Computer simulation of structure and ferroelectric phase transition of vinylidene fluoride copolymers. IV. The factors governing the ferroelectric phase transition of VDF-TrFE copolymers

Abstract

A molecular dynamics simulation was performed to clarify the factors governing the ferroelectric phase transition of vinylidene fluoride–trifluoroethylene copolymers. The vinylidene fluoride (VDF) content dependence of the phase-transition temperature or the temperature of trans-to-gauche conformational change was investigated via models with VDF contents of 50, 70, and 100 mol %. The phase-transition temperatures of 70 mol % VDF models were higher than those of 50 mol % VDF models, consistent with experimental data. Poly(vinylidene fluoride) form I with 100% VDF showed the trans-to-gauche conformational change at a high temperature of about 600 K, although this temperature was tremendously higher than the melting point. The effects of the intermolecular and intramolecular interactions were also investigated. When only the van der Waals interaction was taken into consideration as an intermolecular interaction, the molecular chains in the cell rotated rigidly around their chain axes with the trans conformation kept. However, the consideration of only Coulombic interactions resulted in the collision of chains because of the excessively strong attractive force. The transition behavior was governed by the balance among the torsional potential energy barrier of the skeletal chains, the intermolecular van der Waals interaction, and the Coulombic interaction between the atomic charges. Additionally, the introduction of head-to-head and tail-to-tail abnormal linkages into the molecular chains expanded the cell volume and lowered the phase-transition temperature. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 689–702, 2001