electronics, biosensors, and chemical engineering [33]. The one-dimensional nanofillers are common nanomaterials in electronic and thermal devices due to their shape characteristics.
1.3.2 Two-Dimensional Nanofillers
Two-dimensional fillers are the materials with two dimensions less than 100 nm, and they are mostly in the form of rods [14]. The typical two-dimensional nanomaterials are carbon nanofibers (CNFs), carbon nanotubes (CNTs), halloysite nanotubes (HNTs), nickel nanostrands (NiNS), and aluminum oxide nanofibers (Nafen). In addition, the most common two-dimensional nanofillers in PNCs are nanotubes [34], plant fibers [35–39], nanowires [40], carbon fibers [41–44], oxides [45–55], graphene [56, 57], molybdenum disulfide (MoS2) [58], and hexagon boron nitride (h-BN) [59]. Compared with one- and three-dimensional fillers, two-dimensional fillers have better flame retardancy and striped characteristic, resulting in wide applications in the fields of catalysis, electronics, optics, sensing, and energy [3, 26, 60–62].
1.3.3 Three-Dimensional Nanofillers
Three-dimensional nanofillers are nanomaterials with three dimensions on the nanometer scale, so they are mostly spherical or cube-shaped [63], which is also commonly referred to zero-dimensional particles. The most common three-dimensional fillers are polyhedral oligomeric silsesquioxane (POSS), nanosilicon, nanometal particles, nanometal oxides, and quantum dots (QDs) [33, 64]. Among them, metals and metal oxide nanoparticles have the advantages of high stability, catalytic activity, and easy preparation, and they are often used in the fields of catalysis [65], purification [66–69], coatings [70–74], and biological fields [75, 76], together with various polymers. One-, two-, and three-dimensional nanofillers all have various special properties, and will ultimately promote the remarkable performance of PNCs by loading in compatible polymers.
1.4 The Properties of Polymer Nanocomposites
In PNCs, many properties of the original polymer can be greatly improved, as well as new properties resulting from the addition of nanoparticles. As shown in Figure 1.4, the main properties of PNCs are listed, covering physical, chemical, and biological areas. In general, the improvement level of properties is determined by the size, loading capacity, aspect ratio, dispersion uniformity, and interface interactions of the nanofillers with polymer matrix [4].
Figure 1.4 Significant properties of polymer nanocomposites.
For example, most polymers don't possess conductivity except some conducting polymers, which is due to the covalent bonding of polymers and the lack of electron channels or ion migration. Interestingly, new PNCs formed by adding conductive nanofillers to insulating polymers exhibit many electrical properties. As early as 1994, Ajayan et al. used CNTs as reinforcement materials to prepare PNCs [77]. Since then, there have been a lot of researches on using CNTs as fillers to improve the electrical properties of PNCs. Only a small volume fraction of such fillers is needed to improve the electrical properties of polymers by several orders of magnitude effectively [78].
1.5 Synthesis of Polymer Nanocomposites
In the synthesis of PNCs, it is necessary to uniformly distribute the fillers into matrix in order to realize the functions of fillers. However, due to the fact that the fillers are nanoscale, the uniform dispersion is much different from that of the microscale fillers, which is mainly manifested in the following aspects. First, if the filling operation is carried out according to the volume fraction, much more nanometer fillers than the microfillers are required at the same volume fraction. Therefore, the nanoparticles in matrix are very crowded with greater van der Waals and electrostatic interactions between the particles, making it difficult to distribute evenly. Second, the anisotropic nanofillers have a very high aspect ratio, which makes them more prone to agglomerate. For example, monolayer graphene has aspect ratio of about 104, so they tend to reduce their surface energy by π–π stacking. Third, a large amount of nanofillers with huge surface area is loaded in the polymer matrix, which will produce a large interface area and change the overall performances of the PNC. Therefore, the decisive step in the synthesis of PNCs is the uniform dispersion of nanofillers in polymer matrix. As shown in Table 1.2, the common methods to disperse nanofillers and prevent the aggregation of nanoparticles by using external energy are summarized.
Table 1.2 Summary of common methods for synthesis of polymer nanocomposites.
| Technique | Suitable filler | Suitable matrix | Solvent | Controlling factors |
|---|---|---|---|---|
| Ultrasonication-assisted solution mixing | All types | Liquid or viscous monomers or oligomers of thermosets | Required | Sonication power and time |
| Shear mixing | Nanosheets | Liquid or viscous monomers or oligomers of thermosets | Required | Shapes of the rotor blades, rotating speed and time |
| Three roll milling | Nanosheets and nanotubes | Liquid or viscous monomers or oligomers of thermosets | Not required | Speed of roller, gap between adjacent roller |
| Ball milling | All types | Liquid or solid thermoplastics and thermosets | Not required | Time of milling, ball size, rotating speed, ball/nanofiller ratio |
| Double-screw extrusion | All types | Solid thermoplastics | Not required | Processing temperature, screw configuration, rotation speed |
| In situ synthesis | All types | Liquid or viscous monomers or oligomers of thermosets | Required | Chemical reaction conditions, temperature, condensation rate |
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
