Crystal Orientation and Temperature Effects on Double Hysteresis Loop Behavior in a Poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)-graft-Polystyrene Graft Copolymer

Recently, double hysteresis loop (DHL) behavior, which is advantageous for the high energy density and low loss dielectric application, was achieved in a poly­(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)-graft-polystyrene [P­(VDF-TrFE-CTFE)-g-PS­(14%)] graft copolymer due to the nanoconfinement effect. In this work, we continued to investigate the crystal orientation and temperature effects on the DHL behavior of this graft copolymer. Based on the electric displacement–electric field (D–E) study, crystal orientation had a profound effect on its electrical behavior. For the nonoriented sample, dielectric instead of ferroelectric behavior was observed. After uniaxial stretching, DHLs gradually developed in the oriented films upon increasing the extension ratio. For a fully stretched film, the DHL behavior was stable below 75 °C but gradually disappeared above 100 °C due to enhanced dc conduction and impurity ion migrational loss at elevated temperatures. After subtracting the dc conduction, D–E hysteresis loops from the ion loss were determined for the poling cycles below 100 MV/m. The hysteresis loss from ion migration under an applied field was closely related to ion concentration and diffusion coefficient, which were determined by broadband dielectric spectroscopy. Both parameters were used in a theoretical calculation to obtain hysteresis loops from ion migrational loss. By fitting the theoretical loops with those after dc conduction subtraction, ion mobility was found to be dependent upon both poling field and temperature. This study provides a quantitative understanding of the effects of impurity ions and dc conduction on dielectric and ferroelectric properties of polymers at elevated temperatures.