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-Tphase 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 Tc is well separated from the melting temperature Tm of the paraelectric phase and both transition temperatures increase with increasing pressure: dTc/dP = 0.38 K MPa- 1 and dT~/dP = 0.25 K MPa- t. For the 80/20 composition the Curie temperature Tc is closer to the melting temperature Tin, 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.
Посилання на статтю:
Ferroelectric transition under hydrostatic pressure in poly(vinylidene fluoride-trifluoroethylene) copolymers / E. Bellet-Amalric and J. F. Legrand // Polymer. – 1994. – Vol 35. – P. 34-46.