Operando Investigation of the Molecular Origins of Dipole Switching in P(VDF-TrFE-CFE) Terpolymer for Large Adiabatic Temperature Change

Relaxor ferroelectric polymers exhibiting a giant electrocaloric effect (ECE) can potentially be used to create next-generation solid-state coolers. Under an electric field, poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer goes through a large dipolar entropy change producing a high adiabatic temperature change (Delta TECE). This work resolves the molecular origins of the large entropy change behind the electric field-induced dipole switching. A Fourier transform infrared spectroscopy equipped with a high voltage source is used to operandoly observe the characteristic molecular vibrational modes. A short-range trans (T) conformation of the CF2-CH2 dyads interrupted by a gauche (G) conformation, e.g., TTTG in the terpolymer chain, undergoes a dynamic transformation that leads to a corresponding Delta TECE whenever an electric field is applied. The molecular dynamics simulation also proves that the energy barrier that the transformation from TTTGs into a long T sequence overcomes is smaller than that for all other conformations. A mixed solvent system is used to obtain T3G-enriched terpolymer films exhibiting a 4.02 K Delta TECE at 60 MV m-1 and these films are employed to manufacture a 2-layer-cascaded cooling device that achieves a 6.7 K temperature lift, the highest reported value for a 2-layer cascaded device made of fluoropolymers. The dynamic dipole switching in the relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer, P(VDF-TrFE-CFE), under an externally applied electric field is studied with operando Fourier transform infrared spectroscopy. Short trans sequences (T3G) along the polymer chains are found to be responsible for the reversible entropy change with the field, accounting for a large electrocaloric effect. image