1+z1=100%;    R'=SO2 or OP-Ph;    T8=a group of formula (ic);    m=40-90%;    n=0-40%;    o=5-40%;    p=0-30%;or    m+n+o+p=100%.