Ferroelectric transition under hydrostatic pressure in poly(vinylidene fluoride-trifluoroethylene) copolymers

Abstract

A clear ferroelectric transition of first-order type is observed in poly(vinylidene fluoride-trifluoroethylene) (P(VDFTrFE)) random copolymers with TrFE contents higher than 20% and lower than 40%. This structural phase transition which takes place in the crystalline regions of the polymeric material has been analysed using neutron diffraction under hydrostatic pressure for three copolymer compositions: 80 20 , 70 30 and 60 40 . The P- T phase diagrams have been determined in the ranges 0.1–300 MPa, 300–500 K both upon heating and upon cooling, and several thermodynamic parameters have been evaluated for the different crystalline phases: compressibility and thermal expansion coefficients, enthalpies of transition, etc. For the middle composition ( 70 30 ) the ferroelectric transition temperature T C is well separated from the melting temperature T m of the paraelectric phase and both transition temperatures increase with increasing pressure: dT C dP = 0.38 K MPa −1 and dT m dP = 0.25 K MPa −1 . For the 80 20 composition the Curie temperature T C is closer to the melting temperature T m, and under increasing pressure the two transition lines are expected to merge at a triple point ( P∗ ≈ 500 MPa , T∗ ≈ 560 K ), above which a single line corresponds to the melting of the ferroelectric phase. For the 60 40 composition a new ‘low temperature disordered’ phase appears and coexists with the ferroelectric phase but with a volume fraction which depends on temperature and thermal history.