The invention mainly pertains to a process for heat treating a composition [composition (C)] which contains at least one melt-processible perfluoropolynner [polymer (F)] formed of tetrafluoroethylene (TFE) copolymer with one or more perfluorinated comonomers [comonomer (F)] containing at least one unsaturation of ethylene type in amounts from 0.5% to 13% by weight, preferably from 0.6% to 11% by weight, and more preferably from 0.8% to 9% by weight; the process comprising at least the step of heat-treating the composition (C) at a temperature of at least 260° C. and in the absence of a melt flowable polytetrafluoroethylene. The invention is also related to a melt-processible perfluoropolynner [polymer (F1)] formed of tetrafluoroethylene (TFE) copolymer with one or more perfluorinated comonomers [comonomer (F)] containing at least one unsaturation of ethylene type in amounts from 0.5% to 13% by weight, preferably from 0.6% to 11% by weight, and more preferably from 0.8% to 9% by weight, wherein a particular relation between comonomer (F) content and first melting temperature of polymer (F1) is satisfied.