TECHNOLOGY FOCUS - POLYMERS - Preparation (Claimed): Preparation of (I) comprises mixing the antioxidant with the polymer before the casting of the membrane.Preferred Process: The layer of mixture of polymer and antioxidant is deposited on the membrane during drying.Preferred Components: The antioxidant agent is a photostabilizer of the amino group. The photostabilizer is an amine compound of formula (Ia). The photostabilizer has a molecular mass of 300-600 g/mol.The antioxidant is present in a proportion of 0.5-1% by weight compared to the weight of the non-fluorinated or partially fluorinated polymer. The membrane is entirely constituted by a mixture of polymer and antioxidant.The membrane comprises on one of its surfaces, a fine layer of a mixture of polymer and antioxidant (where the layer has a thickness of 2-10 microns). The non-fluorinated or partially fluorinated polymer is a polyaromatic polymer comprising arylsulfonic groups. The non-fluorinated polymer is: polymers of polyether ketone sulfone comprising motifs of formula (IIa); polymers of sulfonated polyimide comprising motifs of formula (IVa); polymers of sulfonated polyarylene ether sulfone comprising k motifs of formula (Va); or polymers of polystyrene divinylbenzene sulfonic acid comprising motifs of formula (VIa). The partially fluorinated polymer is a irradiation grafted polymer comprising motifs of formula (IIIa).R = H or an alkyl, acyl or alkoxy (preferably H or methyl);X = 1-9 (such that the X/Y ratio is 2/8 or 6/4);k, n = 20-500;m = 5-10; andp = 3-10. EXAMPLE - The synthesis of sulfonated polyimide of block type was done in 2 stages in the same reactor. The first stage consisted of synthesizing a hydrophilic block by the polycondensation of dianhydride with sulfonated diamine. The imidation occurred thermally at 180degreesC for 15 hours and the diamine used was not in the acid form. The synthesis of the hydrophobic block was done by introducing the hydrophobic diamine and the dianhydride into the reaction (where the imidation took place at 180degreesC for 20 hours). The block polymer was obtained in solution using the synthesis solvent (e.g. phenol). The polymer is cooled to room temperature and it became viscous. The antioxidant (soluble in the synthesis solvent) was introduced once the synthesis of the polymer was done. The obtained homogenous solution was then set by heating, casting and drying.