Preparation of single-walled carbon nanotube (SWNT) gel composites using poly(ionic liquids)
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- 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
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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.
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 [1–3]. 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 . 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 [5–7]. 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 [8–14]. Recently, the developments on the functionalization of CNTs with ionic liquids to form composite gel have attracted considerable attention for various applications [15–25]. 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 . CNT-based bucky gels were also demonstrated as precursor in electrochemical materials for application to electrochemical actuators [27–29] and sensors [30–32].
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.
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 . 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
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.