Shear Orientation in Nematic Carbon Nanotube Dispersions: A

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Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation. Franco Tardani, Luigi Gentile, Giuseppe A. Ranieri, and Camillo A La Mesa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4015349 • Publication Date (Web): 27 Mar 2013 Downloaded from http://pubs.acs.org on April 7, 2013

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The Journal of Physical Chemistry

Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation.

Franco Tardani1, Luigi Gentile 2, Giuseppe A. Ranieri 2, and Camillo La Mesa 1*

1

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, Dept. of Chemistry, “La Sapienza” University, P.le A. Moro 5; I-00185, Rome, ITALY;

, Dept. of Chemistry, Calabria University, Via P. Bucci s.n.c.; I-87030, Arcavacata di Rende (CS), ITALY.

ABSTRACT. Carbon nanotubes were dispersed in a sodium dodecylsulfate/decanol/water nematic fluid. The long-term stability of the dispersions is ensured by the small density gradients existing between nanotubes and the nematic fluid, and by its viscosity, as well. Presumably, surfactant or nematic micelles adsorb onto nanotubes and concur to stabilize them. A Rheo 2

H-NMR characterization was performed. It was supported by classical 2H quadrupole

splitting and pulsed field gradient spin-echo NMR, allowing to ascertain the diffusive trends therein. The nematic fluid shows uniaxial spectral profiles and marked diffusion anisotropy. No such effects were observed in nanotube-containing nematic dispersions. In addition, the measured water self-diffusion values are substantially lower than the pure nematic fluid. In absence of shear, dispersed nanotubes do not modify the quadrupole splitting amplitude, but affect the spectral profiles. The reasons for the observed behavior are briefly outlined. In presence of shear, the spectral modifications are substantial and lead to the onset of isotropic dispersions, after long-time shearing. KEYWORDS Carbon nanotubes, nematic fluids, dispersions, shear-induced orientation, 2H NMR, self-diffusion.

* Corresponding Author. Camillo La Mesa, Dept. of Chemistry, “La Sapienza” University, P.le A. Moro 5, I-00185, Rome, Italy. Tel. +39-06-49913707. E-mail: [email protected]

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1. INTRODUCTION. Much interest is currently devoted to investigate dispersions containing carbon nanotubes, NTs.1,2 The rationale underlying such research line arises by the potentialities that NTs have as electron conductors and mechanical reinforcers of the hosting matrices.3,4 The above potentialities, however, are hardly optimized, since NTs poorly dissolve in common solvents,5 surfactant and/or polymer solutions,6 and in polymer melts.7 The above drawbacks led material scientists to look for new and properly dispersing matrices. On this regard, anisotropic fluids are promising, since nanotubes may orient therein along well-defined directions, dictated by the local organization of these fluids. In this way, the NT potentialities can be optimized and composite materials with strongly anisotropic character may be built up. In presence of magnetic fields, thus, significant elasticity or conduction anisotropy occurs along the directions dictated by NT orientation in the hosting matrix. This fact gives the opportunity to prepare materials with directional character, making them useful for many practical purposes. Efforts were formerly made to get NT-based ordered phases. Accordingly, nanotubes were dispersed in strong acids,8 where they form solutions or anisotropic fluids, depending on composition. In addition, lyotropic phases,9,10 DNA-based anisotropic fluids,11-13 or lyotropic nematic mixtures14 were proposed as dispersants. Lyotropic nematic mixtures are promising matrices, because of their moderate viscosity.15 The moderate viscosity of such matrices easily allows nanotube orientation in presence of magnetic fields. This fact may induce chemo-mechanical effects in NT-based nematic dispersions. To date, however, the above possibility was not considered; that is why the long-term stability of NT-based nematic dispersions is not fully understood. Previous studies on nematic nanotube dispersions gave information on SWCNT size, but also on the rheology and macroscopic organization of the nematic dispersion.14 The onset of percolation thresholds, with subsequent elastic effects, was


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inferred too. We focused on mixtures containing single-walled carbon nanotubes, SWCNTs, dispersed in a sodium dodecylsulfate /decanol /water nematic fluid. In controlled temperature and concentration regimes, two uni-axial phases are observed therein;16-19 the phases are connected by a biaxial region. For reasons to be clarified later, we focused on the discotic,20-22 uni-axial nematic phase. To shed light on the above aspects, a nuclear magnetic resonance, NMR, investigation was performed on a nematic fluid dispersing NTs. To the best of our knowledge, little or no NMR investigations on nematic dispersions of carbon nanotubes were previously reported. This is why systematic studies by deuterium NMR, 2H NMR, Rheo 2HNMR and three-dimensional NMR self-diffusion are reported and discussed. The former technique characterizes the system in static (or rotating) conditions; Rheo-NMR is a powerful tool to investigate shear-induced deformations. It has been used by some of us, and its intrinsic potentialities are well acquainted.23-25 Self-diffusion anisotropy, finally, indicates whether the nematic fluids preferentially orient in magnetic fields. The above methods jointly allow characterizing in detail some properties of NT-based nematic fluids. NMR provides useful information on local order and dynamics. In particular, water selfdiffusion gives information on obstruction effects. Applied shear, conversely, modifies the 2H spectral profiles and indicates whether mechanical effects induce loss of order. Studies in presence of applied shear, or without it, clarified the stability and supra-molecular organization modes occurring in NT-based nematic fluids. To account for the above effects, studies were performed at rates close to the shear-thinning threshold. In this way, the poor reversibility of shear-induced phase transitions was put in evidence.

2. EXPERIMENTAL SECTION. 2.1. MATERIALS AND PREPARATION PROCEDURES.


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2.1.1.Chemicals. Sodium dodecylsulfate, SDS, and 1-decanol, DEC, are Sigma high purity products. The surfactant was dissolved in absolute ethanol, filtered, and precipitated by cold acetone. The procedure was repeated twice. The resulting white solid was vacuum dried at 70°C. The surfactant purity was inferred by surface tension and ionic conductivity of its aqueous solutions. Decanol, 98% nominal purity, Sigma-Aldrich, was used as received. Single-walled carbon nanotubes, s, were purchased from Unydim, (Houston, TX). They are of the HiPCO type, with 98% nominal purity. Their diameter, D, and length, L, are 2-4 and 100-1000 nm, respectively. According to DLS and TEM, the average hydrodynamic radius of s, , is in the 800-900 nm size range, when D spans between 2 and 4 nm.14,26 Thus, the axial ratios, M, (M = L/D), are hundred units large. Freshly prepared doubly distilled water (χ Dy (= 1.06+0.05·10-9 m2s-1) ≈ Dx (= 1.04+0.06·10-9 m2s-1), as indicated in Figure 2. There are also reported, on the right hand side of the figure, data relative to SWCNT dispersions in the nematic fluid.

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). Bars indicate the confidence limits for each point.


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Since Dx ≈ Dy, the average value along these directions is indicated as [Dx,y]. Clearly [Dx,y] < Dz indicates oblate particles. In the case of the SWCNT dispersion Dx ≈ Dy ≈ Dz, this is probably due to the presence of both prolate (nanotubes) and oblate particles. The anisotropy factor, R, can be inferred by an empirical relation stating36

where the meaning of symbols is as before. The values calculated accordingly are about -0.11 that indicates the presence of thin discs.36 Water self-diffusion complements information from 2

H quadrupole splittings. The former gives information on the preferred orientation of the

objects with respect to the magnetic field. This implies a preferential orientation of slightly anisometric disks, with their axis oriented normal to the magnetic field director. 1.2. Rheo-NMR. Given the preferred orientation of domains with respect to the magnetic field, the role that applied shear has on local order may be easily detected.


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Figure 3. Temporal evolution of 2H NMR spectral profiles during the shear-induced nematicisotropic phase transition, at 25.0°C. The applied shear rate was 1.0 s-1.

Shear-induced effects are immaterial on the local organization modes, and 2H splittings do not depend on shear rate. This is the reason why we have chosen discotic phases as dispersants. Measurements performed at 1.0 s-1 shear rate indicate progressive loss of orientation. The kinetics of the process is associated to shear-induced anisotropic-isotropic phase transitions. Data indicate loss of nematic order 20 min. under shear flow is applied. A two-state regime, with coexistence of different signals, is observed from 40 min. The transition ends when only isotropic signals occur, at 70 min. An isotropic dispersion is met

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


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thereafter, Figure 3 and Figure 4. Long time standing of the samples outside the magnet (from 15 to 120 hours) and subsequent determination of 2H splittings implies partial recovery of the quadrupolar signals. Isotropic signals are still present in the spectra, Figure 5. Very presumably, this is because shear randomizes the average orientation of nematic micelles. A significant amount of time is needed to partially recover orientation. Hence, the effect of

Figure 5. Recovery of 2H spectral profiles, at 25 °C, 15 and 120 hours after shear was stopped. The composition is as above. Samples are kept outside the NMR unit, until 2H measurements are run.


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

applied shear is irreversible. In presence of SWCNTs, the reorientation process occurs in less than 20 min., whereas 70 min. are required for complete randomization, Figure 6. The transition needs a slightly higher strain, compared to systems without SWCNTs. In presence of 0.10 SWCNT wt%, the 2

H spectral profiles are slightly different from the original conditions, Figure 1, and the

diffusion anisotropy vanishes. Accordingly, the transitions are irreversible. 2

H spectral profiles indicate a randomization of domain orientation compared to the

original conditions. The 2H quadrupole splitting indicate the average bond order parameter of water with respect to the disk axis. Accordingly,


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where ∆νq is the quadrupole splitting between the inner wings, θ the average orientation angle with respect to the magnetic field, and δ the quadrupole coupling constant. In such conditions, single crystals, with the main axis oriented at 90° with respect to the magnetic field, are observed. Randomly oriented crystallites in the dispersions show powder pattern spectra. The doublet width observed in the nematic fluid, Figure 1a, does not substantially change when SWCNTs are added, Figure 1b. The Pake doublet and the quadrupolar wings observed in the second figure are, thus, due to a random distribution of domains. It is questionable to assume any modification in bond order parameter. Presumably, SWCNTs only induce partial randomization in the orientation of domains. SWCNTs reduce self-diffusion coefficients, Fig. 2. Typical D values are 2.92+0.06·1010

m2 s-1, that is 2-3 times lower than the pure nematic phase. In addition, they are nearly the

same along the x, y, z axes, and R values calculated by Eq. (2) are close to zero. High obstruction to water diffusion is reasonable, given the large size of SWCNTs compared to nematic micelles. The possibility of a substantial SWCNT entanglement in the nematic fluid is realistic. As it is well known, SWCNTs form entangled networks at low volume fractions.37 The volume fraction threshold required for entanglement, φC*, decreases in inverse proportion to their axial ratios.38 Estimates for the present SWCNTs indicate that φC* is in the range 23·10-3. The amount of SWCNTs present in this system is substantially higher. It is supposed, thus, that nematic micelles are dispersed in a SWCNT entangled network. In such conditions, micelles keep a preferred orientational order with respect to the magnetic field director, but are sensitive to local heterogeneities dictated by the SWCNT frameworks. In the volume fraction regimes considered here, the system is presumably nanotube-continuous rather than nematic-continuous. In words, nematic micelles are located (and oriented) in a framework dictated by the reciprocal SWCNT arrangement. ACS Paragon Plus Environment

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4. DISCUSSION. Ordered phases in SWCNTs were predicted and experimentally verified.39,40 In all water-based fluids, carbon nanotubes form ordered phases when their volume fraction, φ, exceed a critical value, φC*. In such regimes, their reciprocal orientation is controlled by excluded volume interactions. Unfortunately, it is cumbersome getting ordered phases in pure form, unless surface functionalization procedures are used.41 Different organization modes are possible to SWCNTs, namely: formation of ordered phases, bundles and precipitation, or entangled networks. A delicate balance of attractive and repulsive forces between SWCNTs controls the above processes. The possible organization modes are dictated by the interaction entropy, by the solvent quality, and by the presence of stabilizers. The latter hinder the formation of flocks or gels. The main requirements for an effective dispersion are: a) homogeneous surface coverage; b) significant adsorption energy; c) formation of adsorbed layers, preventing SWCNTs coming closer than distances required for vdW interactions to occur. Polymers, biopolymers and/or surfactants are generally used on that purpose. However, depletion, an unbalanced osmotic effect, is rather common in colloidal dispersions and induces macroscopic phase separation.42 These are the reasons why nematic fluids were considered. According to experiments, in fact, they do not induce significant depletion. It is possible that surfactant and/or nematic micelles adsorb onto SWCNTs, with subsequent stabilizing effects. In addition, the density of the nematic fluid (ρNT) and its viscosity avoid sedimentation; this is why these dispersions are stable for long times. In the present systems the nanotube length is much higher than nematic micelles. This is a rather common feature met in dispersing anisometric particles in ordered fluids, and was extensively described in the literature.43-45 The above mixtures can be modeled as a nematic continuum in which anisotropic particles orient. The average nanotube orientation reflects the effect of elastic torque and shear sensed by the matrix. At low volume fraction regimes and ACS Paragon Plus Environment

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very low deformation rates, applied shear does not substantially affect the system stability. In words, field-induced ordering is not modified when shear rates are moderate. When shear is substantial, and close to (or in) the thinning regime, departures from the ideal behavior are noticeable. This is what has been experimentally observed. The phenomenon is ascribed to the high aspect ratios of long anisometric SWCNTs. In the present conditions, in fact, the volume fraction is close to φC*, and it is possible that entanglement between SWCNTs occurs. It is supposed, accordingly, that nanotubes form a framework in which nematic micelles are embedded. Therefore, the continuum theory accounting for the behavior of the dispersing fluid must be modified. This is why information on the local order sensed by 2H Rheo-NMR, detecting loss of orientation, is useful. Quadrupole splittings in Figure 1a and 1b are very similar, although the profiles are different. Changes are essentially ascribed to the orientational distribution of domains. This is because micelles are sensitive to the local fields dictated by SWCNTs. Therefore, micelle orientation in proximity of nanotubes is perturbed and a random distribution of the domains occurs. The geometrical and/or orientational constraints dictated by SWCNTs compel micelles to assume different orientations with respect to the magnetic field director. Thus, Pake doublets occur and the well-developed iso-oriented splittings pertinent to nematic fluids vanish. As to the combined action of magnetic field ads shear the following consideration apply. We choose a shear rate implying progressive loss of orientation with respect to the applied shear direction. There is a velocity gradient sensed by micelles, which is normal to the direction of shear. If the shear operates along the x-axis, there will be a velocity gradient along the y one. In the same time, the nematic micelles are subjected to the force fields exerted by the magnet. If the magnetic field director is located along the z-axis, there will be a distribution of the nematic axes director along the x-y plane. As a result of the above effects, it is supposed that the phases subjected to shear will have a overall distribution of orientations ACS Paragon Plus Environment

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surely different from that observed in static conditions. This is the rationale underlying the spectral changes that are observed. For reasons related to the SWCNT volume fraction, this behavior holds to be true irrespectively as to whether they are contain nanotubes, or not. SWCNTs play another substantial effect; they interconnect randomly oriented domains and act as junctions between them. In words, the original order and the marked diffusion anisotropy are lost; in the same time, the hindrance to water molecular motion increases, because of excluded volume effects. The final organization in the fluid is consistent with a substantial reduction in self-diffusion. Low and quasi-isotropic self-diffusion is, therefore, possible. This is because of the random distribution of both nanotubes and nematic micelles in the composites. It is obvious, therefore, that hindrance to water diffusion is significant in the latter systems.

5. CONCLUSIONS. The possibility to use nematic fluids as dispersants was experienced. According to experiments, nanotubes are dissolved in the nematic fluid and the resulting dispersions are stable for long times. Very presumably, stabilization occurs because of surfactant or micelle adsorption therein. Also the viscosity and density of the nematic fluid are relevant. The resulting dispersions retain liquid crystalline order, although the original nematic appearance is partly lost. The same holds for water self-diffusion anisotropy, which vanishes when s are present in substantial amounts. Presumably, better performances could be obtained at lower NT volume fractions. To optimize orientation effects, presumably, φ < 2.5-3.0·10-3 should be considered. In such conditions, in fact, the transition towards entangled states and the onset of elastic regimes have been observed.14 Alternatively, very low content could be experienced, or lower aspect ratios could be considered. Presumably, multi-walled carbon nanotubes could be useful on this regard.


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The applied shear modified the 2H spectral profiles of the nematic dispersions and induces modifications, with subsequent loss of order. Thereafter, a two-phase regime follows and a transition to an isotropic dispersion takes place at longer times. The effect is irreversible and the original spectral profiles are never recovered, even after long-time standing in the magnet. This is because the distribution of SWCNTs is disturbed by the applied shear and the original frameworks built up by nanotube entanglement are destroyed.

ACKNOWLEDGMENTS. La Sapienza University is acknowledged for financing the present research line. The authors wish to thank Cesare Oliviero-Rossi and Isabella Nicotera (both at the Dept. Chemistry, Unical), for fruitful discussion on some aspects of the manuscript.


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TOC Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation. Franco Tardani, Luigi Gentile, Giuseppe A. Ranieri, and Camillo La Mesa*


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Shear Orientation in Nematic Carbon Nanotube Dispersions: A

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