Spontaneous depolarization current in different mole ratio VDF/TrFE ferroelectric copolymers

Vinylidene fluoride (VDF)/trifluoroethylene (TrFE) ferroelectric copolymer samples with different mole ratio of 56/44, 70/30 and 77/23 were fabricated by casting from methyl ethyl ketone and followed by slowly cooling from the melting state. Trapped delocalized charges during circulating thermo-stimulating have been observed as spontaneous depolarization current (SDC). For the VDF(56)/TrFE(44) copolymer, SDC spectra show a positive current peak at 72°C during heating and a negative peak at 58°C during cooling. For the VDF(70)/TrFE(30) copolymer, SDC spectra display a positive current peak at 114°C during heating and present a negative peak at 58°C and a positive one at 52°C during cooling, respectively. As for the VDF(77)/TrFE(23) sample, SDC show a negative peak at 71°C and a positive one at 63°C during cooling. By comparison with DSC measurements, the peaks are ascribed to the conformation change during ferro-to-paraelectric or para-to-ferroelectric phase transition in heating or cooling processes. This is also an evidence of two coexistent ferroelectric phases in the VDF/TrFE copolymers