The invention concerns a polybenzazole block copolymer suitable for solid polymer electrolyte membranes application comprising: at least one block (B1) comprising recurring units (R), more than 50% of said recurring units comprising at least one benzimidazole group (formula 1a and 1b) and at least one sulfonic acid group (formula 2) [recurring units (R1)];  wherein E in formula 1a can be a substituted or non-substituted carbon atom or nitrogen atom, and Q in formula 1 can be a hydrogen atom or a group comprising a sulfonic acid group; and at least one block (B2) comprising recurring units (R′), more than 50% by moles of said recurring units comprising at least one benzazole group chosen among benzoxazole group (formula 3a and 3b) and benzothiazole group (formula 4a and 4b) [recurring units (R2)],  wherein E′ and E″ in formula 3a and 4a, respectively, can be a substituted or non-substituted carbon atom or nitrogen atom, and the block (B2) is free from sulfonic acid groups. The invention also relates to a polybenzazole polymer composition thereof, a solid polymer electrolyte membrane therefrom, a solid polymer electrolyte doped membrane therefrom, and the fuel cell therefrom.