Annealing effects on the Curie transition temperature and melting temperature of poly(vinylidene fluoride/trifluoroethylene) single crystalline films

The effect of reannealing on the Curie transition temperature TC, melting point Tm, and structures of single crystalline (SC) films of ferroelectric copolymer of poly(vinylidene fluoride/trifluoroethylene) (P(VDF/TrFE)) (75:25 mol%), which were prepared by uniaxial stretching of solvent cast films followed by crystallization/annealing at 140°C, was investigated using differential scanning calorimetry and X-ray diffraction. Accumulative reannealing in the ferroelectric phase just below TC caused a large increase in TC by 17°C without distinctive change in Tm, while prolonged reannealing in the hexagonal phase caused an increase in Tm by 5°C with a slight decrease in TC. From the X-ray diffraction study it is found that the reannealing at a temperature below TC removes the gauche conformational defects existing in molecular chains, and that the reannealing in the hexagonal phase for long periods leads to rearrangement of chain molecules through their sliding motion along the chain axis so as to eliminate crystal defects and to improve the preferential orientation distribution of the crystal axes perpendicular to the (110)/(200) planes in the film. The effects of such structural changes on TC and Tm are explained on the basis of the Gibbs free energy-versus-temperature diagram.