The effect of an external electric field on solid-state phase transition of P(VDF/TrFE)(72/28)

The copolymers of vinylidene fluoride and trifluoroethylene (P(VDF/TrFE)) with VDF content of 50-80 mole% can be applied to the field of nonvolatile ferroelectric polymeric random access memory (FePoRAM) devices, since they exhibit stable ferroelectric β-phase at room temperature with spontaneous polarization of the C-F dipoles towards an external electric field greater than the coercive field. Many researchers have already reported the molecular structures and dynamics of the ferroelectric (F) crystalline phase and the unique change in chain conformation between polar F phase and non-polar paraelectric (P) phase near their Curie transition temperature (T_c) which is dependent on factors such as VDF content and annealing treatment conditions. The effect of external electric field strength on the F⇔P crystalline phase transition in P(VDF/TrFE)(72/28) random copolymer samples of nanometer thickness was investigated. Capacitance of 250 nm thick sample measured as a function of heating-cooling under varying external electric field strength exhibited increasing T_c's during heating (T_c ^↑) and cooling (T_c^↓) under an applied electric field of more than 0.03 MV/cm. Applying cyclic bias electric field (+1 to -1 MV/cm) for samples kept isothermally at just above their T _c↓ during cooling, we were able to observe the field-induced P→F phase transition. With increasing cycles of the applied electric field for sample maintained just above T_c↓, the bistable C-E hysteresis was observed and the phase change from P→F is irreversible even after the electric field is removed. However, for samples kept well above T_c↓ and near T_m, (100°C and 120°C respectively) during cooling, the F-phase initially formed through the field-induced phase transition is reversibly transformed to the P-phase when the applied electric field is removed. Drastic changes were observed in both coercive field (E_c) and remanent polarization (P_r) values during heating and cooling near the T_c range due to the F⇔P phase transition and the results are reported in detail here.