Structural changes in ferroelectric phase transitions of vinylidene fluoride-tetrafluoroethylene copolymers: 1. Vinylidene fluoride content dependence of the transition behaviour

Abstract

Structural changes occurring in the ferroelectric phase transitions of vinylidene fluoride-tetrafluoroethylene (VDF-TFE) copolymers with VDF contents of 81-0 mol% have been investigated by X-ray diffraction and i.r. and Raman spectroscopies. In a VDF 81%-TFE sample, the conformational change between the trans (low temperature) and gauche (high temperature) phases occurs in the temperature region close to the melting point with thermal hysteresis. The VDF 75%-TFE sample shows a similar transition between the low and high temperature phases but via the disordered cooled phase: the behaviour is close to that observed for vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer with VDF 65 mol%. As the VDF content decreases, the transition becomes more diffuse: for the VDF 64%-TFE sample the trans-gauche conformational change occurs gradually over a wide temperature region with almost no detectable hysteresis. The X-ray fibre diagram taken at room temperature is of the cooled phase, which reversibly transfers into the low temperature phase by the application of tensile force. The same phenomenon is observed for VDF-TrFE copolymer with VDF 37%. In this way the transition behaviour of VDF-TFE copolymers with VDF contents of 60–80% corresponds to that of VDF-TrFE copolymers with lower VDF content. As the VDF content decreases, the low temperature phase is stabilized more due to the effect of the TFE monomeric units. The as-drawn VDF 41%-TFE sample shows an X-ray fibre pattern characteristic of the low temperature phase, which transforms irreversibly to the cooled phase by heating above 110°C. Tensile force causes the transition to the original low temperature phase irreversibly. VDF 23%-TFE sample exists in the stable ‘untilted’ all- trans chain conformation above room temperature. Below −40°C, the X-ray fibre diagram changes into an overlap of the patterns of the poly (vinylidene fluoride) planar zigzag structure and of the poly(tetrafluoroethylene) helical structure. The transitional behaviour has been discussed in comparison with that of VDF-TrFE copolymers and the factors governing these structural phase transitions have been investigated.