1. Introduction
The excellent mechanical, electrical, thermal, and chemical properties of carbon nanotubes (CNTs) and graphene have attracted considerable attention for applications at the macroscale [1,2]. Polymer composite fabrication is a common method used to assemble CNT- and graphene-based macroscale products [3]; however, this method has been limited by the agglomeration of carbon nanomaterials in the polymer matrix. Sufficient dispersion of individual CNTs and graphene in the polymer host is required to retain the intrinsic properties of the nanofillers. Various dispersion methods have been investigated, including surfactant-treated dispersion [4-6], surface-modified dispersion [7], and polymer wrapping [8-10]. Surfactant-treated dispersion requires that the surfactant be eliminated for subsequent processing while surface-modified dispersion of carbon nanomaterials reportedly generates surface defects. In contrast, the polymerwrapping method minimizes defect formation on the conjugated structures of CNTs and graphene, preserving their superior electrical conductivity [11]. However, to date, lack of control over the wrapping configuration due to intrachain and weak interactions between the polymers and CNT/graphene has limited the effectiveness of the polymerwrapping method [12,13].
Various CNT wrapping agents have been studied in efforts to improve the solvent stability. Conjugated polymers, such as poly(3-hexylthiophene) (P3HT) [14] and poly(m-phenylene vinylene) [15], have been investigated, on the basis of their strong π–π interaction with CNTs; however, these polymers have limited solubility and stability, as well as a high material cost. Block copolymers, consisting of conjugated P3HT and non-conjugated polymers (polystyrene, PS), have also been used as wrapping materials [16,17]. The van der Waals interactions and dispersion stabilization of poly(acrylic acid) (PAA), a non-conjugated polymer, have been examined [9,18]. A numerical molecular dynamics study demonstrated that non-conjugated poly(acrylonitrile) (PAN) interacts with and wraps single-walled CNTs (SWCNTs) via the PAN cyano groups [12]. The polymer wrapping method has seldom been applied to graphene, however, due the difficulty in wrapping graphene’s platelet-like surface. One study used PS and poly(methyl methacrylate) polymers to wrap graphene but the graphene content was very low (e.g., 0.0090 wt%) [19], making it difficult to evaluate the effectiveness of the polymer wrapping.
In this study, the polymer-wrapping method was used to wrap multi-walled CNTs (MWCNTs) and graphene to enhance their dispersion. To reduce processing and material costs, a non-conjugated polymer, PAN, was selected, based on its ability to interact with the graphene layer of the MWCNTs and graphene via the cyano groups in its side branch. The physical wrapping of CNTs and graphene with PAN was investigated for various PAN concentrations, in an attempt to simplify and improve the polymer-wrapping process. Transmission electron microscopy (TEM) analysis confirmed wrapping of the MWCNTs and graphene with a PAN layer and was also used to determine the layer thickness. Proton nuclear magnetic resonance (1H-NMR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and Raman spectroscopy revealed that the cyano groups of the PAN molecules facilitated adhesion to the MWCNTs and graphene for polymer wrapping by physical interaction. Shelf-life and zeta potential measurements verified the stability and enhanced dispersion of MWCNTs and graphene.
2. Experimental
2.1. Materials
PAN (Mw = 200,000 g mol−1; Polysciences Inc., Warminster, PA, USA) was used to wrap MWCNTs (CM-95; Hanwha Nanotech Co., Seoul, Korea; length: 10–20 μm, diameter: 10–20 nm) and graphene (XGNP M-5; XG Sciences, Lansing, MI, USA; average diameter: 5 μm, thickness: 6–8 nm). N,N-Dimethylformamide (DMF, 99.5%; Daejung Chemical Co., Siheung, Korea) was used as the solvent.
2.2. Preparation of PAN-wrapped MWCNTs and graphene
PAN was dissolved in DMF at 90℃ at concentrations of 0, 0.1, 0.25, 0.5, 1, 2, and 4 wt%. Next, 300 mg of MWCNTs and graphene, respectively, was added to 30 g of each PAN solution. The resultant solutions were then ultrasonicated for ~3 h and stirred for over 1 d. Centrifugation (Sorvall Primo R; Thermo Scientific, Scoresby, Australia) was carried out at 5000 rpm for 15 min to eliminate aggregation of the MWCNTs and graphene. PAN-wrapped MWCNTs (Pw-MWCNTs) and graphene (Pw-G) were recovered from the supernatant solutions.
2.3. Characterization
The surface morphology of the Pw-MWCNTs and Pw-G were observed by field-emission scanning electron microscopy (FE-SEM; JSM-7600F, JEOL, Tokyo, Japan). 1H-NMR spectroscopy (500 MHz, DMF-d7, Avance 500 Spectrometer; Bruker, Billerica, MA, USA) was carried out to investigate the interaction between PAN and MWCNTs and graphene. Pw-MWCNTs and Pw-G were dried in a vacuum oven and redispersed in DMF-d7 for 1H-NMR analysis. UV-Vis spectroscopy (Cary 100; Agilent Technologies, Santa Clara, CA, USA) spectra were recorded using quartz cuvettes with a path-length of 10 mm. Pw-MWCNTs and Pw-G solutions were diluted to prepare samples whose maximum absorption became around unity and were scanned in a range of 200–800 nm. FT-IR (TENSOR27; Bruker) spectra were obtained between 600 and 4000 cm–1 with a signal resolution of 2 cm–1. The wrapping morphologies of Pw-MWCNTs and Pw-G and the PAN layer thickness were examined using TEM (JEM-3010; JEOL). TGA was carried out using a Q-5000 IR system (TA Instruments, New Castle, DE, USA); measurements were conducted in an N2 atmosphere, over a temperature range of 50℃–800℃, with a heating rate of 10℃ min−1. About 1 mL of Pw-MWCNT, Pw-G solutions was dried and Raman spectra (514 nm laser, T64000; Horiba, Kyoto, Japan) of Pw-MWCNTs and Pw-G were recorded in a wavenumber range between 1000 and 3000 cm–1. The MWCNT and graphene concentrations in the decanted solutions were measured by drying the solutions on a hot plate (60℃).
The zeta potentials of Pw-MWCNTs and Pw-G were measured using an electrophoretic light-scattering spectrophotometer (ELS-8000; Otsuka Electronics, Osaka, Japan) to investigate the dispersion characteristics of the solutions. Particle size measurements of the Pw-MWCNTs were performed using dynamic light scattering (DLS) (DLS-7000 Photometer; Otsuka Electronics). A particle size analyzer (S3500; Microtrac, San Diego, CA, USA) was used to measure the particle size of Pw-G.
MWCNTs and graphene were modified by the same polymer, facilitating their hybrids. Hybrid buckypapers of MWCNT/graphene were prepared by mixing Pw-MWCNT and Pw-G solutions and then characterized. Pw-MWCNT (0.1 wt%) and Pw-G (0.25 wt%) solutions were blended at 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10 in volume ratio to make hybrid buckypapers. Each solution of 5 mL was vacuum-filtered using a nylon membrane filter (47 mm diameter, 0.45 μm pore; Whatman, Marlborough, MA, USA). The hybrid buckypapers were then dried in a vacuum oven at 60℃ overnight. The 4-point probe method was used to measure the sheet resistance of the hybrid buckypapers and their resistivity on the basis of their thickness was measured by SEM images.
3. Results and Discussion
3.1. Morphologies of the Pw-MWCNTs and Pw-G
The morphology of the Pw-MWCNTs was examined using SEM and TEM. Fig. 1a and c show SEM and TEM images of pristine MWCNTs, revealing a smooth and clean surface. After PAN wrapping, the MWCNT surface changed significantly: the average thickness of MWCNTs increased (Fig. 1b) and PAN molecules were visible on the outermost layers of the MWCNTs (Fig. 1d). The thickness of Pw-MWCNTs was characterized based on TEM images. The thickness of the wrapped PAN layer on the MWCNTs increased with the PAN concentration (Table 1): a significant increase in the PAN layer thickness was observed for a PAN concentration of 0.5 wt%. TEM images could not be obtained for Pw-MWCNTs having a PAN concentration exceeding 4 wt%, due to the high polymer concentration. Fig. 2 shows that higher PAN concentrations induced aggregation of Pw-MWCNT bundles, as opposed to single Pw-MWCNTs, i.e., PAN molecule aggregation.
Fig. 1.Scanning electron microscopy images of (a) pristine MWCNTs, (b) Pw-MWCNTs prepared with 0.1 wt% PAN and transmission electron microscopy images of (c) pristine MWCNTs, (d) Pw-MWCNTs. MWCNT, multi-walled carbon nanotubes; Pw-MWCNTs; poly(acrylonitrile) (PAN)-wrapped MWCNTs.
Table 1.PAN, poly(acrylonitrile); MWCNTs, multi-walled carbon nanotubes.
Fig. 2.Transmission electron microscopy images of Pw-MWCNTs prepared using (a) 0.1 wt% and (b) 0.5 wt% PAN. Pw-MWCNTs, poly(acrylonitrile) (PAN)-wrapped multi-walled carbon nanotubes.
Fig. 3 shows SEM and TEM images of pristine graphene (Fig. 3a and c) and Pw-G (Fig. 3b and d). Interestingly, Pw-G exhibited a striped pattern of PAN chains on its surface (Fig. 3d), in contrast with pristine graphene (Fig. 3b). The cross-sectional view of Pw-G (Fig. 3e) confirmed the presence of a PAN coating on the graphene surface. A simulation study reported that the cyano groups in PAN tend to align along the longitudinal direction of SWCNTs; the results showed that the cyano groups preferentially interacted with graphitic layers, as opposed to building intrachain coils [12]. In this study, 1H-NMR spectroscopy was used to investigate the interaction between the cyano groups in PAN and the graphitic structure, as well as the striped pattern formation on the graphene surface. This is discussed in detail in section 3.2.
Fig. 3.Scanning electron microscopy images of (a) pristine graphene, (b) poly(acrylonitrile) (PAN)-wrapped graphene (PW-G), transmission electron microscopy images of (c) pristine graphene and PW-G: (d) surface and (e) cross-section.
As discussed earlier, a simple process was used to wrap MWCNTs or graphene with PAN to eliminate van der Waals interactions. Thus, PAN polymer wrapping was expected to enhance the dispersion of the MWCNTs and graphene. Fig. 4 shows shelf-life test results of Pw-MWCNT-DMF and Pw-G-DMF solutions after 1 wk. Pristine MWCNTs became aggregated within just a few hours after dispersion in DMF. In contrast, despite some precipitation, most of the Pw-MWCNTs remained well dispersed in DMF after 1 wk, regardless of the PAN concentration. To eliminate the precipitate, centrifugation was carried out after sonication. Once the centrifugation step was added, no aggregates were observed from the resultant solutions for up to 1 mo. The Pw-G solution exhibited similar dispersion as that of the Pw-MWCNT solution, regardless of PAN concentration, i.e., uniform dispersion without precipitate for 1 mo.
Fig. 4.Shelf-life photographs of (a) the Pw-MWCNTs and (b) Pw-G solutions after 1 wk. The PAN concentrations used to prepare the solutions shown in the photographs are as follows, from left to right: 0, 0.1, 0.5, 1, 2 and 4 wt% for Pw-MWCNTs and 0, 0.25, 0.5, 1, 2, and 4 wt% for Pw-G. Pw-MWCNTs, poly(acrylonitrile) (PAN)-wrapped multi-walled carbon nanotubes; Pw-G, PAN-wrapped graphene.
3.2. Interactions between PAN and MWCNTs and graphene
1H-HNR spectroscopy was used to investigate the interaction between PAN and MWCNTs and graphene (Fig. 5). The 1H-NMR spectra of Pw-MWCNTs and Pw-Gs exhibited a significant decrease in the peak intensity of hydrogen in PAN; this was attributed to the interaction between PAN and the MWCNTs and graphene, which not only affected the magnetic resonance, but also created large diamagnetic ring currents in the MWCNTs [20]. Shifting and broadening of the NMR peaks have been observed between the conjugated polymer, poly(aryleneethynylene)s, and single-walled carbon nanotube (SWNTs) [21]. Changes in the NMR peaks were also observed for PAA, a non-conjugated polymer, and MWCNTs, as a result of hydrophobic forces and van der Waals interactions between the two [18]. From these results and the results of our study, we concluded that the interaction between the π bondings of the PAN cyano groups and the conjugated structure of the MWCNTs led to the wrapped configuration of PAN on the MWCNTs, resulting in good dispersion of Pw-MWCNTs in the DMF solution.
Fig. 5.Proton nuclear magnetic resonance spectra of (a) PAN, (b) Pw-MWCNT, and (c) Pw-G. DMF, N,N-Dimethylformamide; PAN, poly(acrylonitrile); Pw-MWCNTs, PAN-wrapped multi-walled carbon nanotubes; Pw-G, PAN-wrapped graphene.
UV-Vis spectroscopy results in Fig. 6 show the blue-shift and peak broadening of the PAN characteristic peak. Pw-MWCNTs solution showed a 7 nm shift in all PAN concentrations, while the shift of the Pw-G solution was 4 nm in 0.25 wt% PAN, gradually decreased to 1 nm in 4 wt% PAN. This blue-peak shift was caused from a strong π–π interaction between C≡N bonds and the carbon graphitic structures [22]. The tubular structure of MWCNTs brought about easier wrapping of PAN compared to graphene, which is supported by the blue-peak shift.
Fig. 6.Ultraviolet-visible spectra of (a) PAN, MWCNTs, and Pw-MWCNTs and (b) PAN, graphene, and Pw-G. The number in front of ‘Pw’ represents the weight percent of PAN used for MWCNT-and graphene-wrapping. PAN, poly(acrylonitrile); MWCNTs, multi-walled carbon nanotubes; Pw-MWCNTs, PAN-wrapped MWCNTs; Pw-G, Pw-graphene.
Fig. 7 shows FT-IR spectra of pristine MWCNTs, graphene, PAN, Pw-MWCNTs, and Pw-G, verifying that the peaks of PAN characteristic bonds (C≡N stretch at 2250 cm–1, C-H stretch at 2850–3000 cm–1, C-H bend at 1460 and 1380 cm–1, etc) and CNT and graphene characteristic peaks (2360 and 2336 cm–1) were not affected by the wrapping processes. These results imply that there was no chemical bonding between PAN and MWCNTs (and graphene), confirming that the interaction between PAN and the nanocarbons came from the physical wrapping force.
Fig. 7.Fourier transform infrared spectroscopy spectra of (a) PAN, MWCNTs, and Pw-MWCNTs and (b) PAN, graphene, and Pw-G. PAN, poly(acrylonitrile); MWCNTs, multi-walled carbon nanotubes; Pw-MWCNTs, PAN-wrapped MWCNTs; Pw-G, Pw-graphene.
Fig. 8 shows the TGA curves of PAN, MWCNTs, Pw-MWCNT, graphene, and Pw-G. PAN undergoes cyclization and dehydrogenation processes to form a ladder-like structure below 300℃ in air [23], but degrades significantly above ~300℃. MWCNTs and graphene degraded slightly up to 800℃ under air. Interestingly, in the early stages of the thermal analysis, Pw-MWCNTs and Pw-G exhibited a smaller weight loss than PAN. This was attributed to the enhanced crystallization from wrapping of the MWCNTs and graphene with PAN, similar to the results reported in Reference [24]. Stabilization of the PAN molecules thereby resulted in higher residual Pw-MWCNT and Pw-G content. Additionally, the decomposition temperature of Pw-MWCNTs and Pw-G, ~265℃ (Fig. 8a) and 280℃ (Fig. 8b), respectively, increased compared with pristine MWCNTs and graphene. After decomposition, the Pw-MWCNTs and Pw-G exhibited a rapid weight loss, due to the high thermal conductivity of the carbon materials [25], i.e., heat transfer to the PAN molecules, bringing about their rapid degradation.
Fig. 8.Thermogravimetric analysis curves of (a) PAN, MWCNTs, and Pw-MWCNTs and (b) PAN, graphene, and Pw-G. The number in front of ‘Pw’ represents the weight percent of PAN used for MWCNT-and graphenewrapping. PAN, poly(acrylonitrile); MWCNTs, multi-walled carbon nanotubes; Pw-MWCNTs, PAN-wrapped MWCNTs; Pw-G, Pw-graphene.
The attractive force of polymer chains to the surfaces of MWCNTs can be analyzed by Raman spectra. The upshift of the Raman peak of surface-modified SWCNTs [26-28] and MWCNTs [18] with surfactants or polymers has been investigated. The elastic constant of the harmonic oscillator of MWCNTs when attached to polymer induces the upshift of the Raman characteristic peaks [18]. In Fig. 9a, the characteristic peaks of pure MWCNTs and Pw-MWCNTs are observed at 1350, 1579, and 2688 cm–1 and at 1355, 1580, and 2691 cm–1, respectively, which represent the D-band, G-band, and G’-band, respectively. All of the characteristic peaks were upshifted after the polymer wrapping. In addition, the intensity of the D-band of MWCNTs, which is attributed to defects in sp2 carbons [29], increased after the PAN wrapping. These results confirm that the disruption of graphitic conjugation in MWCNTs occurred due to the interaction between PAN and MWCNTs. In Fig. 9b, pristine graphene shows a strong and sharp G-band (1584 cm–1) and G’-band (2730 cm–1), as compared with the D-band (1353 cm–1) and D’-band (1637 cm–1), due to highly ordered graphitic structures. The D-band (1365 cm–1) and D’-band (1869 cm–1) intensities of Pw-G remarkably increased and upshifted due to the strong physical interaction between the PAN molecule and the graphitic layers. After polymer wrapping, upshift of the G-band of Pw-G (1656 cm–1) was observed, while the G’-band was downshifted (2678 cm–1). The G’-band wavenumber of graphene decreases when the laser energy weakens [30]. Therefore, it can be argued that Pw-G took lower energy than graphene due to the polymer layer near the graphene, producing a downshifted G’-band.
Fig. 9.Raman spectra of pristine and PAN-wrapped (a) MWCNTs and (b) graphene. PAN, poly(acrylonitrile); MWCNTs, multi-walled carbon nanotubes; Pw-MWCNTs, PAN-wrapped MWCNTs; Pw-G, Pw-graphene.
3.3. Characteristics of Pw-MWCNTs and Pw-G
The mass ratio of wrapped PAN contents to that of MWCNTs and graphene was determined to be 800℃; aggregated Pw-MWCNTs and Pw-G were removed via centrifugation before TGA. TGA results confirmed that the MWCNTs and graphene interacted with the PAN molecules, leading to crystallization of the PAN molecules. A high PAN concentration created more MWCNTs or graphene dispersed throughout the solution, indicating that more of the carbon nanomaterials became wrapped with PAN as the concentration of PAN increased (Table 2).
Table 2.MWCNTs, multi-walled carbon nanotubes; Pw-MWCNT, poly(acrylonitrile)-wrapped MWCNTs; Pw-G, poly(acrylonitrile)-wrapped graphene; PAN, poly(acrylonitrile).
The dispersability of surface-modified CNTs [31,32] and ionic surfactant-treated CNTs [4] can be evaluated using the zeta potential, based on the observation that colloidal particles of modified CNTs had charges on their surfaces. Although the PAN polymer, MWCNTs, and graphene used in this study had no charge, alignment of the PAN cyano group on the basal plane of the carbon crystals can create a dipole moment on the surface of the dispersed particles [33]. To evaluate the dispersability of Pw-MWCNTs and Pw-G, we measured the zeta potential using the Helmholtz-Smoluchowski equation [34]. The PAN concentration significantly influenced the zeta potential (Fig. 10). In general, the zeta potential of the Pw-MWCNT and Pw-G colloidal particles decreased as the PAN concentration increased. This trend can be explained by the sizes of the colloidal particles. Because the outermost molecules of the particles dispersed in the solution have a significant effect on the zeta potential, the zeta potential of the larger particles was smaller. As the concentration of PAN molecules increased, more PAN molecules attached to the surface of the MWCNTs or graphene; this phenomenon was supported by the Pw-MWCNT and Pw-G particle sizes measured by DLS and the particle-size analyzer, respectively (Table 2), where the particle size for both increased with PAN concentration.
Fig. 10.Zeta potential of (a) Pw-MWCNT and (b) Pw-G solutions as a function of the PAN concentration. Pw-MWCNTs, poly(acrylonitrile)-wrapped multi-walled carbon nanotubes; Pw-G, Pw-graphene.
Finally, the sheet resistance and resistivity of the hybrid buckypapers of Pw-MWCNT and Pw-G were measured to investigate the contribution of the common modifier (PAN) on the distribution of both MWCNTs and graphene in the hybrid (Fig. 11). The hybrid buckypapers showed the highest electrical conductivity at a volume ratio (9:1) of Pw-MWCNT and Pw-G, demonstrating that the randomly dispersed MWCNTs and the graphene together provide a synergistic effect by increasing the conducting network [35].
Fig. 11.Electrical conductivity (left) and the sheet resistance (right) of the hybrid buckypapers of Pw-MWCNTs and Pw-G. Pw-MWCNTs, poly(acrylonitrile)-wrapped multi-walled carbon nanotubes; Pw-G, Pw-graphene.
4. Conclusions
A simple polymer-wrapping method was used to wrap MWCNTs and graphene with PAN, in an attempt to improve their dispersion. The method was based on the premise that the cyano group in the PAN molecule interacts with the graphene layers of the carbon crystals, to induce wrapping or coating of the MWCNTs and graphene with PAN. TEM analysis revealed wrapping of MWCNTs and graphene with PAN molecules. The Pw-MWCNTs and Pw-G dispersed in a DMF solution displayed solution stability over 4 wk, i.e., no aggregation or precipitation. The interaction between the graphitic structure and cyano groups in the PAN molecules was characterized using 1H-NMR spectroscopy, UV-Vis spectroscopy, FT-IR, TGA and Raman spectroscopy; the results confirmed the physical wrapping/coating of PAN molecules on CNTs and graphene, and, consequently, their enhanced dispersion in solutions, as quantified using the zeta potential. The advantage of the common modifier effect was demonstrated by the electrical conductivity of the hybrid buckypapers of Pw-MWCNT and Pw-G.
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