NOVELTY - The method comprises subjecting a membrane containing protonic conducting polymer to a chemical treatment at 80 degrees C for 5 minutes to 100 hours for grafting polar functional groups on the conducting polymer and optionally performing partial cleavage of chains of polymer. The chemical treatment consisting of a thermochemical treatment is carried out using oxidation. The oxidation is carried out: in vapor phase or liquid phase; and using an oxidizing agent with a constant concentration. The chemical treatment is performed using one of acid oxidation, heat treating and/or photochemical oxidizing.    USE - The method is useful for manufacturing a membrane containing protonic conducting polymer (claimed), where the membrane is useful in a fuel cell.    ADVANTAGE - The method is capable of increasing protonic conductivity and/or diffusion coefficient and improving core performances and durability of the membrane and economically manufacturing the membrane with properties such as low permeability to gases and good chemical and mechanical stability.    DETAILED DESCRIPTION - The method comprises subjecting a membrane containing protonic conducting polymer to a chemical treatment at 80 degrees C for 5 minutes to 100 hours for grafting polar functional groups on the conducting polymer and optionally performing partial cleavage of chains of polymer. The chemical treatment consisting of a thermochemical treatment is carried out using oxidation. The oxidation is carried out: in vapor phase or liquid phase; and using an oxidizing agent with a constant concentration. The chemical treatment is performed using one of acid oxidation, heat treating and/or photochemical oxidizing, and treatment with x-ray and oxygen gas (O2), ozone, O2 plasma and corona and O2. The chemical treatment is stopped when the treated membrane has one of: a stress and/or rupture and/or elastic limit that is reduced to a maximum of 10-20%; a swelling of less than or equal to 60 vol.% in water; a protonic conductivity of greater than or equal to 10-1 S/cm in water measured at a room temperature; and a diffusion coefficient of greater than or equal to 10-5 cm2/seconds in water measured at the room temperature. An INDEPENDENT CLAIM is included for a membrane.