Enhanced dielectric property and energy density in poly(vinylidene fluoride-chlorotrifluoroethylene) nanocomposite incorporated with graphene functionalized with hyperbranched polyethylene-graft-poly(trifluoroethyl methacrylate) copolymer

Polymer film capacitors are attractive as promising candidates for potential applications in compact and efficient electric power systems. The development of a method for producing a polymer nanocomposite with enhanced dielectric property and a high energy density is a fundamental solution for electric storage in a film capacitor. In this work, a hyperbranched polyethylene-graft-poly(trifluoroethyl methacrylate) (HBPE-g-PTFEMA) copolymer was synthesized to exfoliate and functionalize graphene from natural graphite in chloroform on the basis of the CH-pi non-covalent stacking between the HBPE-g-PTFEMA stabilizer and graphene. Examination of the morphologies by transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirmed that the resulting graphene had a lateral size of 0.2-0.6 mu m and a thickness of around 4 layers. The presence of peaks due to F in the X-ray photoelectron spectroscopy (XPS) spectra of graphene indicates that the fluorinated copolymer was attached to the surface of nanosheets. Few-layer graphene was introduced into a poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE)) matrix via simple solution casting. The relative content of the electroactive phase in the nanocomposite film increased because the phase transition from the alpha- to the beta-phase was induced by the addition of graphene. The dielectric constant increased to 24.8 with a low dielectric loss of 0.06 at 100 Hz for a 0.8 vol% nanocomposite, and a released energy density of 4.6 J cm(-3) with a charge-discharge efficiency of 62% at 250 MV m(-1) was achieved with a 0.1 vol% nanocomposite, which was attributed to the combination of the large content of the electroactive phase and interfacial polarization. This strategy based on a nanocomposite with graphene exfoliated by a fluoropolymer sheds light on the mechanism of interfacial polarization and exhibits commendable prospects for applications in flexible film capacitors.