The advantages and disadvantages of these methods are discussed. In addition, a variety of nanocomposites with different properties, such as mechanical properties, electronic conductivity, thermal conductivity and thermal properties, are summarized comprehensively. The potential applications of these nanocomposites in conductive materials, electromagnetic shielding materials, photocatalytic materials and so on, are briefly presented. Future research directions of the nanocomposites are also presented.
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Keywords: graphene ; nanocomposites ; performance ; polymer ; review. In recent decades, polymer nanocomposites, which refer to the size of the dispersed phase being less than nm, have been paid much attention in the field of materials science. A variety of nanocomposites have been developed by blending nanoparticles with polymer matrixes. Nanoparticles, such as carbon nanotubes and nano silica, can exhibit their own nanometer-size effects and endow the polymer matrix with excellent mechanical properties, thermal properties, electronic performance and so on.
At the same time, they also can maintain the good processing property, light weight and favorable stability of the polymer substrates [ 1 ], [ 2 ]. Because of its excellent mechanical, electrical and thermal performance, graphene, since its discovery, has been considered an ideal inorganic nanofiller for the preparation of polymer nanocomposites.
Because it does not contain any active groups, the surface of graphene is inert and the force acting between the polymer matrix and graphene is weak. It causes some difficulties in processing, and hence graphene is modified through oxidization to convert it into graphene oxide GO. However, because of the large number of hydrophilic groups on the surface of GO, most hydrophobic polymers are incompatible with it.
The critical step is to disperse graphene in the polymer matrix. Good dispersion can ensure that the contact interface between the polymer matrix and graphene is maximized, thereby affecting the overall performance of the nanocomposites. Much effort has been made to disperse modified or unmodified graphene and GO in the polymer matrix evenly, achieving some good results. In this method, the chemically or thermally reduced GO can be well dispersed in organic solvents because of the presence of oxygen-containing groups which are not removed completely.
There are three steps involved in solution blending. First, graphene is dispersed in a suitable organic solvent under sonication. Second, the polymer is added to the solvent and dissolved. Third, the solvent is removed. This method is simple and effective. This method can be modified or improved.
For example, an effective route for synthesizing poly 3-hexylthiophene P3HT -coated GO P-GO based on an ethanol-mediated mixing and solvent evaporation was reported by Zheng et al. It is noteworthy that much attention has been given to make graphene disperse uniformly in the polymer matrix for this method. For instance, graphene in the form of chemically reduced GO r-GO was homogeneously dispersed in organic media by Zhang et al.
Aqueous colloids of GO sheets were synthesized in an alkaline solution. By using NaCl as flocculant, the resultant r-GO aggregates could be easily redispersed in appropriate organic media as individual nanosheets with the aid of sonication, which is a facile and economical approach for the large-scale production of graphene dispersions in organic solvents with high concentrations. The homogeneous graphene organic dispersion facilitates solution processing for incorporating graphene materials into novel composites and other applications.
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However, it should be noted that the used organic solvents are commonly adsorbed between the graphene layers, which adversely affects on the performance of the nanocomposites. Some polar and nonpolar solvents adsorbing between the graphene layers were studied systematically through solid-state NMR nuclear magnetic resonance and elemental analysis by Barroso et al.
This is a shortcoming of the solvent blending method. It was more evident in the aromatic and alcohol series. Chlorobenzene was the most retained of all chloride solvents, followed by dichloromethane, chloroform and carbon tetrachloride. NMR experiments confirmed the presence of solvents within the GO layers after drying procedures, demonstrating that a strong adsorption took place in these graphitic materials. On the other hand, the poorly retained CCl 4 0.
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This could be due to the fact that the molecular diameter of CCl 4 5. The corresponding solvent spectra are shown in grey color. Schematic representation of the solvent intercalation in the GO interlayer and the maximum expansion obtained in our results [ 38 ]. Besides, solution blending is not environmentally friendly because large quantities of organic solvents need to be used in the preparation when the process is used for industrial production.
The use of organic solvents also boosts the production cost. Therefore, this method is not common for the large-scale production in the industry, as compared to the melt blending method. Compared to the other methods, melt blending is more attractive for the preparation of commercial products because it is more convenient and more environmentally friendly because of the nonuse of organic solvents. For this method, the polymer and graphene are directly mixed in a twin-screw extruder, and then the parameters of the extruder screw, including the screw speed, temperature and blending time, are adjusted.
The downside of this approach is that the density of graphene becomes very small after GO is thermally reduced. It leads to difficulty in feeding and poor dispersion of graphene in the polymer matrix, which will make the mechanical properties and other performance parameters poorer. The procedure for the preparation of PCG nanocomposites is given in Figure 4. Transmission electron microscopy TEM observations Figure 5 showed that the morphology of pristine TRG and TRG0 was similar, and no obvious characteristic or peculiar distribution could be observed, suggesting that that no PC chains were adsorbed on the surface of TRG.
However, when the melt-blending time was increased from 10 to 30 min, the number of dark elliptical spots on the TRG30 surface also increased. These spots should be the wrapped PC domains, which were mostly formed during the melt-blending process, as indicated in the Fourier transform infrared FT-IR spectra. The transesterification between the carbonate groups in PC and TRG is one possible mechanism for the enhanced interfacial interaction. Shown in Figure 6 are the proposed schematic diagrams of the formed chemical bonding between PC and TRG, where the carboxyl groups on TRG sheets could react with the carbonate groups of PC through transesterification during melt blending.
Schematic diagrams of the transesterification between the carbonate groups in PC chains and the carboxyl groups on TRG sheets at high temperature during melt blending [ 45 ].
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In general, without the use of solvents, melt blending method is not only environmentally friendly but also simple and economical. It has been widely used for the fabrication of polymer nanocomposites. Especially, this method is often preferred by many polymer manufacturers for commercial production. However, the high temperature required to achieve a homogenous mix can lead to occasional degradation of the polymer, therefore requiring care during the compounding process. In this method, the monomer or prepolymer is mixed with graphene, sometimes in a suitable solvent, and then polymerization is carried out.
The surface of the chemically or thermally reduced GO contains some functional groups that can be directly connected to the polymer. These groups can also be used as the reaction points of graphene for further modification, such as being covalently grafted on polymer chains by atom transfer radical polymerization ATRP [ 46 ], [ 47 ], [ 48 ], [ 49 ], [ 50 ].
The polymers that were used as the matrix included polyurethane [ 51 ], [ 52 ], [ 53 ], [ 54 ], PS [ 55 ], [ 56 ], PMMA [ 57 ], [ 58 ], epoxy resin [ 59 ], [ 60 ], [ 61 ], poly lactic acid [ 62 ], poly vinyl alcohol PVA [ 63 ] and so on. The procedure for the preparation is shown in Figure 7. Process of chemical grafting modification of GO during in-situ emulsion polymerization and alcoholysis [ 63 ].
The advantage of in situ polymerization is that a strong interfacial interaction is formed between the polymer and graphene. It is not only advantageous to the stress transfer but also can make graphene to disperse evenly in the polymer matrix. However, the viscosity of the system will usually increase during polymerization, which can cause some problems to the subsequent processing and material-forming. However, the research on the nanocomposites fabricated via in situ polymerization should not be limited to the final morphology, structure and performance.
The effect of graphene on the polymerization or the curing reaction should also be taken into account. In general, graphene is dispersed in the monomer first. On initiation of the reaction, the monomer present in and out of the interlayers polymerizes to generate the nanocomposite. Because the control of the polymerization in and out of the graphene layers is difficult and high levels of exfoliation cannot be achieved for the large-scale production, this method is still not applied in the industry.
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For instance, in the study of Li et al. By concisely duplicating the sponge-like, highly ordered three-dimensional architectures from MF, the resulting MF-G with an interconnected graphene-based scaffold and tunable nanostructure was explored as compressible, robust electrodes for efficient energy storage. A thin layer of pseudocapacitive polypyrrole PPy was then attached and uniformly coated on MF-G, resulting in a well-defined core—double shell configuration of the MF-G-PPy ternary composite sponges.
These encouraging results thus provide a new route toward low-cost and easily scalable fabrication of lightweight and deformation-tolerant electrodes. The hollow nanostructured design of PANI-HS greatly enlarges the specific surface area, providing high electroactive regions and short diffusion lengths for both charge and ion transport. Many nanocomposites of polymers and carbon nanotubes have been studied in order to develop and utilize their excellent mechanical properties or to introduce some new properties to the polymer matrix such as electrical and thermal conductivity.
However, there still existed some problems, although many studies were focused on these materials. Compared to carbon nanotubes, graphene is more suitable for the nanocomposite system because of the structure and function of graphene.