alt="(a, b) The PEO/DNA nanofiber webs containing 5% DNA-dispersed DWNTs. (c) Stress–strain curves for nanofiber webs of pure PEO, PEO/DNA, and PEO/DNA/DWNT. Source: (a)–(c) Reproduced with permission. [57] Copyright 2013, American Chemical Society."/>
Figure 2.2 (a, b) The PEO/DNA nanofiber webs containing 5% DNA-dispersed DWNTs. (c) Stress–strain curves for nanofiber webs of pure PEO, PEO/DNA, and PEO/DNA/DWNT. Source: (a)–(c) Reproduced with permission. [57] Copyright 2013, American Chemical Society.
2.2.3 In Situ Polymerization
In situ polymerization was firstly proposed by Imai et al. where polyimide (PI)/CB composite was obtained by dispersing the CB into the polymer salt monomer [66]. In fact, this is a unique solution based processing technology for preparing the CPCs, and chemical reaction is usually involved during the polymerization [67, 68]. The efficient polymer-chain graft onto the filler surface could form a perfect interface interaction between the filler and polymer matrices, which also improves the homogeneous dispersion of the fillers in the polymer matrix and influences the crystallization of polymer chains to some extent [69–71]. As a result, CPCs with a high weight content of the nanofillers can be obtained by this method [69, 72].
Zhu and coworkers prepared reduced graphene oxide (r-GO)/PI composites with different loadings of GO by in situ polymerization and the maximum content of GO can reach 30 wt%. During polymerization, a relatively high temperature was set to reduce GO into conductive r-GO in the polymer matrix. The electrical property of the obtained r-GO/PI was greatly enhanced, because of the conductive network formed by r-GO in the composite film and the conductivity could reach as high as 1.1 × 101 S m−1, which is about 1014 times that of pure PI film [73]. The mechanical properties of GO/PMMA composites fabricated by in situ polymerization were also tested by Potts et al. [74]. It is found that the elastic modulus and tensile strength of the GO/PMMA composites could be improved even with 1 wt% loading of fillers.
2.3 Morphologies
Different fabrication techniques may lead to different morphologies of conductive networks in CPCs, which significantly influence the electrical properties of these composites [30]. Various morphologies including uniform dispersion of nanofiller in the polymer matrix, segregated structure, and selective decoration of the nanofiller on the skeleton of porous polymer materials are reported [75].
2.3.1 Random Dispersion of Nanofiller in the Polymer Matrix
The most straightforward and popular strategy to fabricate CPCs is the incorporation of conductive fillers (e.g. graphene, CNTs) in the polymer matrix by either melting or solution method. Naturally, the nanofillers are randomly located in the polymer matrix, and some of them may be squeezed out to polymer surface in particular at a high filler concentration [76]. The nanofillers can be uniformly distributed in the fibers, film, and three-dimensional polymer materials, depending on the processing technology used. Wet-spinning [77, 78] and electrospinning [75, 79] are two most common means to prepare one dimensional CPCs. Yu et al. [80] have successfully prepared conductive poly(styrenebutadiene-styrene) (SBS)/CNT fiber through wet-spinning the mixed solution of CNT and SBS elastomer. The schematic diagram of the fabrication procedure of SBS/CNT and the pictures of flexible SBS composite with different content of CNTs are shown in Figure 2.3a. Figure 2.3b–d show the SEM micrographs of fracture SBS/CNT fibers with a content of 0.5%, 0.75%, and 1%, respectively. Uniformly dispersed CNTs in SBS matrix can be observed even at a CNT content of 1%. Also, the conductivity of the obtained 1-D conductive fiber composite improves with the increase of CNT content (Figure 2.3e).
Figure 2.3 (a) Schematic diagram illustrating the preparation procedure for SBS/CNT fibers (SCFs) and photograph of SCFs with the various content of CNT. (b–d) The cross-sectional morphologies of SBS/CNT fibers containing different content of CNT. (e) The specific conductivities of SFCs as a function of different loading content of CNT. Source: (a)–(e) Reproduced with permission. [80] Copyright 2018, Elsevier Ltd. (f) Schematic illustration of the PU-PEDOT:PSS/SWCNT/PU-PEDOT:PSS with sandwiched structure on a polydimethylsiloxane (PDMS) substrate. (g) Transmittance of the integrated composite in the visible wavelength range from 350 to 700 nm. Source: (f)–(g) Reproduced with permission. [81] Copyright 2015, American Chemical Society.
Thin film-like CPCs can be fabricated by facile casting or template molding [82, 83] or polymer encapsulation [81, 84–86]. A stretchable, transparent, patchable nanohybrid conductive polymer film was reported by Roh et al. [81]. The CNTs were encapsulated in the interlayer of polyurethane (PU)-poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) forming a sandwich-like structure, which is schematically illustrated in Figure 2.3f, where the CNTs functioned like a bridge that connects the conductive PEDOT to the PEDOT phases. The obtained sandwich-like composite with 5 mg ml−1 SWCNT dispersion is optical transparent and shows the optical transmittance of 62% in the visible range (Figure 2.3g).
In terms of three-dimensional composite, nanofillers are often distributed in a foam composite that is usually obtained by freezing drying method [87, 88]. For example, Huang et al. [87] fabricated a novel aligned porous CNT/thermoplastic polyurethane (TPU) foam composite by using a directional-freezing method. During the freezing–drying process, the solvent of the mixture would form directional crystal due to the low temperature and then the ice crystal of the solvent would be sublimated, leaving aligned interconnected pores.
2.3.2 Selective Distribution of Nanofillers on the Interface
To reduce the content of conductive fillers in polymer matrix and at the same time maintain a relatively high conductivity of the CPCs, researchers try to locate conductive fillers on the interfaces of the polymer granule (i.e. segregated structure) or on the skeleton (surface coating) of the porous materials. Also, when used as sensors, the specially distributed conductive paths are easier to destruct upon external stress compared with conventional CPCs with relatively strong and dense conductive paths.
2.3.2.1 Segregated Structure
The study about construction of segregated structure was first reported in 1971 [89], and to date much work have been done on this topic [15, 30, 90]. In fact, segregated structure is a unique dispersion state of the conductive fillers in the polymer matrix, at which conductive fillers are dispersed at the interfaces between polymer particles. Mechanical blending and hot compression molding technique is usually applied to fabricate CPCs with a segregated structure [91, 92]. Generally,
