The invention relates to the preparation of a crosslinked polymeric film bearing ionic groupings.The process consists in extruding a material comprising at least one prepolymer, in polymerizing said material after extrusion, and in subjecting the extruded material to a chemical reaction for grafting ionic groupings. The prepolymer comprises repeating units each comprising at least one aromatic group GA and at least one functional group GF; it bears at least one group GP which is polymerizable thermally at a temperature greater than the extrusion temperature, or photochemically, and also at least one reactive group GR which allows the grafting of ionic groupings.The films are useful as a membrane for fuel cells or electrodialysis, as an electrolyte of a lithium battery, of a supercapacitor or of an electrochromic device, or as an ion exchange membrane.