EFFECT OF THERMAL HISTORY ON THE MELTING BEHAVIOR AND THE CRYSTAL-CRYSTAL TRANSITIONS OF TETRAFLUOROETHYLENE-PERFLUOROALKYLVINYLETHER COPOLYMERS

By differential scanning calorimetry (DSC), the effect of thermal trea tments on the melting of tetrafluoroethylene-perfluoroalkylvinylether copolymers (PFA) with different contents of perfluoromethylvinylether as a comonomer has been investigated. Two melting peaks can be identif ied for all copolymers, whose presence, value, and extent depend upon the thermal treatments. The higher temperature one, scarcely influence d by the crystallization conditions (i.e., cooling rate and annealing) , can be attributed to more perfect crystals present in the original s amples. The lower temperature one is produced only by annealing (annea ling peak) and can be interpreted as resulting from much poorer crysta ls grown among the larger ones. For the copolymer with the lowest cont ent of comonomer, the effect of thermal treatments on the low-temperat ure (crystal-crystal) transitions has also been studied. The rejection of the counits from the crystals, at least in equilibrium conditions, is confirmed. (C) 1996 John Wiley & Sons, Inc.