Electrical Conductivity Behavior of Polymer Nanocomposite with Carbon Nanofillers

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Electrical Conductivity Behavior of Polymer Nanocomposite with Carbon Nanofillers Ayesha Kausar1 and Reza Taherian2 1

School of Natural Sciences, National University of Sciences and Technology (NUST), Islamabad, Pakistan, 2Faculty of Chemical & Materials Engineering, Shahrood University of Technology, Shahrood, Iran

3.1 Introduction It is generally believed that nanomaterials and polymer together develop unique molecular interactions which play essential role in the development of overall system properties [1]. The key which holds true for the enhancement in electrical, mechanical, and thermal characteristics is the large surface area of nanostructures available for such interactions. Nanodiamond (ND) is a unique carbon-based nanostructure. ND has diameter of few nm, having range of essential features of diamond include chemical and mechanical stability [2]. The additional nanorecompenses such as small size, large surface area, and adsorption capacity are also engrossed by these nanostructures. ND and graphite have now been joined by fullerene, graphene, carbon nanotube (CNT), carbon black (CB)—All are carbon-based nanomaterials [3
5]. Reinforcement of these nanocarbons in polymers has led to various classes of polymeric nanocomposites. According to research, small addition of nanostructures to polymers may remarkably enhance the electrical, mechanical, and thermal properties of the final hybrid [6]. Consequently, nature of interaction between nanocarbon and polymer matrix depends on the dimensionality of nanofiller. The synergistic effect in the properties of matrix and nanofiller define final nanocomposite contour. The small size, large surface area, and dimensionality of nanocarbons facilitate the formation of effective conducting polymer/ nanofiller network. Such network formation assists the electron transport through the conducting nanocomposite at certain percolation threshold values [7]. Key parameters affecting the percolation threshold of carbon nanomaterial reinforced polymer nanocomposites are the Electrical Conductivity in Polymer-Based Composites: Experiments, Modelling, and Applications. DOI: https://doi.org/10.1016/B978-0-12-812541-0.00003-3 © 2019 Elsevier Inc. All rights reserved.

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type, dispersion state, and aspect ratio of nanofiller used [8
10]. This article initially addresses the structure and properties of different nanocarbons indispensable for the development of unique electrically conducting nanomaterials. Moreover, various classes of polymer/nanofiller nanocomposites have been conversed with special emphasis on electrical properties. Towards the end, significance and challenges in the fabrication of conducting nanocarbon-based nanocomposite and their future prospects have been discussed.

3.2 Carbon Nanofiller 3.2.1 Nanodiamond The elemental carbon in ground state is 1 s2, 2 s2, 2 p2. The two electrons in 1 s orbital are core electrons, whereas the 2 s2, 2 p2 four electrons are valence electrons. Carbon atom may undergo sp3 hybridization of carbon atoms to form tetrahedral structure of diamond. In diamond structure, valence electrons forms σ-bond with the neighboring carbon atom to form staked layers of carbon. Due to lack of free electrons in diamond structure render it an inert solid, tough, thermal conductor, and transparent. The structure of ND particle is rather intricate consisting of three parts (i) diamond core, i.e., central part made up of sp3 hybridized carbon; (ii) shell of sp2 hybridized carbon around the core; and (iii) carbon atom on outer surface which may have various functional groups (Fig. 3.1). ND has been produced using different methods such as chemical vapor deposition (CVD), plasma-assisted CVD, laser ablation, high-energy ball milling, autoclave synthesis, electron irradiation of

Figure 3.1 Diamond structure and functionalization at surface.

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carbon onions, etc. [11]. However, the most frequently used is detonation technique. ND is a single crystal of cubic diamond particles [12]. According to X-ray diffraction, the average size of crystals was found to be 4
5 nm [13]. Dynamic light scattering measurements after light sonication also revealed single-digit nanorange sharp distribution and individual particles having diameters of 4
5 nm. Detonation of oxygendeficient explosives in inert medium may produce ultrafine diamond particles. Moreover, larger ND aggregates having diameter of 100-200 nm have also been recovered necessitating deaggregation. Table 3.1 presents classification of particle assemblies in detonation ND. Spherical particles of nanodiamond are present in soot [14]. Differences between ND in detonation soot and commercial ND are highlighted in Fig. 3.2. The diamond nanoparticles are known to graphitize at 1100
1200 C. Onionlike carbons have been obtained by annealing diamond nanoparticles at such a high temperature [15]. High-resolution transmission electron microscopy (TEM) observation of ND nanoparticles depicts size in the range of 2
10 nm (Fig. 3.3). The nanoparticles were found to have clear boundaries and distinct shapes. The lattice fringes corresponding to (111) planes of diamond were also observed (Fig. 3.4). The interlayer spacing of 0.206 nm was experiential and was found in agreement with that of the diamond (111) planes [16
18]. Generally, nanofiller with small particle size play an important role in improving materials properties [19]. The extent of nanofillers to advance the polymer properties relies upon its size, concentration, structure, dispersion, and orientation in polymer matrix [20]. The nanofillers Table 3.1 Classification of Particle Assemblies in Detonation Nanodiamond [14]

Equilibria Primary particle Core aggregate Intermediate aggregation Agglomerate a

Average Diameter(nm) Volume Ratioa

Mode of Assembly

4.4

(1)

Nanoagglutination

100 2000

8000 (1) 9 3 107 10,000 (1)

Nanoaggregation Nanoaggregation

20,000

9 3 1010

107 1000 Micronaggregation

Approximate relative sizes of aggregates compared under three different standards, primary particle (third column), core aggregate (fourth column), and intermediate aggregate (fifth column).

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Figure 3.2 Structural components of detonation soot and commercial nanodiamond product.

Figure 3.3 HRTEM image of initial diamond nanoparticles [16].

features rely on particle shape, geometry, size, and functionalization. The ultrafine ND particles may result in improved electrical conductivity, mechanical properties, thermal stability, transparency, thermal conductivity, and flame retardancy of polymeric systems. The electronic properties of NDs have been premeditated. There are two types of nanoparticles in this regard, i.e., diamondoids formed from admantine cages and spherical H-terminated diamond nanoparticle. In ND, quantum confinement effect was observed up to 27 nm, while in Si

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Figure 3.4 The interlayer spacing of nanodiamond (111) planes [16].

and Ge quantum confinement effect was found to disappear after 5
7 nm [21]. The diamond films has been examined through X-ray technique to presume nanoparticle gap and size [22]. X-ray absorption and emission spectra of detonated ND was found similar to that of bulk diamond. The spectra was found shallower at secondary minimum of 302 eV and exciton broadening of 289.3 eV. Density functional theory (DFT) has been used to find the optical gaps of diamondoids [23]. It was found that for the nanoparticles larger than 1 nm, quantum confinement effect was disappeared. According to DFT, the gaps of small diamondoids were found lower than the gaps of bulk diamonds for size between 1 nm and 1.5 nm. Moreover, the quantum confinement was not found to influence the electronic structure of nanoparticles of 4 nm. The behavior was entirely different than the hydrogen-terminated Ge and Si nanoparticle. It was also anticipated that diamondoids show negative electron affinity [24]. The DFT eigenvalue gap usually relies upon the size of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO). Generally, nanoparticle size directly affect the HOMO-LUMO gap. The quantum confinement models prophesied that HOMO eigenvalue was decreased as nanoparticle size reduces. However, LUMO eigenvalue was nearly independent of size and showed no quantum confinement. In nanoelectronic application, p- and n-type doping of ND clusters is important. Ultrananocrystalline films doped with nitrogen (1%
20%) caused high electrical conductivity of ND films. The increase of nitrogen gas concentration was aimed to high electrical conductivity due to deposition on larger grain boundaries [25].

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Figure 3.5 Structure of graphene.

3.2.2 Graphene Graphene is a unique nanocarbon having hexagonal layered structure. In hexagonal structure, each carbon is connected to three neighboring carbon atoms by σ bonds with sp2 hybridized orbitals. The distance between carbon atoms is B1.42 A (Fig. 3.5). The delocalization of electrons in π-orbitals above and below the atomic layer render graphene with remarkable electrical properties [26]. In other words, it is the building unit of graphite and other nanocarbon structures. Similar to graphite, graphene can conduct electricity within each layer via delocalized electrons, while it has low electrical conductivity perpendicular to the graphene plane. The modulus and strength of graphene has been found as B1 TPa and B130 GPa, respectively [27]. The exclusive crystalline structure and mechanical, thermal, and electrical features of graphene rely on several parameters such as defects, grain boundaries, vacancies, etc. Different routes have been used for the preparation of graphene such as epitaxial growth of graphene CVD, gamma irradiation to graphite particles, micromechanical exfoliation of graphite, scotch tape method, and several chemical routes. However, production of high-quality graphene on large scale is still challenging. Preparation of oxidized form of graphene, i.e., graphene oxide (GO) from graphite and then its reduction to graphene has attracted considerable attention (Fig. 3.6). Reduction of GO to graphene has been achieved using several approaches such as thermal annealing and chemical, photocatalytic, and plasma reduction. The extraordinary physical properties of graphene has rendered it a remarkable filler for polymer nanocomposites. Polymer/graphene nanocomposites have been found potential in sensors, solar cell, lithium ion batteries, supercapacitors, biomedical, etc.

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Figure 3.6 Formation of graphene from reduction of graphene oxide.

Figure 3.7 Fullerene molecules.

3.2.3

Fullerene

Fullerene are symmetrical nanocarbon molecules [28]. These are caged compounds having high temperature stability and electrical conductivity to be employed in nanocomposites. Fullerenes are composed of different number of polygones, and these molecules are named according to the number of carbon atoms present in their structure (Fig. 3.7). Fullerene molecule possess sp2 and sp3 hybridized carbon atoms. The angle strain in these molecules is due to sp2 carbon atoms in structure. These molecules were initially discovered experimentally in 1985 and the scientists (Richard Smalley, Robert Curl, and Harry Kroto of Rice University) were awarded with Nobel Prize for this discovery. Fullerenes are hollow molecules. A commonly known fullerene is C60. It was named as Buckminster fullerene, after the name of an American architect, Buckminster. It is made up of 20 hexagons and 12 pentagons. The average C
C bond length in C60 is found as 1.44 A , while molecule has diameter of 7.09 A . It has been prepared using

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Figure 3.8 Endohedral fullerene.

Figure 3.9 Different types of carbon nanotube.

laser-ablated graphite and arc-discharge techniques. Due to delocalized π electrons, fullerenes has large nonlinear optical responses, high electron affinity, and high transport charge ability. Endohedral fullerene molecules have also been generated with an atom (usually noncarbon atom) inserted inside molecule (Fig. 3.8). Ions or molecules are usually accelerated and implanted in fullerene cage. Fullerene molecules has exceptionally high strength to harden metals and alloys deprived of compromising ductility [29]. Fullerenes have also found potential in interdigitated capacitors, sensors, molecular wires, bulk heterojunction (BHJ) solar cell, and magnetic resonance imaging (MRI).

3.2.4 Carbon Nanotube CNT is an 1D nanocarbon material. It is a cylindrical macromolecule having radius of few nanometers, while length of several μm. The nanotube wall is made up of carbon atoms network similar to that of the atomic planes of graphite (Fig. 3.9).

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Figure 3.10 Schematic diagram showing how a hexagonal sheet of graphene is rolled to form a CNT with different chiralities (A: armchair; B: zigzag; C: chiral) [30].

The π-orbitals on carbon atom in CNT may lead to a weak attraction forces resulting in agglomeration. After the discovery of fullerene, CNT was proposed by Iijima in 1991 [30]. On the basis of arrangement and number of cylinders, CNT are of numerous types. Single-walled CNT (SWCNT) has diameter of 0.4
3 nm, with single rolled graphene sheet with simple geometry. CNT structure was identified by chiral vector (n,m). Consequently, nanotubes are also categorized on the basis of its chirality as armchair (n 5 m), chiral (m 6¼ 0), or zigzag (m 5 0), according to hexagonal carbon arrangement (Fig. 3.10) [31]. The chirality of nanotubes has noteworthy influence on the electronic properties of CNT. The nanotube is metallic if (2 n 1 m) is a multiple of 3, otherwise it is a semiconductor. Double-walled CNT (DWCNT) own two overlapping nanotube cylinders. Consequently, multiwalled CNT (MWCNT) possess carbon rings having diameter of about 100 nm. As MWCNT consists of multilayer of graphene, each layer having different chiralities, therefore, electrical and physical properties are intricate than SWCNT. Various methods have been designed for CNT synthesis. The most efficient one is CVD. CVD involves catalytic decomposition of hydrocarbon or carbon monoxide with transition metal catalyst. Thermal CVD or microwave plasma CVD have been used. This method has found success for product purity and large scale production. Electric arc discharge has also been employed. This method involves generation of inert atmosphere of helium or argon between two electrodes of graphite. Another influential technique to attain high yield is laser ablation. However, the CNT produced may have 30 %
40 % impurities. CNT possess excellent electrical, mechanical, and thermal properties rendering it an ideal candidate as advance nanofiller in nanocomposites. MWCNT is sufficiently electrically conductive having high electrical

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conductivity up to 2 x 103 S/cm. Tensile strength of 100 GPa and Young modulus up to 1000 GPa are also remarkable mechanical features. SWCNT also has high thermal conductivity .200 W/Mk and electrical conductivity .104 S/cm [32]. The SWCNT nanocomposites are transparent and conducting, however, optical transparency may be lost in MWCNT owing to the development of conducting mesh. The exceptionally high aspect ratio, electronic features, and mechanical properties of CNT has led to applications in solar cell, sensors, devices, biomedicine, and nanocomposites [33]. The essential properties of CNT has been effectively enhanced via using various modification techniques to further increase their utilization in technical fields [34]. Another very similar nanocarbon to CNT is carbon nanofiber (CNF) [35]. It is also an 1D allotrope of graphene. The only difference CNF own with respect to CNT is its length. The CNF is several time longer as compared to CNT. This nanocarbon has also been prepared using CVD. The electrical conductivity of CNF may be as high as 105 S/m with controllable degree of crystallinity. The significant electrical, thermal, mechanical, and other physical properties of CNF have rendered it a good nanofiller for nanocomposites.

3.2.5 Carbon Black CB is also an important type of nanocarbon [36]. CB is almost pure elemental carbon and has been effectively used as carbon filler for polymers [37]. It generally consists of small carbon particles ,100 nm. In CB, small carbon particles may aggregate to form chain or network like structure (Fig. 3.11). Its microstructure is often referred as quasigraphitic, in which nonparallel CB layers may conglomerate to generate circular particles of 100
300 nm. The accretion of particle is known as nucleation process. Consequently, the primary particles may further agglomerate to form grape-like clusters of up to 500 nm. The CB particles are in contact with each other due to van der Waals forces. Thus, there are small interaggregate distances making CB highly electrically conducting. Due to high surface area of CB particles, electrical percolation and conductive carbon network are achieved at lower filler concentrations. Polymer/CB nanocomposites have fine antistatic, electrostatic dissipative, and conducting properties. These materials may have electrical resistivity of 1026
1029 S/m and even higher depending on CB concentration and its mode of blending in polymers. The electrically conducting polymer/CB nanocomposites have been efficiently employed in electromagnetic shielding materials.

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Figure 3.11 Fusion of carbon black primary particles to form aggregates.

3.3 Electrical Conductivity in Polymer/Carbon Nanofiller Nanocomposite 3.3.1

Polymer/Nanodiamond Nanocomposite

Detonation ND has been found as an exceptional filler for nanocomposite materials by developing optimum interface interaction with the matrix. ND has been incorporated in thermoplastic as well as thermosetting polymers. Moreover, conducting polymers have been successfully reinforced with ND. Consequently, different polymers have been composited with ND using various techniques. Poly(vinyl alcohol)/ND nanocomposite have been produced by solution method. Polydimethylsiloxane/ND and polyacrylonitrile/ND nanocomposite have been fabricated via electrospinning technique [38,39]. Both the noncovalent and covalent functionalization of ND and solubilization in polar media have been explored to improve the final nanocomposite properties. Different surface modification methods have been designed and developed to attain fine compatibility between matrix and nanoparticle. Epoxy resin has also been reinforced with dispersed ND using solution method. The nanocomposite exhibited enhanced mechanical properties at low nanofiller content [40]. The 1 wt.% ND in diglycidyl ether of bis-phenol A caused 25% increase in Young’s modulus, while 0.5 wt.% ND enhance the strength up to 47%. Hardness of surface functionalized epoxy/ND nanocomposite was also found to increase. The surfactant wrapped ND having fine dispersal was reinforced in poly

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(methyl methacrylate) (PMMA) resin [41]. Glass transition temperature of PMMA/ND nanocomposite was improved with nanofiller loading. Low concentration of ND was found effective to enhance the overall performance of these nanocomposite. Poly(phenyleneisophthalamide) was also modified with detonation ND using solid-phase dispersion method [42]. The transport properties of nanocomposite with nanofiller up to 5 wt.% have been studied in the methanol and methyl acetate. The mass transfer processes was observed using sorption and pervaporation methods. The ND membranes facilitated selective mass transport of methanol
methyl acetate mixture. Various π-conjugated polymers have also been successfully explored with ND. The high-performance polymer/ND nanocomposite prepared have found potential in various advanced engineering and biomedical applications. ND has been considered as an effective reinforcement due to nanosize, spherical shape, and excellent physiochemical characteristics [43
45]. In polymeric materials, conducting phase polymer as well as conducting fillers may ensure the presence of conductivity domains throughout the nanocomposite. π-conjugated polymers usually referred as conducting polymers is an important class of materials having range of significant features such as electronic properties, mechanical strength, and optical characteristics. π-conjugated polymers have found application in photovoltaics, organic transistors, light emitting diodes, batteries, electrochromic devices, anticorrosion coatings, etc. [46]. Among commonly known conducting organic polymers are polyaniline (PANi), polythiophene (PTh), and polypyrrole (PPy). These conducting organic polymers have been reinforced with ND and found to revel remarkable mechanical, thermal, and electrical characteristics. Formation of conducting matrix/filler network is most likely observed in the case of π-conjugated polymer/ND materials. In this regard, functional (F-NDs) and nonfunctionalized ND (NF-NDs) reinforced nanocomposite have been prepared using chemical oxidative in situ polymerization route [47]. The layered polymerization technique was used involving PANi, PPy, PTh, and polyazopyridine (PAP) as layered matrices. The in situ polymerization of 2,6-diamino pyridine, aniline, thiophene, and pyrrole monomers on ND surface was opted for the synthesis of NF-NDs/PAP/PANi/PPy, F-NDs/PAP/PANi/PPy, NF-NDs/ PANi/PPy/PTh, and F-NDs/PANi/PPy/PThnanocomposites. Physical characteristics of the prepared nanocomposites were studied using appropriate techniques. Electrical conductivity of heteroaromatic NDs/ PAP/PANi/PPy nanocomposites with functionalized and nonfunctionalized filler have been analyzed. The conductivity of NF-NDs/PAP/

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PANi/PPy was 3.8 S/cm, which was improved to 5.4 S/cm with functionalized ND in F-NDs/PAP/PANi/PPy. In the case of NF-NDs/PANi/ PPy/PTh and F-NDs/PANi/PPy/PTh nanocomposites, the electrical conductivity was found to slightly decrease as 2.9 and 3.7 S/cm, respectively. The results have shown that the combination of PANi, PPy, and PAP with ND formed better conducting network compared with the PANi, PPy, and PTh nanocomposite. Consequently, ND with high surface area and tunable surface structure doped with conducting polymers have shown noteworthy increase in conductivity due to synergetic effects of layered conducting matrices and nanofiller. Functionalized ND was, thus, more effective to improve the electrical conductivity compared with the nonfunctionalized filler due to better interaction with polymers [48]. Conducting PANi/detonated ND (PANi-DND) nanocomposite fibers have been prepared using chemical oxidative precipitation polymerization [49]. Interaction between ND and PANi was found to enhance the electrical conductivity. The conductivity measurements were performed using two-probe and 4-point conductivity measurements. The sample was imaged in contact mode by applying a DC voltage ΔV between Ag electrode and the tip, so acquiring current flowing from tip to the sample (Fig. 3.12A). The current signal was recorded at each point of the scanned area. Atomic force microscopy (AFM) images and resultant current maps obtained by applying ΔV 5 2 V between AFM tip and PANI-DND fibers are shown in Fig. 3.12B. The current map revealed that the electric current flow through the fibrils, whereas no current signal was observed above the noise. The inversion of the direction of electric current flow was observed on reverse ΔV (Fig. 3.12D). The conductibility of PANI fiber and nanocomposite fiber yielded high values of S/m. Besides conducting polymers, thermoplastics have also been reinforced with ND. In the case of nanostructures in insulating polymers, nanoparticle are dispersed in nonconducting matrix depending on the interaction between matrix and nanofiller. ND may become wrapped with a thin layer of insulating polymer to form conducting setup. The extent of the formation of conducting polymer/ND network depends on covalent interaction between the phases. Poly(vinylidene fluoride) (PVDF) nanocomposite with different ND content and functionalization have been processed by solvent casting method [50]. Variations in electrical, optical, thermal, and morphological properties of nanocomposite have been investigated for treated and nontreated ND in the range 0.1
1 wt.% loading. The effect of ND content and oxidation (NDox) on electrical properties of nanocomposite have been presented in Fig. 3.13. The

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Figure 3.12 Conductivity imaging of individual PANI-DND fibers: (A) Schematic diagram of the measuring method based on a conductive Pt tip of an AFM apparatus; (B) AFM image and (C) current maps of two isolated PANI-DND fibers; (D) current I(nA) flowing from tip to PANI-DND fibers sample recorded at each point of the scanned area [49].

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PVDF1010 0.1 wt% ND 0.5 wt% ND 1 wt% ND

(A)

10

0.10 0.1 wt% NDox 0.5 wt% NDox 1 wt% NDox

(B)

PVDF 0.1 wt% ND 0.5 wt% ND 1 wt% ND 0.1 wt% NDox 0.5 wt% NDox 1 wt% NDox

0.08

tan (δ)

ε'

9 8

0.06

0.04

7 6

0.02

5 103

104

105

106

103

104

f (Hz) PVDF

–8

10

106

ND

NDox

10 (D) 8 6 10–15

10–9

σ (S/cm)

σ' (S/cm)

10–7

PVDF 0.1 wt% ND 0.5 wt% ND 1 wt% ND 0.1 wt% NDox 0.5 wt% NDox 1 wt% NDox

ε'

(C)

105

f (Hz)

10–10

10–16 10–17

103

104 f (Hz)

105

106

0.0 0.1

0.5

1.0

Filler amount (%)

Figure 3.13 Room temperature (A) dielectric constant (ε0 ); (B) dielectric loss tan (δ); (C) AC electrical conductivity as function of frequency; and (D) ε0 (1 kHz) and DC conductivity as function of filler type and content [50].

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properties have been studied by the measurement of dielectric response (Fig. 3.13A & B), AC (Fig. 3.13C), and DC (Fig. 3.13D) electrical conductivity. Fig. 3.13A reveals decrease in dielectric constant with increasing frequency from 500 Hz to 1 MHz due to lower mobility of polymer chain dipoles with increasing frequency [51]. The nanocomposite films with treated and nontreated ND depicted increase in ε0 with respect to neat PVDF film. The dielectric loss tan (δ) of samples revealed analogous frequency dependence between 500 Hz and 1 MHz with values lower than 0.1 (Fig. 3.13B). The dielectric loss behavior has shown slight decrease in frequencies up to 104 Hz followed by a sharp increase for higher frequencies due to microBrownian movement of amorphous polymer chains. Generally, AC electrical conductivity increases with the increasing frequency [52]. However, no relevant differences were observed between the pristine polymer and nanocomposite films of ND and NDox (Fig. 3.13C). Fig. 3.13D shows the dependence of dielectric constant at 1 kHz and DC electrical conductivity with filler concentration. The ε0 was found to increase with the NDs addition, however, significant changes were not observed for 0.5 and 1 wt.% NDox nanocomposite as ε0 varies between B8.6 and 9.6, respectively. Accordingly, oxidation treatment was not found to produce significant effects on dielectric properties of nanocomposite. The DC electrical conductivity has shown differences between the two types of nanocomposites. The NDox nanofiller caused slight increase in DC conductivity due to increase in number of charge carriers promoted by the oxidation treatment of functional ND. The studies on conductivity profile of polymer/ ND nanocomposite have shown better increase in electrical properties of conjugated polymer-based systems. The success of π-conjugated polymer and ND may be attributed to better conducting network formation and synergetic effect of the two phases.

3.3.2 Polymer/Graphene Nanocomposite Polymer/graphene nanocomposites reveal superior electrical, mechanical, thermal, gas barrier, and flame retardant properties compared with neat polymers [53]. Graphene has revealed fine potential for electrical conductivity enhancement of polymers [54]. Due to high aspect ratio, flexibility, and exceptional conductivity render graphene a promising choice as conducting nanofiller for conducting materials. The polystyrene/graphene nanocomposites has been prepared by chemical modification and reduction of graphene in polymer solution. The

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nanocomposite exhibited low percolation threshold of 0.1 vol.%. The percolation threshold was significantly low matching the polymer/ SWCNT nanocomposites [55]. Compared with exfoliated graphite having much higher threshold ( . 0.6 vol.%) at low content, graphene is an exclusively beneficial material [56]. The graphene nanosheets [twodimensional (2D) flat layers] are localized in polymer matrix and at surface resulting in close contact between the particles. Consequently, very low percolation has been attained. To further improve the conductivity properties, graphene has been modified and used in the fabrication of nanocomposites with different polymer matrices. Graphene nanosheet and ultrahigh-molecular weight polyethylene (UHMWPE) composite have been fabricated using solvent-assisted dispersion and hot compression methods [57]. The low percolation threshold of 0.070 vol.% was achieved due to the formation of a 2D conductive network (Fig. 3.14). At nanofiller content of 0.070 vol.% (percolation threshold), the conductivity was increased sharply (1024 S/m). However, at low 0.06 vol.%, the conductivity was only 1028 S/m. The conductivity of nanocomposite was increased from 0.076 to 0.15 vol.%. At 0.6 vol.% nanofiller, high conductivity of B1021 S/m was archived, i.e., suitable for several 10–1 10–2 –1.0 –1.5

10–4

t = 1.26t 0.06 Log conductivity (s/m)

Conductivity (s/m)

10–3

10–5 10–6 10–7 10–8 10–9 0.0

0.1

–2.0 –2.5 –3.0 –3.5 –4.0 –4.5

0.2

–4.0

–3.5 –3.0 Log (j–jc)

0.4 0.3 CNS content (vol%)

0.5

–2.5

0.6

–2.0

0.7

Figure 3.14 Electrical conductivity as a function of graphene nanosheet content for UHMWPE/GNS nanocomposites. The inset is a log
log plot of the conductivity as a function of ϕ 2 ϕc with an exponent t 5 1.26 and critical volume content ϕc5 0.070 vol.% [57].

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Figure 3.15 TEM images of graphene under low (A) and high (B) magnifications. The scale bar is 5 nm. The inset is SAED pattern of graphene [58].

electrical applications. Graphene nanosheets were also prepared by thermal exfoliation and reduction of graphite [58]. Polyethylene terephthalate (PET)/graphene nanocomposites were developed using melt compounding. TEM images revealed an average thickness B1.57 nm of graphene sheets (Fig. 3.15). The selected area electron diffraction (SAED) pattern indicated loss of long range ordering between graphene sheets and the ultrathin structure of graphene. Fig. 3.16 shows plots of electrical conductivity vs filler content for PET/graphene nanocomposite. The integration of graphene in polymer matrix greatly affected the electrical conductivity. There was a sharp transition from electrical insulator to semiconductor with low percolation threshold of 0.47 vol.%. At 3.0 vol.% graphene, considerably high electrical conductivity of 2.11 S/m was attained. In polymer/graphene nanocomposite, low percolation threshold and superior electrical conductivity have been credited to high aspect ratio, large specific surface area, and uniform dispersion of graphene in matrix.

3.3.3 Polymer/Fullerene Nanocomposite Fullerenes have been considered as unique polymer nanofillers and special dopants for π-conjugated polymers [59]. The conductivity of these nanocomposite has been increased due to electron transfer from

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Figure 3.16 Plots of electrical conductivity vs filler content for PET/graphene nanocomposites and PET/graphite composites. The inset is double-logarithmic plot of volume electrical conductivity vs (φ
φc) [58].

the valence band of conducting polymer to dopants. Consequently, electrical and optical properties of conducting polymer/fullerene nanocomposite have gained significance [60]. In this regard, new junction devices exploiting effective charge separation at the interface of conducting polymer/C60 have been developed. The conductivity was found to increase with increasing the volume fraction of fullerene, which can be elucidated in terms of conduction by percolation process. Moreover, improvement in photoconductivity of nanocomposite films has been observed near percolation threshold. The photo-induced charge transfer between polymer and C60 occurred at the interface region due to dissociation of neutral photo excitations. Moreover, the charge was not influenced by electric field. The polystyrene sulfonate doped poly (3, 4-ethylenedioxythiophene)/fullerene films were deposited on indium tin oxide [61]. The current
voltage characteristics of solar cells were measured in the temperature range 125
320K under variable illumination between 0.03 mW/cm2 and 100 mW/cm2. The short-circuit current density and the fill factor was increased monotonically with temperature till 320K. The power conversion efficiency of 1.9% at T 5 320K and 100 mW/cm2 were observed. Moreover, electron mobility characteristics of polymer/fullerene materials had low effective charge
carrier mobility and strong temperature dependence. The active layer thickness was also kept low for improved efficiency of solar cell.

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3.3.4 Polymer/Carbon Nanotube Nanocomposite Electrical percolation in mixtures of electrically conducting polymer/ CNT is a widely investigated field of recent research [62]. The conductivity threshold in polymer/CNT composites has activated worldwide activities. Experimental and theoretical investigations have shown that the parameters such as CNT type, content, synthesis method, dispersion technique, CNT modification, filler dimensionality as well as polymer type affect the percolation threshold and conductivity of these nanocomposite. Polymer/CNT nanocomposites offer higher sensitivity and superior electrical properties relative to conventional smart materials. Due to high surface area and unique structure, MWCNT may form conductive path at low content compared with the use of CB or carbon fiber as conductive reinforcements. In this regard, functionalization of CNT may cause better dispersion in polymeric matrix for improved conductivity of functional materials. Both the physical and electrical properties of these nanocomposites have been influenced by the chemical treatment of MWCNT embedded in the polymeric materials [63]. The electrical conductivity of oxidized MWCNT-filled epoxy nanocomposites was found dependent on chemical treatment using various reagents [64]. Strong acids were used for the oxidation of nanotube. MWCNT oxidized by H2O2/NH4OH solution resulted in higher conductivity of composites compared with nitric acid treatment. It was observed that damage to nanotube structure influenced the electrical properties of nanocomposites. Thus, electrical conductivity was lowered at low nanofiller content and percolation threshold was raised. The nonlinear curve fitting curves of DC conductivity as a function of MWCNT content results for p . pc were obtained (Fig. 3.17). The best fitted constants were obtained in terms of r0, pc and t (Table 3.2). The stronger oxidation conditions resulted in higher percolation threshold of nanocomposite. However, A0 and B0-MWCNT nanocomposite (H2O2 treatment, different solutions) showed very low percolation. The A4-MWNT nanocomposite (HNO3 treatment) showed exceptionally high percolation threshold. The exponents obtained were higher than the critical exponent (t 5 1.44 6 0.30) of typical epoxy/MWCNT nanocomposite [65]. It was believed that there is contact resistance among nanotube and aggregates. In neighboring conductive clusters separated by insulating polymer, the conduction of electrons can be explained on the basis of quantum tunneling effect. Consequently, the strain sensor based on polymer/MWCNT has also been fabricated [66]. The piezoresistivity of nanocomposite strain

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10–4

10–1

10–6

DC conductivity (S/cm)

DC conductivity (S/cm)

10–2

10–8 10–10 10–12 10–14

10–2 10–3 10–4 10–5 t = 1.7457

10–6 10–7 10–8 10–9

10–3

10–2

10–16 0

10–1 P-Pc

100

1 2 Volume percent of A4-MWNT

(A)

101

3

10–4

10–1

10–6

10–2

DC conductivity (S/cm)

DC conductivity (S/cm)

10–2

10–8 10–10 10–12 10–14

10–3 10–4 t = 1.8176

10–5 10–6 10–7 10–2

10–1

100

101

P-Pc

10–16

1 2 Volume percent of B0-MWNT

0 (B)

3

Figure 3.17 Plot of DC conductivity of (A) A4-MWNT and (B) B0-MWNT composite as a function of MWCNT content. Each inset shows the log
log plot of DC conductivity with p
pc for p . pc[64].

Table 3.2 Comparison of Percolation Threshold in Composite Systems [64] Nanocomposite

σ

pc (vol.%)

t

Epoxy/A0-MWCNT Epoxy/A1-MWCNT Epoxy/A2-MWCNT Epoxy/A4-MWCNT Epoxy/B0-MWCNT

1.9 3 1024 1.5 3 1024 6.0 3 1025 4.8 3 1023 2.0 3 1023

0.017 6 0.024 6 0.028 6 0.077 6 0.021 6

1.71 6 2.01 6 1.74 6 1.75 6 1.83 6

0.06 0.03 0.05 0.02 0.02

0.03 0.04 0.06 0.05 0.06

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(A)

(B)

Tunneling current Rtunnel

Polymer

Rcn

J

CNT

Figure 3.18 Modeling of tunneling effect in the resistor network: (A) SEM image of possible tunneling effect among CNTs in the nanocomposites; (B) modeling of tunneling resistance in the resistor network [65].

sensor was studied using 3-D model with CNT percolation network. Owing to the short distance between adjacent MWCNT, tunneling effect and its influence on electrical conductivity were investigated. The CNT aggregates have been observed using SEM (Fig. 3.18A). The tunneling resistance between two neighboring CNT was also estimated. The model to study the resistance is shown in Fig. 3.18B. The tunneling effect was considered as a probable mechanism for small strains in sensor. Low nanofiller volume fraction caused weak nonlinear piezoresistivity in these sensors. Moreover, high sensitivity was obtained for nanocomposite sensors with MWCNT volume fraction close to percolation threshold.

3.3.5 Polymer/Carbon Black Nanocomposite The characteristics of CB-filled polymer nanocomposites have been studied in the last few decades [67]. Consequently, their electrical properties have gained considerable attention [68]. Models have been proposed to the electrical resistivity of polymer/CB composites. The electrical properties have been explained on the basis of the formation of network of interparticle tunneling conduction. The tunneling effect in turn formed basis of classical percolation theory [69]. According to this theory, particles were not connected geometrically, nevertheless, the

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106

ρ (Ohm cm)

105

104

103

102

101 6

7 8

2

0.1

3

4

5

6 7 8

1 (ρ-ρc)

Figure 3.19 The measured dependence of the resistivity on the proximity to the percolation threshold in a particular low-structure carbon black
polyethylene composite. The values derived from the power-law fit were v 539 vol% and t 56.4 [70].

two conducting particles in the system are connected electrically by tunneling. The percolation was found dependent on the particular type of CB. The more spherical the CB particle is, lower is its percolation threshold. Thus, interparticle tunneling conduction and percolation were reliant on CB particle type and content. Dependence of resistivity on percolation threshold in polyethylene (PE)/ CB nanocomposite has been studied [70]. The resistivity of the composite was explained by power law ρ α (υ-υc)2t. Here, υ is vol.% of CB phase and υc is its value at percolation threshold. The resistivity depends on the proximity to threshold (ρ-ρc) 5 (υ-υc)/(100-υc). According to percolation theory υc516 vol.% and t 5 2.0, however, both v and t were found very different than universal values (Fig. 3.19). The PE/CB composites were prepared using different type of CB particle. Fig. 3.20 shows the average interparticle distance in different networks. The CB particles having narrow distribution of interdistances due to mutual friction may yield entangled-particle structures. The CB particles having longer interdistances will not have significant CB clusters in them. The structure with small interparticle distances may result in a universal percolation-like behavior. According to literature [71], formation of poignant CB particles network is not always constant, but it relies on fabrication

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Figure 3.20 An illustration of an almost close-packed network of spheres, that are “connected” by nearest-neighbor tunneling. The black circles represent the carbon particles while their gray shells nearest represent the effective tunneling distance [70].

technique. The resulting electrical conductivity depends inter-particle interaction, aggregation, and filler concentration. Low percolation threshold of 0.5 vol.% has been explained in CB-filled resins. The optimization of processing and particle parameters, may decrease the threshold to 0.06 vol.%, so increasing the overall conductivity of the system.

3.4 Comparative Evaluation of Electrical Conductivity Characteristics in Nanocomposite Electrical conductivity (S/cm) of pristine ND is 102
1015, graphene about 102
105, fullerene around 105, SWCNT B102
106, and MWCNTB103
105. However, the electrical conductivity of the resulting polymer/ND, polymer/graphene, polymer/fullerene, polymer/CNT, and polymer/CB nanocomposite depends strongly on the ability of the nanocarbon to interlink for the formation of conducting network at certain percolation threshold. Owing to large and accessible surface area, small size and functionalizable surface, ND holds excellent potential for conducting as well as strengthened polymeric systems. These significant features render ND distinctive nanomaterial for nanocompositing via strong covalent bond formation with polymer matrix. However, ND is relatively less documented nanocarbon for polymers. Initially, ND has been reinforced in lubricants and rubbers to enhance wear resistance. Presently, conducting polymer/ ND has gained interest of materials

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Figure 3.21 Fabrication of PANi-g-ND and gold nanoparticle nanocomposite.

research. In polymer nanocomposite, ND has shown fine nanofiller dispersion without agglomeration, however, nonfunctional ND nanocomposites have shown nanoparticle clusters. In this regard, choice of processing technique is also an imperative footstep. ND has been dispersed in polymer matrix using various techniques such as in situ polymerization, solution casting, and melt processing method. In situ polymerization has been considered as an effective method for fine nanoparticle dispersion, nevertheless, melt method may cause meager dispersion and aggregation in the matrix. Addition of ND to epoxy has shown increase in conductivity, young modulus, hardness, and thermal properties of the nanocomposite [72]. Such nanocomposite have found potential in various technical fields. A biosensor of PANi-grafted-ND and gold particles has been reported (Fig. 3.21) [73]. The nanocomposites possess fine dispersal of gold nanoparticle and polyaniline fiber with ND. The biosensor revealed good electrocatalysis towards the identification of nitrite ion. The rechargeable polymer lithium-ion battery are promising power source for electronic devices, electric vehicles, and energy storage systems due to high conductivity, energy density, and long life cycle. Moreover, amalgamation of ND and conducting polymer yielded improved features for Li-ion battery electrodes [74]. Polymer/ ND especially with π-conjugated polymers may

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act as conducting materials. In the case of graphene-filled polymer nanocomposites, the electrical properties strongly depend on the method of graphene preparation as well as its dispersion technique in matrix. Solution dispersion of polymer graphene has shown high conductivity of B1021 S/m [57]. The percolation threshold as low as 0.070 vol.% has been attained due to development of 2-D conducting mesh of nanofiller in matrix. However, spontaneous reagglomeration of the 2D nanoparticles may cause disturbance of conducting network and low conductivity. The tendency of graphene reaggregation and decrease in the conductivity of nanocomposite is occasionally greater than the nanotube-filled nanocomposite. Polymer/fullerene nanocomposites have been explored focusing the electron mobility in conducting polymer/fullerene BHJ solar cell [60,61]. Thermomechanical properties of polymer/ fullerenes have also been explored, however, the electrical conductivity measurements on conducting nanocomposites have not been focused [75,76]. CNT is among the most widely explored form of nanocarbon with polymers. Nevertheless, its discovery comes after that of fullerene. Consequently, electrical conductivity of SWCNT and MWCNT has gained attention. Formation of percolation network and tunneling effect has been clearly observed in the case of polymer/MWCNT. Depending on appropriate nanotube functionalization and dispersion techniques, higher electrical conductivities can be achieved compared with polymer/graphene and polymer/CB materials. However, nanotube modification is quite challenging due to the formation of surface defects and agglomeration, so increasing the percolation threshold [64]. CB is also an essential nanocarbon to form high performance nanocomposite [77]. Low percolation threshold of polymer/CB nanocomposite materials has been achieved due to better charge transport among CB aggregated network (Fig. 3.22). An interesting comparison has been found between the electrical properties of CB, MWCNT, CB/MWCNT-filled polypropylene (PP) nanocomposite [78]. The electrical percolation of PP/CB, PP/MWCNT and PP/CB/MWCNT, fabricated through melt mixing were studied. The percolation threshold of PP/CB and PP/MWCNT was 4.7 vol% and 2.7 vol%, respectively, nevertheless threshold of PP/CB/MWCNT was 3.7 vol.%. However, again the conductivity of resulting nanocomposite strongly relies on nanofiller content, modification, dispersion, dispersal method, and processing parameters. In summary, polymer/nanocarbon nanocomposites may result in novel structural, electrical, and physical features. The synergistic effect of polymer and nanofiller are responsible for unique topographies and low percolation threshold of the

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Volume resistivity (ohm.cm)

1015 PC-0 PCM-0 PM-0

1013 1011 109 107 105 103 101 0

2

4 6 Filler content (wt%)

8

Figure 3.22 Dependence of electrical resistivity on the filler content in the PP/CB (PC), PP/MWCNT (PM) and PP/CB/MWCNT (PCM) systems [78].

resulting nanocomposite. The research scenario depicts high-tech future applications of conducting polymer/nanofiller nanocomposites.

3.5 Summary Research on nanocarbon-based polymer nanocomposite is a rapidly advancing field due to exceptional electrical and physical properties. The possibilities to form different type of nanocomposites by reinforcing polymers with ND, graphene, fullerene, CNT, CB, etc. is a forthcoming way to develop materials science. This chapter addresses fundamentals about the structure and properties of these nanocarbon forms and their effect on electrical properties of polymer/ND, polymer/ graphene, polymer/fullerene, polymer/CNT, and polymer/CB nanocomposite. Hybrid materials of various polymers and nanocarbons have been fabricated using range of processing methods. With respect to the electrical properties, more success can be achieved using appropriate type, content, and functionalization of nanofillers and employing suitable dispersion technique and processing parameters. The nanocomposite properties have revealed nanocarbons as promising candidates for various technical relevance demanding improved electrical conductivity. Thus, polymer/nanocarbon materials possess various challenges for future investigations. The enhanced in electrical properties of nanomaterials point toward brilliant technological and industrial future.

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