TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The main chain is an ethylene/tetrafluoroethylene or propylene/tetrafluoroethylene copolymer and the side chains consist of a styrene/divinylbenzene or styrene/alpha-methylstyrene/divinyl- benzene copolymer with sulfonic acid groups.The membrane has a side chain content of 10-150 parts by weight (per 100 parts main chain); it shows an ion exchange capacity of 1.0- 3.5 m-equivalents/g and a water content of 30-200%.Preferred Processes: In process (A), the copolymer formed in stage (a1) has the above preferred composition without sulfonic acid groups; the acid groups are introduced (step a2) by treatment with fluoro- or chloro-sulfonic acid.In process B, the acid group-containing preferred structure is formed directly by reaction with sulfonic acid monomers.The radiation dose used in both processes is 1-100 kGy, and the grafting step is carried out at a temperature no higher than the boiling point of the olefinic aromatic hydrocarbon. EXAMPLE - An ethylene/tetrafluoroethylene copolymer film (thickness = 50 microns) was irradiated with gamma-rays at a dose of 5 kGy in air at room temperature (RT), and then contacted for 2 hours at 60degreesC with an oxygen-free (nitrogen- purged) mixture of 95 parts styrene and 5 parts divinylbenzene. The film was washed, dried, contacted with a mixture of 5 parts chlorosulfonic acid and 60 parts dichloroethane for 1 hour at room temperature under nitrogen and then washed with water at 90degreesC for 1 hour. The modified film showed an ion exchange capacity of 1.75 m-equivs./g and a water content of 66% at RT.    The film was used as the polyelectrolyte membrane between two gas diffusion electrodes (PTFE-coated carbon paper strips coated on one side with platinum) in a fuel cell which was tested at 80degreesC with hydrogen and air at 2.5 atm.    As shown by the curves in Fig. 1, this cell could be operated at high current densities (1 A/cm2) while a similar cell containing a membrane with a water content of 180% (made by grafting with styrene only) could not be operated in this range due to its marked fall in voltage at high current densities.