producing radicals able to react with the monomer therefore both the grafting temperature and time are important parameters the following examples illustrate the invention a hydrophilic material described in wo03 023890 was used graft polymers were produced using ethylene tetrafluoroethylene film etfe low density polyethylene film ldpe styrene and chlorosulphonic acid pieces of polymer film approx 0 3 m u00d7 0 5 m were cut weighed and rolled in a u2018 swiss roll u2019 configuration with a non woven interlayer the roll was placed in a glass vessel and the vessel filled with monomer solution the solution was allowed to soak into the roll for approximately 10 minutes the oxygen in the vessel was then either removed by purging with nitrogen or by evacuation using a water pump the whole vessel was then placed in a cobalt 60 gamma source to be irradiated for a predetermined time the distance of the tube from the source determining the dose rate the temperature of the irradiation cell was measured at 23 u00b1 1 u00b0 c after irradiation the vessel was emptied the roll untied and the grafted films unwrapped from the interlayer the copolymerised film was washed in toluene for 24 h to remove unreacted or homopolymerised styrene washed in methanol to remove the toluene and dried.