Tunable Encapsulation Structure of Block Copolymer Coated Single

May 15, 2015 - In this work, we have fabricated a molecular building block of single-walled carbon nanotubes (SWNTs) coated by PPO–PEO–PPO block c...
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Tunable Encapsulation Structure of Block Copolymer Coated SingleWalled Carbon Nanotubes in Aqueous Solution Youngkyu Han,† Suk-kyun Ahn,‡ Zhe Zhang,†,§ Gregory S. Smith,† and Changwoo Do*,† †

Biology and Soft Matter Division, Neutron Sciences Directorate, and ‡Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Jülich Center for Neutron Science, Forschungszentrum Jülich, Jülich, NRW 52425, Germany S Supporting Information *

ABSTRACT: Nanosized and shape-tunable molecular building blocks can provide great opportunities for the fabrication of precisely controlled nanostructures. In this work, we have fabricated a molecular building block of single-walled carbon nanotubes (SWNTs) coated by PPO−PEO−PPO block copolymers whose encapsulation structure can be controlled via temperature or addition of small molecules. The structure and optical properties of SWNT block copolymers have been investigated by small-angle neutron scattering (SANS), ultraviolet−visible (UV−vis) spectroscopy, atomic force microscopy (AFM), and molecular dynamics (MD) simulation. The structure of the hydrated block copolymer layer surrounding SWNT can be controlled reversibly by varying temperature as well as by irreversibly adding 5-methylsalicylic acid (5MS). Increasing hydrophobicity of the polymers with temperature and strong tendency of 5MS to interact with both block copolymers and π orbitals of the SWNTs are likely to be responsible for the significant structural change of the block copolymer encapsulation layer, from loose corona shell to tightly encapsulating compact shell. Our result shows an efficient and simple way to fabricate and manipulate carbon-based nano building blocks in aqueous systems with tunable structure.

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carbon’s hexagonal sp2 conformation and the one-dimensional (1D) geometry.9,14 Pristine CNTs, however, exhibit poor solubility and the strong tendency to bundle together in polar solvents, which are the major obstacles in the advance of CNT application toward environmental and biomedical systems and devices,11,12,14−17 and thus, polymers are utilized in CNT− polymer composites playing a crucial role in the practical use of CNT-based materials by providing the CNT-friendly media as well as the structural stability to the as-assembled architectures. By taking advantage of the rich phase behavior of polymers, various self-assembled architectures using CNTs have been demonstrated,18,19 and tunable self-assembled CNT structures have been also reported.20 However, much less attention has been devoted to utilizing the CNTs as a tunable 1-D nanobuilding block, which can potentially provide the capability of controlling self-assembled nanostructures at a much finer level.21,22 Here we report our progress toward the fabrication of structure-tunable SWNT-based nano building blocks in aqueous solution using noncovalent functionalization of Pluronic triblock copolymers. Pluronic polymers, which consist of two hydrophilic poly(ethylene oxide) (PEO) chains at both

ottom-up nanofabrication via hierarchical molecular selfassembly has been shown to be a powerful and effective way to create functional nanostructured materials.1,2 A variety of self-assembled architectures with distinct functionalities can be achieved by changing constituent building blocks and their diversified interactions.3−6 In particular, “smart” materials, which are responsive to surrounding environmental conditions due to weak noncovalent interactions involved in self-assembly, are of great interest in a wide range of applications, such as catalyst, sensors, and drug delivery.7,8 Easy and novel methods to fabricate such functional nanostructures with well-defined geometries and precisions still remain challenging. The careful design and utilization of nanosized and shape-tunable molecular building blocks for hierarchical self-assembly provide great opportunities for the fabrication of precisely controlled nanostructures. One promising and robust strategy for fabrication of a novel functional building block is to construct nanoparticle−polymer nanocomposite materials, which possess both the outstanding functionalities of nanoparticles and the unique advantages of polymers, such as easy production and manipulation, light weight, and flexibility. Carbon nanotubes (CNTs), one fascinating choice as fillers of the nanocomposites, have been used in a wide range of nanotechnology applications9−13 because of their tremendous mechanical, thermal, and electrical properties as well as the low percolation threshold based on the © XXXX American Chemical Society

Received: March 3, 2015 Revised: April 17, 2015

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DOI: 10.1021/acs.macromol.5b00456 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules ends and one hydrophobic poly(propylene oxide) (PPO) chain at the center, exhibit rich thermodynamic phase behavior23,24 due to their sensitivity to temperature, concentration,23 salt,25 or pH26 in aqueous solutions and extraordinary biocompatibility due to the minimal toxicity and immune response of PEO.27,28 The introduction of a stimuli-responsive encapsulation layer onto SWNTs can lead to the fabrication of smart nano building blocks with tremendous mechanical and electrical properties, reversibly tunable structures by controlling temperature or pH, and environment-friendly and biocompatible interfaces. Additionally, such noncovalent surface modification enables us not only to stabilize the dispersed CNTs using steric or electrostatic repulsion between the hydrophilic parts of block copolymers29−32 but also to minimize any surface-treatment-induced suppression in intrinsic CNT functionality.33 In this work, we have used Pluronic F127 triblock copolymers (PEO100-PPO64−PEO100, MW = 12.6 kg/mol, 70% w/w PEO, BASF) as a dispersing agent of SWNTs (Unidyn, Pure HiPCO34 ). The F127/SWNT aqueous suspension, where the debundled SWNTs are stabilized by F127, was prepared by adding the as-syntheized SWNTs of 0.01 g into F127/D2O solutions (0.25 wt %) of 35 g and by taking the supernatant after sonication (1 h, Cole-Parmer 130 W processor) and centrifugation (2 h, 9820g, Sorvall 6 plus centrifuge, Thermo Scientific). Two stable F127/SWNT solutions (called first and second samples) were prepared. In the second solution, 0.1 wt % of 5-methylsalicylic acid (5MS, TCI America) was added. Both dispersions are stable over several months. The structure of such loosely attached polymers on SWNTs was controlled either by varying temperature (20 °C → 60 °C → 20 °C) or by adding a small aromatic additive, and the structure and optical property of the F127/SWNT building blocks have been investigated by combining small-angle neutron scattering (SANS), ultraviolet− visible (UV−vis) spectroscopy, atomic force microscopy (AFM), thermogravimetric analysis (TGA), and molecular dynamics (MD) simulation. The details are described in the Supporting Information. The first step toward sample characterization was to determine how well SWNTs are dispersed and what is the proportion of each component in the final samples via UV−vis spectroscopy and TGA. In Figure 1A, the UV−vis spectra of the first and second samples are present, which show distinct peaks representing the sub-band electronic transitions of SWNTs. The absorption spectral features differ from those of other 3D materials as well as bundled CNTs and thus indicate the presence of individual SWNTs dispersed in our samples.35−38 This result emphasizes that the SWNTs, which are individually dispersed by the block copolymers through the sample preparation process, maintain their dispersity without any other bundling or aggregation regardless of temperature and the presence of 5MS. The final mass of F127 and SWNT, MF127 and MSWNT, respectively, in the supernatant after sonication and centrifugation is also obtained via the TGA measurement after freeze-drying; only 3.6% (±1.2%) of the total mass of the dried F127/SWNT powders, which corresponds to the mass of SWNTs, remained after increasing the sample temperature to about 750 °C under N2 gas as shown in Figure 1B. The measured mass ratio m (MSWNT:MF127 ≈ 1:26) differs from that of actually added amounts (≈1:9), and the mass fraction of F127 in the sample (0.25 wt %) has nearly unchanged. Since the samples are prepared at 20 °C, which is

Figure 1. Characterization of F127/SWNT nanoparticle suspensions via UV−vis spectroscopy, TGA, and AFM. (A) UV−vis absorption spectra of F127/D2O (solid line), F127/SWNT/D2O (triangle, at 20 °C; dashed line, at 60 °C), F127/SWNT/5MS/D2O (+). The presence of individual SWNTs dispersed in suspensions is indicated by sub-band transition peaks independent of temperature or 5MS concentration. (B) TGA result, which provides the actual mass ratio of F127, SWNT, and D2O in the suspension. (C) Tapping mode AFM image of F127-coated SWNTs on an Si-wafer (scale bar = 1 μm). (Inset) Distributions of length (top) and diameter (bottom) of F127coated SWNTs measured from AFM images. (D) Zoomed-in image of a F127-coated SWNT (scale bar = 200 nm).

below the critical micellization temperature (CMT) at the given F127 concentration,39 no micellization is expected during the sample preparation. We also directly observed the polymercoated SWNT particles via AFM measurements as shown in Figure 1C,D. AFM images of the spin-coated samples on Si wafers were acquired under ambient condition by using the tapping mode (Innova AFM, Bruker). From the AFM images, well-stabilized SWNTs by F127 polymers are observed, and the length and diameter distribution of these F127-coated SWNT nanoparticles are derived with a resolution of 20 and 0.1 nm, respectively. In combining these observations, it is assumed that (i) most of the F127 has attached on the surfaces of individual SWNTs for stabilization and (ii) the F127/SWNT samples are stable even upon either increasing temperature to 60 °C or adding 5MS without the bundled or nonstabilized SWNTs. However, the AFM image of dried samples provides neither real morphologies of F127/SWNT complexes in solutions nor enough spatial resolution for structural characterization of polymer encapsulating a single SWNT core. To investigate the structural aspect of F127/SWNT hybrid nanoparticles, SANS measurements were conducted on the EQ-SANS instrument at Spallation Neutron Source at Oak Ridge National Laboratory. B

DOI: 10.1021/acs.macromol.5b00456 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The SANS intensities of F127/SWNT samples in the absence and presence of 5MS were obtained in a temperature range of 20−60 °C by using neutrons of two wavelength bands 2 Å < λ < 6.1 and 9.1 Å < λ < 13.2 Å at the fixed sample-to-detector distance (1.3 m). By subtracting the scattering intensity contribution of a small amount (15% with respect to the added amount) of free F127 polymers (see Supporting Information), the scattering intensities of F127/SWNT nanoparticle suspension with the q−1 behavior at low q regions (≤0.02 Å−1) were obtained (Figure 2). This is consistent with a

Figure 3. Schematic diagram for our modified core−shell−chain form factor (inset) consisting of an SWNT core, an inner hydrophobic shell including 5MS and parts of F127, and an outer layer including a set of hydration blobs of water and most of F127. Partial contributions of the scattering intensity from both the self-correlation and the cross-term of a “core−shell” cylinder and “chain” blobs are calculated using the fitted parameter of the F127/5MS/SWNT sample at 60 °C.

3). To simply describe the scatting density profile of loosely adsorbed polymer “chains”, the Debye function for a Gaussian coil42 was used in our model by adopting the form factor widely used for similar systems,40,41,43,44 and the adsorbed F127 molecules are assumed to form a set of polymer blobs obeying a Gaussian coil density profile. It is also assumed that most of the added 5MS penetrates into the inner shell because of the combined effect of its low solubility in water, strong tendency to bind with F127,44 and strong adsorption affinity of aromatic rings on NTs in the presence of hydroxyl or carboxyl groups.45−48 The main model fit parameters for the SANS data are summarized in Table 1. The number of fitting parameters is reduced by fixing the scattering length densities (SLDs) of the components40,44 and using the volume fraction of SWNTs, SWNT/F127 mass ratio, and 5MS/F127 mass ratio obtained from the TGA measurements. Since our SANS data do not cover the very low q region corresponding to the length of the F127/SWNT nanorods, an arbitrary unit length (1000 Å) was used in the model fit analysis, based on the measurement of length and diameter distribution of noncovalently stabilized HiPCo SWNTs in aqueous suspensions using a high-power tip sonication and centrifugation.49,50 The model function successfully fits not only the overall shape of each data curve in the entire region of q but also the temperature- and 5MSinduced changes of scattering intensities in each q-region (Figure 2). The model fit results are self-consistent with our assumptions in SANS data analysis. The diameter of SWNT cores (= 7.2 ± 0.2 Å) corresponding to a single HiPCo SWNT indicates the dispersion of individual SWNTs encapsulated by F127 in the solutions,49,51 consistent with the UV−vis spectroscopy result. The results also show that each SWNT is surrounded by an inner shell, where about 12% of the adsorbed F127 on average form a hydrophobic layer of 3−5 Å thickness, and an outer shell, where hydrated chains of the other F127 part occupy 10−12% of the water-rich shell volume, comparably to the polymer volume fractions (10−20%) in corona layers of micelles self-assembled by Pluronic poly-

Figure 2. Scattering intensities of F127/SWNT suspensions in (A) the absence and (B) the presence of 5MS at various temperatures (= 20, 30, 40, 50, and 60 °C) where free 127 contributions are subtracted. The solid lines are the model fit curves using the modified core− shell−chain form factor. The curves are vertically shifted for clarity.

cylindrical particle dispersion regardless of temperature. The scattering intensities exhibit clear distinctions either by changing temperature or by adding 5MS. By increasing temperature, I(q) of the F127/SWNT system shows stronger intensity at low q-region, a shift to higher q at mid q-region (0.02−0.05 Å−1), and a development of a small peak at high q (≈ 0.11 Å−1) as shown in Figure 2A. Interestingly, the effects induced by adding 5MS into the F127/SWNT system are analogous to the temperature-induced change in the scattering intensity, and moreover, increasing temperature in the presence of 5MS causes further change in the same direction. The structure of F127/SWNT nanoparticles was characterized in detail by fitting the SANS data using the core−shell− chain cylinder model form factor, in which an SWNT core cylinder and an additional encapsulating layer of F127 chains and 5MS are considered.40,41 The structure factor from interparticle interference was ignored because our samples are dilute enough. In our model, the additional layer has two shells as shown in the inset of Figure 3: the inner cylindrical shell (shell 1) around a core, where 5MS molecules or a part of F127 (mostly PPOs) is adsorbed on the SWNT surface and no water molecule exists, and the outer chain shell (shell 2), where most of adsorbed F127 extends into water. The total scattering intensity of this core−shell−chain model can be described by the sum of four partial contributions: the self-correlation terms of water-excluding “core−shell cylinder” and water-rich “chains”, the cross-term between “core−shell cylinder” and “chains”, and the cross-term between different “chains” (Figure C

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Macromolecules Table 1. SANS Model Fit Parameters no 5MS

with 5MS

ta (°C)

RCOREb (Å)

T1c (Å)

Rgd (Å)

Nblobe

polymer in shell 1f (%)

D2O fraction in blobsg

20 30 40 50 60 40h 20h 20 30 40 50 60 40h 20h

3.6

4.7 4.5 2.4 4.2 3.4 3.1 4.8 2.7 2.2 3.4 3.2 2.5 2.5 3.7

45.3 44.3 34.5 31.5 29.1 32.8 44.1 32.2 30.4 30.1 28.6 27.0 29.1 33.0

6.1 7.4 17.0 24.5 32.0 20.4 6.6 23.4 28.9 35.3 43.1 50.7 33.0 21.5

14.4 13.7 6.1 12.6 9.4 8.5 15.1 2.8 2.2 3.7 3.5 2.5 2.6 4.2

0.567 0.618 0.626 0.675 0.671 0.638 0.571 0.646 0.655 0.715 0.728 0.720 0.660 0.645

3.6

a

Temperature. bRadius of SWNT cores. It is assumed that the radius is constant regardless of temperature control. cThickness of inner shell. dRadius of gyration of Gaussian chain blobs consisting of F127 molecules. eNumber of Gaussian chain blobs per unit length, which is an arbitrary unit length for cylindrical particles (1000 Å). fPercentage of F127 polymers in the inner shell, vshell 1/vF127, where vshell 1 is the volume of F127 located in the inner shell and νF127 is the total volume of F127. gVolume fraction of D2O in Gaussian chain blobs. hCooling processes. Mass ratios between each component are fixed in model fit, based on TGA measurements (SWNT/F127 = 0.0436, 5MS/F127 = 0.42). Scattering length densities are obtained from the literature.40,44

a temperature range between 30 and 40 °C and kept changing continuously with increasing temperature to 60 °C. Interestingly, addition of 5MS to the F127/SWNT sample causes structural changes of the outer shells similar to that of increasing temperature. By adding 5MS at 20 °C, the structure of outer shells is already close to that of the F127/SWNT at 50 °C without 5MS. In the temperature range between 20 and 60 °C, the reversible changes of Rg and Nblob are still observed in the presence of 5MS. However, any sudden drop or jump in Rg and Nblob, which was shown in the absence of 5MS, is not observed (Figure 4A,B). The penetration of 5MS molecules into the nanoparticles is verified by the inner shell thickness T1 of the model fit results. As shown in Figure 4C, T1 is approximately 4 Å in the absence of 5MS, and it does not significantly change with increasing temperature. By adding 5MS, T1 decreases to around 3 Å with a smaller fitting uncertainty. This thickness of inner shells is close to the interplanar distance between an aromatic ring and the surface of CNT or graphene45,48 and the distance between 5MS and SWNT predicted in our MD simulation (Figure 4D). All these results are in good agreement with the prediction that added 5MSs are likely to occupy the SWNT surface and form a layer with a relatively uniform thickness. The reduced F127 amount (12% → 3%) of inner shells in the total F127 molecules is also self-consistent in the model fit result. While the size of blobs becomes smaller during the structural change induced by either controlling temperature or adding 5MS, the water volume fraction in the blobs does not change significantly enough to contribute to this drastic size change. Nblob, however, increases to compensate the reduced volume of the blobs. This implies that the original blobs at 20 °C become the sets of smaller blobs with increasing temperature or adding 5MS, instead of being dehydrated and condensed on the SWNT surfaces. The larger polymer blob size (Rg ≈ 45 Å), when compared with the radius of gyration for free F127 random coil chains (≈ 30 Å),54 suggests that the SWNT is encapsulated by polymer blobs of several F127 molecules at 20 °C. As temperature increases or in the presence of 5MS, each blob seems to be divided into smaller blobs consisting of only a

mers52,53 (see Supporting Information for more detailed results). Two noticeable changes are observed in the structure of polymer chains located in the outer shells of F127/SWNT nanoparticles by increasing temperature from 20 to 60 °C. As presented in Figure 4A,B, the radius of gyration of the F127 blobs, Rg, was reduced from 46 to 29 Å during the heating process, while the number of the blobs, Nblob, was increased from 6 to 31; they were also reversible by cooling the system down to 20 °C. In particular, both were dramatically changed in

Figure 4. Results of SANS model fit for the F127/SWNT systems in the absence (diamond) and presence (square) of 5MS. (A) Radius of gyration of chain blobs regarded as Gaussian chains in the outer shell. (B) Number of chain blobs per unit length (1000 Å). (C) Thickness of inner shell, T1. The symbols marked by × refer to the cooling process from 60 to 20 °C. (D) Cross-sectional 5MS distribution around an SWNT in water from MD simulation. D

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Macromolecules

SWNT surfaces at a fixed distance provide a new interface that is friendly to both SWNT and F127 because of the benzene ring and hydrophilic groups respectively, making the F127 increase the interfacial area with 5MS by forming a compact layer of individual polymer blobs. In addition, the CNT-free self-assembled morphology of F127 can be also related to the observed structural transformation. Previously, three different models of adsorbed block copolymers on CNT surfaces have been reported, including a random, a micellar, and a cylindrical model.59,60 It is known that the self-assembled morphology induced by the natural curvature is essential to determine the CNT-solubilizing stability, and the “cylinder-forming” block copolymers are the best CNT-solubilizing agents, followed by “sphere- or lamellae-forming” ones.59 Whereas F127 intrinsically self-assemble into spherical micelles in aqueous solution, the addition of 5MS molecules into F127/water systems alters the self-assembled morphology from sphere to cylinder.61 These results suggest that the change of the preferred CNTfree morphology of F127 due to 5MS additive also leads to the structural change of adsorbed polymers from micellar to dense cylindrical morphology, which is analogous to what we observed. Therefore, it can be concluded that stronger interaction of polymer blocks with the core surface, rather than water, drives the structural transition of encapsulating polymers upon temperature control or 5MS addition, and the change of the preferred aggregation morphology of F127 from a sphere to a cylinder in the presence 5MS may also enhance such densification of encapsulation layer. In conclusion, we have successfully fabricated the SWNTbased stimuli-responsive nano building blocks by utilizing triblock copolymers in aqueous solution. The structural changes were determined as a function of temperature with and without aromatic additives using a combination of UV−vis spectroscopy, TGA, MD simulation, AFM, and SANS. We observe the reversible structural transition of F127 layers from loosely attached polymers to more compact individual polymer blobs in the temperature range from 20 and 60 °C. Moreover, adding 5MS enables us to produce the F127/5MS/SWNT nanorods in the suspension, which have compact F127 layers at room temperature and still exhibit the thermosensitive encapsulation layers. Our finding provides a method for more flexibility and opportunity in utilizing and manipulating SWNTs in aqueous solution for self-assembly based fabrication of higher-order tunable nanostructures.

single chain (Rg ≈ 27−30 Å). In short, this transition can be understood as the structural evolution of the encapsulation layer from a loose and uneven polymer shell to more uniform and compact shell, as presented in Figure 5.

Figure 5. Schematic cross-sectional view diagrams for (A) the suggested model of F127/SWNT hybrid particles (top) and (B) its structural change upon either controlling temperature (bottom, left) or adding a small aromatic additive (bottom, right).

Variations in intermolecular interactions between the block copolymer and water and between the block copolymer and SWNT are likely to be responsible for this structural change upon temperature control or aromatic additives. First, the hydrophobicity of both PPO and PEO in aqueous solutions is known to be enhanced as a function of temperature, and the polymer solutions exhibit phase separation above the lower critical solution temperature (LCST).55,56 The thermosensitive hydrophobicity of PPO and PEO leads not only to the micellization of Pluronic polymers, in which relatively hydrophilic PEOs form a spherical shell surrounding a hydrophobic core of PPOs, but also to the growth of micelle size with increasing temperature57,58 (see Supporting Information). In the presence of SWNTs, however, the more hydrophobic SWNT surface restricts the degrees of freedom of adsorbed polymers so that the formation of polymeric micelles is hindered. This is because aggregation between polymers exposes the energy-costing hydrophobic surfaces of SWNTs to water molecules. Instead, the adsorbed polymers lower the interfacial free energy by forming smaller blobs, which can cover the SWNT more efficiently, at high temperature. Thus, the drastic change around 30−40 °C can be related to the hydrophobicity increase of F127 above the LCST of PPO55 and the CMT of F127.39 The fact that the temperature range used in this experiment (20−60 °C)was below the LCST of PEOs56 can also explain why the polymer blobs maintain the water volume fraction and the main feature of a loosely adsorbed structure without collapsing. The significant structural change after adding 5MS is explicable by the strong interaction between 5MS and the other components in our system,44,45,48 which enables the 5MS molecules to occupy the inner shell. The 5MS molecules on

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ASSOCIATED CONTENT

S Supporting Information *

Sample preparation, characterization, MD simulation, and details of SANS experiments and analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00456.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research at Oak Ridge National Laboratory’s Spallation Neutron Source and Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office E

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of Basic Energy Sciences, U.S. Department of Energy. Zhe Zhang gratefully acknowledges the financial support from Jülich Center for Neutron Science, Research center Jülich. The authors thank Ms. Jin Hee Kim, the student for the Oak Ridge Science Semester, for DLS measurements.

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ABBREVIATIONS CNT, carbon nanotube; SWNT, single-walled carbon nanotubes; PEO, poly(ethylene oxide); PPO, poly(propylene oxide); 5MS, 5-methylsalicylic acid; EQ-SANS, extended Qrange small-angle neutron scattering; TGA, thermogravimetric analysis; UV−vis, ultraviolet−visible.

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DOI: 10.1021/acs.macromol.5b00456 Macromolecules XXXX, XXX, XXX−XXX