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

VI Sultanov and VV Atrazhev and DV Dmitriev and NS Erikhman and DU Furrer and SF Burlatsky, MACROMOLECULES, 54, 3744-3754 (2021).

DOI: 10.1021/acs.macromol.0c02465

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 beta-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 beta-phase. A high entropy change at the transition from the beta 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-chloro- fluoroethylene) (poly(VDF-ter-TrFE-ter-CFE)) and poly(vinylidene fluoride-ter-trifluoroethylene-ter-chloro-trifluoroethylene) (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 beta and condis phases from the pair histograms of dihedral angles. Both techniques predict similar temperature drops that validate the proposed molecular mechanism.

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