Modeling Impact of Regiodefects on the Electrocaloric Effect in Poly(VDF-co-TrFE) Copolymers

Impact of regiodefects in the ferroelectric poly(vinylidene difluoride-co-trifluoroethylene) copolymer [poly(VDF-co-TrFE)] on the electrocaloric effect was studied with use of a developed analytical model and molecular dynamics (MD) simulations. It was shown earlier that the electrocaloric effect in these polymers is caused by the first-order phase transition from the ferroelectric β phase to the paraelectric conformationally disordered (condis) phase. MD simulations performed in the current work show that the presence of regiodefects in polymer chains makes this phase transition more gradual, the second order. The proposed analytical model is based on the Landau phenomenological theory of phase transitions, modified to ensure the correct behavior of polymer polarization at low temperatures. The model calculates the polymer polarization, P, and temperature change under adiabatic electric field variation, ΔT, as functions of temperature, applied electric field, and regiodefect concentration. Parameters of the free energy functional are calibrated through MD simulations of the poly(VDF-co-TrFE) crystal. The obtained results show that presence of regiodefects substantially changes the form of dependencies P(T) and ΔT(T), which significantly shifts them closer to experimental data.