and graphene were adsorbed to the surface of polymer microspheres by chemical or physical methods, and then the temperature and pressure of hot pressing were controlled, guaranteeing that the conductive fillers were only distributed at the interface between polymer microspheres instead of evenly dispersed in the whole polymer matrix [15, 93, 94].
For example, Wu et al. [95] added amino-functionalized PS microspheres suspension into the GO solution. Graphene was tightly coated on the surface of PS microspheres after a series process of flocculation, filtration, washing, and hydroiodic acid reduction. The composite with a low percolation value of 0.15 vol% was obtained after hot press of the graphene coated PS microsphere. Also, the conductivity of the composite could reach as high as 1024.8 S m−1 when the volume content of graphene is 4.8%, which is much higher than that of PS/graphene and PS/CNT composite made by solvent blending.
An ultralow percolation threshold of 0.047 vol% was achieved by Cui and Zhou [92]. In their work, the conductive PS/graphene and PS/MWCNTs composites with segregated structures were obtained by hot press surface sulfonated PS microspheres and protonated triethylenetetramine functionalized perylene bisimide (HTAPBI)-stabilized nanocarbon. The formation of conductive PS composites with a segregated network is schematically demonstrated in Figure 2.4a. The SEM micrographs of the fracture surface of PS composites containing 0.94 vol% graphene sheets and 0.94 vol% MWCNTs are displayed in Figure 2.4b,c, respectively. Due to the strong electrostatic attraction between the negatively charged PS and positively charged nanocarbon, the interconnected conductive pathways could be preserved regardless of a high hot press temperature, leading to an ultralow percolation threshold.
Figure 2.4 (a) The fabrication process of PS–nanocarbon composite with interconnected networks. Cross-sectional SEM images of PS composites with (b) 0.94 vol% graphene sheets and (c) 0.94 vol% MWCNTs. Source: (a)–(c) Reproduced with permission. [92] Copyright 2017, The Royal Society of Chemistry. Morphologies of double-segregated (d) CNT/PMMA/UHMWPE (0.2/7.8/92.0 by volume) and (e) CNT/PMMA/UHMWPE (0.5/16.2/83.3 by volume) composites. The inset transmission electron micrograph (TEM) image in (e) shows the state of the segregated CNT conductive network in the CNT/PMMA layers. (f) The variation of electrical conductivity for the double segregated CNT/PMMA/UHMWPE composites with different CNT content. Source: (d)–(f) Reproduced with permission. [96] Copyright 2013, The Royal Society of Chemistry.
Pang et al. reported CPCs with double-segregated structure [96, 97]. CNTs wrapped around the small-sized PMMA particles to form the first layer of segregated structure, and CNTs/PMMA were distributed at the interface of the large-sized ultrahigh molecular weight polyethylene (UHMWPE) particles to construct a second layer of segregated conductive network throughout the whole system. The optical micrographs of CNT/PMMA/UHMWPE composite with 0.2 and 0.5 vol% CNTs are shown in Figure 2.4d,e, respectively, showing perfect double-segregated structure. Figure 2.4f shows the electrical conductivity of CNT/PMMA/UHMWPE composites as a function of CNT content. The percolation value of the CPC was calculated to be only 0.09 vol% [96].
Another kind of segregated structure can be constructed by infiltration of flexible elastomers into conductive foams. Carbon based foams (e.g. graphene and CNTs) and carbonized polymer foams are usually selected as the conductive skeleton and polydimethylsiloxane (PDMS) ink was impregnated in the foam to prepare the CPCs [98–100].
2.3.2.2 Surface Coating
To prepare CPCs with a low percolation, a high conductivity at a low filler content, and meanwhile no evident fillers aggregation in polymer matrix, decoration of conductive nanofillers on the skeleton (out surface) of the polymer material tends to be an effective solution, which includes dip coating [101, 102], spray coating [103, 104], and ultrasonication [105, 106]. Note that the polymer materials usually possess a porous structure (e.g. fabric and foam), which facilitates the nanofillers penetration into the interior of the material during the surface coating.
In dip coating, the polymer scaffold was immersed into the ink of conductive fillers (graphene, CNTs, AgNWs) [102, 107, 108] or the precursor solution [109, 110] followed by drying or in situ reduction and drying. Cui and coworkers [108] produced highly conductive textiles with conductivity reaching 125 S cm−1 by immersing textile into single-walled carbon nanotube (SWNT) ink (Figure 2.5a). The color of the textile becomes black after immersed in SWNT ink (Figure 2.5b), suggesting that the SWNT has been successfully decorated on the surface of textile fibers. Also, SWNT can be observed on the surface of textile from SEM image in Figure 2.5c. The interaction of van der Waals forces and hydrogen bonding guarantees the tightly binding of single walled CNTs to the cellulose. A conductive multifilament fiber with conductive Ag shell and elastomer polymer core was reported by Lee et al. [111]. The multifilament elastomer fiber was first dipped in the AgCF3COO solution to adsorb Ag precursor through ion–dipole interaction. Then the Ag+ ions absorbed in the fibers was in situ reduced by the reduction solution. The microstructure of the core–shell structure can be observed in Figure 2.5d,e. The interfacial interactions like electrostatic interaction [111], hydrophobic interaction, and hydrogen bonding [108, 114] are considered to be the leading driving force for the successful decoration of conductive nanofillers on the polymer substrate surface.
Figure 2.5 (a) Conductive textiles fabricated by dipping coating aqueous SWNT ink. (b) Picture of obtained conductive textile. (c) SEM image showing SWNT coating on the surface of fabric fibers. Source: (a)–(c) Reproduced with permission. [108] Copyright 2010, American Chemical Society. (d) Cross-sectional SEM images of the electrically conductive elastomer fiber composite. (e) Magnified SEM image of (d) showing the conductive Ag-rich shell on the outer surface of elastomer
