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

摘要

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 that 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~-, T_3G~+ and T_3G~- sequences. The population of TG~+ and TG~- was higher and that T_3~+ of T_3G~- 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.