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Article Cite This: ACS Omega 2018, 3, 1367−1373
Highly Conducting, Sustainable, Nanographitic Rubber Composites Katerina Kampioti,†,‡ Carolina F. Matos,§ Fernando Galembeck,∥ Christèle Jaillet,†,‡ Alain Derré,†,‡ Aldo J.G. Zarbin,§ and Alain Pénicaud*,†,‡ †
CNRS, Research Center Paul Pascal (CRPP), UPR 8641, F-33600 Pessac, France University of Bordeaux, UPR 8641, F-33600 Pessac, France § Department of Chemistry, Federal University of Paranà (UFPR), CP 19032, Centro Politecnico, CEP 81531-980 Curitiba, Paranà, Brazil ∥ Institute of Chemistry, University of Campinas (UNICAMP), CP 6154, CEP 13083-970 Campinas, São Paulo, Brazil ‡
S Supporting Information *
ABSTRACT: Environmentally friendly multifunctional rubber composites are reported. Graphitic nanocarbon (NC) deriving from cracking of biogas (methane/carbon dioxide) and natural rubber extracted directly from the Hevea brasiliensis tree are the two components of these composites produced via latex technology. While maintaining and enhancing the intrinsic thermal and mechanical characteristics of rubber, the presence of NC shows a significant improvement on the electrical response. For a 10 wt % NC content, a 1010-fold increase in conductivity has been achieved with a conductivity value of 7.5 S·m−1, placing these composites among the best obtained using other carbon fillers. In addition, the piezoresistive behavior has also been verified. These promising green composites have a potential use in a variety of applications such as sealing of electronic devices and sensors.
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INTRODUCTION Natural rubber (NR) is a biopolymer composed mainly of poly(cis-1,4-isoprene). Elastomers, such as NR, are harnessed in high-volume products such as tires, pipes, and belting.1 It is a strategic material that cannot be replaced by synthetic rubbers for a variety of applications because of its unique characteristics such as outstanding elasticity, flexibility at low temperatures, and resistance to abrasion.2 It has been used in several fields, from medicine and personal protective equipment to asphalt paving roads.3−5 The ultimate properties of NR composites depend not just on the polymers, but also on the fillers, such as nanocarbon, silica, and others.1 Carbon black (CB) is one of the materials principally utilized to reinforce polymer properties.6−8 A lot of research work has been performed to replace total or partial CB. There are several academic studies that substitute CB by harnessing allotropes from the carbon family such as carbon nanotubes (CNTs), graphene oxide, reduced graphene oxide (RGO), and carbon nanoplatelets. 7−17 However, to reinforce the properties of synthetic rubber and NR, the rubber industry still widely utilizes CB.1 Its costeffectiveness combined with its intrinsic properties makes CB the principle candidate to address rubber limitations. However, rather high loads of CB are necessary to reach optimal performances with the associated problems of color (black), cost, and eventual diminution of the matrix properties/ behavior. Therefore, a replacement candidate should be both economical and effective. Graphitic fillers have long been tested as additives for rubber.18 The graphitic nanocarbons under the © 2018 American Chemical Society
study have two important peculiarities: (i) it originates from methane, that is, potentially from food waste and thus would contribute to more sustainable materials and (ii) it has a narrow size distribution around 50 nm lateral size and 5 nm thickness. Food waste can be transformed into high-value graphitic nanocarbon (NC) and renewable hydrogen.19 Methane/carbon dioxide mix, originating from anaerobic digestion of food waste, is split into NC and hydrogen via a microwave plasma process. The resulting graphitic NC needs to be assessed as a valid candidate to substitute the petroleum-based carbon additives (CB) in polymer composites.19 In this work, homogeneous multifunctional composites of NR and graphitic NCs were produced, combining synergistically the properties of the two materials, thereby extending their application possibilities while maintaining their low cost and sustainability. The resulting composites gain 11 orders of magnitude conductivity with only 10 wt % NC loading. Furthermore, they show a good piezoelectric behavior and might be used to monitor mechanical deformation.12
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RESULTS AND DISCUSSION Characterization of the NC in Dispersions and of the NC/NR Composites. Previous analysis of the NCs and their dispersions had showed the NC particles to be multilayer Received: November 24, 2017 Accepted: December 15, 2017 Published: February 1, 2018 1367
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microscopy (TEM). Figure 3 illustrates the images of the neat polymer, a representative composite (with 5 wt % NC),
graphene particles, in turbostratic packing (uncorrelated layers) with narrow lateral size distribution around 60 and 3 nm average thickness (ca. nine graphene layers). Brunauer− Emmett−Teller method shows a specific surface area of 153 m2·g−1.19 Raman spectra of pure NR, a representative composite, purified NC powder, and a dried NC dispersion are shown in Figure 1.
Figure 1. Raman spectra of pure NR, a representative composite (7.5 wt % NC/NR), dried NC dispersion, and initial purified NC powder.
The powder exhibits the G, D, and 2D band characteristics of its graphitic nature. Detailed Raman analysis of the NC powder has been performed in two previous studies.19,20 In the spectrum of the dried NC dispersion, a slight shift of D, G, and 2D bands of 2−3 cm−1 is revealed. This shift could occur due to the surfactant that surrounds the particles. The Raman spectrum of the rubber composite shows both of the bands derived from the NC and NR. Further evidence of the presence of both NR and NC in the composites was acquired by performing X-ray diffraction (XRD) analysis. Diffractograms of the pure polymer, composites, dry NC dispersion, NC powder, and surfactants were measured, as shown in Figure 2.
Figure 3. TEM images of NR, 5 wt % NC/NR, and 10 wt % NC/NR. Inset: TEM image of 10 wt % NC/NR in higher magnification.
and the composite with the highest amount of filler (10 wt % NC) at two different magnifications (TEM images of all composites are in the Supporting Information). The presence and absence of the NC particles are obvious from the images of the composites and pure rubber, respectively. The TEM image at higher resolution provides evidence of the presence of fully exfoliated graphene sheets in the matrix (insetgraphene layers appear almost transparent). The difference in the filler concentration results in the formation of a network, most prominent for 10 wt % NC/ NR composite. These morphological differences influence the composites properties as discussed below. Further characterization of the composites was provided via peak force atomic force microscopy (AFM). In Figure 4, the images of the 10 wt % NC/NR composite are depicted. The AFM topography (Figure 4a) gives an indication of the roughness of the material. Figure 4b,c shows, respectively, the difference in Young’s modulus and the adhesion between the soft material (NR) and the harder one, the NC fillers. The lighter parts in Figure 4b represent materials with higher mechanical response whereas the darker parts are of lower modulus. In contrast, the lighter parts in Figure 4c indicate higher adhesion with the tip, and the darker areas indicate the part where there is less tip−sample interaction. Peak force modulus image of 10 wt % NC/NR composite is shown in Figure 4b. The polymer matrix presents a low modulus structure (dark-colored areas). Many bright-colored areas
Figure 2. XRD spectra of pure NR, composites, dried NC dispersion, and initial purified NC powder.
NR shows a characteristic halo of amorphous materials with a maximum at around 18.5°. All of the composites show the same halo-shaped spectrum. However, as the filler concentration increases, the peak that derives from NC (ca. 25.6°) becomes obvious, and its intensity increases as a function of the amount of additive. The presence of the surfactant peaks (18.99° and 23.13°) in the spectra of the composites with higher carbon concentrations (7.5 and 10 wt %) is noticeable. The morphology and the state of dispersion of the NC particles in the rubber were assessed by transmission electron 1368
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Figure 5. TGA thermograms of neat NR and the NC/NR composites.
range; hence, the inclusion of NCs neither perturbs nor improves the thermal stability of the NR. At around 360 °C, a slower degradation rate occurs. The samples decompose slowly by increasing the NC concentration in the composite, an indication of the superior thermal properties of the composites than that of pure polymer. Electrical Properties. Rubbers have inherently low electrical conductivity. A way to overcome this limitation is the addition of conductive additives into the polymer matrix which leads to the conductive materials. A number of works with remarkable results have been carried out using fillers from the family of carbon allotropes.6−8 Figure 6 (top) shows the ac conductivity
Figure 4. Peak force AFM images of 10 wt % NC/NR composite. In all three images, the NC and polymer component are marked with white and blue circles, respectively. (a) Topography, (b) Young’s modulus, and (c) adhesion.
ascribed to the NC particles that are located close to the sample surface can be seen. The NC network can be detected by the peak force AFM technique because of their significantly higher Young’s modulus than that of NR matrix. The adhesion image (Figure 4c) clearly differentiates the substrate from the NC particles. The soft polymer interacts with the tip (lighter colored areas), whereas the harder material (NC) does not (darker colored areas). The corresponding topography image (Figure 4a) does not show much contrast. Furthermore, from Figure 4c by a rough estimation, the NC particles size has been found to be about 50 nm which is in excellent agreement with the size analysis results of the NC aqueous dispersion.19 Properties of the NC/NR Composites. Thermal Properties. A thermogravimetric analysis (TGA) under air was carried out to estimate the influence of the NC additive on the thermal stability of the polymer. Three to five samples of each composite have been tested to obtain reliable results. All thermograms are shown in Figure S2 where the same trend of the thermal decomposition rate is presented for each composite. Figure 5 illustrates the thermograms of NR and composites. The global compartment is similar for NR and all four composites. Initial degradation starts within the same 20°
Figure 6. Top: ac conductivity as a function of frequency for the neat NR and NC/NR composites. Bottom: ac conductivity as a function of filler concentration at a frequency of 10 Hz.
as a function of frequency of NR and the NC/NR composites. The ac conductivity of the rubber is very low, approximately 10−10 S·m−1. A nearly linear increase is observed for the conductivity as a function of frequency for NR under log−log conditions due to the contribution of bound charges. Owing to the absence of conductive paths deriving from the fillers, the conductivity is dominated by polarization effects occurring in 1369
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(iii) Brij S 100 surfactant here leads to a nonoptimal dispersion and hence partial aggregation leading to better conductivity. Finally, it has previously been reported that the films made by filtering the suspensions of these nanocarbons showed very good conductivity up to 4000 S·m−1 and in an average of 100 S· m−1.19 Mechanical Properties. The effect of the NC fillers on the mechanical response was estimated by tensile testing. In Figure 8, the stress−strain curves of the composites with low particle concentrations (2 and 5 wt %) follow the same behavior as that of the neat NR.
the polymer, which highly depends on the frequency. Therefore, a frequency-dependent ac conductivity is observed. By adding as little as 2 wt % of NCs, the increase of conductivity with frequency, typical of a dielectric matrix, is transformed to an almost constant ac conductivity, associated with a conducting behavior. Likewise, the bulk ac conductivity is increased by 4 orders of magnitude. Further addition of NCs leads to higher and higher conductivity values, the 10 wt % composites reaching σ = 7.5 S·m−1, that is, 11 orders of magnitude higher than the pristine rubber (Figure 6, bottom). By adding NC fillers, a conductive network is formed breaking down the dielectric nature of the elastomer, even at low concentrations, therefore inducing an increase of the conductivity via the contribution of free charges brought by the fillers. Clearly, the results indicate that NC additives enhance the electrical conductivity of rubber. Moreover, NC fillers compare very well with commercial fillers, showing conductivity that is several orders of magnitude higher for the same amount of loading (Figure 7).6−8 The conductivity data do not follow
Figure 8. Representative stress−strain curves of NC and NC/NR composites. Inset: Young’s modulus as a function of NC concentration.
An increment in the tensile strength is observed with further NC addition, and the highest value for stress at the break of 0.6 MPa (double than the NR) appears for 10 wt % filler. Moreover, the elastic modulus increases. The Young’s modulus of the 7.5 and 10 wt % composites, extracted from the initial slope of the stress−strain graphs, is twice that of the neat polymer (inset of Figure 8). Table 1 discloses all mechanical results.
Figure 7. Comparison of our observed conductivity with values from the literature. CB: carbon black, NR: natural rubber, SBR: styrene− butadiene rubber, CNTs: carbon nanotubes, RGO: reduced graphene oxide, GnPs: graphene platelets, FGS: functionalized graphene sheets.
Table 1. Mechanical Properties of the Composites and Neat NRa
percolation behavior but rather a continuous increase of conductivity with filler fraction as has been previously reported by Parant et al. where it was ascribed to fractal filler networks.21 This behavior shows that the conductivity probably follows a more complicated law than a simple percolation law.22,23 In Figure 7, the values of conductivity have been plotted for composites containing different carbon concentrations of various carbon fillers. It is immediately apparent that NCs appear far more efficient than other low anisometry fillers such as CB, with 5 to 7 orders of magnitude better bulk conductivities. Moreover, NC performs similar to multiwalled CNTs, when their anisometry is far lower (10 against 1000). Finally, to reach the conductivity values as high as that obtained with 10 wt % NC, composites with commercial fillers must include as much as 25 wt % filler content,6,7 with the resulting penalties associated with high filler content, such as cost and loss of the matrix elastic (or other) properties. There are several factors that contribute to the high electrical conductivity values of the NR/NC composites: (i) before its dispersion, the NC has been thermally purified to remove the amorphous part and therefore increase its conductivity,19 (ii) these few layered graphene particles have no point defects at all20 and thus constitute a kind of ideal conductive filler, and
sample NR 2 wt % NC/NR (0.83 vol %) 5 wt % NC/NR (2.1 vol %) 7.5 wt % NC/NR (3.2 vol %) 10 wt % NC/NR (4.34 vol %)
tensile strength (MPa)
strain at break (%)
E (MPa) measured
E (MPa) calculated
0.31 (3) 0.22 (5)
292 (10) 153 (7)
0.44 (6) 0.44 (2)
0.44 0.49
0.34 (2)
277 (6)
0.60 (2)
0.53
0.41 (3)
191 (10)
1.10 (4)
0.80
0.59 (5)
299 (7)
0.94 (4)
1.03
a Values were averaged over 2−5 measurements (the numbers in brackets represent the standard deviation). Values of E were calculated from eq 1.
The addition of particles in the composites enhances the hardness of the polymer. This is due to the hydrodynamic effect arising from the inclusion of rigid particles into the soft rubber matrix. In this perspective, the polymer is considered as a continuum, and attention is focused on the effect of NC without taking into account the rubber behavior at the molecular level. Therefore, the tensile strength is higher by increasing the filler concentration, since more force is needed 1370
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factor is observed. However, a better electrical sensitivity is obtained for higher filler concentrations with very interesting results for the 10 wt % NC/NR composite. Indeed, with a gauge factor of 20 (with a stretchability of up to 225%.), the 10 wt % NC/NR composite could be placed among the recently reported values of strain sensor devices (variation from 14 to 62) which often required complicated preparation procedures.28−31 These strain sensor devices have been used for realtime human motion detection28,29 or for sensing various emotions.30 Moreover, possible applications in which strain sensors could be used are in stretchable electronics, robotics, medical diagnostics, and healthcare.28−31
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CONCLUSIONS The NC/NR composites were successfully prepared from methane by an environmentally friendly and sustainable procedure. NC appears as a viable competitor compared to petrol-based CB. In particular, highly conducting rubbers were obtained with only 10 wt % of NCs. Furthermore, piezoresistive experiments have shown that the rubber composites may be used as stress sensors with a gauge factor up to 20. The present availability of NCs actually coming from the methane/CO2 mix originating from food waste anaerobic digestion opens exciting perspectives about sustainable multifunctional rubber composites.
where f is the shape factor: 16.7 (50 nm lateral size and 3 nm widthconsidering individualized particles) and φ is the volume fraction of the filler. The weight fraction was transformed into the volume fraction by using the densities of NR and graphite (for NR, the density was d = 0.93 g·cm−3 and for NC, d = 2.1 g·cm−3, close to that of graphite). The theoretical and experimental values are in rather good agreement (cf. Table 1 and inset of Figure 8). Piezoresistive Properties. The dependence of the electrical properties with mechanical deformation was determined by measuring the change in resistance as the samples were elongated by applying force in a tensile deformation. Figure 9
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EXPERIMENTAL SECTION (MATERIALS AND METHODS) Materials. Nanocarbons were produced by FGV Cambridge Nanosystems via the cracking of CH4/CO2 by a microwave plasma process. NCs were purified using heat treatment at 500 °C in air for 6 h to remove amorphous carbon.19 All of the water was deionized using a Milli-Q system, and a nonionic surfactant, Brij S 100, was used as received from Sigma-Aldrich to disperse purified NC. The polymer used to prepare the composite is an NR latex (NR, Tg: −62 ± 1 °C, ζ: −80 ± 4 mV, solid content: 62.4 ± 0.1 wt %) kindly supplied by Talismã (Mirassol, Brazil). The NR latex has not been cured. Equipment. TEM: the internal structure and the state of dispersion of the filler particles within the elastomeric matrix were assessed via TEM using a Hitachi 7650 operating at 80 kV. Ultrathin sections (70 × 70 nm2) of the composites were microtomed using a cryo ultramicrotome Leica UC7 equipped with a cryo diamond knife DiATOME at −120 °C and with a knife angle at 35°. The thin cuts were put on holey carbon grids. Peak force microscopy: mechanical response and adhesion differences between the matrix and the additives were evaluated via peak AFM in ambient air in tapping mode. A peak force AFM on Icon microscopy was used. The tip was calibrated just before the experiment. An ultrathin section was used and prepared as described above for the TEM analysis. The cut was placed onto an aluminum substrate. Raman spectroscopy was carried out on a HORIBA JobinYvon Xplora microscope with a cooled Andor CCD detector and a laser spot size of ∼1 μm. The spectra were recorded at an excitation wavelength of 532 nm and calibrated with highly oriented pyrolytic graphite. XRD: X-ray diffractograms were acquired with a PANalitycal X’pert MPD-PRO Bragg−Brentano θ−θ geometry diffractometer equipped with a secondary monochromator. The Cu Kα radiation (λ: 1.5418 Å) was generated at 45 kV and 40 mA. The
Figure 9. Ratio of the change in resistance and the initial resistance and gauge factor as a function of strain for 10 wt % NC/NR composite. Inset: schematic representation of piezoresistive measurement. The initial maximum of the gauge factor is reproducible. It may be linked to an initial reorganization of the NC network as has been observed in the case of CNT/poly(vinyl alcohol) fibers.27
shows the change in resistance as a function of strain for the composites with 10 wt % NC. Data were fitted according to the model used by Knite et al.26 Tunneling conductivity dominates with no apparent contribution of the destruction of conducting networks upon strain (see the Supporting Information, Figure S7 and eq S1). Curves for the other samples as well as a video that shows a representative piezoresistive measurement are included in the Supporting Information. In all samples, an increase in the relative resistance (R − R0)/ R0 is observed. To define the sensitivity of the composites as sensors of deformation, the gauge factor (defined as ΔR/(R0·ε), where ΔR is the change from zero-strain resistance (R0) and ε is the strain) has also been plotted in Figure 9 for the 10 wt % NC/NR composites. The gauge factors of the other composites are presented in the Supporting Information. For the samples with NC concentration up to 5 wt %, a relatively low gauge 1371
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min a total energy of 14.4 kJ in an interval mode (pulse on: 0.5 s and pulse off: 0.2 s). The resulting dispersion was then centrifuged for 20 min at 4000 rpm (2486g) with a Fisher Bioblock Scientific centrifuge to remove the largest particles (aggregates) to obtain a monodisperse suspension of NC. After this step, the supernatant was freeze-dried to determine the NC yield. The final concentrations were 17 mg·mL−1 of NC (1.7 wt %, 53% yield) and 5 mg·mL−1 of Brij S 100 (0.5 wt %). The electrical conductivity value of the films made from such NC dispersions has been reported in a previous work and is approximately 1 Ohm·cm.19 To prepare NR composites, adapted volumes of supernatant were directly used. NC/NR Composites. Multifunctional composites with NR were produced via an environmentally friendly route that some of the coauthors of this article have developed with other carbon species as CNTs or graphene, utilizing latex technology.10,13,35 For the preparation of the composites, appropriate volumes of NC aqueous dispersion (ca. 4 to 19 mL) were directly mixed with NR latex (solid content 62.4 wt %) under magnetic stirring (30 min, 2000 rpm). The mixture was then sonicated using a bath sonicator (Branson 5200) for 10 min to remove the large bubbles created by magnetic stirring. Afterward, the slurries were placed onto molds and dried in an oven under vacuum at room temperature to remove the bubbles for an hour and at 70 °C for 48 h. Five different samples were prepared as control samples: an NR film without any additive, referred as NR, and four samples with the following NC concentrations: 2, 5, 7.5, and 10 wt % referred to as: x wt % NC/NR (x = 2, 5, 7.5, and 10 wt %).
spectra were obtained within a range 2θ: 8°−80° with an acquisition time of 2 h and 5 min. The samples were placed on a silicon wafer (“zero background”) sample holder and flattened with a piece of glass when required. TGA was performed using a SETARAM instrument equipped with a two-balance setup for equilibration under air. The samples of weights around 10 mg were collected on a Ptholder and heated at a rate of 5 °C/min. AC conductivity of the composites was measured as a function of frequency with a model M2 Materials Mates 7260 Impedance Analyzer at room temperature (frequency range: 10 Hz to 1 MHz). Prior to the measurements, samples of 10 × 10 × 0.5 mm were gold-metallized and then sandwiched between the two electrodes in a cell. A parallel resistor−capacitor equivalent circuit was used to describe the samples which could be considered as plane capacitors. The dc resistance value was extrapolated from the measured ac resistance values. The conductivity was deduced from eq 2.32−34 σ = z′·(z′−2 + z″−2 ) ·S ·d −1
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where z′ and z″ are the real and imaginary parts of the impedance, respectively; S is the surface area (cross-sectional area); and d is the distance between the two electrodes. The mechanical response of the neat NR and the composites was estimated from the stress−strain curves acquired using a Zwick Roell tensile machine at a strain rate of 1 mm·min−1. Stress is defined as the force applied to elongate the sample divided by the section and, strain is the change in the length while the specimen is stretched divided by the initial length. Three to four specimens were tested for each sample and the reported values were the representative results. The Young’s modulus was extracted from the slope of the stress−strain plots in the Hooke’s region (elastic part). Moreover, the change on resistivity as a function of deformation (piezoresistivity) was determined after following the change in resistance (R) with a Keithley 6517A multimeter during the elongation of the rubbers until breakage. The connection of the samples with the electrodes was achieved by using silver paint and copper wire (inset to Figure 9). The gauge factor, g, (ratio of the change in resistance to the strain) was determined from the slope of the elastic part of the curve as shown in eq 3.
R − R0 g= R 0· ε
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01848. Dispersion and characterization of the NC and analysis of the composites (PDF) Representative piezoresistive measurement (AVI)
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S = V ·l
−1
*E-mail:
[email protected]. Fax: +33 556845600 (A.P.). ORCID
(3)
Aldo J.G. Zarbin: 0000-0002-4876-2464 Author Contributions
The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Funding
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Funding from the European Union’s Seventh Framework Program for Research, Technological Development, and Demonstration under grant agreement no. 603488 (Plascarb project).
(5)
From eqs 4 and 5, we obtain eq 6
ρ = R · V · l −2
AUTHOR INFORMATION
Corresponding Author
where ε is the strain and R0 is the resistance for the initial length. The resistivity (ρ) was calculated by taking into account the dimensions of the samples and considering the volume (V) constant (eqs 4 and 5) (safe assumption for high strains). ρ = R · S · l −1
ASSOCIATED CONTENT
Notes
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The authors declare no competing financial interest.
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where l is the length. Preparations. NC Dispersion. Purified NCs (0.32 g; 3.2 wt %) were dispersed in 9.67 g of water using 0.05 g of Brij S 100 (0.5 wt %) as the surfactant. The dispersion was homogenized with a Branson Digital Sonifier tip sonicator, applying for 30
ACKNOWLEDGMENTS INCT Nanocarbon, CNPQ, and CAPES-COFECUB are gratefully acknowledged. We thank FGV Cambridge Nanosystems for providing us with nanocarbons. We would also like 1372
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(18) Chen, G.; Zhao, W. Rubber/Graphite Nanocomposites. Rubber Nanocomposites: Preparation, Properties, and Applications; John Wiley & Sons, Ltd, 2010; pp 527−550. (19) Hof, F.; Kampioti, K.; Huang, K.; Jaillet, C.; Derré, A.; Poulin, P.; Yusof, H.; White, T.; Koziol, K.; Paukner, C.; et al. Conductive inks of graphitic nanoparticles from a sustainable carbon feedstock. Carbon 2017, 111, 142−149. (20) Cançado, L. G.; da Silva, M. G.; Ferreira, E. H. M.; Hof, F.; Kampioti, K.; Huang, K.; Pénicaud, A.; Achete, C. A.; Capaz, R. B.; Jorio, A. Disentangling contributions of point and line defects in the Raman spectra of graphene-related materials. 2D Mater. 2017, 4, 025039. (21) Parant, H.; Muller, G.; Le Mercier, T.; Tarascon, J. M.; Poulin, P.; Colin, A. Flowing suspensions of carbon black with high electronic conductivity for flow applications: Comparison between carbons black and exhibition of specific aggregation of carbon particles. Carbon 2017, 119, 10−20. (22) Hu, L.; Hecht, D. S.; Grüner, G. Percolation in Transparent and Conducting Carbon Nanotube Networks. Nano Lett. 2004, 4, 2513− 2517. (23) 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, 3273−3287. (24) Othman, A. B.; Gregory, M. J. Stiffening effect of carbon black: Its interpretation by a modified Guth-Gold equation. J. Nat. Rubber Res. 1988, 3, 7−20. (25) Gordon, M. The Physics of Rubber Elasticity (Third Edition). L. R. G. Treloar, Clarendon Press, Oxford. 1975 pp. xii + 370. Price: £14.00. Br. Polym. J. 1976, 8, 39. (26) Knite, M.; Teteris, V.; Kiploka, A.; Kaupuzs, J. Polyisoprenecarbon black nanocomposites as tensile strain and pressure sensor materials. Sens. Actuators, A 2004, 110, 142−149. (27) Alexopoulos, N. D.; Jaillet, C.; Zakri, C.; Poulin, P.; Kourkoulis, S. K. Improved strain sensing performance of glass fiber polymer composites with embedded pre-stretched polyvinyl alcohol−carbon nanotube fibers. Carbon 2013, 59, 65−75. (28) Amjadi, M.; Pichitpajongkit, A.; Lee, S.; Ryu, S.; Park, I. Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire− Elastomer Nanocomposite. ACS Nano 2014, 8, 5154−5163. (29) Park, J. J.; Hyun, W. J.; Mun, S. C.; Park, Y. T.; Park, O. O. Highly Stretchable and Wearable Graphene Strain Sensors with Controllable Sensitivity for Human Motion Monitoring. ACS Appl. Mater. Interfaces 2015, 7, 6317−6324. (30) Roh, E.; Hwang, B.-U.; Kim, D.; Kim, B.-Y.; Lee, N.-E. Stretchable, Transparent, Ultrasensitive, and Patchable Strain Sensor for Human−Machine Interfaces Comprising a Nanohybrid of Carbon Nanotubes and Conductive Elastomers. ACS Nano 2015, 9, 6252− 6261. (31) Wang, S.; Zhang, X.; Wu, X.; Lu, C. Tailoring percolating conductive networks of natural rubber composites for flexible strain sensors via a cellulose nanocrystal templated assembly. Soft Matter 2016, 12, 845−852. (32) Al-Saleh, M. H.; Al-Anid, H. K.; Husain, Y. A.; El-Ghanem, H. M.; Jawad, S. A. Impedance characteristics and conductivity of CNT/ ABS nanocomposites. J. Phys. D: Appl. Phys. 2013, 46, 385305. (33) Dichiara, A. B.; Yuan, J.; Yao, S.; Sylvestre, A.; Zimmer, L.; Bai, J. Effective synergistic effect of Al2O3 and SiC microparticles on the growth of carbon nanotubes and their application in high dielectric permittivity polymer composites. J. Mater. Chem. A 2014, 2, 7980− 7987. (34) Jin, Y.; Xia, N.; Gerhardt, R. A. Enhanced dielectric properties of polymer matrix composites with BaTiO 3 and MWCNT hybrid fillers using simple phase separation. Nano Energy 2016, 30, 407−416. (35) Regev, O.; ElKati, P. N. B.; Loos, J.; Koning, C. E. Preparation of Conductive Nanotube−Polymer Composites Using Latex Technology. Adv. Mater. 2004, 16, 248−251.
to acknowledge Dr. Deniz Pekin for her assistance with the video production.
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ABBREVIATIONS NR, natural rubber; NC, nanocarbon; SLS, static light scattering; AFM, atomic force microscopy; XRD, X-ray diffraction; TGA, thermogravimetric analysis; TEM, transmission electron microscopy
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REFERENCES
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DOI: 10.1021/acsomega.7b01848 ACS Omega 2018, 3, 1367−1373
