Colloid and Polymer Science

, Volume 288, Issue 9, pp 1013–1018

Preparation of single-walled carbon nanotube (SWNT) gel composites using poly(ionic liquids)

Authors

  • Seung Hyun Hong
    • Department of Materials Science and EngineeringKorea University
  • Tran Thanh Tung
    • Department of Materials Science and EngineeringKorea University
  • Le Kim Huyen Trang
    • Department of Materials Science and EngineeringKorea University
    • Department of Materials Science and EngineeringKorea University
    • Department of Materials Science and EngineeringKorea University
Original Contribution

DOI: 10.1007/s00396-010-2229-3

Cite this article as:
Hong, S.H., Tung, T.T., Huyen Trang, L.K. et al. Colloid Polym Sci (2010) 288: 1013. doi:10.1007/s00396-010-2229-3

Abstract

This paper reports a new and practical route for synthesizing nanotube-polymeric ionic liquids gel by non-covalent functionalization of oxidized single-walled carbon nanotube (SWNT) surfaces with imidazolium-based poly(ionic liquids) (PILs), using in situ radical polymerization method. A black and homogeneous precipitate SWNTs was obtained as a gel form, which is well dispersed in aqueous solution without any aggregation. The formation of SWNT gels is explained by the electrostatic attractions or π-bonds between the SWNT surface and the PIL matrix. By anion-exchange reaction of PIL bound to SWNTs, hydrophilic anions in PIL were substituted with hydrophobic anions, resulting in an effective transfer of SWNT-PIL hydrogels to organogels. The result also showed that SWNTs can effectively improve the conductivity along with the thermal stability of nanocomposite gels.

Keywords

Carbon nanotubesNanocompositeHydrogelsOrganogels

Introduction

During the past few years, carbon nanotubes (CNTs) have been widely used in a variety of fields including nanoelectronics, nanocomposite, and biomedical application due to their remarkable electrical, mechanical, and chemical stability [13]. However, since CNTs are intrinsically bundled and heavily entangled due to van der Waals forces of attraction between adjacent tubes, their fine dispersion in solvent or organic binder is the most challenging for their practical application [4]. To overcome these problems, there has been much progress in the functionalization of CNTs with various organic molecules for solubilization and dispersion in different solvents [57]. In this regard, functionalization of CNTs with ionic liquids is an interesting topic because ionic liquids could provide a facile and promising method to control the surface properties of materials [814]. Recently, the developments on the functionalization of CNTs with ionic liquids to form composite gel have attracted considerable attention for various applications [1525]. Fukushima et al. found that grinding single-walled carbon nanotubes (SWNTs) in imidazolium-based ionic liquids led to SWNT gels (bucky gels), afforded by weak cross-linking interactions between the π-electronic nanotube surfaces and the imidazolium cations [26]. CNT-based bucky gels were also demonstrated as precursor in electrochemical materials for application to electrochemical actuators [2729] and sensors [3032].

Herein, we report a new strategy for the preparation of SWNT gel nanocomposites by non-covalent wrapping of oxidized SWNT surfaces with poly(ionic liquids) (PILs). Hydrophilic PILs derived from 1-vinyl-3-ethylimidazolium bromide was introduced on the SWNT surface through in situ radical polymerization method, leading to an SWNT hydrogel. Then, this SWNT hydrogel was subjected to an anion-exchange reaction of PILs with lithium bis(trifluoromethanesulfonyl)amide (Li+TFSI), in which substitution of the hydrophilic anions (Br) in PIL with hydrophobic anions (TFSI) occurs. As a consequence, SWNT organogels were produced, and these organogels can be readily dispersed in a variety of organic solvents such as acetone, propylene carbonate (PC), dimethylformamide (DMF), tetrahydrofuran (THF), N-methyl-pyrrolidone (NMP), and nitromethane (NM) and methyl ethyl ketone (MEK). The morphological aspects of PIL-induced SWNT bucky gels will be discussed as well as their thermal and electrical properties.

Experimental section

Materials

SWNTs were purchased from Carbon Solutions Inc. and subjected to an acid treatment as follows: The SWNTs (0.2 g) were suspended in a 100-ml acid mixture of HNO3-H2SO4 (3:1, volume ratio) solution. The mixture was then stirred at 80 °C for 3 h under reflux. After cooling to room temperature, it was diluted in a 1,000-mL beaker with 500 mL of deionized (DI) water and then vacuum-filtered through a 0.2-μm polycarbonate membrane (PC). The solid was washed with a large amount of DI water until the pH of the filtrate reached 7. The filtered solid was then dried under vacuum at room temperature for 24 h.

1-vinyl-3-ethylimidazolium bromide (ViEtIm+Br) was synthesized in our laboratory according to previous literature [33]. Li+TFSI ((CF3SO2)2NLi) was purchased from Aldrich and used as received. All other regents and solvents were used as received from Aldrich.

Preparation of SWNT-PIL hydrogels

The SWNT hydrogels were prepared as follows: In a typical procedure, a mixture of 3.0 g of imidazolium bromide monomer and 15 mL of ethanol was added into a 250-mL three-necked round bottom flask. Then, 30 mg of SWNT-COOH in 80 mL of DI water, kept in an ultrasonic bath for 15 min, was added to the mixture. After stirring for 1 h at room temperature, 30 mg of azobis(2-methylpropionitrile) (AIBN) was dissolved in 5 mL of ethanol and then added into the mixture. The flask was stirred at 70 °C for 10 h under nitrogen atmosphere. After polymerization, the excess amount of acetone was poured into the solution form to precipitate the ionic liquid polymer wrapped SWNT as gels, which was separated by filtration through a PC membrane (0.2 μm pore size) to produce hydrogels consisting of SWNT-PIL(Br). The concentration of SWNT in a hydrogel was varied from 0.5 to 1.5 wt.%. It should be noted that when the SWNTs concentration is increasing over the SWNTs loading level of 2 wt.%, the complex did not form gels. Typical preparation procedure for the SWNT gels was as follows.

Phase transfer into SWNT-PIL organogels

An aqueous dispersion of SWNT-PIL(Br) was subjected to a phase transfer process with (Li+TFSI). A 1.2 equivalent of TFSI with respect to the repeating units of PIL cations was added to complete the anion-exchange reaction of PIL. Immediately after the addition of Li+TFSI into the aqueous suspension of SWNT-PIL(Br), hydrophilic Br was substituted with hydrophobic TFSI. As a result of the anion-exchange, the product was precipitated as a gel in a mixture solvent water–alcohol. The SWNT-PIL(TFSI) organogels showed a range of solubility in common organic solvents such as acetone, PC, DMF, THF, NMP, and NM and MEK.

Characterization and instrumentation

Raman spectra were recorded on RFS-100/S Raman spectrometer equipment. Spectra were recorded over the range of 500–4,000 cm−1 and an excitation wavelength 642.8 nm. UV–Vis absorption measurements were carried out with a Scinco S-3100 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed with ESCA2000 (VG Microtech) system using a monochromatized aluminum Kα anode. Thermal gravimetric analysis (TGA) was carried out on a NETZSCH STA 409 PC/PG instrument with a heating rate of 10 °C min−1 under nitrogen. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6700F instrument. High-resolution transmission electron microscope (TEM) was conducted on a TECNAI 20 microscope operated at 200 kV. The surface resistance was obtained using a CMT series JANDEL four-point probe at room temperature.

Results and discussion

The SWNT-PIL nanocomposite gel was obtained through a procedure illustrated in Scheme 1. The first step involves acid treatment of SWNTs to give carboxylic groups (COOH) on their surfaces (SWNT-COOH). In the second step, hydrophilic PIL(Br) was introduced on the SWNT surfaces by in situ polymerization of imidazolium monomer with SWNT-COOH. These PIL(Br)-modified SWNTs form an SWNT-PIL hydrogel by pouring acetone in a reaction mixture as shown in the photographs (Scheme 1), and these hydrogels were found to disperse well in water. It should be noted that the concentration of SWNT in hydrogels was varied from 0.5 to 1.5 wt.%, since the gel type composite did not form with the loading level of SWNTs higher than 2 wt.%. In the next step, a phase transfer process of SWNT-PIL hydrogel into organogel was carried out by substitution of the hydrophilic anions (Br) with hydrophobic anions (TFSI) in the PIL. These SWNT-PIL organogels were found immiscible with water, but disperse well in a range of organic solvents and remain stable for over 4 months. Therefore, our procedure to produce SWNT bucky gels is rather simple without the need to grind SWNT in mortar.
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Scheme 1

Schematic illustration of synthetic process for the single-walled carbon nanotube (SWNT)-poly(ionic liquid) (PIL) gels. Photographs show SWNT-PIL gels and their suspensions in water and organic solvent. Inset in the photograph shows an SWNT-PIL gel scooped up with a spoon

The morphology of as-prepared SWNT-PIL gel composites was characterized by SEM, as shown in Fig. 1. SEM images display that SWNT-PIL gel composite produces a more flat film morphology with a smooth surface, while SWNT-COOH has groovy and rough morphology. This suggests that PIL uniformly covers the outer surface of the SWNT in SWNT-PIL, and the SWNT bundles are weekly interlocked with one another, which allow the formation of gels [34].
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Fig. 1

Scanning electron microscopy images of (a) single-walled carbon nanotube (SWNT)-COOH and (b) SWNT-poly(ionic liquid)(Br) hydrogels

A change in morphology of SWNT-PIL gel composite was also confirmed by TEM measurements. In Fig. 2, the SWNT-COOH exists as thick bundles and is heavily entangled with one another to form 3D networks. In contrast, TEM image of SWNT-PIL(Br) gel composite revealed the presence of untangled and individualized SWNTs embedded in PIL(Br) matrix. These results also support that PIL(Br) helps the effective exfoliation of SWNT bundles which enables the gel formation.
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Fig. 2

Transmission electron microscope images of (a) single-walled carbon nanotube (SWNT)-COOH and (b) SWNT-poly(ionic liquid)(Br) hydrogels

Figure 3 illustrates the absorption spectra of SWNTs before and after the coupling reaction with PIL. A typical absorption spectrum exhibited the characteristics of electronic transitions corresponding to metallic (M11, 400–600) and semiconducting (S22 and S11, 600–900 and 1,200–1,500 nm, respectively) SWNT sample. Broad peaks relative to the baseline would have indicated the existence of aggregated SWNTs because inter-tube van der Waals interactions in bundles disturb the electronic structure of SWNTs (solid line) [35]. Therefore, the well-resolved electronic absorption bands observed in SWNT-PIL sample indicate the typical van Hove singularities of individually dispersed SWNTs (dash line).
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Fig. 3

UV/Vis/NIR absorption spectra of oxidized single-walled carbon nanotube (SWNT)-COOH and SWNT-poly(ionic liquid) gels

To investigate the association of PIL(Br) with SWNTs in hydrogels, XPS was employed. In Fig. 4, the presence of N 1s (401 eV) and Br (68 eV) corresponding to the PIL(Br) proves the effective adsorption of PIL(Br) on the surface of SWNT. As mentioned earlier, the SWNT-PIL(Br) hydrogels can be switched into SWNT-PIL(TFSI) organogels by anion-exchange reaction of PILs, since hydrophilicity of PIL is strongly dependent on the type of anions in PIL [36]. This interest is rarely seen for common SWNTs composites and could be very useful for the phase transfer of SWNTs in various solvents and their self-assembly at interface [37,38].
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Fig. 4

X-ray photoelectron spectra of single-walled carbon nanotube-poly(ionic liquid)(Br) hydrogels. a Survey with a spectral region from 0 to 1,100 eV, (b) N 1s spectrum, and (c) Br 3d spectrum

In Fig. 5, the XPS spectrum of SWNT-PIL organogel was also studied to confirm the successful transfer of SWNT-PIL hydrogels into organogels. A complete disappearance of the peak attributed to the Br 3d3/2 is observed for SWNT-PIL organogels, and two new peaks corresponding to the S 2p and F 1s also appeared at around 688.8 and 169.06 eV, respectively. This result indicates that the hydrophilic Br-anions in PIL have been completely replaced with hydrophobic TFSI, which allows the creation of SWNT-PIL organogels.
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Fig. 5

X-ray photoelectron spectrum of single-walled carbon nanotube-poly(ionic liquid) organogel

Raman spectroscopy was employed to investigate a change in the structure of SWNT-PIL gel compare with SWNT-COOH, as shown in Fig. 6. The studied SWNT film had been deposited on PET substrate and drying in convection oven. The Raman spectrum shows a typical D- and G-peaks for both SWNT-PIL and SWNT-COOH, which corresponds to the presence of sp3 defects and vibration of sp2 carbon atoms in SWNT sidewall, respectively. However, the D/G ratio in SWNT-PIL gel composite was found to be much lower, indicating that SWNT-PIL have significantly lower density of defects compared with SWNT-COOH. In addition, a G-peak position of SWNT-PIL gels was down shifted by 8 cm−1 compared with that of SWNT-COOH, suggesting a non-covalent functionalization of PILs on the graphitic structure of SWNTs. Since the SWNT-COOH carries a negative charges and PIL has imidazolium-based polycations, the driving force for the effective functionalization of SWNT-COOH with PIL may come from the electrostatic attraction between the SWNT and PILs. Other possible interactions between PIL and SWNTs may include the cation–π and/or π–π interactions, which were reported in the literature [26,39]. Although the exact mechanism for the interaction between PIL and SWNTs needs to be elucidated, however, in any cases, it is clear that a portion of imidazolium-based polycations in PIL function to interact with SWNT surface for functionalization and debundling, and the rest of them act as phase-transferring medium from hydrogel to organogels.
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Fig. 6

Raman spectra of single-walled carbon nanotube (SWNT)-COOH (solid line) and SWNT-poly(ionic liquid) gels (dash line)

A strong interaction of PIL with CNTs was found to induce a change in thermal properties of SNWT-PIL gel composites, as shown in Fig. 7. TGA curve was obtained on the SWNT-PIL gel composite with different loading levels of SWNT (0, 0.5, 1, and 1.5 wt.%). While a decomposition of PIL was observed for SWNT-PIL gel composite and pristine PIL at a temperature range of 300–400 °C, the onset points of decomposition was found to shift to a higher temperature with increasing SWNT contents in the gel composites. This result implies that the thermal degradation of PIL in SWNT-PIL composites gel is delayed by the incorporation of SWNTs into PIL, which might be attributed to the strong interaction of PIL with SWNTs.
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Fig. 7

Thermal gravimetric analysis thermogram of single-walled carbon nanotube (SWNT)-poly(ionic liquid) organogels as a function of SWNT loading level (0, 0.5, 1, and 1.5 wt.% SWNTs in the gel composite)

To evaluate the electrical properties of SWNT-PIL gel composites, surface resistivity of SWNT-PIL gel composite film was measured using four-point probe system. Figure 8 is a plot of surface resistivity with respect to the loading content of SWNT in SWNT-PIL gel composites, which shows a decrease in surface resistivity of gel composite (from 3.8 × 107 to 1.7 × 103 Ω/square) with a increase of SWNT contents (from 0 to 1.5 wt.%). A significant drop in the surface resistivity is attributed to the individualized SWNTs in the PIL matrix and the formation of effective conducting network in the gel composites.
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Fig. 8

Surface resistivity of nanocomposite single-walled carbon nanotube (SWNT)-poly(ionic liquid) gels at various SWNTs content

Conclusions

This work demonstrated an effective approach for the preparation of SWNT-PIL gel nanocomposites by in situ polymerization of imidazolium-based ionic liquid monomer with SWNTs. The SWNT-PIL hydrogels, in which SWNT is effectively individualized in the PIL matrix, were afforded by strong, non-covalent interactions of PIL with SWNT surfaces. These hydrogels can be switched into organogels through the anion-exchange process of PILs bound to SWNTs, and the resulting SWNT-PIL organogels were found to have the improved thermal stability along with a good electrical conductivity. This will provide a simple synthetic route to the switchable SWNT bucky gels, which may be applied in many applications such as sensors and actuators.

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© Springer-Verlag 2010