Vibrational spectroscopic study on ferroelectric phase transition of vinylidene fluoride-trifluoroethylene copolymers: 2. Temperature dependences of the far-infrared absorption spectra and ultrasonic velocity

Abstract

The temperature dependence of far infrared absorption spectra was measured for vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers with various VDF contents. For 55, 72 and 82 mol% VDF samples, the trans-gauche conformational transition was detected clearly. Near the ferroelectric phase transition temperature of the 55 mol% VDF sample, the band at 116 cm −1, the skeletal torsional mode of the trans zig-zag conformation, was observed to transfer continuously to the position of the torsional band of the gauche type skeletal conformation at 108 cm −1, suggesting a softening behaviour of the skeletal vibrational mode in the structural transition region. The ultrasonic modulus was found to exhibit an unusual minimum in the same temperature region. This anomalous behaviour was interpreted in terms of the coupling between the optical phonon of the skeletal torsional mode and the acoustic phonon through the piezoelectric and electrostrictive effects.