1 Shear Orientation in Nematic Carbon Nanotube Dispersions: A2 Combined NMR Investigation3 Franco Tardani,† Luigi Gentile,‡ Giuseppe A. Ranieri,‡ and Camillo La Mesa*,†

4†Department of Chemistry, “La Sapienza” University, P.le A. Moro 5, I-00185 Rome, Italy

5‡Department of Chemistry, Calabria University, Via P. Bucci s.n.c., I-87030 Arcavacata di Rende (CS), Italy

6 ABSTRACT: Carbon nanotubes were dispersed in a sodium dodecylsul-7 fate/decanol/water nematic fluid. The long-term stability of the dispersions8 is ensured by the small density gradients existing between nanotubes and9 the nematic fluid, and by its viscosity, as well. Presumably, surfactant or10 nematic micelles adsorb onto nanotubes and concur to stabilize them. A11 Rheo 2H NMR characterization was performed. It was supported by12 classical 2H quadrupole splitting and pulsed field gradient spin−echo NMR,13 allowing to ascertain the diffusive trends therein. The nematic fluid shows14 uniaxial spectral profiles and marked diffusion anisotropy. No such effects15 were observed in nanotube-containing nematic dispersions. In addition, the16 measured water self-diffusion values are substantially lower than the pure17 nematic fluid. In the absence of shear, dispersed nanotubes do not modify18 the quadrupole splitting amplitude, but affect the spectral profiles. The reasons for the observed behavior are briefly outlined. In19 the presence of shear, the spectral modifications are substantial and lead to the onset of isotropic dispersions, after long-time20 shearing.

1. INTRODUCTION

21 Much interest is currently devoted to investigate dispersions22 containing carbon nanotubes, NTs.1,2 The rationale underlying23 such research line arises by the potentialities that NTs have as24 electron conductors and mechanical reinforcers of the hosting25 matrixes.3,4 The above potentialities, however, are hardly26 optimized since NTs poorly dissolve in common solvents,5

27 surfactant, and/or polymer solutions,6 and in polymer melts.7

28 The above drawbacks led material scientists to look for new and29 properly dispersing matrixes. On this regard, anisotropic fluids30 are promising since nanotubes may orient therein along well-31 defined directions, dictated by the local organization of these32 fluids. In this way, the NT potentialities can be optimized, and33 composite materials with strongly anisotropic character may be34 built up. In the presence of magnetic fields, thus, significant35 elasticity or conduction anisotropy occurs along the directions36 dictated by NT orientation in the hosting matrix. This fact gives37 the opportunity to prepare materials with directional character,38 making them useful for many practical purposes.39 Efforts were formerly made to get NT-based ordered phases.40 Accordingly, nanotubes were dispersed in strong acids,8 where41 they form solutions or anisotropic fluids, depending on42 composition. In addition, lyotropic phases,9,10 DNA-based43 anisotropic fluids,11−13 or lyotropic nematic mixtures14 were44 proposed as dispersants. Lyotropic nematic mixtures are45 promising matrices because of their moderate viscosity.15 The46 moderate viscosity of such matrixes easily allows nanotube47 orientation in the presence of magnetic fields. This fact may48 induce chemo-mechanical effects in NT-based nematic49 dispersions. To date, however, the above possibility was not

50considered; that is why the long-term stability of NT-based51nematic dispersions is not fully understood. Previous studies on52nematic nanotube dispersions gave information on SWCNT53size and also on the rheology and macroscopic organization of54the nematic dispersion.14 The onset of percolation thresholds,55with subsequent elastic effects, was inferred, too.56We focused on mixtures containing single-walled carbon57nanotubes, SWCNTs, dispersed in a sodium dodecylsulfate/58decanol/water nematic fluid. In controlled temperature and59concentration regimes, two uniaxial phases are observed60therein;16−19 the phases are connected by a biaxial region.61For reasons to be clarified later, we focused on the62discotic,20−22 uniaxial nematic phase. To shed light on the63above aspects, a nuclear magnetic resonance, NMR, inves-64tigation was performed on a nematic fluid dispersing NTs. To65the best of our knowledge, little or no NMR investigations on66nematic dispersions of carbon nanotubes were previously67reported. This is why systematic studies by deuterium NMR,68

2H NMR, Rheo 2H NMR, and three-dimensional NMR self-69diffusion are reported and discussed. The former technique70characterizes the system in static (or rotating) conditions;71Rheo-NMR is a powerful tool to investigate shear-induced72deformations. It has been used by some of us, and its intrinsic73potentialities are well acquainted.23−25 Self-diffusion anisotropy,74finally, indicates whether the nematic fluids preferentially orient

Received: February 12, 2013Revised: March 27, 2013

Article

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© XXXX American Chemical Society A dx.doi.org/10.1021/jp4015349 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

vnh00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.5.i1:3915 | 2.0 alpha 39) 2012/12/04 10:21:00 | PROD-JCAVA | rq_2292032 | 4/08/2013 14:53:57 | 7

75 in magnetic fields. The above methods jointly allow character-76 izing in detail some properties of NT-based nematic fluids.77 NMR provides useful information on local order and78 dynamics. In particular, water self-diffusion gives information79 on obstruction effects. Applied shear, conversely, modifies the80

2H spectral profiles and indicates whether mechanical effects81 induce loss of order. Studies in the presence of applied shear, or82 without it, clarified the stability and supramolecular organ-83 ization modes occurring in NT-based nematic fluids. To84 account for the above effects, studies were performed at rates85 close to the shear-thinning threshold. In this way, the poor86 reversibility of shear-induced phase transitions was put in87 evidence.

2. EXPERIMENTAL SECTION

88 2.1. Materials and Preparation Procedures.89 2.1.1. Chemicals. Sodium dodecylsulfate, SDS, and 1-decanol,90 DEC, are Sigma high purity products. The surfactant was91 dissolved in absolute ethanol, filtered, and precipitated by cold92 acetone. The procedure was repeated twice. The resulting white93 solid was vacuum-dried at 70 °C. The surfactant purity was94 inferred by surface tension and ionic conductivity of its aqueous95 solutions. Decanol, 98% nominal purity, Sigma-Aldrich, was96 used as received.97 Single-walled carbon nanotubes, s, were purchased from98 Unydim (Houston, TX). They are of the HiPCO type, with99 98% nominal purity. Their diameter, D, and length, L, are 2−4100 and 100−1000 nm, respectively. According to DLS and TEM,101 the average hydrodynamic radius of s, ⟨RH⟩, is in the 800−900102 nm size range, when D spans between 2 and 4 nm.14,26 Thus,103 the axial ratios, M, (M = L/D), are hundred units large.104 Freshly prepared doubly distilled water (χ < 10−7 Ω−1 cm−1,105 at 25.00 °C) was degassed before use. Deuterium oxide, D2O,106 Aldrich 99.95% isotopic enrichment, was used as such. Water107 contains 20.0 D2O wt % as cosolvent, and the mixture108 composition was varied, keeping fixed the mole ratios of the109 single components. In D2O−water mixtures, samples having110 the same (SDS/H2O +D2O) and (DEC/H2O + D2O) mole111 ratios as the corresponding water-based ones were used.112 According to optical polarizing microscopy (see below), the113 anisotropic textures of the resulting nematic fluids in the two114 solvents are equivalent.115 2.1.2. Material Preparation. The nematic solvent was116 prepared by mixing 20% D2O−water mixtures and SDS in117 due amounts. Thereafter, DEC was added dropwise, until a

118composition located at the center of the discotic phase was119achieved.21 The mixtures were kindly stirred to attain120macroscopic homogeneity. Thermal equilibration for one121week followed. Typical Schlieren optical textures ascertained122the presence of the nematic phase. The region of existence of123the nematic phase as a function of temperature, T, is moderate124and care was taken to keep the samples at 25.0 ± 0.1 °C.125Two different procedures were used to disperse dry126SWCNTs. In the former, they were directly dissolved in the127nematic fluid; the resulting dispersions were homogenized by128stirring, sonication, and prolonged centrifugation. According to129optical microscopy, nanotube bundles are sometimes present;130in such cases, the samples are discarded. In the second131procedure, SWCNTs were dispersed in 3.00 SDS wt %, and132due amounts of the same surfactant were rapidly added. Finally,133decanol was added, under mild stirring. The latter procedure134ensures better homogeneity and substantial stability. When the135samples resulted to be optically homogeneous, they were kept136undisturbed at 25.0 °C for two weeks and periodically checked137by polarizing microscopy.1382.2. Methods. 2.2.1. Optical Microscopy. A Ceti Laborlux139optical microscope detected the state of the dispersions and of140the original nematic phase in polarized light, at 25.0 °C.141Samples were put on accurately cleaned glass slides. To142optimize the textures quality, different sample thicknesses were143tested. Thin Teflon spacers (0.1 to 0.3 mm in width) were144eventually inserted between glass slides. Optical conoscopy was145also performed, to ascertain if the system is truly uniaxial.1462.2.2. Deuterium Rheo-NMR. Experiments were performed147on 2H nuclei, using a cylindrical cuvette having 9.0 mm inner148radius and 1.0 mm gap. The cell is integrated in a microimaging149probe of a wide-bore superconducting magnet. The long axis of150the cuvette is parallel to the magnetic field director. Shear is151applied by rotating an inner cylinder fitted in the cuvette,152generally at shear rate of 1.0 s−1. The cylinder motion is driven153by an external stepper-motor gearbox, mounted on the top of154the magnet. 2H spectra were recorded by a Bruker Avance 300155unit, working at 46.073 MHz. The measuring temperature was156set to 25.0 ± 0.1 °C. During the shearing procedures, 2H157spectra were repeated at 200 s intervals. More information on158the setup and measuring procedures is reported elsewhere.27

1592.2.3. NMR Self-Diffusion. Self-diffusion coefficients were160measured by a microimaging probe having 3-axes gradient161facilities and a maximum gradient strength of 100 G·cm−1.162They were determined on a Bruker Avance 300 spectrometer

Figure 1. (a) 2H NMR quadrupole splittings of the SDS/DEC/H2O discotic phase and (b) in the presence of 0.10 wt % SWCNTs, at 25.0 °C.

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163 operating at 300.0 MHz on 1H nuclei. The optimal164 experimental conditions were Δ = 90 ms and δ = 5 ms. To165 improve the signal resolution, G values were varied from 1.0 to166 12.0 G·cm−1, in 16 gradient steps. The working temperature167 was set to 25.0 ± 0.1 °C by an air flow thermostatting unit.168 A 1H NMR Fourier transform pulsed-gradient spin−echo,169 FT-PGSE, method was used.28 The spin−echo decays were170 analyzed according to Stejskal and Tanner, using the relation-171 ship29

= γδ δ− Δ−I I e G D0

( ) ( 3 )2

172 (1)

173 where D = Dx, Dy, or Dz. In eq 1, I and I0 represent the water174 resonance peak intensity in the presence and absence of field175 gradients, respectively. δ is the gradient pulse length, and Δ the176 separation between the leading edges of two subsequent177 gradients. γ is the proton gyro-magnetic ratio and G the178 magnitude of the field gradient. Dz is measured along the179 magnetic field director, z, whereas Dx and Dy are detected along180 the x and y axis, respectively. No spurious effects due to181 SWCNTs sedimentation or local heterogeneities were182 observed.

3. RESULTS183 3.1. 2H NMR. Deuterium spectral profiles characterize the184 orientation of the phase director with respect to the magnetic

f1 185 field. In Figure 1a is reported the spectrum of a pure nematic186 phase, with quadrupole splittings, Δνq, close to 200 Hz, and the187 spectrum in the presence of SWCNTs, Figure 1b. Spectra in188 rotating conditions were also run to ascertain whether the189 splitting amplitude under rotation conforms to type II190 (discotic-nematic) phases.30 The location of the samples in191 the phase diagram, the 2H NMR spectral profiles (both in static192 and rotating conditions), and the results from conoscopy193 indicate discotic order.31 Information from Figure 1a,b194 indicates the average orientation of the OD bond relative to195 the static magnetic field. Wennerstrom et al.32 explicitly196 accounted for this. They found that the average OD bond197 orientation is normal to the aggregate surface (a lamella in that198 case). Thus, the director axis of disks and OD bonds are199 oriented along the same direction. Further support comes from200 a previous paper by Thiele et al.33 on the same discotic nematic201 fluid.202 3.2. Water Self-Diffusion. It allows determining the local203 state of organized solutions, such as micelles, microemulsions,

204gels, and liquid crystals. In the former systems, the hindrance to205water diffusion is ascribed to hydration and excluded volume206effects.34

207In anisotropic fluids, conversely, water motion is essentially208hindered by geometrical barriers. These do not allow water to209diffuse along definite directions. Lyotropic systems, therefore,210are totally, partially, or not water-continuous along the211directions dictated by the phase structure. It is well acquainted,212for instance, that water self-diffusion in lamellar phases is213hindered in the direction normal to the lamellae.35 Thus,214hindrance to diffusion gives indication on the domains215arrangement. Self-diffusion anisotropy gives information on216the structure of nematic liquid crystals. It gives a fingerprint of217the reciprocal location of discotic micelles in the fluids. In the218present fluid, water self-diffusion is markedly anisotropic. The219diffusion coefficients fulfill the sequence: Dz (=1.51 ± 0.05 ×22010−9 m2 s−1) > Dy (= 1.06 ± 0.05 × 10−9 m2 s−1) ≈ Dx (= 1.04221 f2± 0.06 × 10−9 m2 s−1), as indicated in Figure 2. There are also222reported, on the right-hand side of the figure, data relative to223SWCNT dispersions in the nematic fluid.224Since Dx ≈ Dy, the average value along these directions is225indicated as [Dx,y]. Clearly, [Dx,y] < Dz indicates oblate226particles. In the case of the SWCNT dispersion Dx ≈ Dy ≈227Dz, this is probably due to the presence of both prolate228(nanotubes) and oblate particles.229The anisotropy factor, R, can be inferred by an empirical230relationship stating36

⟨ ⟩ −⟨ ⟩ +

=⎡⎣⎢⎢

⎤⎦⎥⎥

D D

D DR

2x y z

x y z

,

, 231(2)

232where the meaning of the symbols is as before. The values233calculated accordingly are about −0.11 and indicate the234presence of thin discs.36 Water self-diffusion complements235information from 2H quadrupole splittings. The former gives236information on the preferred orientation of the objects with237respect to the magnetic field. This implies a preferential238orientation of slightly anisometric disks, with their axis oriented239normal to the magnetic field director.2403.3. Rheo-NMR. Given the preferred orientation of domains241with respect to the magnetic field, the role that applied shear242has on local order may be easily detected.243Shear-induced effects are immaterial on the local organ-244ization modes, and 2H splittings do not depend on shear rate.245This is the reason why we have chosen discotic phases as

Figure 2. Water self-diffusion, obtained by FT-PFGSE decay, in the nematic fluid (a) and in presence of 0.10 wt % SWCNTs (b) at 25.0 ± 0.1 °C.Measurements were run along x, y, and z axes, as indicated in the text. Values reported in the insets are obtained by a logarithmic fit of eq 1. Barsindicate the confidence limits for each point.

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246 dispersants. Measurements performed at 1.0 s−1 shear rate247 indicate progressive loss of orientation. The kinetics of the248 process is associated to shear-induced anisotropic−isotropic249 phase transitions. Data indicate loss of nematic order 20 min250 before shear flow is applied. A two-state regime, with251 coexistence of different signals, is observed from 40 min. The252 transition ends when only isotropic signals occur, at 70 min. An

f3f4 253 isotropic dispersion is met thereafter; Figures 3 and 4. Long

254 time standing of the samples outside the magnet (from 15 to255 120 h) and subsequent determination of 2H splittings implies

256partial recovery of the quadrupolar signals. Isotropic signals are257 f5still present in the spectra; Figure 5. Very presumably, this is

258because shear randomizes the average orientation of nematic259micelles. A significant amount of time is needed to partially260recover orientation. Hence, the effect of applied shear is261irreversible.262In presence of SWCNTs, the reorientation process occurs in263less than 20 min, whereas 70 min are required for complete264 f6randomization; Figure 6. The transition needs a slightly higher

265strain, compared to systems without SWCNTs. In the presence266of 0.10 SWCNT wt %, the 2H spectral profiles are slightly267different from the original conditions, Figure 1, and the268diffusion anisotropy vanishes. Accordingly, the transitions are269irreversible.270

2H spectral profiles indicate a randomization of domain271orientation compared to the original conditions. The 2H

Figure 3. Temporal evolution of 2H NMR spectral profiles during theshear-induced nematic−isotropic phase transition, at 25.0 °C. Theapplied shear rate was 1.0 s−1.

Figure 4. 2H spectra showing nematic, complex, and isotropic systems,at 25.0 °C. Data were taken at different measuring times, as indicatedin the legends. Conditions are as in Figure 3.

Figure 5. Recovery of 2H spectral profiles, at 25 °C, 15 and 120 h aftershear was stopped. The composition is as in Figure 3. Samples are keptoutside the NMR unit until 2H measurements are run.

Figure 6. Evolution of 2H spectral profiles in the presence of 0.10 wt% SWCNTs, at 25.0 °C. The applied shear rate is 1.0 s−1.

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272 quadrupole splitting indicates the average bond order273 parameter of water with respect to the disk axis. Accordingly,

δΔ = ϑ −⎜ ⎟⎛⎝

⎞⎠v

34

((3 cos ) 1)q2

274 (3)

275 where Δνq is the quadrupole splitting between the inner wings,276 θ the average orientation angle with respect to the magnetic277 field, and δ the quadrupole coupling constant. In such278 conditions, single crystals, with the main axis oriented at 90°279 with respect to the magnetic field, are observed. Randomly280 oriented crystallites in the dispersions show powder pattern281 spectra. The doublet width observed in the nematic fluid,282 Figure 1a, does not substantially change when SWCNTs are283 added, Figure 1b. The Pake doublet and the quadrupolar wings284 observed in the second figure are, thus, due to a random285 distribution of domains. It is questionable to assume any286 modification in bond order parameter. Presumably, SWCNTs287 only induce partial randomization in the orientation of288 domains.289 SWCNTs reduce self-diffusion coefficients, Figure 2. Typical290 D values are 2.92 ± 0.06 × 10−10 m2 s−1, that is 2−3 times291 lower than the pure nematic phase. In addition, they are nearly292 the same along the x, y, and z axes, and R values calculated by293 eq 2 are close to zero. High obstruction to water diffusion is294 reasonable, given the large size of SWCNTs compared to295 nematic micelles. The possibility of a substantial SWCNT296 entanglement in the nematic fluid is realistic. As it is well-297 known, SWCNTs form entangled networks at low volume298 fractions.37 The volume fraction threshold required for299 entanglement, φC*, decreases in inverse proportion to their300 axial ratios.38 Estimates for the present SWCNTs indicate that301 φC* is in the range 2−3 × 10−3. The amount of SWCNTs302 present in this system is substantially higher. It is supposed,303 thus, that nematic micelles are dispersed in a SWCNT304 entangled network. In such conditions, micelles keep a305 preferred orientational order with respect to the magnetic306 field director but are sensitive to local heterogeneities dictated307 by the SWCNT frameworks. In the volume fraction regimes308 considered here, the system is presumably nanotube-continu-309 ous rather than nematic-continuous. In other words, nematic310 micelles are located (and oriented) in a framework dictated by311 the reciprocal SWCNT arrangement.

4. DISCUSSION

312 Ordered phases in SWCNTs were predicted and experimentally313 verified.39,40 In all water-based fluids, carbon nanotubes form314 ordered phases when their volume fraction, φ, exceed a critical315 value, φC*. In such regimes, their reciprocal orientation is316 controlled by excluded volume interactions. Unfortunately, it is317 cumbersome getting ordered phases in pure form, unless318 surface functionalization procedures are used.41

319 Different organization modes are possible for SWCNTs,320 namely, formation of ordered phases, bundles and precipitation,321 or entangled networks. A delicate balance of attractive and322 repulsive forces between SWCNTs controls the above323 processes. The possible organization modes are dictated by324 the interaction entropy, by the solvent quality, and by the325 presence of stabilizers. The latter hinder the formation of flocks326 or gels. The main requirements for an effective dispersion are327 (a) homogeneous surface coverage; (b) significant adsorption328 energy; and (c) formation of adsorbed layers, preventing329 SWCNTs coming closer than distances required for van der

330Waals interactions to occur. Polymers, biopolymers, and/or331surfactants are generally used on that purpose.332However, depletion, an unbalanced osmotic effect, is rather333common in colloidal dispersions and induces macroscopic334phase separation.42 These are the reasons why nematic fluids335were considered. According to experiments, in fact, they do not336induce significant depletion. It is possible that surfactant and/or337nematic micelles adsorb onto SWCNTs, with subsequent338stabilizing effects. In addition, the density of the nematic fluid339(ρNT) and its viscosity avoid sedimentation; this is why these340dispersions are stable for long times.341In the present systems, the nanotube length is much higher342than nematic micelles. This is a rather common feature met in343dispersing anisometric particles in ordered fluids and was344extensively described in the literature.43−45 The above mixtures345can be modeled as a nematic continuum in which anisotropic346particles orient. The average nanotube orientation reflects the347effect of elastic torque and shear sensed by the matrix. At low348volume fraction regimes and very low deformation rates,349applied shear does not substantially affect the system stability.350In other words, field-induced ordering is not modified when351shear rates are moderate. When shear is substantial, and close352to (or in) the thinning regime, departures from the ideal353behavior are noticeable. This is what has been experimentally354observed. The phenomenon is ascribed to the high aspect ratios355of long anisometric SWCNTs. In the present conditions, in356fact, the volume fraction is close to φC*, and it is possible that357entanglement between SWCNTs occurs. It is supposed,358accordingly, that nanotubes form a framework in which359nematic micelles are embedded. Therefore, the continuum360theory accounting for the behavior of the dispersing fluid must361be modified. This is why information on the local order sensed362by 2H Rheo-NMR, detecting loss of orientation, is useful.363Quadrupole splittings in Figure 1a,b are very similar,364although the profiles are different. Changes are essentially365ascribed to the orientational distribution of domains. This is366because micelles are sensitive to the local fields dictated by367SWCNTs. Therefore, micelle orientation in proximity of368nanotubes is perturbed and a random distribution of the369domains occurs. The geometrical and/or orientational con-370straints dictated by SWCNTs compel micelles to assume371different orientations with respect to the magnetic field372director. Thus, Pake doublets occur, and the well-developed373iso-oriented splittings pertinent to nematic fluids vanish.374As to the combined action of magnetic field ads shear, the375following consideration applies. We choose a shear rate376implying progressive loss of orientation with respect to the377applied shear direction. There is a velocity gradient sensed by378micelles, which is normal to the direction of shear. If the shear379operates along the x-axis, there will be a velocity gradient along380the y one. In the same time, the nematic micelles are subjected381to the force fields exerted by the magnet. If the magnetic field382director is located along the z-axis, there will be a distribution of383the nematic axes director along the x−y plane. As a result of the384above effects, it is supposed that the phases subjected to shear385will have an overall distribution of orientations surely different386from that observed in static conditions. This is the rationale387underlying the spectral changes that are observed. For reasons388related to the SWCNT volume fraction, this behavior holds to389be true irrespectively as to whether they contain nanotubes.390SWCNTs play another substantial effect; they interconnect391randomly oriented domains and act as junctions between them.392In other words, the original order and the marked diffusion

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393 anisotropy are lost; in the same time, the hindrance to water394 molecular motion increases because of excluded volume effects.395 The final organization in the fluid is consistent with a396 substantial reduction in self-diffusion. Low and quasi-isotropic397 self-diffusion is, therefore, possible. This is because of the398 random distribution of both nanotubes and nematic micelles in399 the composites. It is obvious, therefore, that hindrance to water400 diffusion is significant in the latter systems.

5. CONCLUSIONS

401 The possibility to use nematic fluids as dispersants was402 experienced. According to experiments, nanotubes are dissolved403 in the nematic fluid, and the resulting dispersions are stable for404 long times. Very presumably, stabilization occurs because of405 surfactant or micelle adsorption therein. Also, the viscosity and406 density of the nematic fluid are relevant. The resulting407 dispersions retain liquid crystalline order, although the original408 nematic appearance is partly lost. The same holds for water self-409 diffusion anisotropy, which vanishes when s are present in410 substantial amounts. Presumably, better performances could be411 obtained at lower NT volume fractions. To optimize412 orientation effects, presumably, φ < 2.5−3.0 × 10−3 should413 be considered. In such conditions, in fact, the transition toward414 entangled states and the onset of elastic regimes have been415 observed.14 Alternatively, very low content could be experi-416 enced or lower aspect ratios could be considered. Presumably,417 multiwalled carbon nanotubes could be useful on this regard.418 The applied shear modified the 2H spectral profiles of the419 nematic dispersions and induces modifications, with subsequent420 loss of order. Thereafter, a two-phase regime follows and a421 transition to an isotropic dispersion takes place at longer times.422 The effect is irreversible and the original spectral profiles are423 never recovered, even after long-time standing in the magnet.424 This is because the distribution of SWCNTs is disturbed by the425 applied shear, and the original frameworks built up by nanotube426 entanglement are destroyed.

427 ■ AUTHOR INFORMATION

428 Corresponding Author429 *(C.L.M.) Tel: +39-06-49913707. E-mail: camillo.lamesa@430 uniroma1.it.

431 Notes432 The authors declare no competing financial interest.

433 ■ ACKNOWLEDGMENTS

434 La Sapienza University is acknowledged for financing the435 present research line. We wish to thank Cesare Oliviero-Rossi436 and Isabella Nicotera (both at the Department of Chemistry,437 Unical) for fruitful discussion on some aspects of the438 manuscript.

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