How to make graphene evenly dispersed?

Graphene is a two-dimensional honeycomb carbon material composed of carbon atoms arranged in a hexagonal shape. Carbon-carbon atoms are formed by sp2 hybridization, and their structure is very stable. The special structure of graphene results in many excellent properties. Graphene is the hardest material found at present, and has excellent mechanical properties (1060GPa). Its theoretical specific surface area is up to 2600m²/g, with outstanding thermal conductivity, up to 3000W/(M·K). In addition, graphene also has good electrical conductivity. At room temperature, its electron mobility can be as high as 20,000 cm²/ (V·s). Due to the excellent properties of graphene, researchers have considered adding it as reinforcement to the matrix material to improve the performance of the matrix material.

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 performance improvement of graphene reinforced composites. Moreover, such agglomeration is irreversible unless it is uniformly dispersed by applying an external force such as ultrasonication and vigorous stirring. In order to obtain graphene reinforced composites with excellent performance, researchers have done some research in overcoming graphene agglomeration.

1 method for uniform dispersion of graphene in a matrix

The large specific surface area of graphene makes it easy to irreversibly agglomerate in the matrix, which affects the performance of the graphene reinforcement. In general, due to the hydrophobicity and chemical inertness of graphene, its dispersibility is relatively low relative to graphene oxide. Therefore, the phenomenon of agglomeration of graphene in the matrix has also attracted more and more attention from researchers. Various methods have been tried to overcome the problem of graphene agglomeration.

Uniform dispersion of graphene in the matrix [Methods mainly include physical dispersion and chemical dispersion. Here, the in-situ polymerization method, functionalization of graphene (covalent bond functionalization and non-covalent bond functionalization), and graphite are mainly introduced Alkenes and other modification methods.

1.1 In-situ polymerization

The in-situ polymerization method firstly disperses the nanoparticles uniformly in the monomer, and then initiates polymerization with an initiator to uniformly disperse the nanoparticles or molecules on the polymer matrix and form an in-situ molecular polymeric material. The in-situ heterogeneous polymerization not only maintains the nano-characteristics of the particles, but also achieves uniform dispersion of the filled particles, and can form nano-shaped particles with a core-shell structure with an elastic coating layer. Since the outer layer is an organic polymer, it can increase the affinity of the material to the organic phase.

Lan Liu used in-situ polymerization to polymerize polyamide-amines between layers of graphene to open the graphene sheets, which increased the interlayer spacing, which prevented the agglomeration of graphene sheets to some extent. . Since this method does not undergo an oxidation step, the original sp2 structure of graphene is less damaged, and the resulting product has good stability and hardly precipitates.

Huang et al. used in-situ polymerization to solve the problem of uniform dispersion of graphene in the matrix. Figure 2 is a transmission electron microscope (TEM) photograph of a polypropylene composite material having different graphene contents.

Opel also used in-situ polymerization to solve the problem of graphene dispersion. They found that nanocomposites can increase the spacing of graphene sheets and prevent the agglomeration of graphene sheets to achieve uniform dispersion of graphene. In addition, the stability of the product was relatively good, and it exhibited good solubility in an organic solvent (formic acid), and the solution was stable and long-lasting, and no delamination was observed for 6 months.

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 both raw materials and inorganic materials, it will inevitably increase the research time and Cost will also cause environmental pollution. In addition to this, the addition of graphene increases the viscosity of the polymer, making the polymerization more complicated.

1.2 Functionalization of graphene

The functionalized graphene can be uniformly dispersed in the matrix, which contributes to the excellent performance of graphene as reinforcement. In order to enable the excellent properties of graphene to be applied in more fields, it is necessary to adopt certain methods and measures to functionalize it. The principle of functionalization is to modify the defects or groups on the surface of graphene by covalent and non-covalent methods, to give some new properties to graphene, and to further study and expand its application fields. Functionalization is an important method to improve the solubility and dispersibility of graphene and make it easier to process and shape.

The functionalization of graphene has evolved to produce a material of a particular property or to address the performance of certain aspects of graphene. Functionalized graphene not only maintains the original properties of graphene, but also exhibits the reactivity of the modifying group, which provides a possibility for the dispersion and reaction of graphene, further increasing the application range of graphene.

The structure of graphene is a benzene ring, so it is relatively stable. However, the benzene ring defect site and the edge have relatively high activity, and the surface of the oxidized graphene contains a large amount of active epoxy groups, such as a carboxyl group and a hydroxyl group, so that various chemical reaction methods can be used for grapheme covalent bond modification treatment.

Some researchers have found that due to the lack of functional groups on the surface of graphene, the dispersibility of graphene can be improved by adsorbing certain special functional groups on the surface of graphene. Xiaoyue Xu uses a silane coupling agent to silanize graphene to improve the dispersibility of graphene. Untreated graphene was placed in water for 1 h, and graphene was deposited substantially at the bottom of the vessel. The agglomeration phenomenon was more serious, as shown in Fig. 2(a). As shown in Fig. 2(b), the silanized graphene (PS-gG) was uniformly and stably dispersed in water, and after one day of standing, the modified graphene hardly agglomerated. There was also no precipitation at the bottom of the container, which indicates that the graphene subjected to the silane coupling agent coupling treatment can be stably dispersed in water. Due to the introduction of the silane coupling agent, a large amount of reactive functional groups are present on the surface of the graphene, which increases the hydrophilicity of the graphene so that it can be stably dispersed in the solution.

Coskun adhere polyvinyl alcohol to the surface of graphene oxide by covalent grafting through esterification, so that the dispersibility of graphene in aqueous solution is greatly improved. Graphene functionalized by covalent bonds greatly improves its processing properties and gives it some new and superior properties.

Disadvantages of functionalized graphene:

However, graphene functionalized by covalent bonds also has some obvious shortcomings. The covalent bond modification of graphene destroys the intrinsic structure of graphene and changes the chemical and physical properties unique to graphene itself.

1.3 graphene modification

Graphene with stable benzene ring structure, its chemical stability is high, the surface presents an inert state, the interaction with other media is weak, and there is a strong inter-layer between graphene. The intermolecular forces cause the sheets to be easily stacked and difficult to disperse.

Haijiao Zhang improved the dispersibility of graphene by surface modification of expanded graphite by ionic liquid. This modification is a modification of the physical method, which can reduce the influence of the modification process on the structure and functional groups of the graphene. They observed that the modified graphene sheets have a small particle size and exhibit a wrinkled state; the graphene modified by the ionic liquid can maintain a uniform dispersion state in the acetone solution for a long time, and can be uniformly distributed in the silicone rubber. In the (SR) matrix, the ionic liquid chain length is increased to allow the sample to be more uniformly dispersed.

Li found that the modified graphene was uniformly distributed in the matrix. He modified graphene through organic small molecular isocyanate and found that graphene can be stably dispersed in N,N-dimethylformamide solvent, which is beneficial to improve the uniformity of graphene in the process of compounding with polyvinylidene fluoride. Dispersibility avoids agglomeration of graphene in the matrix.

However, this method also has its disadvantages: the isocyanate molecules on the surface of the graphene cannot function to separate the graphenes from each other between the graphene sheets, and thus some properties of the graphene are not improved.

The modification of graphene can increase the dispersibility of graphene in the matrix to some extent, but the performance in other aspects is degraded. We should study the effect of modification on other properties in order to obtain an optimized result.

In addition to improving the dispersibility of graphene by means of ionic liquid modification and small molecule modification, there are other methods to prevent the agglomeration of graphene, such as the co-sulfonation precipitation process and the way of branching functional groups. The co-sulfonation precipitation process directly blends the modified graphene and the unsulfonated polyphenylene ether in chloroform, and by controlling the addition rate of the chlorosulfonic acid, the composite is simultaneously precipitated under ultrasonic conditions, and the graphite can be effectively prevented agglomeration of alkenes.

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 a dispersing agent to make graphene more dispersed in the solution. He found that in the prepared composite film, the long chain of PVA wrapped the graphene sheet, and the two phases were very tightly combined, so that the graphene was uniformly dispersed in the solution.

However, the manipulation of this method is difficult, and it is necessary to further accurately study the mechanism of film formation, thereby improving the application range and cost of the material.

Tianjiao Qi and others used a charge attraction method to solve the dispersibility of graphene. He used the Hummers method to prepare graphene oxide containing a large amount of oxygen-containing groups, which made the graphene oxide have a strong negative charge. Then, the surface of the aluminum powder is positively charged, and finally the problem of the dispersibility of graphene is solved by the positive and negative charge attraction. It was found that graphene has no obvious agglomeration phenomenon and achieves uniform dispersion of graphene to a certain extent.

However, this approach results in a significant decrease in composite elongation compared to pure aluminum. The charge attraction method causes other functions to decline, and it is also a problem that cannot be ignored. This requires improving and solving this problem under certain circumstances.

1.5 Other methods of dispersion

Mingjie Zhou improved the dispersion properties of graphene by ultrasonically treating the graphene suspension. Due to the action of the critical fluid, the carbon nanotubes are more uniformly mixed with the graphene. Because the instantaneous release pressure of the ultrasonic waves destroys the van der Waals force between the graphene layer and the layer, the graphene is less likely to agglomerate together, thereby uniformly mixing the carbon nanotubes and the graphene together.

Li et al added graphene to the matrix to make the graphene uniformly dispersed in the matrix. By adding graphene to the aluminum matrix, they form a "graphene/aluminum alloy" master alloy that allows graphene to be added to the molten aluminum solution by means of an intermediate alloy, maximizing grapheme uniform dispersion in aluminum liquid. However, this method increases the process and cost of preparing the graphene composite material, which requires finding a relatively simple way and method to reduce the cost and the like.

Jing Hu solved the problem of poor dispersion of graphene by in-situ reduction method. However, this method uses toxic substances such as hydrazine hydrate, which brings difficulties for the safety of industrial production operations and wastewater treatment.

Zhou et al. used a method that does not require the addition of a surfactant to increase the dispersibility of graphene, and reduced graphene oxide in dimethylformamide by solvothermal heat during the reaction. The dispersion concentration of graphene in the solution can reach 0.3 mg/ml, and this stable dispersion can be maintained for more than one year. In this way, it is not necessary to add a reducing agent and a stabilizer during the solvothermal reduction, but the graphene oxide is reduced by the pressure spontaneously generated by the high temperature and high pressure during the reaction.

Chong et al. found that agglomeration of graphene nanosheets can be avoided when chemically reducing in ABS resin groups. Graphene can be uniformly dispersed in the styrene-acrylonitrile matrix, and as the filler content increases, graphene forms a stable network structure in the styrene-acrylonitrile matrix, thereby preventing the agglomeration of graphene. Other methods for dispersing graphene have been studied less, and some mechanisms are not well understood. This requires strengthening research in this area, and proposes a more efficient and simple method to make the potential application of graphene a reality.

2 Research direction and exploration of uniform dispersion of graphene composites

After some introductions to increase the dispersibility of graphene in the matrix, it is found that the uniform dispersion of graphene is still in its infancy, and less research has been done. A lot of research is focused on one aspect, and does not consider whether the treated graphene will affect its excellent performance. There are still many problems concerning the uniform dispersion of graphene in composite materials, such as the problem of wettability of graphene and matrix, and the large specific surface area of graphene.

Weizong Ye believes that the wettability of graphene in the solvent will affect its sedimentation volume and further affect its dispersibility. If there is good compatibility between the solvent and the graphene, then the graphene has good dispersing properties in the medium, is not prone to agglomeration, is dispersed in the solvent, and the graphing rate of the graphene is relatively small. The settling volume formed is relatively small. On the other hand, if the wettability of graphene in a solvent is not good, agglomeration is easily formed between graphene to reduce the specific surface area, and the sedimentation effect reflected in the solvent is that the sedimentation rate is fast and the sedimentation volume is large.

In view of the above problems, such as the wettability problem of graphene and matrix, it may be considered to add other elements to optimize the matrix component or to chemically treat the surface of the material by microwave plasma chemical vapor deposition (CVD), in situ growth CVD or electroless plating. Graphene is functionalized or modified.

The problem of large specific surface area of graphene can prevent physical contact between graphene by surface coating of graphene. In recent years, the method of computational simulation has received much attention and is being applied more and more widely in this field to solve some problems. Computer simulation can be used to establish a mathematical model to simulate the experimental process, to find the best experimental solution through computer simulation, and to verify the experimental results; to combine the theory and practice to develop an optimized production process to prepare graphene composites with excellent properties.

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