Experimental Evidences for Field Induced Phase Transition of P(VDF/TrFE)(72/28)

Abstract

P(VDF/TrFE) (VDF 50~80 mole %) has significant applications in 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 below their Curie transition temperature (Tc). 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 Tc which is dependent on factors such as VDF content and annealing treatment conditions. In this paper, we have investigated the effect of external electric field strength on the FP crystalline phase transition in P(VDF/TrFE)(72/28) random copolymer samples of nanoscale thickness subjected to varying heat treatment conditions. Capacitance (C-T) of 250 nm thick sample measured as a function of heating-cooling under varying external electric field strength exhibited increasing Tc during heating (Tc ↑) and cooling (Tc ↓) under an applied electric field of more than 0.03MV/cm. Applying bias electric field (+1 to -1 MV/cm) for samples kept isothermally at just above their Tc 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 Tc, 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 Tc and near Tm (100oC and 120oC 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 (Ec) and remnant polarization (Pr) values during heating and cooling near the Tc range due to the F⇔P phase transition and the results are reported in detail here.