Dissolving and Aligning Carbon Nanotubes in Thermotropic Liquid

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Dissolving and Aligning Carbon Nanotubes in Thermotropic Liquid Crystals Yan Ji, Yan Yan Huang,* and Eugene M. Terentjev Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom

bS Supporting Information ABSTRACT: It has been widely recognized that the combination of carbon nanotubes (CNTs) and low molar mass thermotropic liquid crystals (tLCs) not only provides a useful way to align CNTs, but also dramatically enhances the tLC performance especially in the liquid crystal display technology. Such CNT-tLC nanocomposites have ignited hopes to address many stubborn problems within the field, such as low contrast, slow response, and narrow view angle. However, this material development has been limited by the poor solubility of CNTs in tLCs. Here, we describe an effective strategy to solve the problem. Prior to integrating with tLCs, pristine CNTs are physically “coated” by a liquid crystalline polymer (LCP) which is compatible with tLCs. The homogeneous CNT-tLC composite obtained in this way is stable for over 6 months, and the concentration of CNTs in tLCs can reach 1 wt %. We further demonstrate the alignment of CNTs at high CNT concentrations by an electric field with a theory to model the impedance response of the CNT-tLC mixture.

’ INTRODUCTION Since their discovery, carbon nanotubes (CNTs) have attracted overwhelming attention across various disciplines, ranging from nanoelectronics to biochemical sensors and other material-related high-performance applications. For a large number of the proposed applications, the challenge lies in the dispersion and alignment of CNTs to take full advantage of their highly anisotropic thermal, electrical, and optical characteristics. Driven by scientific interest as well as commercial demands, incorporating CNTs into low molecular mass thermotropic liquid crystals (tLCs) has become an active research front in recent years.1
4 On one hand, the flexible orientational order of tLCs provides a facile approach to efficiently align CNTs5 and the CNT alignment could be dynamically manipulated by an electric or magnetic field.6
8 On the other hand, CNTs could induce distinctive changes to the physical properties of the tLC matrix, leading to enhanced performances of tLC. In the field of display technology, profound benefits of introducing CNTs into a tLC matrix include increased dielectric anisotropy, decreased threshold voltage, restrained field-screening effect, and much accelerated electrooptical response, to name just a few.9
14 Using photorefractive devices as an example, even a very small amount of CNTs have been shown to produce a enhanced nonlinear optical effect accompanied by a 1000 enhancement in refractive index.15 These findings also raise expectations in other areas of novel optical and optoelectronic materials.1
4 Although initial results seemed to be promising, further performance enhancement is inhibited by the immiscibility of CNTs and tLCs.1
3,16 Vigorous van der Waals interaction and r 2011 American Chemical Society

high aspect ratio compel CNTs to aggregate fervently whenever possible. In spite of many efforts to facilitate dispersion, agglomerates in the form of bundles or clusters are often easily visible in the CNT-tLC mixture, by an optical microscopy and even often by naked eyes.7 In fact, researchers found that, when equilibrium is reached in the CNT-tLC dispersions, the CNT concentration is as low as 10
9 wt %, even though the CNTs have been chemically functionalized to have better solubility in tLCs than pristine CNTs.15 When the CNT concentration is above the equilibrium concentration, the CNT-tLC mixtures are invariably unstable. Phase-separation starts immediately after preparation. As a result, the physical properties of CNT-tLCs become complicated and not reproducible. It is in fact difficult to assess whether the variety of reported physical phenomena resulted from the effects of the dispersed CNTs or the packed aggregates. Some guidelines have been recently developed to optimize the dispersion quality.19 Later, as the best case in literature, 0.01% CNTs could be homogenously dispersed in tLCs with the aid of some small molecular stabilizers.20 With these stabilizers, the CNT-tLC could remain stable for one week. Long-term stability and high loading of CNTs remain an unsolved challenge. Even though high concentrations of CNTs in polymeric and oligomeric liquid crystals have been achieved,21,22 the dispersion of CNTs in low molecular mass tLCs still lags behind. In comparison with the case of ordinary solvent or polymer matrix, high Received: July 20, 2011 Revised: September 9, 2011 Published: September 18, 2011 13254

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Figure 1. (a) Chemical structure of the polymer surfactant PCMSC. (b) Homogenous dispersion of MWNTs in chloroform when PSMSC is used. (c) SEM showing the dispersion morphologies of the MWNTs dispersed in (b), after solution drop-casting and surfactant removal by washing the sample with chloroform. (d) SEM image showing MWNTs in droplets of tLCs on a conducting ITO substrate. MWNT dimension: diameter 60
100 nm, as-produced length 5
15 μm.

loadings of CNTs in tLCs become even more difficult because adding too much conventional dispersant17 would disrupt the ordering of the liquid crystalline phase. Here, we solve the immiscibility problem between CNTs and tLCs with the help of a properly designed liquid crystalline polymer (LCP), which acts as a multifunctional dispersion promoter (although a term “surfactant” would be more appropriate, in spite of the system not involving any water). In general, LCPs are made of mesogens which have chemical structures resembling low molar mass tLCs and form liquid crystalline phases following similar principles to those of tLCs. With proper chemical structure design, LCPs could be tailored to have good compatibility with tLCs while preserving the liquid crystalline phases of the host tLCs. We demonstrated recently that debundled CNTs may be encapsulated by LCPs after solvent evaporation, thus preventing the isolated tubes from reaggregating.21 Therefore, it is reasonable to suppose that CNTs may be dispersed well into tLC matrix after they first form complexes with LCPs. On the basis of this hypothesis, we developed a LCP that not only has a high affinity to CNTs, but also can help homogenously disperse and stabilize pristine CNTs into tLCs. Meanwhile, the alignment of CNTs by electric field was demonstrated and analyzed with the obtained CNT-tLC nanocomposites.

’ EXPERIMENTAL SECTION LCP Synthesis. The chemical structure of the designed LCP (labeled as PCMSC, shortened from pyrene-cinnamate-MBB Side Chain) is shown in Figure 1a. Monomers 4-methoxyphenyl-40 -buteneoxy benzoate (MBB) and undec-10-enyl 4-(pyren-2-yl) butanoate (UPB) were synthesized according to ref 21. The undec-10-enyl cinnamate (UC) was synthesized by a similar procedure to UPB. For preparation of the PCMSC, MBB, UPB, and UC (85:5:10 molar ratio) were added to the polymethyl hydrogen silxoane (PMHS, Aldrich) with a total ratio of vinyl in the mesogens to the Si
H in PMHS of 1.1:1. Dry toluene was used as a solvent. The mixture was heated to 80
C under nitrogen with magnetic stirring. After the addition of Karstedt’s platinum catalyst, the reaction was carried out at 80
C until the infrared spectra

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showed no Si
H peak. After the mixture was cooled down to room temperature, PCMSC was precipitated by the addition of methanol. The crude product was boiled with silica gel and activated carbon to get rid of the catalyst. The polymer was then further purified by a short column (first petroleum ether/ethyl acetate = 9:1, then acetone) to get rid of all the unreacted monomers. Preparation of CNT-tLC Composite. Two types of MWNTs were tested. The first type is of larger diameter. They are obtained from the Nanostructured & Amorphous Materials, Inc. (CVD, purity 95+%, diameter 60
100 nm, as-produced length 5
15 μm) with no functionalization. The other type of MWNTs was purchased from Cheaptubes. com (carbonaceous purity of 90%, diameter 10
20 nm, length 500 nm to 2 μm). To prepare the CNT
solvent solution for the fabrication of tLC composites, 0.1 g PCMSC and 0.01 g CNT were added in 13 mL chloroform. This solution was then tip-sonicated for 30 min. The resulting dispersion was dried in a fume cupboard overnight. The residue was added with 3 mL of chloroform and held still for 2 days. Subsequently, the upper 90% layer was carefully taken out the vial. The concentration of such CNT solution was estimated by the absorption at 750 nm (see the SI for detailed measurement). The LCP to CNT ratio was calculated to be about 13:1. It is noted that further optimization may give an even better result such as using less PCMSC. An Ultrasonic Processor (Cole Parmer, 750 W) with titanium microtip was used for sonication. The following sonication parameter settings were maintained throughout all experiments: pulse rate with 5 s on and 3 s off, probe temperature held at 15
C, and vibration amplitude at 25%. An appropriate volume of PCMSC/CNT—chloroform solution from above was mixed with tLCs. The solvent was evaporated by heating with a hot-gun while the sample was stirred mechanically. The resulting mixture was further vacuum-dried at 50
C overnight. Scanning Electron Micrographs (SEM). SEM images of all CNT samples were obtained on FEI XL30-SFEG high-resolution scanning electron microscope. For all SEM characterizations, the MWNTs are of the larger type (diameter 60
100 nm, as-produced length 5
15 μm). The sample in Figure 1c was prepared by first dropcasting the as-sonicated CNT-PCMSC solution onto an ITO glass or a metallic stub, after which a few drops of chloroform were used to dissolve and wash off the excess PCMSC, leaving the residue chloroform evaporating in open air. In order to reveal the nanotubes, the top surface of the film was first lightly etched by chloroform. For the CNTcontaining cyclic cholesteric siloxane LC islands (Figure 1d), the sample was prepared by drop-casting the solution onto a conducting indium tin oxide glass (ITO, polyimide-free). The sample was then placed in an oven (at 70
C) to age overnight before the SEM imaging. Differential Scanning Calorimetry (DSC). We prepared samples with the concentrations of CNTs of 0.2 wt %, 0.5 wt %, 0.75 wt %, and 1 wt % in tLC using the PCMSC to CNT ratio of 13:1 w/w. The pure PCMSC and tLC mixture at the same ratio as those samples was also prepared as blank reference samples. Short MWNTs (diameter 10
20 nm, length 500 nm to 2 μm) were used. The equilibrium transition temperatures given in SI Table S-1 were determined on a Perkin-Elmer, Pyris 1 differential scanning calorimeter with a cooling rate of 10
C/min. AC Impedance Spectroscopy. Complex resistivity was analyzed by Wayne Kerr Precision Component Analyzer 6440A, where in-phase and out-of-phase currents were measured at a supplied voltage (rootmean-square voltage) of 0.1 V, obtaining the complex impedance and the phase angle values. The dielectric properties of the CNT-tLC mixtures were tested using the liquid crystalline cells filled with the same samples used for DSC test. The concentration of PCMSC in the reference PCMSC-tLC mixture is 13 wt %. Short MWNTs (diameter 10
20 nm, length 500 nm to 2 μm) were used. 13255

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’ RESULTS AND DISCUSSION When we design the chemical structure of the LCP, a polysiloxane backbone is used instead of the acrylate analogues, or the main-chain LCP varieties, because it has greater chain flexibility, which leads to reduced viscosity, glass transition, and surface tension. The polysiloxane backbone in PCMSC is decorated with three different moieties having different functionalities. The pyrene group is introduced to improve the affinity of the LCP to CNTs, which acts like a “docking” moiety on the surface of CNTs due to the π-stacking. The cinnamate group allows initiator-free photo-cross-linking of polymer chains into an elastomer matrix, if future applications require the composite to permanently maintain the alignment of CNTs. The main mesogenic group used for PCMSC is 4-methoxyphenyl-40 -buteneoxy benzoate (MBB). According to polarized optical microscopy (POM) and differential scanning calorimetry (DSC), PCMSC forms a nematic phase below 39
C with a glass transition temperature (Tg) of around
3
C in the heating cycle. The PCMSC is soluble in many common organic solvents such as chloroform, tetrafuran, dichloromethane, and toluene. To introduce CNTs into a tLC host, CNTs were first dispersed in an organic solvent in the presence of PCMSC. Pristine CNTs exhibit extremely low solubility in these solvents; chloroform, for example, can only dissolve CNTs up to a maximum concentration of 0.001 mg/mL.23 Here, we use multiwalled carbon nanotubes (MWNTs). PCMSC acts as an effective surfactant which significantly enhances the solubility of MWNTs in organic solvent. For example, after 0.01 g PCMSC and 0.001 g MWNTs were mixed with 10 mL chloroform, 15 min ultrasonication24 gave a homogeneous solution of MWNTs (Figure 1b). As seen by SEM, MWNTs were individually dispersed (Figure 1c). Similar to previous studies, we found that, when MWNTs were present, there was a strong suppression of the fluorescence spectrum characteristic to the pyrene attachment in PCMSC.21 This effect signifies the close interaction and association between pyrene groups and the MWNTs. Subsequently, the MWNT-PCMSC in a solvent was mixed with tLCs. Simply, after the solvent evaporation, we obtained tLC composites with well-dispersed MWNTs. Due to our longlasting interest in siloxane-related materials, the PCMSC was designed here for the dispersion of MWNTs into organosiloxane liquid crystals. Siloxane moiety improves the complexity of tLC molecules, reduces crystallization, and stabilizes the desired mesophases into wide temperature ranges.25 In addition to organosiloxane tLCs, we found that PCMSC is also suitable for the dispersion of MWNTs in some fluorinated tLC see the SI for the chemical structures of tested tLCs). We suppose that the solubility of CNTs in tLCs depends on the CNT types (especially single vs multiwalled CNTs, polydispersity of CNTs, and their dimensions), the chemical structure of tLCs, the ratio of PCMSC to CNTs in the solution and the sample preparation details (such as sonication time, solvent type, and so on). Further systematic investigation is necessary to fully elucidate the relationship between CNT solubility in tLC and the related controlling factors. In terms of assessing the dispersion quality of CNT-tLC, very limited characterization techniques are currently available in this field. Normally, macroscopic observation is a primary method that researchers use to differentiate good dispersion from bad aggregation,19,20 because, commonly, the aggregates of CNTs in tLCs could be easily observed by eyes. To simplify the evaluation

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Figure 2. (a) MWNT-SiCB mixtures of different concentrations: upper row, mixtures with no PCMSC; bottom row, mixtures with PCMSC. The samples are at the bottom of small glass vials, which are inverted in this photo. Polarized microscopy images show the stardard liquid crystal cells (5 μm gap) with (b) pure SiCB; (c) pure SiCB - PCMSC 100:13 w/w mixture; (d) SiCB with PCMSC/MWNT complexes at a CNT concentration of 1 wt % MWNT, where the top left corner shows the image taken immediately after preparation, and the bottom right corner shows the same mixture after 6 months standing. The insets of (b), (c), and (d) are the POM image of the samples after SiCB crystallized. MWNT dimension: diameter 10
20 nm, length 500 nm to 2 μm. Scale bar = 1 mm.

of the dispersing efficiency of PCMSC, here, unless otherwise noted, we choose a simple siloxane tLC based on cyanobiphenyl mesogenic moiety as a model tLC for further characterizations. Analogues of this particular tLC have been identified as promising materials for low-cost and energy-saving bistable display, and fast switching storage devices.26 MWNTs with diameters of 10
20 nm and lengths of 500 nm to 2 μm were used unless otherwise stated. The chemical structure of this tLC is shown in Figure 2 and is abbreviated as SiCB. Without PCMSC, pristine MWNTs form aggregates in SiCB shortly after mixing. To make those aggregates easily photographed, we took the picture of inverted sample vials, as shown in the upper row of Figure 2a. In contrast, with PCMSC, no aggregates could be observed any more (Figure 2a lower row). The samples look very homogeneous macroscopically. To further characterize the dispersion, optical microscopy was employed. In the general research area of tLC-nanoparticle composites (including tLC
CNT nanocomposites), few techniques can be used to verify the quality of dispersion of nanoparticles in tLCs. Therefore, it is generally accepted that the dispersion could be regarded as a good dispersion if no aggregates of nanoparticles are observed under optical microscopy.27,28 Without PCMSC, the large aggregates, of sizes of tens of micrometers, are present in the mixtures at a MWNT concentration as low as 0.001 wt % (Figure S-2 left in the Supporting Information). In contrast, SiCB containing the PCMSC/ MWNT complexes remain homogeneous even when the MWNT concentration is as high as 1 wt %. However, at a concentration of 1.1 wt %, we could see phase separation and CNT aggregates under optical microscopy. According to calorimetry and polarized 13256

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Figure 3. (a) Modulus of complex resistivity F* plotted against frequency, on a log
log scale to emphasize the classical dielectric (1/f) response, and deviations from that at low frequency and high CNT loading. (b) Change in the resistivity with voltage in isotropic phase at 10 kHz; the drop in effective resistivity is a reflection of nanotube segments aligned by field. (c) Theoretical plot of the nondimensional mean square orientational factor changing due to the CNT segment alignment by external field (presented in nondimensional form on the x-axis). (d) Plots of the data in (b) in the inverse form, fitted by the eq 3, to illustrate how the crossover voltage determines the polarizability α , while the amplitude of the rise determines the CNT segment density in the matrix. The CNTs are MWNTs with diameter of 10
20 nm and length of 500 nm to 2 μm.

optical microscopy, pure SiCB exhibits smectic-A phase below the smectic
isotropic transition at 27
C in the cooling cycle. The addition of PCMSC reduces the texture grain of the tLC, as shown by comparing Figure 2b and c. In addition, comparing the nanotube-free PCMSC-SiCB mixture and the nanotube-containing counterpart, in Figure 2c and d, respectively, the microstructure was minimally modified for a MWNT concentration as high as 1 wt %. Furthermore, no MWNT agglomerates are visible in all the areas of the samples. Therefore, our proposed dispersion method has improved the solubility of MWNTs in SiCB by 1000 times compared to the intrinsic solubility (which is lower than 0.001 wt %). Among the limited characterization techniques of CNTtLCs,18 SEM is considered as one of the more adequate techniques to directly characterize the dispersion quality of MWNT-tLC. For a tLC “composite” containing MWNTs with a concentration at a sufficiently higher level, it is possible that they can be imaged by SEM, similar to the common practice used for characterizing the dispersion of CNTs in polymers.29 Here, we choose big MWNTs (rather than the smaller sized MWNTs for the impedance measurements) with diameters of 60
100 nm and lengths of 5
15 μm, of which sizes are well above the resolution of conventional SEM. The SEM was captured with a cholesteric cyclic siloxane liquid crystal (LC1; chemical structure is shown in the Supporting Information) with 1 wt % MWNTs. This is because the big MWNTs above can be dispersed in LC1 at a higher concentration than in SiCB (even though the concentration of small MWNTs could reach 1 wt % in SiCB, this is ∼0.25 wt % for the large MWNTs). Due to the poor wetting

between the hydrophilic ITO surface and the hydrophobic LC1, the LC1 formed small droplets after slow solvent evaporation and oven aging at 70
C overnight. Therefore, we were able to image the individual long MWNTs residing in tLC matrix as shown in Figure 1d. PCMSC also provides long-term stability to the MWNT
SiCB mixtures. For MWNT
SiCB without PCMSC, MWNTs precipitate and almost totally sediment overnight. However, for MWNTs with PCMSC, even after SiCB crystallized after standing at room temperature for a long time, the samples still resemble the pure SiCB under POM (inset of Figure 2b,c,d) and no CNT association has been observed. After half a year, during which time the samples were repeatedly cycled between the isotropic and liquid crystalline phases, the smectic-A texture remains almost the same as that of the freshly prepared samples (right bottom corner of Figure 2d). Notably, the mixture is able to remain stable in the isotropic phase at 100
C, which is 70
C above the clearing point. This is in contrast to the ordinary CNT
tLC mixtures, where heating is known to dramatically accelerate CNT clustering, leading to severe sedimentation within days. In the literature, even at a CNT concentration of 0.005 wt %, the CNT
tLC mixture could contain sufficiently large aggregates that could not pass through the 20 μm liquid crystal (LC) cell gaps.9,11 Because the MWNT
tLC is stable at high temperatures, even though the high concentration of MWNTs has a profound effect on the flow properties of the composites (the higher the MWNT concentration, the higher the viscosity and the slower the MWNT
tLCs flow), these tLC composites at 13257

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)

)

)

)

increase of the underlying dielectric constant) saturates very quickly, just like the CNT embedded in other tLCs.9,11 It is wellknown that such alignment is directly related to the polarizability of CNTs in tLCs. However, there is much contradictory research in this area. In some cases, dramatic increase of electric conductivity of many orders of magnitude was observed after electric field exposure,6,31 while in others, such an increase was less obvious and dependent on the type of tLC.33 Meanwhile, the explanation of the drastic increase of electric conductivity varies. Some research attributed it to the alignment of CNTs,6 some associated it to CNT short-circuiting the cells, while others argued that it was due to the dielectric breakdown and local heating effects after the comparison of electric conductivity difference between electric field induced CNT alignment and the magnetic field induced CNT alignment.35 The discrepancy among researchers is understandable considering the complicated percolation behavior of CNT composite, which depends on the dispersion quality, the CNT dimension, the matrix, the orientation of CNTs, and so on.36,37 The percolation of the random network formed can be probed by analyzing the complex resistivity of the mixture at low frequencies (i.e., f = 100 Hz) in an AC impedance measurement when a low voltage level (Vpeak = 0.1 V) is employed. As shown in Figure 3a, an insignificant decrease in resistivity is observed for CNT loading increased up to 1 wt %. Therefore, we propose that the percolation threshold for the PCMSC/CNT-SiCB system is greater than 1 wt %. Meanwhile, unlike the large magnitude increase of electric conductivity with the alignment of CNTs in electric field,6,31,34,35 the increase in our case is relatively small and very similar to the reported conductivity change because of good CNT alignment induced by magnetic field in ref 35. The mild increase of electrical conductivity has also been reported in CNT
polymeric composites.38 One possible explanation is that the electric breakdown was prevented because CNTs were wrapped by insulating PCMSC.39 Another explanation may be that the CNTs in this system are well-dispersed so that they rarely touch each other to form a conductive network as explained in detail in ref 40, where no percolation occurs with CNT concentration below 3 wt %.40 Even though more systematic investigation is necessary to give a precise picture related to the electric conductivity and the alignment, we suppose that the observed electric resistivity decreases with applied voltage from the alignment of CNTs. Finally, we will investigate the origin of the F*
voltage behavior demonstrated by the impedance measurement. In the anisotropic medium, the dielectric permittivity has two principal values: ε along the director orientation and εp perpendicular to the director (εp < ε for our tLC, and most other mesogenic materials). Assuming that our locally conducting CNTs have a much higher polarizability along their axis than perpendicular CNT polarizability, and also that of the SiCB or surfactant PCMSC molecules, the microscopic polarizability α will be taken as solely due to the CNTs.41,42 The average macroscopic dielectric permittivity is then related to the polarizability by ε = 1 + ϕ(α /ε0), where ϕ is the average number density of straight CNT segments. With an individual CNT segment making an angle θ with the external field E, the effective dielectric permittivity along the field (i.e., perpendicular to the measuring cell, as in the experiments plotted in Figure 3) is given by the average )

isotropic phase can easily fill the commercial LC cells with a 5 μm gap. In addition to microscopic analysis, we also examined the effect of PCMSC/MWNTs on the SiCB phase transitions using differential scanning calorimetry (DSC; see DSC results in SI). No additional phase transition peaks characteristic of a phaseseparated system were detected in the calorimetric studies. Upon cooling, the incorporation of pure PCMSC changed the transition temperature from 25 to 29
C. The addition of MWNTs further increased the isotropic
smectic transition temperature to 29.5, 29.8, 30.5, and 30.8
C for 0.2 wt %, 0.5 wt %, 0.75 wt %, and 1 wt % of MWNTs, respectively. The increased transition with the addition of CNTs was normally attributed to the anchoring of tLC mesogens onto the CNT surface through π
π stacking.9,11,30 Since the LC mesogens are strongly bound to the CNTs, they require a greater amount of thermal energy to randomize, leading to a higher transition temperature. Some researcher also found that, when the CNT concentration increased further (e.g., from 0.05 wt % to 0.5 wt %), the transition started to drop because the severe agglomeration of CNTs reduced the contact between tLC and CNTs.31 Since the PCMSC could prevent the aggregation of CNTs, no similar drop of transition temperature is observed, which is also consistent with the theoretical prediction.32 Such a weak increase is also consistent with the idea of individual tube segments promoting the orientational ordering of the mesogenic matrix in their vicinity and the formation of pseudo-nematic domains within the LC media.9,11 Since the dispersion of CNTs is stable in the isotropic phase, we suggest that the compatibility of CNTs with SiCB is not attributed to the self-aggregation of siloxane moiety in the smectic phase. Instead, it is dependent on the particular chemical structure of PCMSC and SiCB. Overall, the available characterization results confirm that PCMSC is able to create a homogeneous, compatible CNTs dispersion in SiCB with long-term stability. By adjusting the structure of the LCP and further optimization of the dispersion ratios, it is possible to increase the CNT content to even higher concentrations, even though this may not be necessary for most practical applications. Also, designing a similar LCP with acrylate backbones may allow better dissolution of CNTs in the common acrylate tLCs. A direct result of the homogeneous and stable dispersion obtained here is that it makes it possible to align considerable amount of CNTs by an electric field in tLC, with at least 1 wt % instead of a limited small quantity as achieved previously.3,5,6,9 Using standard electrooptic cells, we monitored the change in complex impedance response (Z*) to investigate the alignment of a CNT
tLC system using AC impedance spectroscopy. Here, Z(ω)* = Z(ω) exp(
iθ) = Z(ω)[cos(θ) + i sin(θ)], with Z(ω) being the frequency-dependent impedance modulus, θ the phase angle on the complex plane, and f = 2πω the AC frequency. Taking the geometry of the LC cell into account, the complex resistivity F* = Z*A/t, with A being the area of the electrodes, t being the cell gap, is presented here. It is reasonably straightforward to understand the evolution of resistivity F* with the applied field. As Figure 3a shows, at 10 kHz the response is purely capacitive, that is, F* = (2πfεε0)
1. Since realignment of an ordinary smectic-A phase normally requires a very high electric field, we choose to align the CNTs in the high-temperature isotropic phase where, probably, the “pseudo-nematic” domains exist.9,11 Figure 3b shows that the resistivity decreased with voltage, which is a direct sign of alignment upon the application of an electric field.3 The decreased resistivity (associated with the

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ε|| ¼ 1 þ ϕ 13258

α|| Æcos2 θæ ε0

ð1Þ

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)

where ϕ has the unit of number density and is directly related to the CNT loading measured in wt %; the meaning of ϕ is detailed later. The orientational average is evaluated with the Boltzmann probability distribution for an individual induced dipole p = α E cos θ in the external field. The result of this integration is straightforward Z

! α|| E2 cos θ sin θ dθ cos θ exp kB T ! Z π α|| E2 cos θ sin θ dθ exp kB T 0

π

2

ε||

¼1 þ ϕ

α|| ε0


0

2 !2 3 ! α|| 4 2kB T α|| E2 kB T 5 ¼1 þ ϕ 1
coth þ 2 α|| E2 α|| E2 ε0 kB T ð2Þ The average of Æcos2 θæ is plotted in Figure 3c to illustrate how the field-alignment changes from its isotropic value of 1/3 to the maximum of 1 with the increasing electric field. In order to fit our experimental data, let us rewrite the inverse resistivity (this is because 1/F* = 2πfε*) in terms of these underlying material parameters, which also necessarily involves a background dielectric permittivity εb of the matrix without CNTs

8 2 !2 3 9 ! < 1 α|| 4 2kB T α|| E2 kB T 5= 1
coth ¼ 2π f ε0 εb þ ϕ þ 2 : ; F α|| E2 α|| E2 ε0 kB T

ð3Þ

)

)

)

)

)

)

)

)

There are three fitting parameters in this expression: the background dielectric permittivity εb, the concentrationdependent factor ϕ multiplying the average Æcos2 θæ, and the longitudinal polarizability α which is due to CNTs and is the same for all samples. The explicit expression for the concentration-dependent factor ϕ = c 3 MtLC/(McntVcnt), with c being the CNT loading (in units of wt %), MtLc being the density of liquid crystal, Mcnt being the density of carbon nanotubes, and Vcnt being the polarizable volume of a CNT. Let us first take ϕ as one single unknown parameter. In the plots of Figure 3c, there are two features that allow us to estimate these parameters: the characteristic voltage (equal to the electric field E times the constant cell thickness of 5 μm) at which the orientational transition takes place gives a fixed value for on α , while from the amplitude of change in (1/F*), one can determine ϕ. Now, one replots all the data in Figure 3b to (1/F*) vs voltage. Fitting eq 3 to all the curves associated with the CNT loaded tLCs, as illustrated in Figure 3d, gives α ≈ 4  10
32 (SI units). This value corresponds to A ≈ 7.2  10
22 m3, if one prefers to use α = 4πε0A as definition of polarizability;40 this value is close to the one recently reported by Brown et al.43 Another interesting result is obtained from the value of ϕα /(cε0), which is determined to be close to 1400 m
3. Since ϕ = cMtLC/(McntVcnt), one can estimate the polarizable volume Vcnt in one nanotube. Taking MtLC ≈ 1 g/cm3, Mcnt ≈ 1.5 g/cm3, and α = 4  10
32 as calculated previously, we obtain Vcnt = 2.2  10
24 m3 per nanotube. As a comparison, for a MWNT of length L = 1 μm and diameter d = 15 nm, two theoretical limits of polarizable volume Vcnt are found. The lower theoretical limit is based on the assumption that polarizability is only attributed by the external wall of a MWNT, thus Vcnto = πdtL = 1.6  10
23 m3 (with wall thickness assumed to be t = 0.35 nm, the thickness of a graphene sheet). The upper theoretical limit is for the case in which

polarizability is attributed by all layers in a MWNT,44 thus VcntA = π(d/2)2L = 1.7  10
22 m3. Nevertheless, real values of Vcnt are expected to be much lower than the theoretical values, since CVD MWNTs usually contain very defective walls, and they have many bends along their contour (as shown by the SEM images in Figure 1c). Although our estimated value of Vcnt is about 1 order of magnitude lower than Vcnto, it is not a surprising result considering the factors mentioned above. On the other hand, one cannot eliminate the possibility that some small aggregates or bundles are still present in the mixture. It is also not feasible to induce ordering and alignment of the entire CNT population. Any impurities in the CNTs would further lower the extrapolated values of Vcnt.

’ CONCLUSION Although there is a very good chance that CNT
tLC mixtures may give rise to both the anticipated and unforeseeable advances in electrooptics and photonics, the ultimate performance of the system relies on whether a stable and homogeneous suspension of CNT
tLC can be achieved. To solve this longstanding problem of CNT aggregation, we have demonstrated a novel and effective dispersion strategy through designing a suitable liquid crystalline polymeric surfactant. Using a polysiloxanebased liquid crystalline polymer, CNTs were successfully incorporated into organosiloxane tLCs. The dispersion homogeneity of the CNTs in tLC was established by optical microscopy and SEM. No phase separation was observed by DSC. In addition, our result confirmed that the addition of CNTs into tLCs increases the clear-point of tLCs because of the strong attraction between CNTs and tLCs. Our experiment also showed that the CNT
tLCs could remain stable for more than half a year. With high loadings of CNTs in tLCs, a large amount of CNTs could be aligned by electric field. Even though different LCP may be required for tLC hosts with different chemical structure, the strategy developed here is effective in greatly improving the miscibility between CNTs and tLCs. In the past, the imposed solubility limit of CNTs hindered further development of CNT
tLC nanocomposites; the homogeneous and stable dispersion of CNTs in tLC achieved here paves a promising way toward future research and practical applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Chemical structures of tLCs tested (Figure S-1), Figure S-2, Table S-1, and the measurement of CNT concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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