Structural study on ferroelectric phase transition of vinylidene fluoride-trifluoroethylene copolymers (III) dependence of transitional behavior on VDF molar content

Abstract

Structural changes occurring in the ferroelectric phase transition of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers with the various VDF molar contents have been investigated by means of X-ray diffraction and infrared spectroscopic methods. The phase transition occurs among three crystal phases of the low-temperature (regular all-trans conformation), the high-temperature (gauche conformation with a large rotational motion), and the cooled phases (disordered trans conformation). In the copolymer with low VDF molar content less than 40%, the broad transition between the cooled phase and the high-temperature phase is observed and the conformational change from trans to gauche is not so perfect. In the range of VDF 50–60%, the transition becomes more definite, and on the way of heating the low-temperature phase transforms to the cooled phase, which transfers continuously to the high-temperature phase. For copolymers of VDF 70–80%, a clear and discontinuous first order transition between the low-and high-temperature phases is observed and the cooled phase does not practically appear. In this way the transitional behavior (the cooperativity of the transition, the completeness of trans-gauche conformational change, etc.) is dependent on the VDF content, being ascribed to the following several factors which are functions of VDF content; the VDF regular sequence length within the copolymer chain, the trans-gauche energy difference intrinsic of the chain, the role of TrFE monomer units on initiating the segmental motion of VDF sequences, and the magnitude of effective dipole moment of the trans chain.