78 Chemical Science A variety of soft electrical conductors have been developed by doping carbon nanotubes (CNTs) into organic and polymeric materials, where better electrical properties are realized by the dispersion of a larger amount of CNTs. Recently, liquid crystalline (LC) materials have attracted increasing attention for hybridization with CNTs, since LC materials have the potential to orient CNTs for anisotropic electrical conduction. However, the reported LCs possess rather low miscibility with pristine CNTs of less than 0.01 wt%. Here we report that discotic ionic liquid crystals (ILCs) of triphenylene (TP) derivatives, ILC col and ILC cub ( Fig. 1(a) ) [1,2] bearing six imidazolium ion pendants, serve as excellent dispersants for pristine single-walled CNTs (SWNTs) [3]. The resultant composite materials can maintain their LC properties up to the SWNT content of ~ 8 wt%, which is 2 – 3 orders of magnitude greater than those reported previously. Of further interest, the ILC composites, when sheared, display anisotropic conducting properties, since SWNTs are oriented along the shear direction. This orientation is kept for at least half a year. In 2003, we reported that imidazolium ion-based ionic liquids (ILs), when being ground with SWNTs, are transformed into physical gels (bucky gels), where SWNTs are highly dispersed by a p-cation/p-electronic interaction and eventually form a 3D network structure associated with an interionic interaction of ILs [4]. As reported previously [1,2], the discotic ionic liquid crystals ILC col and ILC cub ( Fig. 1(a) ), utilized for the present study, assemble into hexagonal columnar (Col h , 134 – 18°C on cooling) and cubic (Cub, 221 – 18°C on cooling) mesophases, respectively, over a wide temperature range including room temperature [2]. As a typical example of the hybridization of ILCs with SWNTs, pristine HiPco SWNTs were added at 150°C to ILC col (isotropic melt) with an SWNT content of 5 wt%, and the mixture was ground with a pestle for 30 min ( Fig. 1(b) ), whereupon it turned to a viscous black paste ( Fig. 1(b) , inset ). As observed by optical and cross-sectional transmission electron microscopy, only a very small amount of SWNT agglomerates was detected in the black paste. Similar black pastes were obtained when the SWNT contents employed were in the range of 3 –15 wt%. However, upon further increase in the doping level to 30 wt%, the mixture lost its fluidity and became semisolid. Although doped SWNTs affected the phase diagram of ILC col to a certain extent, the LC clearing temperature decreased only by 18°C (134 → 116°C) when ILC col was doped with even 5 wt% SWNTs. Likewise, SWNTs dispersed very well in ILC cub , where the resulting composite again displayed only a marginal decrease in the LC clearing temperature (221 → 205°C on 5 wt% SWNT doping). Of interest, the effects of doped SWNTs on the X-ray diffraction (XRD) profiles of ILC col and ILC cub , observed at BL44B2 , were essentially different from one another ( Fig. 2). In the case of ILC cub , as the doping level of SWNTs was higher, the diffraction peaks shifted toward a lower 2 θ region ( Fig. 2(b) , inset ). In contrast, the diffraction peaks of ILC col hardly shifted upon increment of the content of SWNTs (Fig. 2(a), inset ). These contrasting observations indicate that ILC cub incorporates SWNTs into its cubic lattice, whereas ILC col prefers to maintain its original 2D hexagonal lattice without incorporating SWNTs. In the course of the above studies, we noticed that the discotic LC columns of ILC col , upon being doped with SWNTs, align homeotropically with respect to the substrate surface, while SWNTs are randomly oriented just as they are in the isotropic melt. However, a shear treatment of the ILC/SWNT composites induces a horizontal orientation of SWNTs with respect to the substrate surface. Accordingly, shear treatment in combination with annealing gave rise to three different states States 1 – 3 in terms of the orientations of the LC columns and SWNTs (Fig. 3). Namely, the ILC col / SWNT composite, upon shear treatment, gave State 1 , where both LC columns and SWNTs were coaxially oriented horizontally with respect to the glass plates. Subsequent short annealing of the sheared composite for 5 min gave State 2 , where the LC columns were oriented homeotropically, while maintaining the horizontal orientation of SWNTs. Further annealing Discotic ionic liquid crystals as excellent dispersants for single-walled carbon nanotubes R(CH 2 ) n O R(CH 2 ) n O R(CH 2 ) n O O(CH 2 ) n R O(CH 2 ) n R O(CH 2 ) n R ILC col : n = 10, R = Im, X = Me LC col : n = 5, R = Me IL C 6 : C 6 H 13 Im, X = H ILC cub : n = 14, R = Im, X = H Im = (BF 4 – Salt) + N Me N X (b) (a) Fig. 1. (a) Molecular structures of ionic liquid crystals ILC col and ILC cub , together with those of nonionic liquid crystal LC col and monovalent ionic liquid IL C6 as references. (b) Pictures of an ILC col /SWNT composite. 79 of the composite for 1 h gave State 3 , where SWNTs were oriented randomly in the homeotropically oriented LC columns. The horizontal orientation of SWNTs, once generated by the shear treatment, was maintained for at least half a year at room temperature unless the material was heated. Namely, thermal reorientation of SWNTs occurs much less rapidly than that of the LC columns. The orientation of SWNTs is a dominant factor for charge-carrier transport properties of the ILC col / SWNT composite (Fig. 3). When ILC col doped with 1 wt% SWNTs was sandwiched by indium tin oxide (ITO) electrodes, the DC conductivity across the film at 25°C in State 1 (red circles) was nearly two orders of magnitude smaller than that in State 3 (black squares). In contrast, State 1 and State 2 (blue rhomboids) were comparable to one another in terms of DC conductivity, indicating a negligibly small contribution of the LC columns to the observed conduction profile. On the other hand, at higher doping levels of SWNTs (3 and 5 wt%), the three states showed a much smaller difference (~20 times) in their DC conductivities, possibly owing to an increased probability of carrier hopping between SWNTs. (a) (b) Intensity (arb. units) Intensity (arb. units) 5 10 15 20 5 10 15 20 2 (degree) θ 2 (degree) θ (210) (110) (200) (211) (220) (321) (400) (420) (332) (422) (431) (521) 2.5 3.5 3 3.5 4.5 4 3 Fig. 2. XRD patterns (25°C) of ILC col (a) and ILC cub (b) alone (black), and their composites doped with 1 (blue), 3 (green), and 5 (red) wt% SWNTs. An X-ray beam with a wavelength of 1.08 Å was used. The asterisked diffraction at 2 θ < 2 degrees in each XRD pattern is due to scattering by SWNTs. SWNT Content /wt% Conductivity Across the Film / S cm –1 σ Fig. 3. Plots of conductivities across the film of States 1–3 at 25°C of ILC col films doped with 0, 1, 3, and 5 wt% SWNTs, sandwiched by ITO electrodes with a separation of 12.5 μ m. Jeongho Jay Lee a and Yohei Yamamoto b, * a Department of Chemistry and Biotechnology, The University of Tokyo b Division of Materials Science and TIMS, University of Tsukuba *Email: yamamoto@ims.tsukuba.ac.jp References [1] J. Motoyanagi et al .: Chem. Commun. (2005) 101. [2] M.A. Alam et al .: J. Am. Chem. Soc. 131 (2009) 17722. [3] J.J. Lee, A. Yamaguchi, M.A. Alam, Y. Yamamoto, T. Fukushima, K. Kato, M. Takata, N. Fujita and T. Aida: Angew. Chem. Int. Ed. 51 (2012) 8490. [4] T. Fukushima et al .: Science 300 (2003) 2072.
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