Curie transition, ferroelectric crystal structure and ferroelectricity of a VDF/TrFE ( [formula omitted]) copolymer: 2. The effect of poling on Curie transition and ferroelectric crystal structure

Abstract

Three typical 75 25 VDF/TrFE copolymer samples, differing in the amounts of the three kinds of ferroelectric phases of different thermodynamic stability in the ferroelectric state, were prepared by consecutive annealing below the Curie transition point and were then poled at a field strength of 0.9 MV cm −1. New findings were made based on d.s.c., X-ray diffraction, i.r. spectroscopy and Raman scattering results of the poled samples. D.s.c. results gave indirect evidence that, upon poling, the most stable ferroelectric phase with nearly perfect all- trans conformation is transformed to the unstable ferroelectric phase with some gauche defects resulting from the imperfect propagation of dipole rotation along the chain due to the large steric hindrance against dipole rotation, whereas the less stable ferroelectric phase with considerable gauche defects along the chain shows a reduction of gauche defects through structural reorganization by dipole rotation. The total enthalphy of the Curie transition was greater for all samples after poling. From X-ray diffraction, the average ferroelectric domain size was found to increase in the direction perpendicular to the chain axis upon poling. These results suggest that all samples show increases in the average packing density, the total amount of trans conformers and the degree of ferroelectric crystallinity, irrespective of the initial content of ferroelectric phases of different thermodynamic stability present in the sample prior to poling. From vibrational spectroscopies, however, clear evidence that the total amount of trans conformers increases and the gauche defects are reduced upon poling was not found. No significant differences in the average degree of dipole orientation after poling were observed between the samples from i.r. absorbance spectra in the CH 2 stretching vibration region. This indicates that, contrary to expectation, the most important factor in obtaining the highest degree of dipole orientation, i.e. the highest piezoelectric constant, is not the thermodynamic stability of the ferroelectric phase present in the sample.