Computer simulation of structural changes in the ferroelectric phase transition of vinylidene fluoride–trifluoroethylene copolymers

Structural change in the ferroelectric phase transition of vinylidene fluoride (VDF)–trifluoroethylene copolymers has been simulated by using molecular dynamics (MD) technique. The force field parameters used in this simulation were those determined in the previous paper [Tashiro et al., Ferroelectrics 1995;171:281] but with some modifications to reproduce the molecular and crystal structures and the infrared/Raman spectra of PVDF crystal forms I, II, and III to a higher degree. The MD calculation was made for the crystal structures of VDF 50 and 70 mol% copolymers, where the MD unit cells consisted of 16–36 chains of different monomer sequences under the three-dimensional periodic boundary condition. In the VDF 50% copolymer case, for example, the trans-to-gauche conformational change was found to occur at about 390 K. The transition temperature estimated for VDF 70% copolymer was about 470 K, higher than that of VDF 50% copolymer. This indicates that the copolymer with higher VDF content exhibits the transition at higher temperature, consistent with the observed results. The molecular conformation in the high-temperature phase was found to be a statistical combination of TG⁺, TG⁻, T₃G⁺ and T₃G⁻ sequences. The population of TG⁺ and TG⁻ was higher and that of T₃G⁺ and T₃G⁻ was lower for the copolymer with higher VDF content, which was also consistent with the experimental data. The trans–gauche conformational change was done with large thermal rotation of the chains, resulting in the extinction of the electric polarization of the whole unit cell in the high-temperature phase. The ratio of the a and b axes of the basic unit cell was 1.73, characteristic of the hexagonal-type structure of the high-temperature phase.