Preparation and Characterization of Multilayered Polymer Nanotube

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Preparation and Characterization of Multilayered Polymer Nanotube Dispersions Bin Huang,†,‡ Theo G. M. van de Ven,*,‡ and Reghan J. Hill† †

Department of Chemical Engineering, and ‡Department of Chemistry, Centre for Self-Assembled Chemical Structures, McGill University, Montreal, Quebec H3A 2B2, Canada

bS Supporting Information ABSTRACT: Despite considerable efforts to synthesize nanotubes using porous alumina or polycarbonate membrane templates, few studies have addressed the resulting nanotube dispersion. We prepared dispersions of multilayered polyethylenimine/maleic anhydride alternating copolymer (PEI/ MAAC) nanotubes synthesized with porous alumina templates. After mechanical polishing to remove the residual polymer surface layer from templates and subsequent template dissolution, the multilayered PEI/MAAC nanotubes were easily dispersed in water at neutral pH by polyelectrolyte adsorption, producing nanotube dispersions that were stable for at least 3 months. We characterized the dispersions using phase-contrast optical microscopy, electro-optics, electrophoresis, and viscometry to help understand their colloidal properties in the dilute and semidilute regimes. The dispersions were resistant to salt-induced aggregation up to at least 1 mM NaCl and were optically anisotropic when subjected to an electric field or flow. Interestingly, the electrophoretic mobility of polystyrene sulfonate (PSS)-stabilized nanotubes increases with increasing ionic strength, because of the high surface charge and softness of the adsorbed polyelectrolyte. Furthermore, unlike many rod-like colloid systems, the polymer nanotube dispersion has low viscosity because of weak rotary Brownian motions and strong tendency to shear thinning. At the high shear rates achieved in capillary viscometry experiments, however, we observed a slight shear thickening, which can be attributed to transient hydrocluster formation.

1. INTRODUCTION The discovery of carbon nanotubes (CNT) triggered significant interest in tubular nanostructures over the last 20 years. A wide variety of tubular nanoparticles (i.e., nanotubes) have been prepared using methods such as laser ablation, arc discharge, molecular self-assembly,1 and template-guided growth. Most studies have focused on refining the existing systems or synthesizing nanotubes using new materials/routes or adding new functionalities. However, effective use of nanoparticles in practical applications often requires them to be dispersed in liquids. While progress has been made in dispersing carbon nanotubes in water and organic solvents, drawing upon electrophoresis and rheology as key diagnostics,2 there are few studies of novel nonCTN nanotube dispersions. While the nanotube “inventory” keeps expanding, knowledge on how to disperse them and control dispersion behavior is still lacking. Furthermore, rod-like particles are much less understood as compared to their spherical counterparts. In addition to the more intricate theoretical interpretation, experimental investigations are limited because there are very few high-aspect-ratio, welldefined model rod systems available. The aspect ratios achieved in the classical model rod systems, such as tobacco mosaic virus (TMV) and clay particles,3 are usually limited to 100 μm mean free path22 at the operating pressure of 0.1
0.3 Torr is large compared to the pore diameter), the etching effect is still significantly deep inside the pores. In comparison to plasma etching, wet mechanical polishing provides satisfactory nanotube separation (Figure 4i) and dispersion (Figure 4ii).23 With the exception of some bundles, most nanotubes are well dispersed. Even with extensive polishing, there always remains a small number of bundles containing several nanotubes. These may arise from imperfections in the alumina templates that yield points of lateral attachment and imperfect polishing.24 We conclude that minimal nanotube damage and maximum dispersion is achieved by mechanical polishing with mild sonication during template dissolution, as adopted exclusively for our subsequent nanotube and dispersion characterization.

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3.4. Nanotube Stabilization and Characteristics. There are few studies of nanotube or nanorod dispersions from porous alumina templates reported in the literature.9c Indeed, most authors have concluded their studies at the template dissolution stage, leaving nanotubes dispersed in a concentrated alkaline, aluminate solution. Interestingly, aggregation occurs when such solution is replaced by or diluted with water, while dialysis against concentrated acid or base does not induce aggregation. The stabilization mechanism is still not well understood, but is likely to involve steric influences. Initial attempts to remove the strong base/acid and aluminates by repeated centrifugation and redispersion in water resulted in aggregation, as did dialysis of nanotubes with SMA or PEI exteriors against decreasing concentrations of acid or alkali. We initially suspected this was mainly due to residual aluminum or aluminate ions, which are known to be strong flocculants. However, when citrate, a strong chelating agent for Al,25 was added, aggregation persisted. Evidently, therefore, the outermost polyelectrolyte layer (note: PEI is a cationic polymer, and SMA becomes a negatively charged polyanion) after hydrolysis of these covalently multilayered nanotubes is not sufficiently charged for stabilization as in many electrostatically assembled polyelectrolyte multilayer microcapsules.26 This is because the charge-inducing groups (amines and maleic anhydride) participate in covalent bonding, making them unavailable for charge stabilization. Therefore, charge stabilization must be enhanced by additional polyelectrolyte adsorption. To ensure the aluminum or aluminate ions are sufficiently depleted while avoiding aggregation, the pH was gradually brought to neutral (these ions are almost solubilized under acidic or alkaline conditions) by decreasing the base or acid concentration at each centrifugation/redispersion step. All of the cationic polyelectrolytes tested (PEI, poly-DADMAC, PAH, and polyallylamine) produce nanotube dispersions that are stable against aggregation. However, among the three anionic polyelectrolytes, only PSS prevented aggregation, producing stable dispersions; PAA and P(AA-co-MA) are weak polyelectrolytes that present insufficient charge. As shown in Figure 1iv, the nanotubes have relatively thin walls and spontaneously collapse into flat ribbons upon drying. Thicker walls can be achieved by increasing the number of deposition cycles, but still collapse when the SMA/PEI deposition index increased from 5.5 to 8.5. Further increase hinders the filtration due to narrowing and blocking of surface pore openings (see Figure 11 in the Supporting Information), and significantly increasing the pressure would cause the thin and brittle alumina templates to crack. To discern whether tubes are open or collapsed in dispersion, microtome sections of dispersed (PEI/SMA)5.5 nanotubes were prepared.27 As shown in Figure 5, the majority of the sectioned tubes are not collapsed. The cross sections are sometimes elliptical because the sectioning was not exactly perpendicular to the nanotube axes. It is also possible that nanotubes retain the shapes of the template pores, which are not perfectly circular (see Figure 1i). To ascertain whether dried, collapsed nanotubes are able to regain their original shape upon rewetting, we imaged dry nanotubes using AFM. The block-shaped height profile shown in Figure 6I confirms the flat ribbon morphology from SEM. Upon rewetting, the height is less than 40 nm even after 30 min,28 as shown in Figure 6ii, suggesting that nanotubes are permanently deformed by drying. 11421

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Figure 5. TEM micrographs of microtomed (PEI/SMA)5.5 nanotubes dispersed in Epon (starting from water dispersion by gradual filtration): (i) lower magnification; (ii) higher magnification. Nanotubes are sectioned across their longitudinal axis. Scale bars: 1 μm.

The glass transition temperature Tg of branched PEI is reported to be
55 °C.29 For SMA alternating copolymer, the manufacturer (Aldrich) provides Tg = 250 °C, but our experimentally measured value from differential scanning calorimetry (DSC) is 169 °C. A simple composite of the two polymers was prepared by simultaneously adding PEI 10 mg/mL and SMA 10 mg/mL solutions dropwise to a mixture of 10 mL of isopropanol and 10 mL of THF. The precipitate was dried, and its Tg was determined by DSC to be 131 °C. Thus, although the SMA/PEI multilayers have interlayer covalent bonds, the polymer nanotubes are likely glassy at room temperature. When the water inside the nanotubes is removed by evaporation, a significant capillary pressure difference, given by the Young
Laplace equation:30
 1 1 r1 þ r2 Δp ¼ p0
pc ¼ γ þ ð1Þ ¼γ r1 r2 r1 r2 exists across the nanotube wall. Here, γ is the interfacial surface tension, and r1 and r2 are the principal radii of curvature. The capillary pressure induces deformation that decreases the meniscus radius of curvature and, therefore, enhances the driving force for collapse. The deformation is initially elastic, and finally becomes plastic and irreversible (longitudinal creasing). Therefore, upon rewetting, the contacting surfaces may separate, but the original cross-section is not fully restored. Note that alumina-membrane-templated nanotubes often have blind-ended side branches (see the Supporting Information), as observed by others.31 These are accurate replicas of the branched pores in anodic alumina (see the Supporting Information) arising from unstable growth at high anodization voltages. Sui et al. showed that straight pores prevail with lower anodization voltages,31b thereby eliminating the possibility of side-branched nanotubes. The lengths of dispersed (PEI/SMA)5.5 nanotubes were determined by analyzing phase-contrast optical microscopy micrographs of the dispersion. The skewed length distribution shown in Figure 4iii, based on more than 2000 counts, gives a number average tube length Ln ≈ 29 μm, weight average length Lw ≈ 39 μm, and polydispersity index PDI ≈ 1.3. The narrow

Figure 6. AFM images (4 μm  4 μm) and height profiles of PEIstabilized (SMA/PEI)5.5 nanotubes (i) dried on glass and (ii) 30 min after rewetting.

peak at about 60 μm is ascribed to nonseparated nanotube bundles. Several factors may affect the length measurement: first, optical microscopy cannot distinguish single nanotubes from a bundle of two or three tubes; second, during the count, obvious large tube bundles were only counted as one tube; and third, side branches may break off the main tubes during preparation, contributing to the short tube counts. Nevertheless, the results are reproducible: another independently prepared sample gave similar results (number average tube length Ln ≈ 28 μm, weight average length Lw ≈ 36 μm, polydispersity index PDI ≈ 1.3). We initially expected to the particles to be highly monodispersed, as they are synthesized with essentially the same template length. Evidently, during nanotube dispersion preparation, the brief sonication to improve nanotube separation breaks nanotubes, reducing the average tube length. This effect is amplified by increasing sonication power and duration (an extreme case is presented in the Supporting Information). 3.5. Dispersion Characteristics. The polyelectrolyte-stabilized nanotube dispersions have a hazy, uniform appearance when stationary as shown in Figure 7. At low nanotube concentrations, disturbances produce complex flow patterns32 that resemble the Schlieren effect (due to density-induced refractive index fluctuations) and streaming birefringence (due to shearinduced directionally dependent refractive index33). Elongated particles such as nanotubes scatter light in a much more anisotropic manner than their spherical counterparts. Accordingly, nanotubes in a quiescent dispersion at low concentrations are randomly oriented, yielding statistically isotropic and homogeneous optical properties. Hydrodynamic disturbances, 11422

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Figure 7. PEI-stabilized (SMA/PEI)5.5 dispersion in water. Left: ∼4.5 mL, ∼0.08 wt % (picture taken right after agitation), the complex flow pattern is due to anisotropic light scattering of nanotubes. Right: ∼1 mL, ∼0.6 wt %, ∼1.5 vol %.

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(according to ϕi = 3.3d/L and ϕn = 4.2d/L for rod of length L and diameter d). However, for the current nanotube system, the dispersion is already opaque at a volume fraction of ∼0.015, as shown in Figure 7ii. In addition, lyotropic liquid crystals usually undergo phase transitions over an extended period of time.35 Because the present system also undergoes sedimentation, forming a white paste-like sediment, we did not identify any liquid crystalline phases. Because of their relatively large size (diameter 200 nm, length up to 60 μm), the polyelectrolyte-stabilized nanotubes still settle when dispersed, with an average settling rate estimated to be ∼10
20 nm/s in water.36 White sediment becomes visible at the bottom within several days after preparation, but can be easily redispersed by gentle mixing/shaking. The sedimentation velocity u of a rod is orientation-dependent. For a very long and thin rod of length L and diameter d, u = [Δmg ln(L/d)]/(2πηL) when parallel to the gravitational field, and u = [Δmg ln(L/d)]/(4πηL) when perpendicular. Here, Δm is the tube reduced mass, η is the fluid viscosity, and g is the gravitational acceleration. For random orientations, the orientationally averaged sedimentation velocity37 is
 L Δmg ln d ð2Þ hu i ¼ 3πηL For sedimenting Brownian particles irrespective of their shape, the balance between diffusion and sedimentation gives a Boltzmann distribution of particle concentrations at equilibrium:38
 c2 Δmg ðx2
x1 Þ ln ð3Þ ¼ kB T c1

Figure 8. Electric-field-induced nanotube alignment changes dispersion opacity: PSS-stabilized (PEI/SMA)5.5 nanotube dispersion (0.01
0.05 wt %) in a 11  11  43 cm polystyrene cuvette (i) before and (ii) after applying an electric field (amplitude E0 = 2  104 V/m, frequency f = 20 kHz) perpendicular to the page. The electrodes are two transparent ITO-coated PET films (35 Ω/0, Aldrich) fixed inside the cuvette at two facing walls. To improve contrast, images were acquired using the illumination and camera of a Dataphysics OCA Contact Angle System. A flexible tube is immersed on the left-hand side for agitation. The corresponding nanotube orientations are also indicated schematically.

however, produce inhomogeneous shear alignment that scatters light in a manner that reveals turbulent eddies and vortices, as shown in Figure 7. Similarly, controlled and homogeneous alignment, achieved by applying a uniform electric field, produces a much more uniform change in opacity, as shown in Figure 8, which is further elaborated in an electro-optics study described elsewhere.34 Because the nanotubes are also well dispersed before removing NaOH and aluminates, such pristine dispersions have optical characteristics similar to those of the polymer-stabilized ones. According to Onsager’s theory,11c the isotropic and nematic volume fraction limits ϕn and ϕi are 1.1  10
2 and 1.4  10
2, respectively, for 60 μm long rods

where c2/c1 is the ratio of particle concentrations at two locations separated by a vertical distance x2
x1. Considering 30 μm long (SMA/PEI)5.5 nanotubes, the orientationally averaged sedimentation velocity is 19 nm/s, and the particle concentration ratio over a distance of 5 cm at equilibrium is essentially zero, which means they will ultimately settle to the bottom. Both agree with experimental observations. The polymer stabilizers are physically adsorbed on the nanotube surfaces with no covalent anchoring. Therefore, there is a dynamic equilibrium between polymer adsorption and desorption. Even after repeated centrifugation and redispersion in polymer-free medium, there is still free polyelectrolyte in the system, the amount of which can be roughly estimated from solution conductivity (conductivity measurements are available in the Supporting Information). We evaluated the colloidal stability of PEI- and PSS-stabilized nanotube dispersions against dialysis in milli-Q water with a 1000 kDa molecular weight cutoff (Spectra/Por cellulose ester membranes, Spectrum Laboratories): a PSS-stabilized nanotube system with ∼0.03 mg/mL of remaining free PSS started to aggregate after 9 days of dialysis and could not be redispersed with sonication; in contrast, the hyper-branched PEI-stabilized system remained flocculation-free after extensive dialysis against Milli-Q water or 0.001 M NaCl. Such differences in response to dialysis can be explained by the polymer conformation. During stabilization, PSS adopts extended conformations on the particle surface at high polymer concentrations, producing thick, fuzzy layers; as PSS desorbs and becomes depleted upon prolonged dialysis, the remaining linear molecules begin to lay flat, providing much less charge and steric stabilization. On the other hand, the globular 11423

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Langmuir structure of branched PEI molecules makes them less subject to adsorption conformation changes with changing concentration. Further evidence for the dynamic adsorption/desorption of polymer stabilizers was also found: supernatants removed during the centrifugation/redispersion procedures are richer in the lower molecular PSS than the pristine PSS solution, because the former is found to reach a much lower conductivity level after extensive dialysis (see the Supporting Information; note that commercial PSS has a broad molecular weight distribution). Obviously, polymer adsorption favors longer chains because of the larger number of adsorption sites per molecule.

Figure 9. Examples of electrophoretic mobilities for nanotubes with different stabilizers, maleic anhydride copolymers, and MAAC/PEI cycles: from left to right, PEI-stabilized (MVEMA/PEI)5.5, (PEMA/ PEI)5.5, and (SMA/PEI)4.5 nanotubes; poly-DADMAC, PAH, and polyallyamine-stabilized (SMA/PEI)5.5 nanotubes (dispersed in 0.001 M NaCl at a concentration of ∼0.003 wt %).

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After long-term storage, the nanotubes remain well stabilized in the aqueous polyelectrolyte solutions. Even after a year, the settled particles are easily redispersed by gentle mixing with no visible aggregation. We measured the rotary diffusivity Dr from PSS-stabilized nanotube dispersions that were at least 3 months old, using a light-transmittance-based electro-optical technique.34 The measured Dr values agree with predictions based on the nanotube dimensions, confirming the absence of aggregation. The polyelectrolyte-stabilized nanotubes also have good resistance to salt-induced aggregation: tested in NaCl solutions up to 0.01 M by visual inspection and up to 0.5 mM confirmed by electro-optics.34 However, in 0.02 M phosphate buffer (pH 7.4), the particles immediately flocculate, because multivalent ions (such as trivalent phosphate anions) are much more efficient than Na+ or Cl
at compressing the double layer and flocculating electrostatically stabilized systems. 3.6. Electrophoretic Mobility and Zeta-Potential. Some electrophoretic mobility examples of positively charged nanotubes are shown in Figure 9. There is no significant difference among the four cationic polyelectrolyte polymer stabilizers. As the stabilizing layer dominates the particle surface properties, neither the maleic anhydride copolymer in the multilayers nor the MAAC/PEI cycle number (4.5 or 5.5) has an effect on the mobility. In addition, there is negligible particle-concentration influence over the range tested (Figure 10i), and, as expected, the particles have lower mobility (i.e., surface potential) at higher salt concentrations. Similarly, for anionic polyelectrolyte PSS-stabilized nanotubes, particle concentration does not affect electrophoretic mobility (cf., Figure 10ii). Interestingly, the nanotube electrophoretic mobility

Figure 10. (i) Electrophoretic mobilities of PEI-stabilized (SMA/PEI)4.5 nanotubes dispersed at different wt % and at two NaCl concentrations: 1 mM (b), 0.1 mM (9). (ii) Electrophoretic mobilities (b) of PSS-stabilized (PEI/SMA)5.5 nanotubes dispersion in 50 μM NaCl at various nanotube concentrations. (iii) Electrophoretic mobilities of PSS-stabilized (PEI/SMA)5.5 nanotubes dispersed in electrolyte with various NaCl concentrations (nanotube concentration 0.003 wt %). 11424

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Langmuir increases with the salt concentration (cf., Figure 10iii), contrary to the PEI-stabilized nanotubes. The effect of ionic strength on zeta-potential or electrophoretic mobility is not straightforward to interpret. On the one hand, the zeta-potential (defined as the potential at the shear plane) decreases with increasing ionic strength because of double layer compression. On the other hand, for impenetrable, bare spheres with fixed, low surface-potential, the mobility increases monotonically from the low-salt H€uckel limit of (2/3)εζη
1 to the high-salt Smoluchowski limit of εζη
1 as ka (particle size/ double layer thickness ratio) increases from zero to infinity. Here, ε is the dielectric constant, ζ is the surface potential, η is medium viscosity, k
1 is the Debye length, and a is the particle radius. At high surface potentials (|ζ| > kBT/e ≈ 25 mV, e is the elementary charge), however, the electrophoretic mobility reaches a minimum before increasing to the Smoluchowski limit. Using the Smoluchowski formula, which can be applied to particles with arbitrary shape30 and ka . 1, the estimated zetapotentials are
40 to
60 mV and 40
60 mV for the PSSstabilized nanotubes and cationic polymer-stabilized nanotubes, respectively. Therefore, for these surface potentials, the mobility may rise or fall depending on ka. From numerical solutions of the standard electrokinetic model by O’Brien and White39 for spherical particles in KCl solutions, for the same zeta-potential, the absolute mobility reaches a minimum when ka ≈ 1, and it increases for smaller or larger ka values. For [NaCl] from 5.5 μM to 0.01 M in Figure 10iii, ka varies between 0.78 and 33 (taking nanotube radius as a), which is exactly in the region of increase for absolute mobility verus ka (salt concentration). The above analysis is based on the assumption that the particles have constant surface potential. If the particles have constant surface charge density σ,40 then the H€uckel and the Smoluchowski limits are (2/3)σaη
1 and σk
1η
1, respectively, meaning that particles move more slowly at high salt concentrations. Similar arguments apply with a constant surface charge density as for constant surface potential, because mobility maxima also exist with high surface charge density.34 An adsorbed fuzzy polyelectrolyte layer presents further complications. Here, we provide a qualitative explanation. At low ionic strength, the adsorbed polymer loops and tails protrude from the surface, displacing the shear plane into the solution; as the ionic strength increases, polymers collapse, displacing the shear plane toward the particle, effectively increasing the charge density/surface potential and, therefore, increasing the mobility. This is similar to the mechanism proposed by van de Ven et al.41 explaining the abnormal mobilities of hairy latex particles. Because PEI is a highly branched polymer, its globular conformation is less affected by the ionic strength than linear PSS. This seems to account for the different mobility behaviors of PEI- and PSS-stabilized nanotubes with increasing ionic strength. Numerical solutions of the standard electrokinetic model for soft spheres by Hill et al. demonstrate the possibility of a mobility that increases with ionic strength when particles are coated with polyelectrolyte. Note that this occurs when the double layer is thinner than the polymer layer and the polymer segment mobility is high.34 Thus, the different segment mobilities of PEI and PSS may also contribute to qualitative differences in the particle mobility behavior. A closer examination of the electrophoretic mobility distribution reveals another interesting phenomenon for the PSS- and PEI-stabilized nanotubes (Figure 11i and ii). At low salt concentration, there are multiple mobility peaks spanning a wide

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Figure 11. Electrophoretic mobility distributions of (i) PSS-stabilized (PEI/SMA)5.5 nanotubes (particle concentration 0.0034 wt %) and (ii) PEI-stabilized (SMA/PEI)4.5 nanotubes (particle concentration 0.0025 wt %) dispersed in various NaCl concentrations. y-axis in counts, x-axis in mobility (μm cm/V s). Each graph contains five distribution curves obtained from five measurements.

range. These do not coincide with repeated measurements; however, with increasing salt concentration, these peaks merge, and the repeatability improves. At low salt concentration, with the polymer “hair” on the surface, the nanotubes are not identically stabilized, and they may have different charge densities or adsorbed layer thicknesses. In addition, the laser Doppler electrophoresis instrument used in this study produces periodic slow and fast field reversal during measurements. With such frequent electric-field changes, the adsorbed polymers possibly undergo constant conformation/charge distribution change outside the shear plane at low ionic strength, giving rise to different mobility distribution profiles. However, with increasing ionic strength, the fuzzy layers collapse, and the surface charge density 11425

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Figure 12. (i) Specific viscosity ηsp = (η/η0)
1 and (ii) reduced viscosity ηsp/ϕ of PSS-stabilized (PEI/SMA)5.5 nanotube dispersions (measured by size 0 and 0C Ubbelohde viscometers) in 50 μM and 5 mM NaCl with increasing particle volume fraction ϕ (ϕ is converted from weight percent based on nanotube density, wall thickness, and diameter). 50 μM NaCl higher shear (4); 5 mM NaCl higher shear (0); 50 μM NaCl lower shear (]); 5 mM NaCl lower shear (O). The secondary x- axis (on top) is the volume fraction scaled with ϕ* for 30 μm long tubes. For clarity, the uncertainty bars are only shown for one data series, demonstrating that the second virial coefficient k2 is constant within experimental error.

becomes constant and uniform, leading to one reproducible mobility peak. Multiple peaks reappear when [NaCl] reaches 0.01 M, because the dispersion is likely destabilized at this salt concentration, causing variations in electrophoretic mobility among particles. 3.7. Viscometry of Semidilute Nanotube Dispersions. We measured the viscosities of PSS-stabilized nanotube dispersions using two different shear rates using two capillary viscometers. On the basis of the nominal parameters for Ubbelohde viscometers,42 knowing the drainage time t (typically 303 and 936 s), the shear rates calculated according to43 : 4V γ¼ 3 R πt

ð4Þ

are 2433 and 466 s
1, respectively. Defined as the boundaries dividing the dilute, semidilute, and concentrated regimes for particle dispersions, the critical transition number concentrations for rod-like particles are n* = 1/L3 and n** = 1/(dL2) (L is the rod length, and d is the rod diameter).44 The corresponding critical volume fractions are ϕ* = 3.5  10
5 and ϕ** = 5.2  10
3 for 30 μm long, 200 nm diameter tubes. Therefore, as shown in Figure 12, the nanotube dispersions were examined in the semidilute regime, approaching the dilute regime. The reduction of dispersion viscosity with higher salt concentrations is evident only at higher ϕ (cf., Figure 12i). As diffuse double layers are compressed by added salt (k
1 decreased from 43 to 4.3 nm), the energy dissipation due to double-layer distortion and interactions also decreases, thereby lowering the viscosity. With ϕ approaching the dilute regime, such an effect is diminished (i.e., the specific viscosity curves merge into one). This suggests that, with progressive dilution, the viscosity is dominated by the disturbance of flow due to the uncharged particle. To confirm this, we plot the reduced viscosity ηsp/ϕ against ϕ to furnish the intrinsic viscosity (cf., Figure 12ii). According to:45 η=η0
1 ¼ ½η þ k2 ϕ þ Oðϕ2 Þ::: ϕ ¼ ð½η0 þ pÞ þ k2 ϕ þ Oðϕ2 Þ:::

ð5Þ

the intrinsic viscosity [η] (including the primary electro-viscous effects, denoted by p) is the intercept of the extrapolated curve at ϕ = 0, and the second virial coefficient k2 (sum of the hardbody interaction and the secondary electro-viscous effect) is evaluated from the initial slope. The intrinsic viscosity (extrapolated to ηsp/ϕ at infinite dilution) is around 80
100 for higher shear, and 40
60 for the lower shear, with little effect of ionic strength. This indicates negligible primary electroviscous effects. The reduced viscosities are constant within experimental errors up to ϕ = 0.0002, suggesting k2 ≈ 0. The contribution of the hard body interaction to k2 is expected to be very small, because for such long rods the flow is not much modified to significantly increase the energy dissipation during a two-body collision (as only two small parts of the rods are interacting). Hence, the secondary electroviscous effects must also be negligible. The rotational diffusion coefficient Dr is 6.3  10
4 s
1 for 30 μm long tubes according to:9e "

2 # 3kB T L d d Dr ¼ ln
0:662 þ 0:917
0:050 πηL3 d L L ð6Þ _ r, which is a dimensionThe rotational Peclet number Per ≈ γ/D less parameter that quantifies the competition between shear and particle rotary Brownian motion, is ∼3.8  106 and ∼7.4  105 for 30 μm long nanotubes in size 0 and 0C viscometers, respectively. Evidently, Brownian motion is insignificant as compared to the shear in these capillary viscosity experiments. Brenner46 has shown that, for suspensions of prolate spheroids, the intrinsic viscosity decreases with increasing Per (especially when Per is beyond 1). In the absence of shear (i.e., Per = 0), the intrinsic viscosity can be estimated to be ∼1200 for 30 μm long tubes based on47   re 2 1 1 ð7Þ þ ½η ¼ þ 1:6 ln 2re
0:5 5 3ðln 2re
1:5Þ where re is the aspect ratio of the slender particles. In comparison, 11426

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the limiting value at infinitely large Per is 13, according to:47 ½η ¼

0:312re 0:5 1:872 þ 2

ln 2re
1:5 re ln 2re
1:5

ð8Þ

[η] from Figure 12ii is likely to be close to the range for hard, uncharged cylinders with average aspect ratio ∼150 and Per ≈ O(106), meaning there is no significant electro-viscous contribution. Takamura and van de Ven32 investigated dispersions of charged spherical latex particles in water, and they noticed decreasing colloidal interactions when increasing the shear rate: the viscosity approached limiting values that were independent of ionic strength. As was also noted by Russel and Larson,48 the electro-viscous effect vanishes at high shear rates. At present, there is no complete theory to account for the electro-viscous effects of rod-like particles. The insignificant electro-viscous effect in our system is likely due to the high shear rate. Another interesting result is that slightly higher specific viscosities were obtained from the smaller capillary (i.e., higher shear) viscometer (as shown in Figure 12i), contrary to the expected shear thinning. Because the electro-optics study shows that nanotubes are well dispersed,34 such behavior cannot be due to aggregates. In dispersions of spherical particles, shear thickening results from transient hydroclusters at very high shear rates,49 even in dilute dispersions.50 Similarly, the unusual shear thickening we observe might be due to reversibly formed nanotube clusters at high shear rates. Because hydrocluster-free long rod systems should be highly shear thinning, the observed viscosity increase likely reflects the net result of shear thinning due to flow-induced alignment and thickening due to hydrocluster formation. There have been reports of rod-like particle dispersions that are notably viscous or even gel-like,3c,10b but these behaviors were not observed in our polymer nanotube system, which is essentially free-flowing. Brownian motion restores random particle orientations, increasing the disturbance and energy dissipation. Therefore, Brownian motion considerably increases the shear viscosity, as noted in the literature.51 However, because of the large size and high aspect ratio, the rotary Brownian motions are relatively weak for these polymer nanotubes. Even a very weak shear can still make Per quite large, leading to considerable shear thinning. Considering this and other factors, such as the absence of electro-viscous effects, and the high shear rate in capillary viscometers, the low viscosities of our dispersions are not surprising.

4. CONCLUSIONS We characterized polymer nanotubes prepared by the layerby-layer deposition of maleic anhydride alternating copolymers and PEI in porous alumina templates. The polymer multilayers grow linearly with the number of deposition cycles, as expected for polymer adsorption. Surface-layer supported nanotube arrays produce novel, free-standing films of aligned nanotubes, which have interesting optical properties. To produce nanotube dispersions, polymer surface layers deposited on the template can be removed by plasma etching, but such treatments severely damage the tubes. Rather, mechanical polishing with sonication during template dissolution minimizes damage and optimizes separation and dispersion. Nanotube dispersions that are stable against aggregation were obtained using anionic and cationic polyelectrolyte as a stabilizer. These thin-walled nanotubes remain tubular in dispersion, but collapse and plastically deform

upon drying due to capillary forces. The dispersions are resistant to salt-induced aggregation up to at least 1 mM NaCl. According to the dispersion/supernatant conductivity, PEI and PSS exhibit different adsorption behaviors, with preferential adsorption of higher molecular weight polyelectrolytes. The dispersions are optically anisotropic with flow or an applied electrical field. We observed an unusual increase in electrophoretic mobility with increasing ionic strength for PSS-stabilized nanotubes, which we attribute to a high surface potential and fuzzy surface stabilizing layer. Furthermore, unlike many rod-like colloid systems, the polymer nanotube dispersion has low viscosity because of weak rotary Brownian motions and strong tendency to shear thinning. At the high shear rates achieved in capillary viscometry experiments, electro-viscous effects are negligible; however, we observed a slight shear thickening, which can be attributed to transient hydrocluster formed at very high shear.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures 1 and 2: Chemical structures of polymers used in the nanotube synthesis and stabilization. Figure 3: Schematic drawing of the filtration assembly used for polymer deposition. Figure 4: Polymer layer formed at the template’s top and bottom surfaces. Figure 5: SEM micrographs of nanotubes from (SMA/PEI)5.5 alumina templates after air
plasma etching. Figure 6: TEM image of (PEI/SMA)5 nanotubes after high intensity ultrasonication. Figure 7: Surface and bulk pore structures of commercial 0.02 μm alumina templates. Figure 8: Side branches of the synthesized polymer nanotubes. Figure 9: Branched pores of commercial alumina templates. Figure 10: Surface and bulk pore structures of commercial 0.2 μm alumina templates. Figure 11: Polymer-deposited template surface before and after mechanical polishing. Table 1: Conductivity data of nanotube dispersions, centrifuge supernatants, and pristine polymer solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We would like to thank NSERC and FPInnovations for financing a NSERC Industrial Research Chair and CSACS (Centre of Self-Assembled Chemical Structures) for use of their characterization facilities. ’ REFERENCES (1) (a) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005, 105, 1401–1443. (b) Liu, D.; Park, S. H.; Reif, J. H.; Labean, T. H. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 717–722. (2) (a) Backes, C.; Schmidt, C. D.; Rosenlehner, K.; Hauke, F.; Coleman, J. N.; Hirsch, A. Adv. Mater. 2010, 22, 788–802. (b) Kim, B.; Park, H.; Sigmund, W. M. Colloids Surf., A 2005, 256, 123–127. (c) White, B.; Banerjee, S.; O’Brien, S.; Turro, N. J.; Herman, I. P. J. Phys. Chem. C 2007, 111, 13684–13690. (d) Sun, Z.; Nicolosi, V.; Rickard, D.; Bergin, S. D.; Aherne, D.; Coleman, J. N. J. Phys. Chem. C 2008, 112, 10692–10699. (e) Kinloch, I. A.; Roberts, S. A.; Windle, A. H. Polymer 2002, 43, 7483–7491. (f) Shaffer, M. S. P.; Fan, X.; Windle, A. H. Carbon 1998, 36, 1603–1612. (g) Etika, K. C.; Jochum, F. D.; Theato, P.; Grunlan, J. C. J. Am. Chem. Soc. 2009, 131, 13598. 11427

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