Sulphonic fluorinated ionomers

Sulphonic fluorinated ionomers crosslinked by radical crosslinking of: A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380-1300 g/eq and comprising: from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene (TFE); from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups -SOF; from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula: wherein: m=2-10, preferably 4-8; R, R, R, R, equal to or different from each other, are H or C-C alkyl groups; B) a fluorinated compound as crosslinking radical initiator; C) a fluorinated bis-olefin of structure (I) as crosslinking agent; the radical crosslinking being carried out at a temperature in the range 250°C-310°C, preferaably 260°C-300°C.