Polymer Nanocomposite Materials. Группа авторов
1.5.1 Ultrasonication-assisted Solution Mixing
The most widely used approach to produce PNCs is ultrasonication-assisted solution mixing [79–83]. In this method, the nanofillers and polymer are initially dissolved in a solution. Then the nanofillers are evenly distributed in the matrix in assistant of the ultrasound. Afterwards, the PNCs are obtained by evaporation of the solvent. The nanoparticles are separated from the agglomeration state to the smaller units by the ultrasonic energy, which is higher than the energy of interaction between the nanomaterials in the aggregates. With the increase of ultrasonic time, the aggregates of nanofillers are broken down into smaller ones, and even become individual nanoparticles independent of other nanoparticles in the polymer. In addition, this process often occurs at a high temperature, which can initiate in situ polymerization of reactive monomers or their soluble prepolymers with nanomaterials to enhance interfacial interactions [84].
Due to the simple operation and stable performance, the ultrasonic-assisted solution mixing method has been widely used in the researches of new nanocomposites. However, due to the poor effect of ultrasound in high viscosity solution, most of the polymers need to be dissolved in a high boiling point solvent and maintain a low concentration, which will affect the process of solvent removal and ultimately reduce the quality of the nanocomposites. Therefore, when using this method, it is important to pay attention to the choice of solvent.
1.5.2 Shear Mixing
Compared with the ultrasonic-assisted method, shear mixing is a much more common and simple method, which only requires the stirring process and has the potential for industrial mass production [85]. In the process of stirring, the shear force generated by stirrer rotating is used to separate the aggregates of nanofillers. Due to the low strength of the shear force, the nanoparticles will be separated under stirring and then aggregated again, so it is generally necessary to increase the speed of the agitator to complete the separation. This method generally does not destroy the structure of nanofillers; therefore it is suitable not only for separating loosely bound nanoaggregates, but also for stripping off some layered nanosheets. In addition, this method needs to be carried out in low viscosity solvent just like ultrasonic assisted method.
1.5.3 Three Roll Milling
Three roller milling is a method of dispersing nanofillers by the shearing force between rolls in high viscosity matrix, such as ink, paste material, coating, etc. The machine of three roll milling is composed of three cylindrical rollers with different rotating speeds, and the adjacent rollers rotate in the opposite direction. The particle size distribution and uniformity of the packing can be well controlled as the speeds of the rollers and the gap between them are adjustable. In addition, the shearing force generated between the rollers is higher than that generated by stirring, so the method can be applied to high viscosity materials, and carried out under the condition of little or no solvent. Therefore, this method is often used to disperse some anisotropic nanofillers, such as CNTs [86–89], graphene nanosheets [90–92], nanoclays [93–95], and so on.
However, it should be noted that the distance between adjacent rollers should be at least 1 μm, so the dispersion effect of nanospheres with three-dimensional direction less than 100 nm will not be good. The aggregates of nanospheres can only be turned into smaller units, not broken into individual particle. On the other hand, the rotation of the roller requires the addition of viscous materials, and nanofillers can only be dispersed in the thermosetting matrix but not in the thermoplastic matrix.
1.5.4 Ball Milling
Ball milling is widely used in metallurgy and mineral processing industry [96]. The principle of ball milling is to grind and mix powders in a closed space by using the huge shear force and compression force produced by hard ball collision. In the synthesis of PNCs, this method can disperse CNTs [97], graphene nanoparticles [98–101], silica nanoparticles [102], and BNs [103, 104] into thermoplastic and thermosetting polymers. The high shear force produced by ball milling can peel off some two-dimensional nanostructures, such as graphene, MoS2, and BNs, but may not separate the interlayer structure connected by ionic bonding [105–110]. In addition, ball milling is not only suitable for solvent-free conditions but also solvent-free conditions, so nanofillers can be directly dispersed in some solid thermoplastic matrix, such as polyethylene (PE) [101, 111], polyphenylene sulfide [104, 112], and polymethyl methacrylate (PMMA) [102].
1.5.5 Double-screw Extrusion
Double-screw extrusion disperses nanofillers in thermoplastic matrix by huge shear force generated by high speed rotation of double-screw at high temperature [113, 114]. This method has been widely used in industry due to the advantages of solvent-free and environment-friendly technology. With this method, the fillers can be dispersed into the polymer in a high content way to achieve the well-controlled performance, and applied to different sizes of nanoparticles, such as graphene sheets [115], CNTs [116], and silicon dioxide [117]. This method needs higher temperature, which is helpful to reduce the viscosity of polymer and load more nanofillers, but also has the risk of decomposing polymers and nanofillers. The reason is owing to the existence of low thermal stability functional groups in the materials. When the temperature is too high, the fracture will occur, resulting in the deterioration of the performance of PNCs [118]. Moreover, the gap between the screws is too large to keep some aggregates of nanofillers evenly, which will not achieve the uniform monodispersing of nanofillers. So, it is necessary to combine other technologies to further improve the performance [119, 120].
1.5.6 In Situ Synthesis
In addition to the aforementioned methods of dispersing prepared nanofillers into polymers, another important synthesis strategy is in situ synthesis, which directly generates nanoparticles in polymers through molecular precursors [121]. This method can be divided into chemical and physical in situ synthesis [122]. Chemical in situ synthesis is used to synthesize nanoscale fillers by chemical reaction, such as the hydrothermal method and sol–gel method [123, 124]. The physical in situ synthesis is transforming the precursor of gas phase into inorganic nanoparticles through plasma action, and then condensing the organic compounds on the surface of inorganic particles to cover the polymer shell to form PNCs [125].
1.6 Conclusions and Future Outlook
In this chapter, the basic principles, properties, and synthesis methods of PNCs are clearly described. The composite material has unique structure and performance, and has a wide range of applications in many fields. The particle size, orientation, shape, dispersion, and volume dispersion of nanofillers affect the properties of PNCs. Most of the physical, chemical, and mechanical properties of PNCs depend on the interface interaction between the filler and the matrix. Therefore, the uniform dispersion of nanofillers is the most important consideration in the synthesis of PNCs. PNCs have recently become part of modern technology, but these areas are still in the early stage of development. With more and more scientists and engineers contributing to the understanding of PNCs, these functional materials will be applied in more and more fields.
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