How to solve the problem of graphene dispersion?

Graphene is a two-dimensional honeycomb of carbon atoms arranged in a hexagonal pattern. The carbon atoms are formed by sp2 hybridization and its structure is very stable. Graphene has many excellent properties due to its special structure. Graphene is the hardest substance found so far, with excellent mechanical properties (1060GPa), the theoretical specific surface area up to 2600m2/g, and outstanding thermal conductivity up to 3000W/(m·K). In addition, graphene has good electrical conductivity. At room temperature, electron mobility can be as high as 20000cm2/(V·s). Due to the excellent properties of graphene, researchers consider adding graphene as a reinforcement to the matrix material to improve its properties.

However, the large specific surface area of graphene tends to agglomerate together, which not only reduces its own adsorption capacity but also affects the performance of graphene itself, thus affecting the improvement of graphene-reinforced composites. Moreover, the agglomeration is not reversible unless external forces, such as ultrasound and strong agitation, are applied to disperse it evenly. In order to obtain graphene reinforced composites with excellent properties, researchers have done some research on overcoming graphene agglomeration.

1. Uniform dispersion method of graphene in the matrix

The large specific surface area of graphene makes it prone to irreversible agglomeration in the matrix, which will affect the excellent performance of graphene reinforcement. In general, due to the hydrophobicity and chemical inertness of graphene, its dispersibility is relatively low compared with that of go. Therefore, the agglomeration of graphene in the matrix has attracted more and more attention of researchers, and many methods have been tried to overcome the agglomeration of graphene.

The homogenous dispersion of graphene in the matrix [methods mainly include physical dispersion and chemical dispersion. In this paper, in-situ polymerization, functionalization of graphene (covalent bond functionalization and non-covalent bond functionalization), graphene modification and other modification methods are introduced.

1.1 in-situ polymerization

In situ polymerization is to disperse the nanoparticles evenly in the monomer, and then initiate polymerization with the initiator, so that the nanoparticles or molecules are evenly dispersed on the polymer matrix and form in-situ molecular polymer materials. In-situ polyphase polymerization not only maintains the nanometer properties of the particles but also achieves the uniform dispersion of the filler particles, which can form the nano-shaped particles with the core-shell structure with an elastic coating. Because the outer layer is an organic polymer, it improves the affinity of the material to the organic phase.

Liu LAN et al. used in-situ polymerization to generate polyamide-amine between the layers of graphene, which stretched the graphene sheet layer apart and increased the layer spacing, which to some extent prevented the agglomeration of graphene sheet layers. Since this method did not go through the steps of oxidation, the damage degree to the original sp2 structure of graphene was small, and the resulting products were stable and almost did not fall out.

Huang et al. solved the problem of uniform dispersion of graphene in the matrix by in-situ polymerization. Figure 2 shows the TEM images of polypropylene composites with different graphene contents. It can be seen from figure 2 that graphene is evenly dispersed in the polypropylene matrix, especially when the graphene content is high, the dispersion is more even, indicating that this in-situ polymerization method is indeed conducive to achieve the uniform dispersion of graphene in the polypropylene matrix.

Opal also adopted in-situ polymerization to solve the dispersion problem of graphene. They found that nanocomposites can increase the spacing between graphene sheets and prevent the agglomeration of graphene sheets to achieve uniform dispersion of graphene. In addition, the product has good stability and can show good solubility in the organic solvent (formic acid), the solution can be stable and maintained for a long time, 6 months no stratification.

Limitations of in-situ polymerization:

In-situ polymerization also has its limitations, that is, inorganic nanomaterials must have good compatibility with the selected raw materials. In order to find a suitable solvent to dissolve the raw materials and inorganic materials at the same time, it will inevitably increase the research time and cost, and also cause environmental pollution. In addition, the addition of graphene will increase the viscosity of the polymer, making the polymerization reaction more complex.

1.2 functionalization of graphene

The functionalized graphene can be dispersed evenly in the matrix, which is conducive to the performance of graphene as a reinforcement. In order to apply the excellent properties of graphene in more fields, certain methods and measures must be taken to functionalize graphene. The principle of functionalization is to use covalent and non-covalent methods to modify the defects or groups on the graphene surface and endow graphene with some new properties, making it easier to further study and expand its application field. Functionalization is an important method to improve the solubility and dispersion of graphene and make it easier to process and form.

The functionalization of graphene has developed into material for the preparation of certain special properties or to solve the deficiency of certain properties of graphene. Functionalized graphene not only maintains the original performance of graphene but also shows the reactivity of modifying groups, which provides a possibility for the dispersion and reaction of graphene and further increases the application range of graphene.

Graphene has a benzene ring, so it's relatively stable. However, graphene has high activity at the defect parts and edges of the benzene ring, and the surface of oxidized graphene contains a large number of active epoxy groups, such as carboxyl group and hydroxyl group, etc. so it can be covalently modified by a variety of chemical reactions.

Some researchers have found that due to the lack of functional groups on the graphene surface, the dispersion of graphene can be improved by adsorbing some special functional groups on the graphene surface. Xu Xiaoyu used silane coupling agent to silanize graphene so as to improve the dispersion of graphene. When untreated graphene was placed in water for 1h, the graphene was basically deposited at the bottom of the container, resulting in serious agglomeration. After being silanized, graphene (ps-g-g) was uniformly and stably dispersed in water. After being placed for one day, the modified graphene showed little agglomeration and no precipitation at the bottom of the container, indicating that the graphene treated with silane coupling agent could be stably dispersed in water. Due to the introduction of the silane coupling agent, a large number of active functional groups exist on the surface of graphene, which increases the hydrophilicity of graphene and enables it to disperse stably in solution.

Coskun attached polyvinyl alcohol to the surface of go by means of covalent grafting through esterification reaction, which greatly improved the dispersion of graphene in aqueous solution. Covalently bonded graphene greatly improves its processing properties and gives it some new excellent properties.

Disadvantages of functionalized graphene:

However, there are some obvious deficiencies in the functionalized graphene through covalent bonds. Covalent modification of graphene will destroy the intrinsic structure of graphene and change its unique chemical and physical properties.

1.3 graphene modification

Graphene with stable benzene ring structure has high chemical stability, and its surface presents an inert state, with weak interaction with other media. In addition, there are strong intermolecular forces between the sheets of graphene, which makes it easy to stack the sheets together and difficult to disperse them.

Zhang hai jiao improved the dispersion of graphene by surface modification of expanded graphite by ionic liquid. This modification is a physical modification that reduces the effect of the modification process on the structure and functional groups of graphene. They observed that the particle size of the modified graphene sheet was small and showed a folded state. The graphene modified by ionic liquid can maintain uniform dispersion in acetone solution for a long time, and can be evenly distributed in the silicone rubber (SR) matrix. The increase of ionic liquid chain length makes the sample more evenly dispersed.

Li found that the modified graphene could be evenly distributed in the matrix. He modified graphene with organic small molecule isocyanate and found that graphene could be stably dispersed in N, n-dimethyl-methylamine solvent, which was conducive to improving the homogeneity and dispersion of graphene in the composite process with polyvinylidene fluoride and avoiding agglomeration of graphene in the matrix.

However, this method has its disadvantages: the isocyanate molecules on the graphene surface cannot act as a barrier between graphene sheets, so some properties of graphene are not improved.

The modification performance of graphene increases the dispersion of graphene in the matrix to some extent, but its performance in other aspects declines. We should further study the influence of modification on other properties to obtain an optimized result.

In addition to improving the dispersion of graphene through ionic liquid modification and small molecule modification, there are other methods to prevent the agglomeration of graphenes, such as co-sulfonation precipitation process and grafting of functional groups. Co-sulfonation precipitation process is to directly mix modified graphene and unsulfonated polyphenyl ether in chloroform. By controlling the addition rate of chlorosulfonic acid, the compound can be precipitated simultaneously under ultrasonic condition, which can also effectively prevent agglomeration of graphene.

1.4 adding dispersant and charge attraction

With the further development of modification, the method of adding dispersant to graphene has gradually attracted the attention and research of researchers. Wu USES polyvinyl alcohol (PVA) as the dispersant to make graphene more dispersed in the solution. He found that the long PVA chain wrapped around the graphene sheet in the composite film was so tightly bound that the graphene was evenly dispersed in the solution.

However, the maneuverability of this method is difficult, and it is necessary to further study the mechanism of film formation accurately, so as to improve the application range of this material and reduce the cost.

Qi tianjiao et al. used a method of charge attraction to solve the dispersion of graphene. He used the Hummers method to make go with a large number of oxygen-containing groups, making it highly negatively charged. Then the surface of the aluminum powder was positively charged, and finally, the dispersion of graphene was solved by the way of positive and negative charge attraction. It was found that there was no obvious agglomeration of graphene, and to a certain extent, the uniform dispersion of graphene was achieved.

But this method makes the elongation of the composite material significantly lower than that of pure aluminum. The decrease in other functions caused by charge attraction is also a problem that cannot be ignored. This needs to be improved and solved under certain circumstances.

1.5 other dispersion methods

Zhou mingjie enhanced the dispersion of graphene by ultrasonic treatment of graphene suspension. Under the action of critical fluid, carbon nanotubes and graphene are mixed more evenly. Because the instant pressure released by ultrasonic waves destroys the van der Waals force between the graphene layers, it is more difficult for graphene to agglomerate together, so that the carbon nanotubes and graphene are evenly dispersed and mixed together.

Li jiongli et al. added graphene to the matrix, so that graphene is evenly dispersed in the matrix. By adding graphene to the aluminum matrix, they formed a "graphene/aluminum alloy" intermediate alloy, which enables graphene to be added to molten aluminum in the form of an intermediate alloy to maximize the homogeneity and dispersion of graphene in the aluminum solution. However, this method increases the process and cost of preparing graphene composites, so it is necessary to find a relatively simple way and method to reduce the cost.

Hu jing solved the problem of poor dispersion of graphene by in situ reductions. However, this method USES hydrazine hydrate, a toxic substance, which brings difficulties to the safety of industrial production and wastewater treatment.

Zhou et al. used a method that does not require the addition of surfactant to increase the dispersion of graphene and reduced graphene oxide in dimethylformamide by solvent heat in the reaction process. The dispersion concentration of graphene in the solution can reach 0.3mg/mL, and this stable dispersion can be maintained for more than a year. In this way, there is no need to add a reducing agent or stabilizer in the solvent-thermal reduction process. Instead, graphene oxide is reduced by the spontaneous pressure generated by high temperature and high pressure in the reaction process.

Chong et al. found that when the chemical reduction was carried out in the ABS resin base, agglomeration of graphene nanosheets could be avoided. Graphene can be uniformly dispersed in the styrene-acrylonitrile matrix, and with the increase of the filler content, the graphene will form a stable network structure in the styrene-acrylonitrile matrix, thus preventing the agglomeration of graphene. Other methods for dispersing graphene are less well studied and some mechanisms are not well understood, which requires more research in this area to propose more efficient and convenient methods to make potential applications of graphene a reality.

2. Research direction and exploration of uniform dispersion of graphene composites

After some introduction to increase the dispersion of graphene in the matrix, it is found that the homogeneity of graphene is still at a preliminary stage and relatively little research has been done. Many studies have focused on one aspect, without considering whether the treated graphene will affect its excellent performance. There are still many problems in the study of the uniform dispersion of graphene in composite materials, such as the wettability of graphene and matrix, and the large specific surface area of graphene.

Ye Weizong believed that the wettability of graphene in solvents would affect its settling volume and further affect its dispersion. If there is a good solubility between the solvent and graphene, then graphene has a good dispersion performance in the medium and is not prone to agglomeration. The dispersion distribution in the solvent means that the deposition rate of graphene is relatively small and the settlement volume formed is relatively small. On the contrary, if the wettability of graphene in the solvent is not good, it is easy to form agglomeration between graphene to reduce the specific surface area. The sedimentation effect reflected in the solvent is that the sedimentation rate is fast and the sedimentation volume is large.

For the above problems, such as wettability of graphene and matrix, other elements can be added to optimize the matrix composition, or chemical treatment of material surface by microwave plasma chemical vapor deposition (CVD), in-situ growth CVD or electroless plating, and functionalization or modification of graphene can be considered.

Due to the large specific surface area of graphene, physical contact between graphene can be prevented by a surface coating of graphene. In recent years, the method of computational simulation has been paid more and more attention and has been widely used in this field to solve some difficult problems. Computer simulation can be used to establish a mathematical model to simulate the experimental process, through computer simulation to find the best experimental program, and combined with experimental results to verify; By combining theory with practice, the optimized production process was developed to prepare graphene composites with excellent properties.

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