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Carbon Nanotubes Incorporated within Lyotropic Hexagonal Liquid Crystal Formed in Room-Temperature Ionic Liquids Wenqing Jiang,† Bo Yu,‡ Weimin Liu,‡ and Jingcheng Hao*,†,‡ Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, China, and State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ReceiVed March 30, 2007. In Final Form: May 13, 2007 Multiwalled carbon nanotubes (MWCNTs) were evenly dispersed within hexagonal lyotropic liquid crystals (LLCs) formed in room-temperature ionic liquids (RTILs), ethylammonium nitrate (EAN). Characterization and tribological properties of dispersed system were studied in detail. Polarized optical microscopy images combined with small-angle X-ray scattering (SAXS) results indicate that the MWCNTs are well-dispersed and that the introduction of MWCNTs does not destroy the structure of hexagonal LLCs. The increase of d spacing demonstrates the integration of MWCNTs within the cylinders of the hexagonal LLCs. FT-IR and Raman spectra of the MWCNTs-LLC composites show the characteristic absorption peaks and Raman bands of MWCNTs. The tribological properties were explored to greatly extend the applications of MWCNTs-LLC composites in RTILs as lubricating materials. The rheological measurement results indicate that MWCNTs-LLC composites are highly viscoelastic and that the apparent viscosity is enhanced by the presence of the MWCNTs.
Introduction Carbon nanotubes (CNTs), because of their novel properties such as high surface area, electrical conductivity, good chemical stability and tribological properties, and extremely high mechanical strength,1,2 are important and interesting high-octane materials with wide-ranging applications in research and industry. It is recognized that the strong tendency toward clustering and aggregation due to strong Van der Waals attractions between the individual CNTs, which is responsible for the general insolubility in common solvents such as water, and the random orientation of the nanotubes are the two impediments for the potential applications of CNTs. Dispersion of CNTs used the chemical derivatization,3 acid functionalization,4 and noncovalent adsorption with different dispersant molecules such as surfactants and/ or polymers5,6 was attempted, and strides for alignment of CNTs employing magnetic fluids,7,8 flow-induced alignment,9,10 and mechanical ordering11 were made. Because liquid crystals (LCs) exhibit long-range orientational order along a special direction, known as the director, n, it seems feasible to exploit the selforganizing properties of LCs to induce the alignment of dispersed * Corresponding author. Fax/Tel: +86-531-88366074, E-mail: jhao@ sdu.edu.cn. † Key Laboratory of Colloid and Interface Chemistry. ‡ State Key Laboratory of Solid Lubrication. (1) Iijima, S. Nature (London) 1991, 354, 56-58. (2) Be’guin, F., Ehrburger, P., Eds. Carbon Nanotubes (special issue). Carbon 2002, 40, 1619. (3) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, M. A.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95-98. (4) Song, W. H.; Kinloch, I. A.; Windle, A. H. Science 2003, 302, 1363-1398. (5) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269-273. (6) Mackay, M. E.; Tuteja, A.; Duxbury, P. M.; Hawker, C. J.; Horn, B. V.; Guan, Z.; Chen, G.; Krishnan, R. S. Science 2006, 311, 1740-1743. (7) Smith, B. W.; Benes, Z.; Luzzi, D. E.; Fischer, J. E.; Walters, D. A.; Casavant, M. J.; Schmidt, J.; Smalley, R. E. Appl. Phys. Lett. 2000, 77, 663-665. (8) Garmestani, H.; Al-Haik, M. S.; Dahmen, K.; Tannenbaum, R.; Li, D. S.; Sablin, S. S.; Hussaini, M. Y. AdV. Mater. 2003, 15, 1918-1921. (9) Xin, H. J.; Woolley, A. T. Nano Lett. 2004, 4, 1481-1484. (10) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (11) Bin, Y. Z.; Kitanaka, M.; Zhu, D.; Matsuo, M. Macromolecules 2003, 36, 6213-6219.
CNTs. Recent efforts focused toward using LCs to disperse and align CNTs have been successful.12-15 Room-temperature ionic liquids (RTILs), a class of organic salts with unusually low melting temperature, have attracted considerable attention. Due to their unique characteristics such as thermal stability, negligible vapor pressure, nonflammability, high ionic conductivity, and a wide electrochemical window, RTILs can be used as an environmentally benign solvent for green chemistry, catalysts for synthetic chemistry, and electrolytes for batteries in photochemistry and electrosynthesis.16,17 Recently, it has been realized that RTILs can be used as solvents for the amphiphilic molecular self-assembly structures, such as micelles, reverse micelles, vesicles, and lyotropic liquid crystals.18 Two reports on the formation of lyotropic liquid crystals (LLCs) by amphiphilic triblock copolymer (Pluronic P123, EO20PO70EO20) into hexagonal (H1) and lamellar (LR) phases in [bmim][PF6]19 and nonionic surfactants (polyoxyethylene, CnEOm) in ethylammonium nitrate (EAN)20 into H1, LR, discrete cubic (I1), and bicontinuous cubic (V1) phases have appeared. Small-angle X-ray scattering (SAXS) results have provided evidence for LLCs (H1 and LR phases) of P123 in [bmim][PF6],19 and the phase diagrams of the binary CnEOm (n ) 16 and 18, m ) 6)/EAN systems20 have been mapped out, in which the large domains of existence of LLCs with I1, H1, V1, and LR phases were reported. Here, it is shown that multiwalled CNTs (MWCNTs) can be evenly dispersed in an H1 phase formed in the binary C16EO6/ EAN system.20 First, we used nonionic polyoxyethylene surfactant C16EO6 to disperse the MWCNTs in EAN, and then the EAN (12) Lynch, M. D.; Patrick, D. L. Nano Lett. 2002, 2, 1197-1201. (13) Dierking, I.; Scalia, G.; Morales, P.; LeClere, D. AdV. Mater. 2004, 16, 865-869. (14) Dierking, I.; Scalia, G.; Morales, P. J. Appl. Phys. 2005, 97, 044309044313. (15) Weiss, V.; Thiruvengadathan, R.; Regev, O. Langmuir 2006, 22, 854856. (16) Welton, T. Chem. ReV. 1999, 99, 2071-2084. (17) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792-793. (18) Hao, J.; Zemb, T. Curr. Opin. Colloid InterfaceSci., in press. (19) Wang, L.; Chen, X.; Chai, Y.; Hao, J.; Sui, Z.; Zhuang, W.; Sun, Z. Chem. Commun. 2004, 2840-2841. (20) Araos, M. U.; Warr, G. G. J. Phys. Chem. B 2005, 109, 14275-14277.
10.1021/la700921w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007
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dispersion of MWCNTs was incorporated in the performed hexagonal phase LLCs formed in the binary C16EO6/EAN system. The samples were characterized by polarized optical microscopy, Fourier transform infrared (FT-IR) spectroscopy, and small-angle X-ray scattering (SAXS), and the macroproperties of the composite were investigated by rheological measurements. Moreover, the tribological characteristic of the system was explored, which extends the application of MWCNTs-LLC composites formed in EAN as a lubricating material. This not only repesents a new approach to expanding potential uses of RTILs, but also indicates design of new self-assembly systems in RTILs for dispersing CNTs.
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Figure 1. Micrographs of LLCs (a) and MWCNTs-LLCs composite (b) samples prepared in EAN. The samples were equilibrated 6 months at T ) 25.0 ( 0.1 °C.
Experimental Section Materials. MWCNTs were a gift from FutureCarbon GmbH, Bayreuth, Germany, and were delivered at a purity of 99%. Nonionic polyoxyethylene surfactant hexaethylene glycol monohexadecyl ether (C16EO6, purity > 99% TLC) was purchased from Fluka and used as received. Synthesis of RTIL, Ethylammonium Nitrate (EAN). Ethylammonium nitrate (EAN) was synthesized as described by Evans et al.,21 by slow addition of ∼3 M nitric acid to the ethylamine solution while stirring and cooling it in an ice bath. Most of water was removed with a rotary evaporator; the final amounts of water were removed with a lyophilizer (Martinchrister ALPHA1-2). The product was stored in a vacuum desiccator. Its melting point was about 286 K; the density at room temperature was about 1.2 g‚mL-1. It is in good agreement with a previous report.21 The rudimentary water content was determined by Karl Fischer titration to be around 0.6 wt %. 1H NMR and FT-IR indicate that the resulting product is EAN. 1H NMR: R-H (2H, 3.10 ppm), ω-H (3H, 1.32 ppm). 13C NMR: R-C (34.56 ppm), ω-C (11.36 ppm). IR (liquid membrane, cm-1), 3066.75 (υN-H), 1358.13 (υN-O). Dispersion of MWCNTs. The stock solutions with different concentration of MWCNTs dispersed with C16EO6 were prepared by adding MWCNTs and C16EO6 to EAN. In these systems, the weight ratio between the MWCNTs and C16EO6 was kept constant at 1:1. The solutions were sonicated for about 60 min (50 W, 40 kHz) in an ultrasonicator (Analytical Instrument Inc., Shanghai, KQ-250DB) followed by centrifugation (LXJ-ΙΙ, Analytical Instrument Inc., Shanghai) at 2000 rpm for 10 min to remove nondispersed MWCNTs. In this instance, the precise amount of MWCNTs dispersed in EAN is not known. However, in contrast to the initial MWCNTs added, the amount of precipitates is so small that it can be negligible for the study. So, the calculation of the concentration of MWNTs in weight percent is based on the assumption that all MWCNTs are in the solution. Synthesis of Hexagonal Lyotropic Liquid Crystals in EAN. The phase diagram of the binary C16EO6/EAN system shows the formation of an H1 phase between 43 and 68 wt % C16EO6 at a temperature ranging from 288 to 338 K.16 The H1 phase LLCs was achieved by heating the EAN solution of 50 wt % C16EO6 at 343 K for 20 min, followed by equilibration at 298 K for at least 1 week. Synthesis of MWCNTs-LLC Composites. RTIL dispersions of MWCNTs at different concentrations were added to the preformed LLCs, followed by addition of a certain amount of C16EO6 in order that the total concentration of C16EO6 was maintained at 50 wt % in all samples. This C16EO6 concentration was chosen to avoid phase separation. After an appropriate amount of dispersed MWCNTs was added to the LLCs, the samples were heated to 343 K in order to achieve the homogeneous phase. This procedure was followed by sonicating for 1 h at 343 K (50 W, 40 kHz) and centrifugation for 10 min at 2000 rpm to remove air bubbles. Before measurement, all samples were equilibrated in a constant-temperature cabinet at 298 K for at least 4 week. Characterization. Polarized optical microscopy examination was carried out with a Zeiss Axioskop 40 with cross-polarizers. Samples (21) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89-96.
were placed between thin microslides (all slides were washed in acetone and dried at room temperature before use). In order to avoid the influence of shearing due to contact with the glass, the thin microslides with the sample sandwiched between them were all stored at 298 K for at least 1 week. Small-angle X-ray scattering (SAXS) measurements were carried out at 298 K on a HMBG-SAX X-ray small angle system (Austria) with Ni-filtered Cu KR radiation of 0.1542 nm operating at 50 kV and 40 mA. The scattering patterns were recorded with a linear position-sensitive detector (OED 50M from Mbraun) containing 1024 channels of width 54 mm. The range of scattering vector was chosen from q ) 0.05 to 3 nm-1 (q ) 4π/(λ) sin(θ)/2, with θ and λ being, respectively, the scattering angle and incident X-ray wavelength of 0.1542 nm). The distance from sample to detector was 27.7 cm, and the exposure time was 600 s for each sample. Samples were prepared by filling the composite or LLCs into stainless steel cells and vacuum sealing them. Raman spectra were obtained using NEXUS 670, NXR FT-Raman Modular Laser Raman Spectrometer (detector, InGaAs; beamsplitter, CaF2) with laser operating at excitation wavelength of 1064 nm. The number of sample scans is 1000. The spectral resolution is 4 cm-1. Fourier transform Infrared (FT-IR) spectroscopy was conduced with AVATAR 370 infrared spectrometer (Thermo Nicolet USA). The samples were coated on the KBr plate. The spectra were collected from 400 to 4000 cm-1 with a resolution of 2 cm-1. Rheology measurements were carried out at a Haak Rheostree RS75 stress-control rheometer using a cone-plate fixture (Ti; radius, 20 mm; cone angle, 1°). The cone-plate distance was adjusted to 52 µm for all measurements. The temperature of the samples was kept at 25 ( 0.1 °C. The viscoelastic properties of all samples were determined from the oscillatory measurements in the frequency range from 0.01 to 10 Hz. Frequency sweep measurements were performed in the linear viscoelastic regime, which was determined by strain sweep measurements. The tribological behavior of EAN, LLCs, and MWCNTs-LLC was evaluated on a universal UMT-2MT friction and wear tester under the load of 0.5 N for the steel-steel frictional pair. The plate specimen was an AISI-52100 steel plate, coupled with an AISI52100 steel ball of 3.0 mm diameter. All the tests were conducted at the frequency of 5 Hz and 3 min of test duration. The wear volume was conducted by a MicroMAX surface mapping microscope (resolutions of 1 nm in vertical and 500 nm in lateral, ADE PHASE SHIFT). 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 Hz NMR spectrometer operating in the Fourier transform mode with quadrature detection. D2O served as solvent and calibration.
Results and Discussion The phase-equilibrated samples of LLCs formed in EAN and LLCs on addition of 0.2 wt % MWCNTs are shown in Figure 1a,b. MWCNTs were uniformly dispersed in the LLCs. One does not find any precipitates or phase separation in the MWCNTs-LLC composite after 6 months. As a contrast, the sample in which the MWCNTs were directly dispersed in the LLCs without using C16EO6 to disperse them in EAN beforehand
MWCNTs-LLC Composites
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Figure 2. Polarized optical microscopy images of LLCs (a) and MWCNTs-LLC composites (b).
Figure 5. d-spacing of the (100) peak of MWCNTs-LLC composites as a function of MWCNTs concentration.
Figure 3. Polarized optical micrograph of MWCNTs-LLC composites with a composition of 0.2 wt % MWCNTs at large magnification. The arrow indicates the alignment of MWCNTs along the LC director, n.13,15
Figure 6. FT-IR spectra of LLCs (a), MWCNTs (b), and MWCNTsLLC composites with 0.2 wt % MWCNTs (c).
Figure 7. Raman spectra of the MWCNTs, LLCs, and MWCNTsLLC composites with 0.2 wt % MWCNTs.
Figure 4. SAXS patterns of LLCs (a) and MWCNTs-LLC composites with different concentrations of MWCNTs. The amounts of MWCNTs from b to e are 0.05 wt %, 0.10 wt %, 0.15 wt %, and 0.2 wt %.
was prepared, and a great amount of MWCNTs deposition and aggregation appeared, indicating that MWCNTs were dispersed inhomogeneously in this instance. In the absence of added MWCNTs, the system is a hexagonal LLCs phase, demonstrated by the fanlike optical texture. A typical polarized optical microscopy (POM) image of fanlike texture is presented in Figure 2a. With added 0.2 wt % MWCNTs, the system retains the hexagonal LLCs phase demonstrated by the fanlike texture in Figure 2b, indicating that the introduction of MWCNTs does not destroy the structure of the hexagonal LLCs phase. The orientational behavior of the MWCNTs in LLCs can clearly be observed from the polarizing light micrograph at high magnification, as shown in Figure 3. The great mass of MWCNTs,
the black line in Figure 3, orients parallel to the director n of the hexagonal-phase LLCs,13,15 indicating that the LLCs matrix imposes a certain alignment on the MWCNTs. Further structural information can be obtained by small-angle X-ray scattering (SAXS) measurements. Figure 4 shows the SAXS patterns of LLCs and MWCNTs-LLC composite with different MWCNTs concentrations. All the samples were equilibrated for 6 months at T ) 25.0 ( 0.1 °C before SAXS measurements. Similar scattering curves were obtained, demonstrating that the dispersion of MWCNTs in LLCs does not destroy the structure of the hexagonal LLCs phase. Typically, two scattering peaks appear with relative positions of 1:x3, which corresponds to the 100 and 110 planes of a hexagonal liquid crystals.22 The lattice spacing, d (calculated from the 100 peak position 2π/q100), is 4.85 nm for the LLCs. It can be seen from Figure 4 that by increasing the concentration of MWCNTs in the hexagonal LLCs phase there is a q-shift for the scattering peaks forward small q value, corresponding to the increase of d spacing. The variation in d spacing with MWCNTs concentration is shown in Figure (22) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 1149-1158.
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Figure 8. Rheograms for the LLCs (a) and MWCNTs-LLC composites with 0.2 wt % MWCNTs (b). Plots of the storage modulus G′, loss modulus G′′, and complex vesicosity |η*| as a function of the frequency. T ) 25.0 ( 0.1 °C.
5. The increase in d spacing might be attributed to swelling induced by the dispersed MWCNTs within the cylinders of the hexagonal LLCs. Interestingly, a similar increase of d spacing is also observed for the intergration of single-walled carbon nanotubes (SWCNTs) into hexagonal LLCs formed in aqueous solution.15 Herein, for the first time, the integration of MWCNTs into hexagonal LLCs formed in RTILs is shown, inducing the increase of d spacing. The incorporation of MWCNTs in the LLCs can be detected by FT-IR and Raman spectroscopy. A comparison of the FT-IR spectra of LLCs, raw MWCNTs, and MWCNTs-LLC composites is shown in Figure 6. For LLCs, there is no obvious peak in this spectral region. The FT-IR spectra of raw MWCNTs and MWCNTs-LLC composites are very similar. The strong peak near 1558 cm-1 attributed to -CdC- group vibration in the hexagonal carbon cycle of CNTs23 appears, but the peak of MWCNTs-LLC composites originally around 1558 cm-1 shifts to a slightly lower frequency, completely different from the spectrum of LLCs. The Raman spectra of LLCs, raw MWCNTs, and MWCNTsLLC composites are shown in Figure 7. For the MWCNTs, it shows a strong peak around 1586 cm-1 (G-band) and a weak peak near 1284 cm-1 (D-band), which are the characteristic Raman peaks of MWCNTs.24,25 The G-band corresponds to an E2g mode of graphite and is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice.25 Nanotubes with concentric multiwalled layers of hexagonal carbon lattice display the same vibration.26 The D-band is associated with vibrations of carbon atoms with dangling bonds in plane terminations of disordered graphite or glassy carbons.25 The LLCs have high Raman activity in this range; a strong peak at 1043 cm-1 and three weak peaks near 715, 872, and 1459 cm-1 emerge. For the MWCNTs-LLC composites, besides the peaks emerging in the spectra of LLCs, the strong peak near 1588 cm-1 and weak peak around 1278 cm-1 are present, corresponding to the G-band and D-band of MWCNTs. The combination of FT-IR and Raman spectra results further indicate that the MWCNTs were really incorporated into the LLCs. Moreover, the properties of MWCNTs were not affected by our experimental conditions. The macroproperties of LLCs and MWCNTs-LLC composites with different amounts of MWCNTs were characterized by rheological measurements of the oscillatory shear. We found that all the samples show similar rheological behavior. Two typical rheograms of the oscillatory shear for two samples of LLCs and MWCNTs-LLC composites with 0.2 wt % MWCNTs at 25.0 (23) Zhang, J.; Zou, H.; Qing, Q.; Yang, Y.; Li, Q.; Liu, Z.; Guo, X.; Du, Z. J. Phys. Chem. B 2003, 107, 3712-3718. (24) Chang, J. Y.; Ghule, A.; Chang, J. J.; Tzing, S. H.; Ling, Y. C. Chem. Phys. Lett. 2002, 363, 583-590. (25) Moreno, J. M. C.; Yoshimura, M. J. Am. Chem. Soc. 2001, 123, 741-742. (26) Kasuya, A.; Sasaki, Y. Phys. ReV. Lett. 1997, 78, 4434-4437.
Figure 9. Steady-shear rheological data for LLCs and MWCNTsLLC composites with different concentrations of MWCNTs. All data were collected at 25.0 ( 0.1 °C.
( 0.1 °C are shown in Figure 8. By comparing these two rheograms, one can see that the rheological behavior of these two samples is similar. Both storage modulus (G′) and loss modulus (G′′) are frequency-dependent; they all increase gradually with the increase of frequency, and what’s more, the slope of G′ is larger than that of G′′. In the low-frequency range, G′′ slightly dominates, while G′ is higher than G′′ in the higherfrequency range. The complex viscosity (|η*|) decreases linearly with a slope of -1 over the whole frequency range from 0.01 to 10 Hz. To further explore the influence of the introduction of MWCNTs on the macroscopic properties of the hexagonal LLC phase, steady-shear measurements of LLCs and MWCNTsLLC composites with different amounts of MWCNTs were carried out. Typical steady-shear rheograms (viscosity as a function of shear rate) are shown in Figure 9 for LLCs and MWCNTs-LLC composites with different amounts of MWCNTs. For all samples, the apparent viscosity, η, decreases gradually with increasing shear rate in the whole range measured, indicating shear-thinning behavior. At high shear rate, the curve is irregular with the variation of shear rate, indicating that the structure of the samples has been destroyed by the high shear rate. By comparing the steady-shear rheograms before and after adding MWCNTs, one can see that the apparent viscosity of MWCNTs-LLC is higher than that of LLCs at the same shear rate. Moreover, the η values of the MWCNTs-LLC composites are affected by the concentrations of the MWCNTs, which were shown in Figure 10. The apparent viscosity increases obviously with the increase amount of MWCNTs, which also indicates the incorporation of MWCNTs in LLCs matrix from another aspect. To conclude, the LLCs and MWCNTs-LLC composites are highly viscoelastic. The presence of MWCNTs does not influence the rheological properties of the system obviously; however, the apparent viscosity is enhanced due to the introduction of MWCNTs. The potential application of MWCNTs incorporated in LLCs formed in EAN acts as the lubricating materials to be explored.
MWCNTs-LLC Composites
Figure 10. Variation of apparent viscosity of MWCNTs-LLC composites with the concentration of MWCNTs. The shear rate is 1.5 s-1.
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Figure 12. Wear volumes of EAN, LLCs and MWCNTs-LLC composites with 0.2 wt % MWCNTs on the steel disc after sliding 180 s under the load of 0.5 N.
volume is 2.013 × 105 µm3 by lubricating with EAN. LLCs exhibit better antiwearability than EAN under the same load. It is speculated that an LLCs protecting layer between the two mating wear surfaces makes the shear easy. When the MWCNTs are incorporated into LLCs, the antiwearability of MWCNTsLLC composites become weaker than the LLCs. The wear volume increases to 3.28 × 105 µm3, which might be because the integrity of the LLCs layer is destroyed by the introduction of MWCNTs.
Conclusions Figure 11. Variation of the friction coefficients of EAN (black), LLCs (red), and MWCNTs-LLC composite with 0.2 wt % MWCNTs (blue) with time under the load of 0.5 N.
The typical tribological behavior of three samples: pure EAN, LLCs, and MWCNTs-LLC composites were compared. The variation of friction coefficients (FCs) of the three samples with time under the load of 0.5 N were shown in Figure 11. The FC of EAN is 0.2502. LLCs have the value of FC 0.1173, which is smaller than that of EAN. It is presumed that there is a stronger interaction between the LLCs film and the substrate, but EAN does not moisten the substrate. Thus, in comparison to EAN, the LLCs can form a stronger protecting layer on the substrate. When MWNTs are dispersed into the LLCs, the FC value reduces to 0.1053. It might be attributed to the superior mechanical properties of MWNTs, which might play the role of roller bearing during friction process. To further confirm the tribological behavior of EAN, LLCs, and MWCNTs-LLC composites, the wear volumes (Figure 12) on a steel disc after sliding 180 s under the load of 0.5 N were measured. Under the experimental loading condition, the wear
To conclude, integration of MWCNTs into the hexagonal lyotropic liquid crystals (H1 phase) formed in room-temperature ionic liquid is demonstrated. MWCNTs-LLC composites preserved the typical characteristic fanlike texture of hexagonal LLCs, and SAXS results indicate that the MWCNTs integrated within the cylinders of hexagonal LLCs, as manifested by the increase of d spacing, indicating that the LLCs formed in RTIL can impose a certain degree of alignment on the CNTs along its director. The FT-IR and Raman spectra results further confirm that the MWCNTs have been incorporated in the LLC phase. The rheological results indicate that MWCNTs-LLC composites are highly viscoelastic and that the apparent viscosity is enhanced due to the introduction of the MWCNTs. The research of tribological behavior indicates that MWCNTs-LLC composites show special tribological character, illuminating the potential applications in the tribological field as lubricating materials. Acknowledgment. We are grateful for financial support from the NSFC (grant nos. 20625307, 20533050 (J. Hao), and 20533080). LA700921W
