Microscopic Mechanism of the Large Electrocaloric Effect in Vinylidene Difluoride-Based Polymers

Abstract

The molecular mechanism of the large electrocaloric effect in ferroelectric vinylidene difluoride-based polymers was proposed. The mechanism involves the electric field-induced phase transition between the ferroelectric β-phase and the paraelectric conformationally disordered (condis) phase. The condis phase is characterized by the absence of a three-dimensional crystallographic order: there is only two-dimensional translational symmetry in the plane perpendicular to the chain direction. Due to conformational disorder, the entropy of the condis phase is higher than that of the β-phase. A high entropy change at the transition from the β to the condis phase causes a large electrocaloric effect. Molecular dynamics simulations of the poly­(vinylidene fluoride-co-trifluoroethylene) (poly­(VDF-co-TrFE)) copolymer and poly­(vinylidene fluoride–ter–trifluoroethylene–ter–chlorofluoroethylene) (poly­(VDF-ter-TrFE-ter-CFE)) and poly­(vinylidene fluoride-ter-trifluoroethylene-ter-chlorotrifluoroethylene) (poly­(VDF-ter-TrFE-ter-CTFE)) terpolymers were performed. The electrocaloric effect was determined from simulations using two techniques: (i) direct calculation of the temperature drop in adiabatic conditions and (ii) calculation of the entropy difference between β and condis phases from the pair histograms of dihedral angles. Both techniques predict similar temperature drops that validate the proposed molecular mechanism.

Combined analytical and molecular dynamics model of electrocaloric effect in poly(VDF‐co‐TrFE) copolymer

“…That indicates the first order phase transition. Analysis of the chains conformations above and below the transition temperature performed in Ref 12 indicated that this is the phase transition from the ferroelectric β phase to the paraelectric condis phase. For visualization of fragments of poly(VDF‐co‐TrFE) chains in β and condis phase see Figure 3.…”

“…The object of this study is poly(VDF‐ co ‐TrFE) copolymer with molar monomeric composition VDF:TrFE = 55:45, for which the electrocaloric effect was studied experimentally and both and remanent polarization as functions of temperature are available in literature 9 . As an initial system for the MD simulations, we prepared the β crystal of the copolymer with quasi‐infinite chains as described in Reference 12. The total monomer composition of the system was 1760 VDF +720 ( R )‐TrFE +720 ( S )‐TrFE.…”

“…On the other hand, strong enough external field assists formation of the ordered phase, thus, location of in the middle of the range between transition temperatures on cooling and on heating seems a reasonable assumption (taking into account that at 100 MV/m this range has an approximately constant width of only about 15 K). According to the thermodynamic relation 12 the dependence of on should be approximately linear. Thus, we approximate the dependence by the result of linear regression of average transition temperature values for = 100, 200, and 300 MV/m and extrapolate this linear dependence towards 0 (see Figure 4).…”

“…In our previous work, 12 we used molecular dynamics (MD) simulations to achieve understanding of the molecular mechanism of the electrocaloric effect in ferroelectric polymers. We established that the electrocaloric effect in these fluoropolymers is possible due to the impact of the external electric field on the temperature of phase transition from the ferroelectric β phase to the paraelectric condis phase.…”

“…That makes it difficult to compare quantitatively the MD modeling results for perfect crystal with experimental data for actual polymer system. To take into account the above mentioned factors, in paper 12 we developed a simplified analytical model to estimate the electrocaloric effect of the amorphous‐crystalline polymer. The model utilizes the value of the entropy change under the phase transition calculated in MD simulations.…”