Electrical behavior and positive temperature coefficient effect of graphene/polyvinylidene fluoride composites containing silver nanowires
© He and Tjong; licensee Springer. 2014
Received: 17 July 2014
Accepted: 27 July 2014
Published: 1 August 2014
Polyvinylidene fluoride (PVDF) composites filled with in situ thermally reduced graphene oxide (TRG) and silver nanowire (AgNW) were prepared using solution mixing followed by coagulation and thermal hot pressing. Binary TRG/PVDF nanocomposites exhibited small percolation threshold of 0.12 vol % and low electrical conductivity of approximately 10-7 S/cm. Hybridization of TRGs with AgNWs led to a significant improvement in electrical conductivity due to their synergistic effect in conductivity. The bulk conductivity of hybrids was higher than a combined total conductivity of TRG/PVDF and AgNW/PVDF composites at the same filler loading. Furthermore, the resistivity of hybrid composites increased with increasing temperature, giving rise to a positive temperature coefficient (PTC) effect at the melting temperature of PVDF. The 0.04 vol % TRG/1 vol % AgNW/PVDF hybrid exhibited pronounced PTC behavior, rendering this composite an attractive material for making current limiting devices and temperature sensors.
Polymers with low weight, low production cost, and good corrosion resistance are favorable materials for making adhesives, membranes, circuit boards, electronic devices, etc.. Most polymers are insulators with poor electrical conductivity. Their electrical conductivity can be improved markedly by adding large volume fractions of conductive metal particles and carbon blacks of micrometer dimensions. Polymer composites with large microfiller loadings generally exhibit poor processability and inferior mechanical strength[2–6]. In this regard, nanomaterials can be used as effective fillers for nanocomposite fabrication and property enhancements[7–9]. In particular, electrical properties of polymers can be enhanced greatly by adding low loading levels of graphene with high mechanical strength and electrical conductivity, forming conductive nanocomposites of functional properties[10, 11]. Such nanocomposites have emerged as a promising and important class of materials for the electronics industry.
Graphene is a two-dimensional, monolayer sp2-bonded carbon with remarkable physical and mechanical properties. It has a large aspect ratio, exceptionally high mechanical strength, and excellent electrical and thermal conductivity. Graphene fillers are generally prepared by oxidizing graphite flakes in strong acids to generate graphene oxide (GO). GO sheets are heavily oxygenated, bearing hydroxyl and epoxide functional groups on their basal planes, in addition to carbonyl and carboxyl groups at the sheet edges. As a result, GO sheets can mix intimately with many organic polymers, facilitating the synthesis of GO/polymer composites with homogeneous dispersion of nanofillers. However, GO is an electrically nonconductive material; thus, the oxygenated functional groups must be removed either by reacting with chemical agents to form chemically reduced graphene or heating in a furnace to yield thermally reduced graphene (TRG)[10, 11, 14, 15].
In a previous study, we reported the preparation of electrically conductive TRG/polymer composite by mixing GO in a polymer solution followed by hot pressing. The in situ TRG sheets were dispersed homogenously in the polymer matrix. The main disadvantage of this approach, however, is that the percolated composites have a relatively low electrical conductivity, resulting from incomplete thermal reduction of GO. The low conductivity can greatly limit potential applications of the composites. In the past decade, the synthesis of one-dimensional metal nanomaterials has received great attention from chemists, materials scientists, and physicists. These materials include Ag[17, 18], Cu[19, 20], Au[21, 22], and CuNi nanowires (NWs). The incorporation of those nanowires with unique properties into polymers can yield novel composites with functional characteristics. For example, da Silva et al. incorporated CuNWs into polyvinylidene fluoride (PVDF) and found that the CuNW/PVDF nanocomposites exhibit high dielectric permittivity and low dielectric loss.
Conductive polymer composites generally show a large increase in electrical resistivity by heating near the glass transition or melting temperature of the polymer matrix. This behavior is widely known as the ‘positive temperature coefficient’ (PTC) effect. The mechanisms responsible for the PTC effect are rather complex. PTC effect may arise from a difference in thermal expansion coefficient between the polymer matrix and conductive fillers. In addition, other factors such as the type, size and dispersion state of fillers, and the type of polymers can affect the PTC behavior[25–38]. From the literature, many researchers have extensively studied the PTC behavior of polymer composites filled with carbon blacks (CBs) in the past two decades[26–32]. Kim et al. incorporated 40 to 60 wt % CBs (0.86 and 0.3 μm) into PVDF, polyacetal, polyester, polyamide-11, and polyamide-12. They reported that the PTC intensity of polymer composites was proportional to the polymer crystallinity. For example, the degree of crystallinity of the 50 wt % CB/polyamide-12 and 50 wt % CB/polyester composites was 48.30 and 36.26%, respectively. Thus, the former composite exhibited higher while the latter showed lower PTC intensity. Similarly, the 55 wt % CB (90 nm)/high-density polyethylene (HDPE) composite with large crystallinity exhibited higher PTC intensity than polypropylene (PP) composite at the same filler loading. Recently, Dang et al. reported that the PP and HDPE composites with hybrid fillers of CBs (50 nm) and carbon fibers at 8 vol % loading exhibit strong PTC intensity. They attributed this to the ease of a conducting network formation in the polymer matrix because of the large aspect ratio of carbon fibers. Analogously, hybridization of CBs (24 nm) with multiwalled carbon nanotubes also led to enhanced PTC intensity and reproducibility. In this study, we aimed to improve electrical conduction behavior of TRG/PVDF composites by incorporating AgNWs. The AgNW/TRG/PVDF hybrid composites displayed interesting temperature-dependent electrical properties. PVDF is a semicrystalline polymer with high thermal stability, excellent chemical resistance, and high piezoelectric property.
Graphite flakes, ethylene glycol (EG), N,N-dimethylformamide (DMF), ferrite chloride (FeCl3), and poly (vinylpyrrolidone) (PVP) were purchased from Sigma-Aldrich (St. Louis, MO, USA). PVDF (Kynar 500) pellets were purchased from Arkema Inc. (King of Prussia, PA, USA). Silver nitrate (AgNO3) was obtained from Shanghai Chemical Reagent Company (Shanghai, China). All chemicals were used as received without further purification.
The TRG/PVDF composites were prepared based on our previous strategy. To prepare AgNW/TRG/PVDF composites, the PVDF solution (in DMF), GO solution (in DMF), and AgNW dispersion (in DMF) with appropriate mass ratios were mixed under sonication for 20 min. The mixed suspension was then coagulated into a large amount of stirring water. The precipitated fibrous mixture was washed with distilled water and ethanol and then collected using vacuum filtration. By drying at 70°C overnight, the fibrous mixture was finally hot-pressed at 200°C. This process converted GO to TRG, thereby forming AgNW/TRG/PVDF hybrid composites. The composite samples were pressed into sheets of about 0.5 mm thick for the electrical characterization.
The morphology of AgNWs and AgNW/TRG/PVDF composites were examined in scanning electron microscopes (SEMs; JEOL JSM 820 and JEOL FEG JSM 6335; JEOL Ltd., Akishima-shi, Japan). Static electrical conductivity of the composites was measured with an Agilent 4284A Precision LCR Meter (Agilent Technologies, Inc., Santa Clara, CA, USA). The specimen surfaces were coated with silver ink to form electrodes. Moreover, the specimens were placed inside a computer-controlled temperature chamber to allow temperature-dependent conductivity measurements.
Results and discussion
where p is the filler content and t the critical exponent. Nonlinear fitting in Figure 2 gives pc = 0.12 vol %. We attribute the low pc to the high aspect ratio of TRG sheets, which lead to easier connectivity in forming a conductive network. Although the TRG/PVDF composites have a small pc, their conductivity at pc is quite low, i.e., in the order of approximately 10-7 S/cm. Such a low conductivity renders percolating TRG/PVDF composites can be used only for antistatic applications. From Figure 2, the conductivity reaches approximately 5 × 10-3 S/cm at 1 vol % TRG. As recognized, TRGs still contain residual oxygenated groups despite high temperature annealing. In other words, TRGs are less conductive than pristine graphene. To improve electrical conductive properties, AgNWs are added to the TRG/PVDF composites as hybridized fillers.
Recently, Ansari and Giannelis prepared TRGs by fast heating GOs in a furnace at 1,000°C for 30 s. The PTC effect was not found in solution-mixed 3 to 4 wt % TRG/PVDF nanocomposites. Instead, the resistivity of such nanocomposites decreased from ambient to 170°C, displaying NTC effect behavior. They attributed this to the higher aspect ratio of TRGs such that the contact resistance dominated over tunneling resistance. More recently, Rybak et al. studied electrical conducting behavior of HDPE and polybutylene terephthalate (PBT) filled with Ag spherical nanoparticles (150 nm). The percolation threshold of Ag/HDPE and Ag/PBT nanocomposites was determined to be 17.4 and 13.8 vol %, respectively. Silver spherical nanoparticles exhibited low aspect ratio of unity, leading to large percolation threshold of these nanocomposites as expected. Furthermore, percolated Ag/HDPE and Ag/PBT nanocomposites also displayed PTC characteristics. Comparing with binary Ag/HDPE and Ag/PBT composites, our ternary hybrid composites only require very low AgNW additions, i.e., 1 to 2 vol % to achieve the PTC effect. Such low AgNW additions are beneficial for industrial applications, because AgNWs with high aspect ratio are more cost-effective than Ag nanoparticles of large volume fractions.
AgNW/TRG/PVDF hybrid composites were prepared using solution mixing followed by coagulation and thermal hot pressing. Electrical measurements showed that the bulk conductivity of hybrids was higher than a combined total conductivity of both TRG/PVDF and AgNW/PVDF composites at the same filler loading. This was due to the AgNWs bridged TRG sheets effectively in forming a conductive network in the PVDF matrix, producing a synergistic effect in conductivity. Consequently, electrical conductivity of 2 vol % AgNW/0.08 vol % TRG/PVDF composite was comparable to measured conductivity of graphite paper. Finally, the resistivity of hybrid composites increased with increasing temperature, particularly at the melting temperature of PVDF, generating a pronounced PTC effect. This effect was caused by the volume expansion of PVDF matrix with increasing temperature, which disrupted the synergistic effect and reduced electrical contacts among the conductive fillers.
This work is supported by the project (R-IND4401), Shenzhen Research Institute, City University of Hong Kong.
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