Low percolation threshold of graphene/polymer composites prepared by solvothermal reduction of graphene oxide in the polymer solution
© He and Tjong; licensee Springer. 2013
Received: 7 February 2013
Accepted: 12 March 2013
Published: 22 March 2013
Graphene/polyvinylidene fluoride (PVDF) composites were prepared using in-situ solvothermal reduction of graphene oxide in the PVDF solution. The electrical conductivity of the composites was greatly improved by doping with graphene sheets. The percolation threshold of such composite was determined to be 0.31 vol.%, being much smaller than that of the composites prepared via blending reduced graphene sheets with polymer matrix. This is attributed to the large aspect ratio of the SRG sheets and their uniform dispersion in the polymer matrix. The dielectric constant of PVDF showed a marked increase from 7 to about 105 with only 0.5 vol.% loading of SRG content. Like the other conductor-insulator systems, the AC conductivity of the system also obeyed the universal dynamic response. In addition, the SRG/PVDF composite shows a much stronger nonlinear conduction behavior than carbon nanotube/nanofiber based polymer composite, owing to intense Zener tunneling between the SRG sheets. The strong electrical nonlinearity provides further support for a homogeneous dispersion of SRG sheets in the polymer matrix.
KeywordsGraphene Composite Percolation Electrical conductivity
Recently, there is a great demand for polymeric materials with excellent electrical performance and high mechanical strength in the electronic industries. Conventional polymeric materials are insulators and can be made conductive by adding large volume fractions of conducting fillers in micrometer size such as metal and graphite particles [1–3]. However, high filler loadings generally result in low mechanical strength, heavy weight, and poor processability [4–6]. In this respect, fillers of nanometer dimensions are added to polymers to enhance their mechanical and physical performances [7–10]. Carbonaceous nanofillers such as carbon nanotubes (CNTs) with large mechanical strength and high electrical conductivity have been widely added to polymers to form conductive nanocomposites [11–17]. Their large aspect ratios enable the formation of conductive network in the polymer matrix at low filler contents. However, single-walled carbon nanotubes are very expensive, and the cost of multiwalled CNTs still remain relatively high despite a large reduction in their price in recent years . The high cost of CNTs and their strong tendency to form aggregates have greatly limited their potential applications.
Graphite nanoplatelets (GNPs) prepared from the exfoliation of graphite intercalation compound (GIC) are low-cost fillers for preparing conductive polymer nanocomposites. The GIC can be synthesized by reacting natural graphite with electron-donor agents such as alkali metals or with electron acceptors . However, GNPs consist of tens to hundreds of stacked graphene layers, corresponding to partially exfoliated graphite . In 2004, Geim and co-workers successfully exfoliated graphite into graphene monolayer using the scotch tape method . The monolayer graphene they obtained is believed to be a promising nanofiller for polymers due to its exceptionally high mechanical strength and excellent electrical and thermal properties. It has been reported that graphene/polymer composites exhibit much improved electrical and mechanical properties when compared to CNT/polymer composites [22, 23]. In practice, however, the low yield of mechanically exfoliated graphene has greatly limited its applications. Thus, high-yield graphene oxide (GO) prepared from the chemical oxidation of graphite in strong oxidizing acids is commonly used to prepare graphene [24, 25]. GO is electrically insulating; therefore, chemical reduction or thermal treatment is needed to restore its electrical conductivity [26, 27]. In addition, graphene sheets have a great tendency to aggregate when they are loaded to the polymers. The aggregation is mainly due to the van der Waals attractions between the graphene sheets. This would deteriorate the electrical performance of the resultant composites, and usually, more fillers need to be loaded to form a percolating network in this case. In this study, we fabricated graphene/polymer composites using solvothermal reduction of GO in the polymer solution. This method enables the reduction of GO to graphene and its blending with the polymer matrix in one step. The polymer material used was polyvinylidene fluoride (PVDF). It is a semicrystalline polymer having remarkable thermal stability, excellent chemical resistance, and extraordinary pyro- and piezoelectric characteristics. It has found wide applications in the fields of electronic and biomedical engineering . This study presents the first report on the synthesis and electrical characterization of the solvothermal reduced graphene/PVDF nanocomposites.
Graphite flakes and PVDF (Kynar 500) were purchased from Sigma-Aldrich Inc. (St. Louis, MO, USA) and Arkema Inc. (King of Prussia, PA, USA), respectively.
Graphite oxide was prepared using a typical Hummers method . In a typical composite fabrication procedure, graphite oxide was firstly ultrasonicated in N, N-dimethylformamide (DMF) for 40 min to be exfoliated into GO. PVDF pellets were then dissolved in this suspension at 60°C. Subsequently, the solution mixture was transferred into a 50-ml steel autoclave and placed in an oven at 100°C for 12 h. In this solvothermal reaction, DMF acted as the solvent for dissolving PVDF and also served as a medium to transmit heat and pressure to reduce GO. After the reaction ended, the autoclave was taken out and allowed to cool naturally, and a solution mixture of solvothermal reduced graphene (SRG) sheets and PVDF was obtained. This solution was used to fabricate the SRG/PVDF composites via the coagulation method . In this process, the suspension was dropped into a blender containing a large amount of distilled water. The SRG/PVDF composite mixture precipitated out immediately due to its insolubility in the DMF/water mixture. The obtained fibrous SRG/PVDF mixture was vacuum filtrated and dried and finally hot-pressed into thin sheets of approximately 1 mm thick.
To convert wt.% loading of graphene sheets in the composite samples to vol.% (as used in the text), a density for the GO sheets of 2.2 g/cm3 was assumed . The prepared GO was examined using an atomic force microscope (AFM, Veeco Nanoscope V, Plainview, NY, USA). The morphology of the SRG/PVDF composites was examined using a scanning electron microscope (SEM, Jeol JSM 820, JEOL Ltd., Akishima-shi, Japan). The dielectric constant and electrical conductivity of the composites were measured with a Hewlett Packard 4284A Precision LCR Meter (Hewlett-Packard Company, Palo Alto, CA, USA). The current density-electric field (J-E) characteristic of the composites was measured by a Hewlett Packard 4140B pA meter/DC voltage source (Hewlett-Packard Company, Palo Alto, CA, USA). Silver paste was coated on the specimen surfaces to form electrodes.
Results and discussion
Parameters characterizing percolative behavior of SRG/PVDF composites
AC electrical transport properties of percolated SRG/PVDF composites
2.43×10−9 ± 2.12×10−10
1.42×10−11 ± 7.14×10−12
0.88 ± 0.01
3.40×10−9 ± 8.13×10−10
3.23×10−11 ± 8.04×10−12
0.86 ± 0.01
8.02×10−8 ± 1.34×10−8
6.14×10−11 ± 3.95×10−12
0.83 ± 0.01
2.05×10−6 ± 7.90×10−8
1.44×10−9 ± 8.19×10−11
0.71 ± 0.01
SRG/PVDF composite was prepared by in-situ solvothermal reduction of graphene oxide in the PVDF solution. The large aspect ratio of SRG sheets in combination with uniform dispersion in the polymer matrix led to a relatively low percolation threshold of 0.31 vol.%, which is smaller than graphene/polymer composites prepared by direct blending chemically/thermally reduced GO sheets with PVDF. It is found that only 0.5 vol.% SRG doping will increase the dielectric constant of the material from 7 to about 105, while keeping the conductivity at a low level. Such a dielectric performance is superior to those of carbon nanotube/nanofiber based polymeric composites. The AC conductivity of the composite above pc follows the universal dynamic response, as with many other conductor-insulator systems. Moreover, the electrical nonlinearity of these composites is stronger than the carbon nanotube/nanofiber filled polymer system, resulting from the Zener tunneling effect between the uniformly dispersed SRG sheets.
This work is supported by the project (R-IND4401), Shenzhen Research Institute, City Unversity of Hong Kong.
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