Dielectric properties of graphene/poly(vinyl alcohol)/poly(vinylidene fluoride) nanocomposites films

Abstract

Graphene has been a superstar in the fields ranging from materials science to condensed-matter physics since 2004. Graphene possesses good thermal and mechanical properties, high electron transfer capability and relatively low production cost. As a consequence, graphene has been used in the areas of multi-functional advanced materials and electronics. A direct disperse method has been widely applied to polymers to improve their dielectric properties. Recently, graphene/polymer composites have received much attention. Graphene nanosheets can significantly improve the physical properties of the host polymer at a very low content of conductive filler loading. Poly vinylidene fluoride (PVDF) is a semicrystalline thermoplastic polymer with remarkably high piezo-/pyroelectric coefficient, and excellent thermal stability and chemical resistance. More efforts have been recently devoted to the preparations of high-ε' composites based on PVDF. In this work, a graphene/PVA/PVDF nanocomposite film composed of poly(vinyl alcohol) (PVA), reduced graphene oxide (RGO), and poly(vinylidene fluoride) (PVDF) is fabricated. First of all, graphene oxide (GO) is prepared by the modified Hummers method. GO and PVA are successively dissolved in the dimethyl sulfoxide (DMSO) solution, in order to obtain PVA functionalized GO which is formed via non-covalent bonds. Then PVDF is added into this solution to form a homogeneous three-phase aqueous mixture. According to the solution-casting and thermal reduction processes, the three-phase nanocomposite films are formed. The thickness values of the films are in a range of 0.3-0.4 mm. The square specimens are coated with a silver paste prior to electrical measurements. The obtained products are characterized using X-ray diffraction, UV Vis absorption spectrum, Fourier transform infrared absorption spectrum, and atomic force microscopy. The morphologies of PVDF and RGO/PVA/PVDF films are investigated by a scanning electron microscope. Electrical measurements are conducted in a frequency range from 102 to 104 Hz. Results suggest that GO can be reduced to RGO and phase transition of PVDF from α to β phases is effectively promoted at 120℃. The dielectric properties of the polymer matrix are improved. Furthermore, PVA modified RGO is easier to disperse in the PVDF substrate than the original one, which strongly reduces the spherulite size of PVDF and improves the dielectric property of the composite film. The percolation threshold (fvol*) of RGO/PVA/PVDF film is estimated to be 8.45 vol.%, and the dielectric constant of the film is 238 times as large as that of the pure PVDF films at 102 Hz. In addition, the dielectric constant increases rapidly near the percolation threshold and depends on frequency, which is mainly ascribed to the Maxwell-Wagner-Sillars polarization in the low frequency range. This study provides a low-cost and simple method of preparing polymer nanocomposites with high dielectric properties.