Ultralow Electrical Percolation Threshold in Poly(styrene-co

Article pubs.acs.org/IECR

Ultralow Electrical Percolation Threshold in Poly(styrene-coacrylonitrile)/Carbon Nanotube Nanocomposites Nilesh Kumar Shrivastava, Supratim Suin, Sandip Maiti, and Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology, Kharagpur, Kharagpur 721302, India ABSTRACT: Here, we demonstrate a new method that involves in situ copolymerization of styrene and acrylonitrile monomers in the presence of multiwall carbon nanotubes (MWCNTs) and commercial poly(styrene-co-acrylonitrile) (SAN) beads, for the preparation of electrically conducting SAN/MWCNT nanocomposites with a significantly low percolation threshold of the CNTs. At a constant CNT loading, the conductivity of the nanocomposites was increased with increasing content (weight percent) of the SAN beads, indicating the formation of a more continuous network structure of the CNTs in SAN matrix. Thus, the electrical conductivity (1.38 × 10−6 S·cm−1) of the nanocomposites with 40 wt % SAN beads increased to 8.07 × 10−5 S·cm−1 when the SAN bead content was increased to 70 wt % at constant CNT loading (i.e., 0.1 wt %). The morphology study revealed the dispersion and distribution of the MWCNTs selectively in the in situ polymerized SAN phase of the nanocomposites, leading to an increase in effective concentration of the CNTs in the in situ polymerized SAN phase of the nanocomposites. Thus, the percolation threshold of the nanocomposites was reduced to a lower value (0.032 wt % MWCNT), which was not reported elsewhere for SAN/MWCNT nanocomposites with unmodified, commercial MWCNTs of similar qualities. This report discusses detailed methodology of the process as well as other characteristics of the SAN/MWCNT nanocomposites. nanocomposites. For instance, Lee et al.25 reported an electrical conductivity of ≈4 × 10−4 S·cm−1 with 2 wt % MWCNT loading in SAN/MWCNT nanocomposites, prepared by melt mixing in a twin-screw extruder. Gültneret et al.26 observed a dc conductivity of ≈5 × 10−4 S·cm−1 in melt mixed SAN/aminofunctionalized MWCNT nanocomposites at 1 wt % MWCNT loading. Göldel et al.27 prepared conductive polymer nanocomposites by melt mixing of SAN/MWCNTs and observed dc conductivity at ≈1 × 10−4 S·cm−1 at 1.5 wt % MWCNT loading in SAN matrix. In another work, Göldel et al.28 showed a dc conductivity of ≈3.33 × 10−3 S·cm−1 at 3 wt % CNT loading in SAN/MWCNT nanocomposites prepared through melt mixing. Kim et al.29 used noncovalent functionalized MWCNTs and prepared SAN/MWCNT nanocomposites by solution casting using chloroform. They reported a dc conductivity (≈1 × 10−3 S·cm−1) at 1.5 wt % MWCNT in SAN matrix. Dufresne et al.30 showed a dc conductivity of ≈10−2 S·cm−1 at 15 wt % CNT loading in poly(styrene-co-butyl acrylate)/MWCNT nanocomposites. Singh et al.31 prepared (80/20 w/w) ABS/ PS/MWCNT nanocomposite with variation of MWCNT loading by a melt mixing process. They found an electrical conductivity value of ≈1.27 × 10−6 S·cm−1 at MWCNT loading close to 0.64 wt %. Zhao et al.32 reported very low percolation (0.07 wt % MWCNT) in poly(vinylidene fluoride) (PVDF)/ MWCNT composites using MWCNT coated PVDF spherical particles where coalescence of the spherical particles forms a conduction network in the composites. Etika et al.33 suggested a synergistic stabilization of clay by carbon black (CB) that ultimately influenced the electrical and mechanical properties of

1. INTRODUCTION With regard to the enormous industrial relevance, preparation of electrically conducting polymer nanocomposites by incorporating highly conductive nanofillers into insulating polymer matrixes has experienced a remarkable growth during the past few decades.1−3 The electrical conductivity of an insulating polymer is usually altered by dispersing conducting fillers such as carbon black (CB),4−7 carbon fibers,8 metallic fillers,9−11 intrinsically conducting polymers,12−14 and, recently, carbon nanotubes (CNTs),15 at a certain loading (percolation threshold; minimum loading of the filler to achieve electrical conductivity) into the matrix phase. The electrical and dielectric properties of polymer/conducting filler filled nanocomposites mainly depend on the type, shape, size, and concentrations of the filler. Since efficient dispersion of CNTs is important for the development of electrical conductivity in nanocomposites, chemical modification or functionalization16−18 of the CNTs is resorted to in order to increase their compatibility with the host polymer. The resulting composites have an extensive range of applications in various fields such as electronics, aerospace, military, flexible electronics, electromagnetic interference (EMI) shielding, electrostatic dissipation, and sensors. The improved electrical properties conferred by CNTs to nanocomposites have been a source of interest for developing newer nanocomposites using various thermoplastics such as poly(methyl methacrylate) (PMMA),19 polystyrene (PS),20 medium density polyethylene (MDPE),21 polycarbonate (PC),22 and others.23,24 Among these, PS/CNT nanocomposites have been widely studied. However, studies on another important copolymer of styrene, poly(styrene-co-acrylonitrile) (SAN), have been rarely reported. SAN has better chemical resistance, weathering ability, and surface hardness than PS, but it has a considerable lack of research data, which makes it an ideal proposition to be probed further. Few researchers have worked on SAN/multiwall carbon nanotube (MWCNT) © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2858

September 27, 2012 January 28, 2013 January 28, 2013 January 28, 2013 dx.doi.org/10.1021/ie3026295 | Ind. Eng. Chem. Res. 2013, 52, 2858−2868

Industrial & Engineering Chemistry Research

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epoxy composites. Mu et al. 34 prepared PS/SWCNT composites by hot pressing using CNT coated PS pellets that resulted in a decrease in the electrical percolation threshold compared to composites with well-dispersed SWCNTs. Balogun et al.35 used a partially soluble method for selective dispersion of graphite particle in poly(acrylonitrile−butadiene− styrene) and achieved a percolation concentration of 0.23 vol %. Grunlan et al.36 used latex and water-dispersible powder to reduce the percolation threshold from 15 to 2.5 vol % in CB/ poly(vinyl acetate) (PVAc) composites. Miriyala et al.37 used nanoclay for inducing a network of CB in PVAc latex and reduced the percolation to 0.9 vol % with the addition of 0.2 wt % clay. Liu et al.38 also used clay for good dispersion of SWCNTs in epoxy matrix, which reduced the percolation from 0.05 to 0.01 wt % SWCNT. In summing up, reports on PS and its copolymers dictate a general trend that there is difficulty in dispersion of the CNTs due to the inherent high melt viscosity of the base polymers. This raises the percolation threshold of the CNTs to a higher value than expected. Moreover, various methodologies32−38 have already been reported to develop electrical conductivity in polymer matrixes using nanoparticles (i.e., nanoclay), solvents, and surfactants. On the other hand, as discussed earlier, the chemical modification of CNTs results in structural defects in them which reduce the electrical conductivity. Solvent casting process or surfactants have been used for better dispersion of CNTs although the presence of residual traces of solvent or surfactants in the nanocomposites is a disadvantage which in general is sought to be avoided. In order to avoid the abovementioned disadvantages, we have demonstrated a new in situ polymerization method in which the polymerization of SAN monomers occurs in the presence of CNTs as well as SAN beads. This method effectively achieves high dispersion of CNTs which results in a low percolation threshold and at the same time avoids the need to either chemically modify the CNTs or to use any hazardous solvents or surfactants. Dispersion of CNTs in organic medium (in monomer) is better than that in water without any surfactants, as previously reported. On the other hand, low molecular weight CNTs containing in situ polymerized phase of the nanocomposites flow easily to create a continuous structure during molding, which is essential for electrical conductivity. Thus, compared to the previously reported methods, here we demonstrate an innovative straightforward, industrially feasible, and environmentally safe method for the preparation of polymer/MWCNT conducting nanocomposites. The innovativeness of this method lies in the selective localization of the CNTs in a continuous section of the nanocomposites which results in high electrical conductivity at extremely low loading of CNTs. The minor phase of the nanocomposites in which CNTs are selectively localized is made from the monomers which get polymerized during the polymerization process, while the remaining major phase consists of commercial SAN beads that have been added during a specific stage of the polymerization process. This major phase can be considered to be an “excluded volume” which is devoid of any CNTs and, hence, plays no part in the conductivity.

obtained from Sigma-Aldrich Inc., USA. General-purpose, noncross-linked, commercial grade poly(styrene-co-acrylonitrile) copolymer (SAN) was supplied by ACROS Organics, USA (grade 31076, 25 wt % acrylonitrile and 75 wt % styrene random copolymer, Mw ∼ 165 000 g/mol, specific gravity 1.08 at 25 °C, average diameter of pellets ≈ 2.35 mm and length ≈ 2.70 mm). Industrial grade MWCNTs (NC 7000 series; average diameter of 9.5 nm and length 1.5 μm, surface area 250−300 m2/g, 90% carbon purity) were purchased from Nanocyl S.A., Belgium. The MWCNTs were used as received, without any further chemical modification. 2.2. Preparation of SAN/MWCNT Nanocomposites. Volumes of 250 mL of styrene and acrylonitrile monomers were separately taken in a 500 mL separating funnel and 20 mL of 5% aqueous NaOH solution was added to it. The mixture was shaken for 15 min, and the purified monomers were individually decanted into a 500 mL beaker. This process was continued 5 times. Finally, after washing with deionized water, the purified monomers were collected. Styrene (75 mol %) and 25 mol % acrylonitrile monomers were mixed in a beaker. The desired amount (0.25 g) of MWCNT was dispersed in 40 mL of purified monomer mixture by ultrasonication for 4 h at room temperature. The monomer mixture/MWCNTs was charged to a 250 mL three-neck round-bottom flask connected with a condenser, a thermometer, and nitrogen gas inlet/outlet. Nitrogen gas was bubbled into the flask throughout the reaction. Under magnetic stirring, the required amount (1 wt %) of the benzoyl peroxide, as polymerization initiator, was added to the monomer mixture/ MWCNT dispersion. The reactor temperature was gradually increased to ≈85 °C under constant stirring. During the progress of the polymerization reaction (after ≈45 min of the reaction when the monomer mixture/MWCNTs started developing viscosity), 22 g of commercial SAN beads was added into the reactor. Addition of the SAN beads into the monomer mixture at the initial stage of copolymerization would result in swelling (dissolution) of the SAN beads and thus dispersion of some CNTs inside the SAN beads. The reaction was continued for 5 h under nitrogen atmosphere with constant temperature. The SAN/MWCNT nanocomposites thus obtained through in situ copolymerization of (styrene/ acrylonitrile)/MWCNTs in the presence of SAN beads were air-dried and finally dried in a hot air oven at 60 °C for 24 h. From the weight (≈55 g) of the final product, the calculated loadings of CNTs and SANs bead in the SAN/MWCNT nanocomposites were 0.5 and 40 wt %, respectively. The SAN/ MWCNT nanocomposites with lower CNT loading (0.3 and 0.2 wt %) and higher amount of SAN beads (50, 60, 70, and 80 wt %) were also prepared through the same polymerization route, by varying the amounts of monomer mixture, MWCNTs, and the SAN beads during the polymerization reaction. Through GPC analysis, the weight-average molecular weight (Mw) of the bulk polymerized neat SAN was found to be ≈36 500. The schematic representation for the preparation of the nanocomposites is illustrated in Figure 1. 2.3. Molding of the SAN/MWCNT Nanocomposites. The SAN/MWCNT nanocomposites with various amounts of SAN beads and CNT loading were compression molded at 210 °C in a hot press under constant pressure (10 MPa), and the molded parts were air-cooled to room temperature for further characterization. In spite of the higher loading of SAN beads (Mw ≈ 165 000), bulk polymerized SAN (Mw ≈ 36 500) forms

2. EXPERIMENTAL SECTION 2.1. Materials Details. Synthesis grade styrene and acrylonitrile monomers were procured from Merck, Germany. Benzoyl peroxide (BPO), used as polymerization initiator, was 2859

dx.doi.org/10.1021/ie3026295 | Ind. Eng. Chem. Res. 2013, 52, 2858−2868


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where i = (−1)1/2 and ω = 2πf (f is the frequency) is the angular frequency of the measuring electric field. The real part of the relative permittivity, ε′, commonly known as the “dielectric constant”, is associated with the polarization or capacitance of the material, and the imaginary part, ε″, generally called the “loss factor” or “dielectric loss”, is associated with its conductance. The ratio of the imaginary to the real parts (ε″/ ε′), known as the dissipation factor, is represented by tan δ, where δ is the angle between the voltage and the charging current. This angle δ is called the “loss angle”. From the dielectric data, the ac conductivity (σAC) was calculated using the relation σAC = ωε0ε′ tan δ

3.2. Optical Microscopy. A high resolution optical microscope (HROM; Carl Zeiss Vision GmbH) was used to investigate the distribution of MWCNTs in the SAN matrix. An image from the surface of the compression molded sample was taken in monochromatic light at different resolutions. 3.3. Transmission Electron Microscope (TEM) Analysis. The extent of dispersion of the MWCNTs in the SAN matrix was studied by a transmission electron microscope (HRTEM; JEM-2100, JEOL, Japan), operating at an accelerating voltage of 200 kV. The SAN/MWCNT nanocomposite sample was ultramicrotomed under cryogenic condition with a thickness of 50−80 nm. Since the CNT has a much higher electron density than the polymers, it appeared dark in the TEM images. 3.4. Field Emission Scanning Electron Microscope (FESEM) Study. The phase morphology of the SAN/ MWCNT nanocomposites was studied with a field emission scanning electron microscope (FESEM; Carl Zeiss-SUPRA 40), operated at an accelerating voltage of 100 kV. The specimens were broken under liquid nitrogen, and the fractured surface of the samples was coated with a thin layer of gold to avoid electrical charging. SEM images were taken on the fracture surface of the sample. 3.5. Atomic Force Microscope (AFM) Analysis. The bulk morphology of the nanocomposites was verified by tapping mode AFM analysis of the cryofractured surface of the nanocomposites, using a Nanonics Multiview 1000TM (Israel) SPM system with a quartz optical fiber tip (diameter 20 nm and spring constant 40 N/m).

Figure 1. Schematic representation of the nanocomposite manufacturing process.

a matrix phase during compression molding due to lower viscosity and lower molecular weight.

3. CHARACTERIZATION 3.1. Electrical Conductivity. The dc conductivity measurements were done on compression molded specimen bars of dimensions 30 × 10 × 3 mm3. The electrical conductivity of the conducting composites was measured with a two-probe technique. A minimum of five tests was performed for each sample, and the average data were reported. Room temperature ac electrical conductivity and dielectric properties of the nanocomposite were obtained using a computer-controlled precision impedance analyzer (Agilent 4294A) by application of an alternating electric field across the sample cell in the frequency range 40−3 × 106 Hz. The dielectric permittivity (ε′) was determined with the following equation: Cp ε′ = C0 (1) where Cp is the observed capacitance of the sample (in parallel mode) and C0 is the capacitance of the cell. The capacitance (Cp) and the loss tangent (tan δ) were measured directly. The value of C0 was calculated using the area (A) and thickness (d) of the sample, following the relation C0 =

ε0A d

4. RESULTS AND DISCUSSION 4.1. Electrical Properties. The room temperature dc conductivity of the SAN/MWCNT nanocomposites containing various amounts (40−80 wt %) of SAN beads at a constant CNT loading of 0.1 wt % is shown in Figure 2a. The SAN/ MWCNT nanocomposites with 0.1 wt % MWCNT loading did not exhibit any change in electrical conductivity compared to the pure polymer (i.e., 10−11 S·cm−1). It was noteworthy that a dc conductivity of 7.1 × 10−6 S·cm−1 was achieved in the SAN/ MWCNT nanocomposites even with 0.1 wt % CNT loading, when the composite was prepared in the presence of 40 wt % SAN beads. Moreover, it was noted that, on increasing (up to ∼70 wt %) the weight percent of SAN beads during copolymerization, the dc conductivity of the nanocomposites gradually increased although the CNT concentration remained constant. However, beyond 70 wt % loading of the SAN beads, a decrease in electrical conductivity in the nanocomposites was evident. A similar trend of decreasing electrical conductivity in the SAN/MWCNT nanocomposites was observed beyond 70

(2)

where ε0 is the permittivity of the free space. The dielectric permittivity (ε′) and dielectric loss (ε″) of the samples were determined as a function of frequency. By using eqs 1 and 2, the dielectric permittivity (ε′) is obtained as shown in eq 3. Using eq 4, we get the value of the dielectric loss (ε″) from the dielectric permittivity (ε′) in eq 3. ε′ =

Cpd ε0A

ε″ = ε′ tan δ

(3) (4)

The total complex permittivity or the complex dielectric constant (ε*) can be represented as ε*(ω) = ε′(ω) − iε″(ω)

(6)

(5) 2860



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(30/70 w/w) in situ copolymerized SAN/SAN bead nanocomposites with 0.1 wt % CNT loading, theoretically, assuming all the CNTs are located only in the 30 wt % in situ copolymerized SAN region, the effective concentration of the CNTs in the in situ copolymerized SAN phase would become ∼0.33 wt %. Thus, the conductivity of the nanocomposites with 0.1 wt % MWCNT loading should be in fact comparable to the nanocomposites with 0.33 wt % CNT loading. To explore this, we measured the conductivity of in situ copolymerized SAN/ MWCNT nanocomposites with 0.33 wt % CNT loading, without adding any SAN beads during copolymerization. Interestingly, the conductivity of the in situ copolymerized SAN/MWCNT (0.33 wt %) nanocomposites was 3.27 × 10−5 S·cm−1, which was relatively lower than the conductivity value (8.06 × 10−5 S·cm−1) of the SAN/MWCNT (0.1 wt %) nanocomposites containing 70 wt % SAN beads. Furthermore, the presence of 40 wt % SAN beads in the nanocomposites with 0.1 wt % MWCNT loading should be expected to give rise to an electrical conductivity similar to that of the nanocomposites with 0.166 wt % MWCNTs in SAN matrix without any SAN beads, since all the CNTs are dispersed selectively in the 60 wt % in situ polymerized SAN matrix. However, the electrical conductivity of the in situ polymerized SAN/MWCNT (0.166 wt %) nanocomposites without any SAN beads revealed a conductivity similar to that of the pure SAN. Thus, this clearly indicates that the effective concentration of the CNTs in the in situ polymerized SAN region of SAN/MWCNT (0.1 wt %) nanocomposites with 40 wt % SAN beads is considerably higher than that of the theoretical value. To explain this, we assume that addition of SAN beads after 45 min during copolymerization of styrene and acrylonitrile in the presence of MWCNTs resulted in partial swelling of the SAN beads by the unreacted monomers. Thus penetration and polymerization of a certain amount of unreacted monomers inside the SAN beads resulted in reducing the actual amount of in situ polymerized SAN region in the nanocomposites, which increased the effective concentration of the MWCNTs in the in situ region. Thus, the conductivity of the in situ polymerized SAN/MWCNT nanocomposites remarkably increased in the presence of 40 wt % SAN beads even at 0.1 wt % MWCNT loading. This assumption is also supported by the increased conductivity value of the nanocomposites with increasing bead loading (Figure 2a) at constant CNT loading, due to swelling of more SAN beads by the unreacted monomers. If this assumption is true, then at a particular bead loading one should expect a higher electrical conductivity in SAN/ MWCNT (0.1 wt %) nanocomposites with micrometer-size SAN beads (SAN-s) than that with large SAN beads due to the swelling of more SAN beads (high surface area of SAN-s beads exposed to the monomers). To investigate this, we formulated the nanocomposites with relatively small size SAN beads (SANs) prepared by the solvent agitation method. Commercial SAN beads were dissolved in dichloromethane and the solution was dropwise added into aqueous poly(vinyl alcohol) (3 wt %) solution under constant stirring at ∼70 °C. Finally, after solvent evaporation and filtration, smaller SAN beads (SAN-s; diameter of beads 80−120 μm) were obtained. It was noteworthy (Figure 2b) that, at particular CNT and SAN bead loadings, the electrical conductivity of the SAN/MWCNT nanocomposites with SAN-s beads was relatively higher than that with the millimeter-size commercial SAN beads.

Figure 2. dc conductivity of SAN/MWCNT nanocomposites with weight percent (a) SAN beads and (b) micrometer-size SAN-s beads, in SAN matrix at various MWCNT loadings, and (c) conductivity of SAN/MWCNT (0.1 wt %) nanocomposites with variation of SAN beads added at two different times during copolymerization.

wt % SAN beads in the nanocomposites for all concentrations (0.1, 0.3, and 0.5 wt %) of CNTs. We assume that the presence of SAN beads in the matrix phase increased the effective concentration of the CNTs in the in situ copolymerized SAN phase. The SAN beads in the matrix phase of SAN/MWCNT nanocomposites can be regarded as excluded volume in which the CNTs cannot penetrate. Thus, in 2861



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As can be seen (Figure 2b), the dc conductivity of the nanocomposites with 25 wt % SAN-s beads was ≈1.94 × 10−6 S·cm−1at 0.1 wt % CNT loading, which was almost comparable to the conductivity value (≈1.38 × 10−6 S·cm−1) of the SAN/ MWCNT nanocomposites with 40 wt % commercial SAN beads at a similar CNT loading. Moreover, at 0.1 wt % CNT loading, the dc conductivity of SAN/MWCNT nanocomposites with 40 wt % SAN-s beads was 3.16 × 10−5 S·cm−1, which is significantly higher than that (≈1.38 × 10−6 S·cm−1) with 40 wt % commercial SAN beads in SAN/MWCNT nanocomposites. With increasing number of SAN beads (i.e., the case with SANs), more CNT networks (or branching) may also form throughout the composites due to decreased in situ polymerized SAN region. Thus, establishment of more continuous paths through the in situ polymerized SAN phase may lead to higher electrical conductivity for similar loading of smaller beads into the composites. Now one can expect that addition of the SAN beads at a later stage of polymerization (when almost all the monomers are converted to oligomer) may result in a similar conductivity value of the nanocomposites at constant MWCNT loading and SAN bead content, irrespective of the bead size (assuming insignificant swelling of SAN beads by the oligomers). To check this, we studied the conductivity value of the SAN/ MWCNT (0.1 wt %) nanocomposites with 40 wt % SAN and SAN-s beads, prepared by adding the SAN beads after 90 min of the copolymerization. It was noteworthy, irrespective of the bead size, that both nanocomposites revealed almost similar conductivity values at constant CNT and bead loading (Figure 2c). Moreover, conductivity values of these nanocomposites were significantly lower that those obtained for the nanocomposites with similar compositions (Figure 2a,b) when SAN beads were added after 45 min of the polymerization reaction. However, the nanocomposites with SAN-s beads showed a decreasing trend in dc conductivity beyond 50 wt % SAN-s bead loading at constant CNT loadings (0.1, 0.3, and 0.5 wt %). This trend of decreasing conductivity could be due to the blockage in continuous conducting paths as a result of increase in nonconducting SAN bead content in the nanocomposites. At a particular bead content, the number of beads and, hence, the surface area (or extruded volume) for SAN-s beads is much higher compared to those for commercial SAN beads. Thus, at higher bead content (above 50 wt %), SAN beads may lead to the formation of a continuous phase of the SAN-s beads due to fusing of the molten SAN-s beads during compression molding. This resulted in decreasing the conductivity of the nanocomposites above 50 wt % SAN-s beads. The change in dc conductivity of SAN/MWCNT nanocomposites in the presence of 70 wt % SAN beads with increasing MWCNT content is shown in Figure 3a. A drastic increase in conductivity by several orders of magnitude, from 10−11 to 10−4 S·cm−1, can be observed by varying the CNT content between 0.02 and 0.3 wt %. This radical increase in content increases the electrical conductivity, indicating the formation of a continuous network structure of MWCNTs in the nanocomposites, which is well-known as a percolation network. Thus, the percolation threshold of the MWCNTs in the nanocomposites was found between 0.02 and 0.3 wt % CNT loading. On further addition of CNTs, a slow but gradual increase in conductivity of the nanocomposites was observed. The conductivity of the SAN/MWCNT nanocomposites with 70 wt % SAN beads was measured to be 7.05 × 10−3 S·cm−1 at 1 wt % CNT loading. In a hybrid material where the matrix is

Figure 3. (a) dc conductivity of SAN/MWCNT nanocomposites with MWCNT loading at constant 70 wt % SAN bead loading. Inset: log− log plot for σDC vs (p − pc) for the same nanocomposites. The straight line in the inset is a least-squares fit to the data using eq 4 returning the best fit values pc = 0.032% and t = 2.15. (b) Linear variation of σDC vs P−1/3.

an insulator with conductive inclusions, as is the case in the polymer/CNT nanocomposites, several results predicted for a percolating system can be applied by adding the variation of the dc conductivity with the concentration of the nanofiller. The dependence of the dc conductivity (σDC) on the weight concentration (p) of the nanofiller and the percolation threshold concentration (pc) can be written as a scaling (power) law equation (eqs 7 and 8). To estimate the pc, experimental data were fitted using a scaling law for the nanocomposite conductivity near the percolation threshold.39,40 σDC ∝ (p − pc )t σDC ∝ (p − pc )−s

for for

p > pc p < pc

(7) (8)

where σDC is the dc conductivity of the nanocomposites; s and t are the critical exponents. As shown in Figure 3 (inset), the straight line fit by considering the value of pc = 0.032 wt % (which gave the least error when pc was varied from 0.02 to 0.1). The critical exponent (t) was calculated from the slope and was found to be t ≈ 2.15 with a standard deviation of ±0.103, and the y intercept point is −2.001 for the straight line of the log σDC vs 2862



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log(p − pc) plot in Figure 3 (inset). These values gave an excellent fit with the conductivity of SAN/MWCNT nanocomposites. This result is in agreement with the percolation behavior given in eq 7 and indicates an extremely low percolation threshold at 0.032 wt % MWCNT loading in the SAN matrix. The value t ≈ 2.15 for the CNT nanocomposites is consistent with the theoretical value of t ≈ 2.0 for a percolation network in three dimensions.41,42 The value of t (≈2.15) produced in this work is in good agreement with the theoretical and the experimental data in the literature. The threedimensional percolation theory suggests that values of the critical exponent (t) lower than 2.0 indicate that the percolation take place in a network displaying more “dead arms”. The increase of the critical exponent (t) could be associated with the reduction in the number of “dead arms” present. Many researchers found different critical exponent (t) values for the same polymer applying different processes. Kota et al.43 found critical exponent values of 1.5 and 1.9 for solution blended PS/ MWCNT nanocomposites prepared by using dimethylformamide (DMF) and tetrahydrofuran (THF) as solvents, respectively. Blighe et al.44 experimentally found t ≈ 2.2 ± 0.2 for PS/SWCNT nanocomposites. The values reported for the critical exponent in other polymer/CNT systems show a great dispersion: t = 1.8 for epoxy/MWCNT;45 t = 2.1 for polycarbonate (PC)/MWCNT;46 t = 2.3 for poly(methyl methacrylate) (PMMA)/MWCNT;47 t = 1.6 for polyimide (PI)/MWCNT.48 The different values of the critical exponent (t) depend on various parameters such as the length and diameter of CNTs, carbon purity, structure, type of polymer, and dispersion method. This very low percolation threshold is the indication of excellent dispersion of high aspect ratio MWCNTs in the nanocomposites. A general relationship between the percolation threshold and aspect ratio of the filler for the system in which filler is randomly distributed and assumed to be sticks of length (L) and diameter (D) has been proposed by Balberg et al.49 as L /D = 3/PC

of a thin coating of polymer over the CNTs during the bulk copolymerization of styrene and acrylonitrile. This increases contact resistance, and electron tunneling occurs at the ends of the MWCNTs. The calculated t value does not indicate whether the MWCNTs are coated with a polymer layer or not. However, a higher t value indicates that the thickness of any coating layer between the MWCNTs or the bundles is uniform.53 Good anchoring between polymer and CNT allows a uniform dispersion but also reduces direct contacts of MWCNT−MWCNT. These results lead to a relatively low increment of conductivity above 0.5% CNTs and the low extrapolated value at p → 100. It generates uniform resistance and tunnel barriers between two MWCNTs. The existence of tunneling conduction in polymer−CNT nanocomposites has already been reported in the literature.54,55 The matrix properties affect the characteristics of the energy barrier, the polymer properties, and the fabrication process used. With increasing of the barrier gap, the conductivity of the nanocomposites decreases rapidly. The tunneling assisted conductivity is express as σDC ∝ exp( −Ad)

(10)

where A and d represent the tunnel parameter and tunnel distance, respectively. In case the particles are randomly distributed, then the mean average distance (d) among particles can be assumed to be proportional to P−1/3. Thus, eq 10 can be written as log(σDC) ∝ P−1/3

(11)

In an insulator matrix such as SAN, a tunneling conductive mechanism is expected to occur in the nanocomposites and the dependence on σDC and P−1/3, given in eq 11, is represented in Figure 3b. The linear fitted line shows the existence of a conventional tunneling conduction mechanism. However, very low percolation is difficult to explain because the exact morphology of MWCNT bundles is not known. A high resolution FESEM image (Figure 7c) revealed that MWCNTs dispersed in the polymer matrix uniformly on a nanometer scale, which clearly shows flexible MWCNTs running out of the polymer matrix. All of the visible MWCNTs are seen to have high aspect ratio and physical entanglements. Consequently, it can be safely assumed that the nanocomposites are macroscopically uniform with randomly distributed MWCNTs. There are several factors influencing the percolation threshold pc, which might include some or all of the following: (i) A high aspect ratio of the MWCNT bundles influences the reduction of the pc value. Unfortunately, the mean size and distribution of the aspect ratios of the experimentally observed bundles are not known. (ii) Different lengths and an irregular packing of the nanotubes give a higher structure factor than a smooth stick, enhancing entanglements of CNTs with neighboring tubes. Higher structure factors lead to the lower pc values. (iii) MWCNT bundles have very high surface area. This leads to a greater probability of conductive tunneling between the bundles, and it leads to a lower pc value. (iv) The MWCNT bundles and MWCNTs which extend out beyond a surface of bundles are very flexible; this can make physical entanglements with neighboring tubes and small attractive forces that hold contacting neighboring MWCNTs together. Then pc will be markedly lowered. These contacts can

(9)

where PC is the critical concentration and L/D is the aspect ratio of the filler. This may give an idea for the average aspect ratio of the CNT bundles. These bundles may be made by the aggregation of the individual CNTs, and its size thus indicates the dispersion of nanofiller. If we assume the average length of an MWCNT is 1.3 μm and the diameter is 9.5 nm (i.e., data given by MWCNT supplier), we get L/D ≈ 150 and PC ≈ 0.02, but from practical data we found PC ≈ 0.032, which means the aspect ratio of the bundles is estimated as ≈93.75 in our case. This analysis is complicated by the fact that MWCNTs contain an unknown number of tubes which may result in the bundles being longer than 1.3 μm and having a diameter greater than 9.5 nm. This observation confirms that the conduction in our nanocomposites is mainly due to the CNT bundles. However, the high aspect ratio (i.e., >95) of the MWCNT bundles makes the percolation possible with a very small content of CNTs. The extrapolation of the value of p → 100 gives a conductivity value of 9.97 × 10−3 S·cm−1, which is far lower than the conductivity expected for pure MWCNTs, as previously reported; for example, SWCNT “bucky paper” gives a dc conductivity in the range 102−103 S/cm.50 In other studies,51,52 a very high conductivity of about 50 S·cm−1 was reported for unaligned CNT pellets. Decrease in electrical conductivity of the CNTs in SAN/ MWCNT nanocomposites can be explained by the formation 2863



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be either between the isolated MWCNTs or the MWCNT bundle surfaces or combinations of these, and the contacts can be with or without a tunneling barrier. While (i) appears to be the major factor, it is not by itself sufficient, and therefore some or all of the other three factors also play an important role in lowering the pc value. In the SAN/MWCNT nanocomposites under investigation, interCNT spacing is very low because of high effective loading of CNTs by incorporating the SAN beads. In the range of CNT loading from 0.02 to 0.3 wt %, the conductivity value of the SAN/MWCNT nanocomposites was drastically increased by several orders of magnitude (from 10−11 to 10−4 S·cm−1). At the CNT loading from 0.5 to 1 wt %, the conductivity of the nanocomposites stabilized around 10−3 S·cm−1, which is, to the best to our knowledge, the highest value of conductivity ever reported for SAN/MWCNT nanocomposites at this low level of CNT loading with unaligned, unmodified, commercially available MWCNTs of similar qualities (carbon purity, aspect ratio, etc.). At room temperature, the relationship between ac conductivity and frequency for the nanocomposites containing 0.1 wt % MWCNT loading with varying weight percent of SAN beads is shown in Figure 4a. It is observed that the conductivity of pure polymer increases with increasing frequency. This is similar to the common tendency of insulating materials. At constant CNT loading, an increase in the weight percent of SAN beads results in a gradual increase in the conductivity of the nanocomposites. This increment in weight percent of SAN beads results in the CNT containing part of the nanocomposite (i.e., bulk polymerized SAN phase) getting more concise in a smaller area, which increases the continuous network structure of the CNTs. The ac conductivity remained almost constant, showing a plateau region up to a critical frequency (fc). Beyond this frequency (fc), all the nanocomposites showed an increase in conductivity with further increase in the frequency. With increase in the weight percent of SAN beads, the critical frequency was switched to a higher value. This is because of the increase in the effective concentration of MWCNTs in the in situ polymerized SAN phase with increasing weight percent of SAN beads that led to the formation of a greater amount of network structure. Similar results were also reported in previous reports56,57 where with increase in conducting filler loading the critical frequency shifted to a higher frequency region. The frequency-dependent ac conductivity of the nanocomposites with variation of CNT loading, keeping the SAN bead content (70 wt %) constant, is shown in Figure 4b. In comparison to pure SAN, a higher conductivity is seen in the nanocomposites with increasing weight percent of CNT loading. Similar to the pure SAN, the conductivity of the SAN/MWCNT (0.1 wt %) nanocomposites was increased with the frequency. In the low frequency region, a drastic increase in conductivity was observed on 0.1 wt % CNT loading into SAN matrix. Further increase in CNT loading beyond 0.1 wt % results in a very small improvement in conductivity in the nanocomposites. At higher CNT loading (≥0.5 wt %), the conductivity of the nanocomposites was constant throughout the entire frequency range. This is the behavior of a highly conducting nanocomposite where the flow of current is due to the electron transfer, and not due to the polarization. The variation of the dielectric constant of SAN/MWCNT (0.1 wt %) nanocomposites with frequency at room temperature with increasing weight percent of SAN beads is shown in Figure 5a, and the variation of the dielectric constant of the

Figure 4. ac conductivity of SAN/MWCNT nanocomposites versus frequency at (a) varying weight percent of SAN beads at 0.1 wt % CNT loading and (b) different MWCNT loadings at constant SAN bead content.

nanocomposites at constant SAN bead loading (70 wt %) with increasing weight percent MWCNT from 0.05 to 1 wt % is shown in Figure 5b. In both cases, a decrease in the dielectric constant with increasing frequency can be seen for all the nanocomposites. A higher value of the dielectric constant is due to the presence of many types of polarizations at low frequency. The value of the relative dielectric constant decreases with increase in frequency, which can be attributed to the dominant nature of electronic polarization over others at higher frequencies. The dielectric constant of neat SAN has a low value and behaves almost independent of frequency. At low frequencies (Figure 5a), the dielectric constant of the nanocomposites significantly increased with increasing amount of SAN beads. At higher frequency a relaxation process took place that led to a drastic drop in the dielectric constant with increase in frequency. A similar result of the dielectric response was found by Potschke et al.,58 where the dielectric constant decreased with increasing frequency in elastomer/carbon black composites above the percolation threshold. We can conclude that this behavior is probably due to an increase in the effective loading of CNTs which creates more network structures in the nanocomposites. 2864



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Figure 6. Dielectric loss tangent (tan δ) of SAN/MWCNT nanocomposites (a) with variation of SAN beads at constant CNT loading and (b) with variation of CNTs at constant SAN bead loading.

Figure 5. Dielectric constant of SAN/MWCNT nanocomposites (a) with varying SAN bead loading at constant CNT loading and (b) with variation of CNT loading at constant PMMA bead loading.

marginal effect was found in the higher frequency region. It has already been reported that, with increase in filler loading, dielectric loss increases in the lower frequency region.59 Here, the effective loading of CNTs was increased by increasing the SAN bead loading in the nanocomposites. This result is similar to the tan δ values shown in Figure 6b, which represent the variation in tan δ with frequency at room temperature with increasing CNT loading. 4.2. Morphology. The key to the results of high electrical conductivity obtained lies in the morphology of the nanocomposites formed. Figure 7a−e presents the micrographs of SAN/MWCNT nanocomposites with 0.1 wt % CNT loading in the presence of 70 wt % SAN beads, and Figure 7f presents the micrograph of nanocomposites with 0.1 wt % CNT in the presence of 50 wt % SAN-s beads. Optical microscopy (Figure 7a) showed the presence of two regions: one was transparent and the other was opaque. On closer observation with the FESEM (Figure 7b), we found the CNTs to be selectively concentrated in one region while in the remaining parts we could observe only sparse presence of CNTs. On logical analysis of the above results, we postulate that the CNT-rich region is the bulk copolymerized SAN phase, while the other region, which appears transparent under an optical microscope and CNT devoid in the FESEM, is the SAN beads. This

Figure 5b, which shows the increase in dielectric constant of the SAN/MWCNT nanocomposites with increasing content of CNTs, revealed very high values of the dielectric constant (ε′ > 104) at low frequencies for the nanocomposites with MWCNTs ≥ 0.3 wt %. This may be due to the presence of small gaps between the conducting CNT networks, with various dead ends forming capacitors of various length scales. The charge between polymer and CNTs increased in the dead ends due to their conductivity differences. Thus the overall capacitance was increased, which in turn increased the dielectric constant of the nanocomposites. Figure 6 shows the room temperature variation in the dissipation factor (tan δ) with frequency for the nanocomposites with variation of SAN beads and CNT content. In both cases, all the nanocomposites showed a decrease in tan δ value with increase in frequency and became almost constant at 1 MHz. This behavior may be attributed to the dipole relaxation phenomena, where movement of the electric dipoles was not able to be in line with the frequency of the applied electric field. It can be seen from Figure 6a that there is an increase in the dielectric loss tangent with the increase in SAN beads at constant CNT loading. In the lower frequency region, tan δ increased with the increase in SAN bead loading, while a 2865



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however, it also reveals the penetration of a small amount of CNTs in the SAN bead region which can be attributed to the diffusion of CNTs during the molding process. The extent of dispersion of the CNTs in the selective regions (marked in Figure 7b) of the nanocomposites is shown in the FESEM image at higher magnification (Figure 7c). The formation of highly networked structured at extremely low CNT loading arising due to high dispersion of MWCNT can be seen in this high magnification image. Parts d and e of Figure 7 represent TEM images of the nanocomposites at low and high magnifications, respectively. Figure 7d shows the presence of CNTs in selective regions of the nanocomposites, supportive of the optical and FESEM images. Figure 7e confirms the good dispersion and distributed CNTs forming a continuous network structure. The high magnification TEM image also shows a high individualization of the CNTs as seen in FESEM images. However, the millimeter scale dimension of SAN beads made it impossible to completely capture the beads in a single image; hence a portion of the beads that could be captured has been presented. Figure 7f shows the FESEM image of cryofractured as-formed nanocomposites consisting of 50 wt % SAN-s beads. The inset in Figure 7f represents a high individualization and formation of a highly network structure of CNTs mostly in the in situ copolymerized SAN region. The distribution of the filler particles in the nanocomposites was also verified by tapping mode AFM analysis. Figure 8

Figure 7. (a) Transmitted optical micrograph, (b) low magnification and (c) high magnification FESEM micrographs, (d) low magnification and (e) high magnification TEM micrographs of 0.1 wt % MWCNT 70 wt % SAN bead containing nanocomposites, and (f) FESEM micrograph of 25 wt % SAN-s beads/0.1 wt % MWCNT nanocomposites.

Figure 8. AFM 3D phase diagrams of 70 wt % SAN bead containing composite (a) without MWCNTs and (b) with 0.1 wt % MWCNTs.

demonstrates AFM three-dimensional (3D) phase images of the cryofractured surface of 70 wt % SAN bead composites without MWCNTs and the same with 0.1 wt % MWCNT loading. Figure 8a resembles a smooth surface throughout the matrix phase in the absence of MWCNTs. On the contrary, the presence of MWCNTs in the 70 wt % SAN bead loaded nanocomposites shows a comparatively rougher surface at selective regions of the fractured surface. These AFM images are representative of the fact that the MWCNTs are selectively distributed in the in situ copolymerized SAN phase, excluding the high molecular weight bead portions.

proposition is reasonable considering the fact that, when the SAN beads were added to the reaction mixture, the polymerization had already progressed to the oligomeric stage. This means that it would be difficult for the SAN beads to dissolve as there was not sufficient monomer available. The SAN beads could at most only swell to a certain extent which was evident during the reaction process. Thus it can be concluded that in the nanocomposites there exist two phases: one containing the bulk copolymerized SAN along with the CNTs and the other consisting of the SAN beads with a discernible amount of CNTs lining the surface. It can be inferred from the morphology that the CNTs form a continuous network structure in the in situ copolymerized SAN regions, leaving the SAN beads free of any CNT dispersion. The SAN beads behave as an “excluded volume” having no penetration of the CNTs, leading to an increased overall effective concentration considering the whole nanocomposites. This facilitates the probability of formation of more conducting paths in the nanocomposites. Figure 7b shows the FESEM image of cryofractured compression molded SAN/MWCNT nanocomposite at low magnification. The morphology shows the presence of CNTs in certain selective regions, consistent with optical microscopy;

5. CONCLUSIONS In this article, we report an easy method of preparing SAN/ MWCNT nanocomposites through selective dispersions of MWCNTs in the SAN matrix that shows an extremely lower percolation threshold of MWCNTs than ever reported. This ultralow percolation threshold was found by using unaligned and unmodified MWCNTs, making this process easy, economical, and industrially applicable. Introduction of commercial SAN beads during copolymerization of the monomers acted as “excluded volume” and forced the MWCNTs to be concise in the small region of in situ 2866



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(15) Shrivastava, N. K.; Khatua, B. B. Development of electrical conductivity with minimum possible percolation threshold in multiwall carbon nanotube/polystyrene composites. Carbon 2011, 49, 4571. (16) Mitchell, C. A.; Bahr, J. L.; Arepalli, S.; Tour, J. M.; Krishnamoorti, R. Dispersion of Functionalized Carbon Nanotubes in Polystyrene. Macromolecules 2002, 35, 8825. (17) Bahr, J. L.; Tour, J. M. Covalent chemistry of single-wall carbon nanotubes. J. Mater. Chem. 2002, 12, 1952. (18) Dyke, C. A.; Tour, J. M. Solvent-Free Functionalization of Carbon Nanotubes. J. Am. Chem. Soc. 2003, 125, 1156. (19) Jin, Z.; Pramoda, K. P.; Xu, G.; Goh, S. H. Dynamic mechanical behavior of melt-processed multi-walled carbon nanotube/poly(methyl methacrylate) composites. Chem. Phys. Lett. 2001, 43, 43. (20) Ayewah, D. O. O.; Davis, D. C.; Krishnamoorti, R.; Lagoudas, D. C.; Sue, H. J.; Willson, M. A surfactant dispersed SWCNTpolystyrene composite characterized for electrical and mechanical properties. Composites, Part A 2010, 41, 842. (21) McNally, T.; Pötschke, P.; Halley, P.; Murphy, M.; Martin, D.; Bell, S. E. J.; Brennan, G. P.; Bein, D.; Lemoine, P.; Quinn, J. P. Polyethylene multiwalled carbon nanotube composites. Polymer 2005, 46, 8222. (22) Potschke, P.; Goad, M. A.; Alig, I.; Dudkin, S.; Lellinger, D. Rheological and dielectrical characterization of melt mixed polycarbonate-multiwalled carbon nanotube composites. Polymer 2004, 45, 8863. (23) Manchado, M. A. L.; Valentini, L.; Biagiotti, J.; Kenny, J. M. Thermal and mechanical properties of single-walled carbon nanotubespolypropylene composites prepared by melt processing. Carbon 2005, 43, 1499. (24) Wanjale, S. D.; Jog, J. P. Crystallization and phase transformation kinetics of poly(1-butene)/MWCNT nanocomposites. Polymer 2006, 47, 6414. (25) Lee, M.; Jeon, H.; Min, B. H.; Kim, J. H. Morphology and Electrical Properties of Poly methylmethacrylate/Poly(styrene-coacrylonitrile)/Multi-Walled Carbon Nanotube Nanocomposites. J. Appl. Polym. Sci. 2011, 121, 743. (26) Gültner, M.; Göldel, A.; Pötschke, P. Tuning the localization of functionalized MWCNTs in SAN/PC blends by a reactive component. Compos. Sci. Technol. 2011, 72, 41. (27) Göldel, A.; Kasaliwal, G. R.; Pötschke, P.; Heinrich, G. The kinetics of CNT transfer between immiscible blend phases during melt mixing. Polymer 2012, 53, 411. (28) Göldel, A.; Kasaliwal, G.; Potschke, P. Selective localization and migration of multiwalled carbon nanotubes in blends of polycarbonate and poly(styrene-acrylonitrile). Macromol. Rapid Commun. 2009, 30, 423. (29) Kim, K. T.; Jo, W. H. Noncovalent functionalization of multiwalled carbon nanotubes using graft copolymer with naphthalene and its application as a reinforcing filler for poly(styrene-coacrylonitrile). J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4184. (30) Dufresne, A.; Paillet, M.; Putaux, J. L.; Canet, R.; Carmona, F.; Delhaes, P. Processing and characterization of carbon nanotube/ poly(styrene-co-butyl acrylate) nanocomposites. J. Mater. Sci. 2002, 37, 3915. (31) Singh, B. K.; Kar, P.; Shrivastava, N. K.; Banerjee, S.; Khatua, B. B. Electrical and mechanical properties of acrylonitrile-butadienestyrene/multiwall carbon nanotube nanocomposites prepared by meltblending. J. Appl. Polym. Sci. 2012, 124, 3165. (32) Zhao, Z.; Zheng, W.; Yu, W.; Long, B. Electrical conductivity of poly(vinylidene fluoride)/carbon nanotube composites with a spherical substructure. Carbon 2009, 47, 2112. (33) Etika, K. C.; Liu, L.; Hess, L. A.; Grunlan, J. C. The influence of synergistic stabilization of carbon black and clay on the electrical and mechanical properties of epoxy composites. Carbon 2009, 47, 3128. (34) Mua, M.; Walker, A. M.; Torkelson, J. M.; Winey, K. I. Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes. Polymer 2008, 49, 1332.

copolymerized SAN phase of the nanocomposites. Thus, the effective concentration of MWCNTs eventually increased in the nanocomposites, resulting in an unprecedented shifting in the percolation threshold to a lower value of 0.032 wt % MWCNT. Theoretical studies revealed that the conduction mechanism in the nanocomposites is not only via direct contact between the individual MWCNTs, rather than electron tunneling between the neighborhood MWCNTs, suggesting the polymer coating of individual MWCNTs during the bulk polymerization process. Furthermore, more precise control of bead size, bead adding time during copolymerization, and bead concentration may lead to a phenomenal shift of the percolation threshold to an extremely lower value.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 3222 283982. Fax: +91 3222 255303. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Connor, M. T.; Roy, S.; Ezquerra, T. A.; Calleja, F. J. B. Broadband AC conductivity of conductor-polymer composites. Phys. Rev. B 1998, 57, 2286. (2) Foulger, S. H. Electrical properties of composites in the vicinity of the percolation threshold. J. Appl. Polym. Sci. 1999, 72, 1573. (3) Flandin, F.; Prasse, T.; Schueler, R.; Schulte, K.; Bauhofer, W.; Cavaille, J. Y. Anomalous percolation transition in carbon black-epoxy composite materials. Phys. Rev. B 1999, 59, 14349. (4) Alexander, M. G. Anomalous temperature dependence of the electrical conductivity of carbon-poly(methyl methacrylate) composites. Mater. Res. Bull. 1999, 34, 603. (5) Zois, H.; Apekis, L.; Omastova, M. Electrical properties of carbon black-filled polymer composites. Macromol. Symp. 2001, 170, 249. (6) Jager, K. M.; McQueen, D. H. Fractal agglomerates and electrical conductivity in carbon black polymer composites. Polymer 2001, 42, 9575. (7) Sahoo, B. P.; Naskar, K.; Tripathy, D. K. Conductive carbon black-filled ethylene acrylic elastomer vulcanizates: physico-mechanical, thermal, and electrical properties. J. Mater. Sci. 2012, 47, 2421. (8) Lozano, K.; Rios, J. B.; Barrera, E. V. A study on nanofiberreinforced thermoplastic composites (II): Investigation of the mixing rheology and conduction properties. J. Appl. Polym. Sci. 2001, 80, 1162. (9) Kar, P.; Khatua, B. B. Highly reversible and repeatable PTCR characteristics of PMMA/Ag-coated glass bead composites based on CTE mismatch phenomena. Polym. Eng. Sci. 2011, 51, 1780. (10) Kar, P.; Khatua, B. B. PTCR characteristics of polycarbonate/ nickel-coated graphite-based conducting polymeric composites in presence of poly(caprolactone). Polym. Compos. 2011, 32, 747. (11) Kar, P.; Shrivastava, N. K.; Mallick, S.; Khatua, B. B. PTCR characteristics of poly(styrene-co-acrylonitrile) copolymer/stainless steel powder composites. J. Appl. Polym. Sci. 2012, 124, 607. (12) Cassignol, C.; Cavarero, M.; Boudet, A.; Ricard, A. Microstructure-conductivity relationship in conducting polypyrrole/epoxy composites. Polymer 1999, 40, 1139. (13) Dutta, P.; Biswas, S.; Ghosh, M.; De, S. K.; Chatterjee, S. The DC and AC conductivity of polyaniline-polyvinyl alcohol blends. Synth. Met. 2001, 122, 455. (14) Bhattacharya, S.; Saha, S. K.; Mandal, T. K.; Mandal, B. M.; Chakravorty, D.; Goswami, K. Multiple hopping conduction in interpenetrating polymer network composites of poly pyrrole and poly (styrene-co-butyl acrylate). J. Appl. Phys. 2001, 89, 5547. 2867



Article

(35) Balogun, Y. A.; Buchanan, R. C. Enhanced percolative properties from partial solubility dispersion of filler phase in conducting polymer composites (CPCs). Compos. Sci. Technol. 2010, 70, 892. (36) Grunlan, J. C.; Gerberich, W. W.; Francis, L. F. Lowering the percolation threshold of conductive composites using particulate polymer microstructure. J. Appl. Polym. Sci. 2001, 80, 692. (37) Miriyala, S. M.; Kim, Y. S.; Liu, L.; Grunlan, J. C. Segregated networks of carbon black in poly(vinyl acetate) latex: influence of clay on the electrical and mechanical behavior. Macromol. Chem. Phys. 2008, 209, 2399. (38) Liu, L.; Grunlan, J. C. Clay assisted dispersion of carbon nanotubes in conductive epoxy nanocomposites. Adv. Funct. Mater. 2007, 17, 2343. (39) Du, F.; Scogna, R. C.; Zhou, W.; Brand, S.; Fischer, J. E.; Winey, K. I. Nanotube networks in polymer nanocomposites rheology and electrical conductivity. Macromolecules 2004, 37, 9048. (40) Stauffer, D.; Aharony, A. Introduction to Percolation Theory, 2nd ed.; Taylor & Francis: London, 1992. (41) Wu, T. M.; Chen, E. C. Preparation and characterization of conductive carbon nanotube-polystyrene nanocomposites using latex technology. Compos. Sci. Technol. 2008, 68, 2254. (42) Weber, M.; Kamal, M. R. Estimation of the volume resistivity of electrically conductive composites. Polym. Compos. 1997, 18, 711. (43) Kota, A. K.; Cipriano, B. H.; Duesterberg, M. K.; Gershon, A. L.; Powell, D.; Raghavan, S. R.; Bruck, H. A. Electrical and Rheological Percolation in Polystyrene/MWCNT Nanocomposites. Macromolecules 2007, 40, 7400. (44) Blighe, F. M.; Hernandez, Y. R.; Blau, W. J.; Coleman, J. N. Observation of Percolation-like Scaling-Far from the Percolation Threshold-in High Volume Fraction, High Conductivity PolymerNanotube Composite Films. Adv. Mater. 2007, 19, 4443. (45) Martin, C. A.; Sandler, J. K. W.; Shaffer, M. S. P.; Schwarz, M. K.; Bauhofer, W.; Schulte, K.; Windle, A. H. Formation of percolating networks in multi-wall carbon-nanotube-epoxy composites. Compos. Sci. Technol. 2004, 64, 2309. (46) Pötschke, P.; Dudkin, S. M.; Alig, I. Dielectric spectroscopy on melt processed polycarbonate-multiwalled carbon nanotube composites. Polymer 2003, 44, 5023. (47) Kim, H. M.; Choi, M. S.; Joo, J.; Cho, S. J.; Yoon, H. S. Complexity in charge transport for multiwalled carbon nanotube and poly(methyl methacrylate) composites. Phys. Rev. B 2006, 74, 54202. (48) Jiang, X. W.; Bin, Y. Z.; Matsuo, M. Electrical and mechanical properties of polyimide-carbon nanotubes composites fabricated by in situ polymerization. Polymer 2005, 46, 7418. (49) Balberg, I.; Anderson, C. H.; Alexander, S.; Wagner, N.; Wagner, N. Excluded volume and its relation to the onset of percolation. Phys. Rev. B 1984, 30, 3933. (50) Sreekumar, T. V.; Liu, T.; Kumar, S.; Ericson, L. M.; Hauge, R. H.; Smalley, R. E. Single-Wall Carbon Nanotube Films. Chem. Mater. 2003, 15, 175. (51) Fan, J.; Wan, M.; Zhu, D.; Chang, B.; Pan, Z.; Xie, S. Synthesis, Characterizations, and Physical Properties of Carbon Nanotubes Coated by Conducting Polypyrrole. J. Appl. Polym. Sci. 1999, 74, 2605. (52) Benoit, J. M.; Corraze, B.; Lefrant, S.; Blau, W. J.; Bernier, P.; Chauvet, O. Transport properties of PMMA-carbon nanotube composite. Synth. Met. 2001, 121, 1215. (53) McLachlan, D. S.; Chiteme, C.; Park, C.; Wise, K. E.; Lowther, S. E.; Lillehei, P. T.; Siochi, E. J.; Harrison, J. S. AC and DC Percolative Conductivity of Single Wall Carbon Nanotube Polymer Composites. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 3273. (54) Linares, A.; Canalda, J. C.; Cagiao, M. E.; Garcia-Gutierrez, M. C.; Nogales, A.; Martin-Gullon, I.; Vera, J.; Ezquerra, T. A. Broad-Band Electrical Conductivity of High Density Polyethylene Nanocomposites with Carbon Nanoadditives: Multiwall Carbon Nanotubes and Carbon Nanofibers. Macromolecules 2008, 41, 7090. (55) Laredo, E.; Grimau, M.; Bello, A.; Wu, D. F.; Zhang, Y. S.; Lin, D. P. AC Conductivity of Selectively Located Carbon Nanotubes in Poly(ε-caprolactone)/Polylactide Blend Nanocomposites. Biomacromolecules 2010, 11, 1339.

(56) Fan, J.; Wan, M.; Zhu, D.; Chang, B.; Pan, Z.; Xie, S. Synthesis, characterizations, and physical properties of carbon nanotubes coated by conducting polypyrrole. J. Appl. Polym. Sci. 1999, 74, 2605. (57) Barrau, S.; Demont, P.; Peigney, A.; Laurent, C.; Lacabanne, C. DC and AC Conductivity of Carbon Nanotubes-Polyepoxy Composites. Macromolecules 2003, 36, 5187. (58) Potschke, P.; Dudkin, S. M.; Alig, I. Dielectric spectroscopy on melt processed polycarbonate-multiwalled carbon nanotube composites. Polymer 2003, 44, 5023. (59) Mahapatra, S. P.; Sridhar, V.; Tripathy, D. K. Dielectric studies of conductive carbon black reinforced microcellular ethylenepropylene-diene monomer vulcanizates. J. Appl. Polym. Sci. 2007, 106, 192.

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Ultralow Electrical Percolation Threshold in Poly(styrene-co

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