Theory of ferroelectric phase transition of polymers

We propose a model of ferroelectric polymers constructed through abstraction of ferroelectric properties of copolymers of vinylidene fluoride and trifluoroethylene. Each element in the model can take two different structures whose energy states are split into three levels under an electric field. Interaction between elements in this system is classified into two categories; intermolecular interaction and intramolecular interaction. Fundamental properties of the model are calculated by the Bethe approximation. We find the thermodynamic character of the phase transition changes from the first order phase transition to the diffuse transition with a change in the ratio of intermolecular and intramolecular interactions. We demonstrate a critical phenomenon at a boundary between the first order phase transition and the diffuse transition. The critical temperature is independent of the properties related to the intramolecular degrees of freedom such as intramolecular interaction and multiplicity of an excited conformation state. Furthermore, a smearing out effect that is introduced into the intermolecular interaction influences the transition temperature. The effects of external fields on the phase transition are also discussed.