Significantly improving dielectric and energy storage properties via uniaxially stretching crosslinked P(VDF-co-TrFE) films

Recently, tuning the normal ferroelectric performance of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) to either relaxor ferroelectric or anti-ferroelectric behavior by confining the relaxation of ferroelectric crystal domains physically or chemically has attracted considerable interest to achieve high discharged electric energy density (U_e) and low energy loss (U_l) for energy storage applications in high pulse capacitors. To improve the dielectric and energy storage properties as well as reduce the energy loss induced by the ferroelectric relaxation of P(VDF-co-TrFE), unsaturation containing P(VDF-co-TrFE) films were uniaxially stretched after crosslinking with peroxide in this work. P(VDF-co-TrFE) containing unsaturation was synthesized via controlled hydrogenation and dehydrochlorination of commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)). The properties of the films obtained were characterized with differential scanning calorimetry (DSC), X-ray diffraction (XRD), dielectric constant and electric displacement-electric field (D-E) hysteresis loop measurements. Compared with the as-cast and as-crosslinked films, the stretched films exhibit a significantly enhanced dielectric constant, breakdown field (E_b > 500 MV m^-1) and U_e but depressed energy loss. This could be attributed to the enhanced film quality, optimized crystalline properties, improved orientation uniformity of crystal domains as well as accelerated ferroelectric relaxation induced by the crosslinking and mechanical stretching. The best performance was achieved for the stretched film with a dielectric constant of 15 at 1 kHz, a relatively high U_e of 17.5 J cm^-3 and a low energy loss of about 30% at 575 MV m ^-1.