Polymer Nanocomposite Materials. Группа авторов
with the percolated conductive network constructed by different conductive nanofillers. It is believed that most of the resistance comes from tunneling between adjacent conductive fillers dispersed in polymer matrix [15]. CPCs based on non-conductive polymer elastomers and conductive fillers have caused wide concern of both academic and industrial group and become a hot topic of polymer composites due to its easy preparation, controllable electrical property, excellent flexibility, and stretchability. In this chapter, we systematically introduce the development and prospect of CPCs through several parts including the fabrication method of CPCs, morphology, and microstructure of CPCs and their application as sensors.
2.2 Fabrication Methods for CPCs
As known, conductive fillers, especially nano-sized conductive particles, are easy to aggregate in the polymer due to their high-aspect-ratio, resulting in uneven distribution, which may deteriorate the comprehensive performance of CPCs [16]. Therefore, surface modification of nanofillers and special processing technique are required to enhance the dispersion of conductive nanofillers in the polymer, which mainly includes the following methods: (i) The physical blending [17]. The conductive particles are uniformly dispersed into the polymer melt matrix or polymer solution under the ultra-strong external field forces (shear, tensile, etc.). (ii) In situ polymerization [18]. The conductive particles were firstly dispersed in the organic solvent containing the polymer monomer. After the polymerization reaction, the previously evenly dispersed conductive particles were anchored in the polymer matrix in situ. (iii) Chemical modification of conductive filler [19]. After the chemical reaction, the surface of conductive particles is grafted with functional groups such as hydroxyl group, carboxyl group, amino group, etc. These groups have good interactions with the polymer, such as covalent bond and hydrogen bond, which can effectively avoid the nanofillers aggregation and hence improve their dispersion in the polymer matrix. (iv) Introduction of surfactant [20]. The surfactant will wrap around the conductive particles, increasing their compatibility in polymer solution or melt, thus improving the dispersion of the fillers.
For CPCs, the electrical conductivity is depended on the transportation of charge carriers (current) along the conductive network constructed by conductive fillers in the polymer matrix. Generally, a sudden increase of several orders of magnitude in conductivity (transition from insulator to conductor) can be found as the concentration of conducting phase reaches a critical value in the polymer matrix, which is defined as the percolation threshold. Above this threshold, the concentration dependence of the conductivity of the CPCs (σ) can be described by a scaling law
where σ0 represents the intrinsic conductivity of the fillers, t represents the electrical conductivity exponent, ϕ represents the volume fraction of conductive particles, and ϕc represents the volume fraction at percolation transition [21]. In a random conducting network, the exponent t only depends on the dimensionality of the network [22], which is called universal percolation behavior. Theoretical calculation suggests that t = 1–1.3 corresponds to two-dimensional networks, while t = 2.0 represents a three-dimensional network. The percolation threshold and electrically conductivity are affected by many factors such as nanofiller dimension and microstructure of the CPCs. For example, conductive nanofillers with a large aspect ratio (CNTs, graphene nanosheet) could form well connected conductive network, which is beneficial to a low percolation threshold in CPCs [23–25]. It is also reported that nanofiller filled CPCs exhibit better electrical and mechanical properties than CPCs incorporated with microsized fillers [26, 27]. Also, the electrically conductive behavior of CPCs has strong relationship with the structure of conductive network in the polymer matrix. The perfect interconnection between conductive nanofillers and formation of 3D continuous pathway in the polymer matrix are regarded as the key factor for construction of the conductive network [28]. As a result, the morphology and distribution of nanofiller in the polymer matrix play an important role determining the electrical conductivity of CPCs. The fabrication process and parameters can greatly influence the morphology of conductive network in polymer matrix and thus their electrical properties. There are many methods for fabrication of CPCs have been reported, among which melt blending, solution mixing and in situ polymerization are regarded as three major methods. However, it still remains challenge to prepare high performance CPCs with low percolation threshold while high conductivity [29, 30].
2.2.1 Melt Blending
Melt blending is processed by using kneading machine, molding machine, internal mixer or double screw extruder, etc. [31, 32] to evenly mix the polymer matrix and conductive fillers with the processing temperature above the melting point of polymer. Thus, high temperature and high shear force are needed in melt blending process to ensure the homogeneous dispersion of the conductive nanofiller in the melting polymer matrix. During the melting blending, the nanofillers are forced to disperse by the mechanical shear force and at the same time prevented from re-aggregation by the viscous polymer matrix. After the masterbatch is obtained, the final CPCs can be prepared by using different polymer processing technologies like spinning, hot press, and injection molding [33–35]. Melt blending is an environmental-friendly process method and is feasible for large-scale industrial production of the CPCs. Many recent studies have investigated the effect of fillers introduction, dispersion state, and processing factors on the physical and electrical properties of the CPCs prepared by the melt blending [36].
Kim [37] studied the influence of the CNTs concentration and dispersion state on thermal, rheological, and mechanical properties of the polybutylene terephthalate (PBT) nanocomposites. It was found that the storage and loss moduli of the composite increased with the increase of the CNTs content. The interaction between nanotube–nanotube and polymer–nanotube was regarded as the main cause for the nonterminal behavior of the PBT nanocomposites. Besides, it can be found that the heat distortion temperature and the thermal stability of the composite were substantially enhanced at a low CNTs concentration. During the melt blending, the interaction between conductive fillers and polymer matrix greatly influences the dispersion state of the filler. For example, due to the interaction of π–π stacking between graphene and polystyrene (PS), the graphene could be easily dispersed in PS matrix during the melt blending [38]. It can be seen in Figure 2.1a that the suspension of PS/graphene without melting blending is transparent, suggesting the absence of graphene in the suspension. However, the solubility of graphene in toluene is greatly enhanced by prolong the melt blending time (5–60 minutes). This dark-colored suspension keeps stable and homogeneous even after three months or longer. The forming of π–π stacking between PS and graphene sheet in melt blending process is schematically demonstrated in Figure 2.1b. In melt blending, the PS chains were stretched by shear forces, forming some closely aligned aromatic rings parallel to the graphene sheet. Therefore, the interaction between PS and graphene was enhanced.
Figure 2.1 (a) Photographs of the PS/graphene/toluene suspension prepared by centrifuged at 8000 rpm for 30 minutes. (b) Schematic illustration for the formation of π–π stacking between graphene and PS in melt blending process. Source: (a)–(b) Reproduced with permission. [38] Copyright 2011, American Chemical Society. Microscopic morphology of (c) 1% pristine MWCNTs/PP; (d) 1% pristine MWCNTs/PP-g-MA/PP. (e) Electrical conductivity of various PP