Interactions between Block Copolymers and Single-Walled Carbon

pubs.acs.org/Langmuir © 2010 American Chemical Society

Interactions between Block Copolymers and Single-Walled Carbon Nanotubes in Aqueous Solutions: A Small-Angle Neutron Scattering Study Meirav Granite,† Aurel Radulescu,‡,§ Wim Pyckhout-Hintzen,‡ and Yachin Cohen*,† †

Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel 32000, ‡ Institut f€ ur Festk€ orperforschung, Forschungszentrum J€ ulich GmbH, Germany, and § J€ ulich Centre for Neutron Science at FRM II, Garching, Germany Received August 3, 2010. Revised Manuscript Received November 25, 2010

The amphiphilic copolymers of the Pluronic family are known to be excellent dispersants for single-walled carbon nanotubes (SWCNT) in water, especially F108 and F127, which have rather long end-blocks of poly(ethylene oxide) (PEO). In this study, the structure of the CNT/polymer hybrid formed in water is evaluated by measurements of smallangle neutron scattering (SANS) with contrast variation, as supported by cryo-transmission electron microscopy (cryoTEM) imaging. The homogeneous, stable, inklike dispersions exhibited very small isolated bundles of carbon nanotubes in cryo-TEM images. SANS experiments were conducted at different D2O/H2O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D2O solvent composition, which is much higher than the expected value of 17% D2O that is based on the scattering length density (SLD) of PEO. At this near match point, the data exhibited a q-1 power law relation of intensity to the scattering vector (q), indicating rodlike entities. Two models are evaluated, as extensions to Pederson’s block copolymer micelles models. One is loosely adsorbed polymer chains on a rodlike CNT bundle. In the other, the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. Both models were found to fit the experimental data reasonably well. The model fit required special considerations of the tight association of water molecules around PEO chains and slight isotopic selectivity.

1. Introduction Carbon nanotubes (CNTs) are hollow graphitic cylinders that are known by their remarkable unique mechanical, optical, thermal, and electronic properties. They were discovered in 1991 by Iijima,1 and since then, they have attracted a great amount of scientific and technological interest. One form of CNTs is a singlewalled carbon nanotube (SWCNT) composed of only one graphitic layer with a diameter of about 1-5 nm and a length that can reach a few micrometers. In production, SWCNTs tend to form bundles with parallel alignment held together by van der Waals forces. Such nonhomogenous agglomerates hinder the realization of unique CNT properties in functional composite materials. Thus, the achievement of stable dispersions of exfoliated SWCNT bundles is essential in expressing their properties and using them for beneficial applications. A widely used method to exfoliate bundles is the physical noncovalent approach, which does not involve any chemical reactions and therefore has negligible deleterious effects on the properties of the tubes. For example, many studies explored surfactants as solubilizing agents in water, whereby the hydrophobic segments adsorb onto the carbon nanotube surface via van der Waals interactions and the hydrophilic chains extend into the solvent. It has been considered that each nanotube is covered with a monolayer of surfactant molecules in which the tails are in contact with the nanotube walls whereas the heads form a compact outer surface.2,3 After testing a series of anionic, cationic, *To whom correspondence should be addressed. E-mail: yachinc@ tx.technion.ac.il. (1) Iijima, S. Nature 1991, 354, 56. (2) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (3) Matarredona, O.; Rhoads, H.; Li, Z.; Harwell, J. H.; Balzano, L.; Resasco, D. E. J. Phys. Chem. B 2003, 107, 13357. (4) Moore, V. C.; Strano, M. S.; Haroz, E.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379.

Langmuir 2011, 27(2), 751–759

and nonionic surfactants and polymers, Moore et al.4 found that a longer hydrophilic group improves the SWCNT dispersion because of the enhanced steric repulsion between the nanotubes. Murakami and co-workers5 reported that mild sonication and subsequent centrifugation provided good separation of byproducts such as defective nanotubes and amorphous carbon. Polymers are also found to be good dispersants for SWCNTs, and their interaction with the nanotube surface has been considered using different approaches. One is termed polymer wrapping, which describes a tight compact interaction whereby the polymer tends to coat the nanotubes in close contact.6-10 In particular, single-stranded DNA was shown to wrap around SWCNTs based on atomic force microscope (AFM) images11 and molecular simulations.11-13 A second approach suggests loose adsorption, a weaker interaction between the polymer and the nanotube, so that adsorbed polymers still assume a coiled conformation.14 (5) Murakami, T.; Kisoda, K.; Tokuda, T.; Matsumoto, K.; Harima, H.; Mitikami, K.; Isshiki, T. Diamond Relat. Mater. 2007, 16, 1192. (6) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y. H.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (7) Curran, S. A.; Ajayan, P. M.; Blau, W. J.; Carroll, D. L.; Coleman, J. N.; Dalton, A. B.; Davey, A. P.; Drury, A.; McCarthy, B.; Maier, S.; Strevens, A. Adv. Mater. 1998, 10, 1091. (8) McCarthy, B.; Coleman, J. N.; Czerw, R.; Dalton, A. B.; in het Panhuis, M.; Maiti, A.; Drury, A.; Bernier, P.; Nagy, J. B.; Lahr, B.; Byrne, H. J.; Carroll, D. L.; Blau, W. J. J. Phys. Chem. B 2002, 106, 2210. (9) in het Panhuis, M.; Maiti, A.; Dalton, A. B.; van den Noort, A.; Coleman, J. N.; McCarthy, B.; Blau, W. J. J. Phys. Chem. B 2003, 107, 478. (10) Yang, M.; Koutsos, V.; Zaiser, M. J. Phys. Chem. B 2005, 109, 10009. (11) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; McLean, R. S.; Onoa, G. B.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545. (12) Johnson, R. R.; Charlie Johnson, A. T.; Klein, M. L. Nano Lett. 2008, 8, 69. (13) Willis, M.; Wusheng, Z.; Goran, K. J. Phys. Chem. B 2008, 112, 16076. (14) Dror, Y.; Pyckhout-Hintzen, W.; Cohen, Y. Macromolecules 2005, 38, 7828.

Published on Web 12/14/2010

DOI: 10.1021/la103096n

751

Article

Granite et al. Table 1. Compositions of Pluronic and SWCNT/Pluronic Dispersions

dispersion

composition (w/w)

%D2O in the solvent

Pluronic F108 SWCNT/Pluronic F108 SWCNT/Pluronic F127

5% 0.5% SWCNT þ 5% F108 0.5% SWCNT þ 4% F127

100, 90, 80, 70, 60, 40 100, 90, 80, 70, 60, 40 100, 80, 70, 40

Amphiphilic block copolymers offer another possibility whereby one block has a strong affinity for the solid surface and the other block or blocks have a relatively high affinity for the solvent.15 Yerushalmi-Rozen, Szleifer, and co-workers16-18 studied block copolymers as suitable dispersants for SWCNTs, focusing in particular on the nonionic Pluronic family composed of hydrophilic poly(ethylene oxide) (PEO) end-blocks and a hydrophobic poly(propylene oxide) (PPO) midblock. Molecular dynamic simulations describing the adsorption of pluronic F88 on a SWCNT suggested that the PPO blocks are adsorbed onto the nanotubes whereas the PEO blocks are extended, offering repulsion interactions leading to a stable dispersion of nanotubes. Experimental evidence19,20 indicated that below the critical micellar temperature (CMT) the nanotube surface is “decorated” by only a few polymer chains whereas dense adsorption occurs above the CMT. The importance of debundling the SWCNT agglomerates emphasized the need for a thorough characterization of SWCNT dispersions. Cryo-transmission electron microscopy (cryo-TEM) provides direct images of the local structure, which are indicative of nanotube debundling. Small-angle neutron scattering (SANS) can provide insight into the adsorbed polymer structure and its interactions with the nanotubes.14,21-25 When SWCNT are in a dispersion, because of their rodlike structure, it would be expected that the intensity follows a q-1 power law relation with respect to the scattering vector (q). Intensive investigations on that issue revealed that the configuration of the SWCNT and dispersants is not simple. In most studies, the observed power law qR is close to or even larger than R ≈ -2, which might indicate that the dispersion consists of branched structures of nanotube bundles rather than single nanotubes.21,22 A classification method according to the value of the power law exponent was suggested.23 However, SWCNT dispersions with NaDDBS demonstrated a power law of -1 in a relatively high q range, which indicated a very effective dispersant.24,25 In this case, the crossover-to-crossover q-2 behavior at low q was interpreted to imply a network “mesh” as in a semidilute polymer solution. SANS data from SDS-stabilized SWCNT dispersions were fit with a cylindrical core-shell model, implying a fluidlike coating on the surfactant as in cylindrical micelles.26 Bauer and co-workers27 reported (15) Szleifer, I.; Yerushalmi-Rozen, R. Polymer 2005, 46, 7803. (16) Shvartzman-Cohen, R.; Nativ-Roth, E.; Baskaran, E.; Levi-Kalisman, Y.; Szleifer, I.; Yerushalmi-Rozen, R. J. Am. Chem. Soc. 2004, 126, 14850. (17) Shvartzman-Cohen, R.; Levi-Kalisman, Y.; Nativ-Roth, E.; YerushalmiRozen, R. Langmuir 2004, 20, 6085. (18) Yerushalmi-Rozen, R.; Szleifer, I. Soft Matter 2006, 2, 24. (19) Nativ-Roth, E.; Shvartzman-Cohen, R.; Celine, B.; Marc, F.; Dongsheng, Z.; Szleifer, I.; Yerushalmi-Rozen, R. Macromolecules 2007, 40, 3676. (20) Shvartzman-Cohen, R.; Florent, M.; Goldfarb, D.; Szleifer, I.; YerushalmiRozen, R. Langmuir 2008, 24, 4625. (21) Schaefer, D. W.; Brown, J. M.; Anderson, D. P.; Zhao, J.; Chokalingam, K.; Tomlin, D.; Ilacsky, J. J. Appl. Crystallogr. 2003, 36, 553. (22) Schaefer, D. W.; Zhao, J.; Brown, J. M.; Anderson, D. P.; Tomlin, D. W. Chem. Phys. Lett. 2003, 375, 369. (23) Wang, H.; Zhou, W.; Ho, D. L.; Winey, K. I.; Fischer, J. E.; Glinka, C. J.; Hobbie, E. K. Nano Lett. 2004, 4, 1789. (24) Zhou, W.; Islam, M. F.; Wang, H.; Ho, D.; Yodh, A. G.; Winey, K. I.; Fischer, J. E. Chem. Phys. Lett. 2004, 384, 185. (25) Hough, L. A.; Islam, M. F.; Hammouda, B.; Yodh, A. G.; Heiney, P. A. Nano Lett. 2006, 6, 313. (26) Yurekli, K.; Mitchell, C. A.; Krishnamoorti, R. J. Am. Chem. Soc. 2004, 126, 9902. (27) Bauer, B. J.; Hobbie, E. K.; Becker, M. L. Macromolecules 2006, 39, 2637.

752 DOI: 10.1021/la103096n

scattering patterns from mixtures of nanotubes labeled by covalently attaching -C4H9 or -C4D9 groups. Those dispersions demonstrated a -2.5 power law that indicated that the scattering particles are clusters of branched SWCNTs. Kim et al.28 created a powder of functionalized SWCNTs that were fabricated by the polymerization of cationic surfactants, covering the surface of SWCNTs with water, followed by freeze drying. The material was very dispersible in alcohols. SANS measurements showed an encapsulation of SWCNTs with a swollen polymerized surfactant layer. This research is aimed at evaluating the conformation and packing of nonionic Pluronic triblock copolymers with long PEO end-blocks adsorbed onto the surfaces of SWCNTs, for which they are very successful in providing stable aqueous dispersions.

2. Experiments Materials and Sample Preparation. Single-walled carbon nanotubes (SWCNTs) synthesized by an electric-arc evaporation method, were purchased from Carbolex Inc. (AP grade). Highresolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (TEM) images of the raw CNTs are provided in the Supporting Information. Pluronic F108 [i.e., (ethylene oxide)132(propylene oxide)50(ethylene oxide)132] and F127 [i.e., (ethylene oxide)106(propylene oxide)70(ethylene oxide)106] were received as a gift from BASF and used as received. Solutions of the dispersing polymer were prepared at room temperature, and then the SWCNTs were added to the solutions. For the contrast variation procedure, the solutions were made with different H2O (milli-q)/D2O (Sigma Aldrich) ratios, as specified in Table 1. All dispersions were sonicated for about 45 min in a 43 kHz Delta D2000 sonicator, resulting in a homogeneous stable inklike dispersion. To remove catalyst particles and amorphous carbon without any chemical treatment or purification, the dispersions were centrifuged for 30 min at 11 700 rpm in a 5810R Eppendorf centrifuge. The dispersions were very stable over many months. Cryo-Transmission Electron Microscopy. Vitrified samples were prepared in a controlled environment vitrification system (CEVS) at a controlled temperature of 25
C and 100% relative humidity. The specimen was transfer to the microscope using an Oxford Instruments CT-3500 cryo-specimen cooling holder and transfer procedure. The samples were investigated in a Philips CM120 at an acceleration voltage of 120 kV TEM using a low-dose electron imaging technique. Images were recorded using a Gatan 791 MultiScan CCD camera, and image processing was done with a Gatan DigitalMicrograph 3.9.2 software package. Small-Angle Neutron Scattering. SANS experiments were conducted on the KWS1 instrument at the FRJ-2 reactor (J€ ulich) and the KWS2 instrument at the FRM-II reactor (TU M€ unich) in Germany. In the former, the incident wavelength, λ, was 7 A˚, and an aperture of 10
10 mm2 was used. The experiments were carried out at sample-to-detector distances of 4, 8, and 20 m so that the measurable q range was 0.002-0.080 A˚-1. In the latter, all samples were studied in 2 mm quartz cuvettes under ambient conditions. The data were corrected by subtracting the scattering from an empty cell (Iempty_cell) and the electronic background (Ielectronic_background) from the measured intensities (Isample). The (28) Kim, T. H.; Doe, C.; Kline, S. R.; Choi, S. M. Macromolecules 2008, 41, 3261.

Langmuir 2011, 27(2), 751–759

Granite et al.

Article

Figure 1. Cryo-TEM image of the SWCNT/F108 dispersion, with increasing electron irradiation in the images: minimal exposures of (A) ∼15, (B) ∼30, and (C) ∼45 e-/A˚2. The scale bar is 100 nm. counts from the 2D detector array containing 128 channels
128 channels were averaged radially to attain a 1D scattering curve. The overall net intensity, Inet(q), is thus Inet ðqÞ ¼ ðIsample - Ielectronic -

Tsample ðIempty Tempty cell

cell

background Þ

- Ielectronic

background Þ

ð1Þ

where Tsample and Tempty_cell are the sample and the empty cell transmission, respectively. The absolute scattering cross section (cm-1) was calibrated using a polycarbonate secondary standard:
 dΣ dΣ Ds Ts Ld, s 2 Inet ðqÞ ðqÞ ¼ ð2Þ dΩ dΩ s DTLd 2 ÆIs ðqÞæ Subscript s represents the standard; D is the sample thickness; T is the transmission; Ld is the sample-detector distance; (dΣ/dΩ)s is the known standard absolute scattering cross section; and ÆIs (q) æ is the standard mean value of the scattering intensity.

3. Results and Discussion Figure 1 presents cryo-TEM images of the SWCNT/F108 dispersion. It shows very small isolated bundles of carbon nanotubes, the diameters of which range from 1 to 5 nm. The tubes appear to be straight and longer than the observed image, approximately 500 nm long. A technique of gradual exposure to electron irradiation29 was used, showing that controlled electron etching reveals bare nanotubes within the bundles. The white arrow in Figure 1A indicates a SWCNT bundle that appears as a single entity at a minimal exposure of about 15 e-/A˚2. Upon increasing electron irradiation, as observed in the subsequent images, its inner structure composed of three nanotubes is revealed as indicated by the white arrow in Figure 1C. This is due to the radiation-induced decomposition of the polymer adsorbed on the CNT surface, and the increasing exposure to electron radiation is indicated by the beam damage apparent in the support film in the image background shown by the black arrows in Figure 1B,C. Figure 2 shows very small, similarly isolated bundles of carbon nanotubes in SWCNT/F127 dispersions. The scattering patterns from dispersions of the SWCNT/F108 hybrid, as well as the bare polymer for comparison, are shown in (29) Talmon, Y. Electron Beam Radiation Damage to Organic and Biological Cryospecimen. In Cryotechniques in Biological Electron Microscopy; Steinbrecht, R. A. Zierold, K., Eds.; Springer-Verlag: Berlin, 1987.

Langmuir 2011, 27(2), 751–759

Figure 2. Cryo-TEM image of a SWCNT/F127 dispersion. The scale bar is 100 nm.

Figure 3. SANS patterns at 100% D2O from dispersions of SWCNT/Pluronic F108 (O) and Pluronic F108 (4).

Figures 3 and 4 for two different contrasts (100 and 70% D2O in the dispersing medium). Under the measurement conditions, the polymer is fully soluble in water because micelles are not formed, so the scattering intensity is rather low. At full contrast (Figure 3), DOI: 10.1021/la103096n

753

Article

Figure 4. SANS patterns at 70% D2O from dispersions of SWCNT/Pluronic F108 (O) and Pluronic F108 (4).

Figure 5. SANS patterns from SWCNT/F108 dispersions after background subtraction at solvent compositions (wt % D2O) of 100 (0), 90 (
), 80 (þ), 70 (!), 60 (4), and 40 (O).

significant excess scattering at small angles is observed from the CNT/F108 hybrid. However, the data at 70% D2O in the dispersing medium, shown in Figure 4, exhibits only marginal additional scattering by the hybrid relative to the bare polymer. This surprising result indicates nearly a match in contrast at a relatively high scattering length density (SLD), which will be discussed further below. At large scattering angles (q > 0.03 A˚-1) the curves from the bare polymer and the CNT dispersion nearly coincide. This indicates that the scattering in this q range results primarily from the structure of the bare polymer, which is present in high excess in the CNT dispersion, as well as from incoherent scattering from hydrogens of the polymer and water, if present. Therefore, a background scattering intensity was fit to the data of the bare polymer (as a Gaussian function plus a constant incoherent term) and subtracted from the scattering of the hybrid dispersion. Figure 5 presents the intensity measured for SWCNT/F108 dispersions in six different D2O/H2O mixtures after background subtraction. It is evident that the shape and intensity differ at different contrasts. This signifies that the hybrid entity consists of heterogeneous particles having distinct inner structure. 754 DOI: 10.1021/la103096n

Granite et al.

Figure 6. SANS patterns from SWCNT/F127 dispersions after background subtraction at solvent compositions (wt % D2O) of 100 (0), 80 (þ), 70 (!), and 40 (O).

The scattering pattern with the lowest intensity, measured at 70% D2O, exhibits a nearly q-1 power law dependence, as shown in Figure 5. This power law relation is characteristic of long, thin rodlike objects.22 At other contrasts no clear power law may be discerned, and any apparent exponent is closer to 2. At scattering length densities of the dispersing medium lower than 70% D2O, an increase in the observed intensity is shown in Figure 5. These results are compatible with the conceptual model presented by Yerushalmi-Rozen, Szleifer, and co-workers,16-18 by which the relatively short PPO midblocks are adsorbed onto the CNT surface and the longer PEO chains are extended into the solvent. Thus, matching of the surrounding chains can reveal the rodlike structure of SWCNT bundles. However, we do not expect to match the scattering length density of the hydrogenated polymer at such a high D2O content. A simple calculation shows that such contrast matching is expected to occur at about 17% D2O.30 Cryo-TEM images of the CNT dispersions in different H2O/D2O mixtures were also taken, and no significant differences were observed. A sample image at 70% D2O is presented in the Supporting Information. This issue will be further discussed in the modeling section. A similar phenomenon was observed for the SWCNT dispersions with the F127 polymeric surfactant. As shown in Figure 6, the minimal intensity is observed at contrast with 70% D2O in the dispersing aqueous medium, and the intensity again exhibited a q-1 power law in relation to the scattering vector q. For comparison, we measured the scattering from Pluronic F108 below the CMT. For the same range of contrasts, as shown in Figure 7, not such apparent matching was observed. At 90 and 100% D2O, the scattering pattern is consistent with that expected from a solution of a “short” polymer with a radius of gyration of about 20-25 A˚, consistent with the size of the polymer. Evaluation of Structural Models. We seek a structural model for which the relatively short PPO midblocks are adsorbed onto the CNT surface, forming a shell around the CNT bundle. The long PEO end-blocks form brushes that may extend into the surrounding solvent. Their conformation has a radius of gyration (Rg) that depends on the adsorption density and the polymersolvent interactions, as in the micellized block copolymer. We need to consider that both the PPO shell and the PEO brushes may be highly hydrated because the polymer at this temperature is (30) Lin, Y.; Alexandridis, P. J. Phys. Chem. B 2002, 106, 10834.

Langmuir 2011, 27(2), 751–759

Granite et al.

Figure 7. SANS patterns from Pluronic F108 dispersions after background subtraction at solvent compositions (wt % D2O) of 100 (0), 90 (
), 80 (þ), 70 (!), 60 (4), and 40 (O).

Article

factor of the SWCNT/polymer hybrid entity of model B, as illustrated in Figure 8, have been provided by Dror et al.14 and are listed in the Supporting Information. The main parameters of this model are Rcore, the radius of the core nanotube bundle; Rg, the radius of gyration of the adsorbed polymer chains; and Nagg, the number of adsorbed polymer chains per arbitrarily chosen length of 100 A˚. When we tried to fit this model, no matching set of parameters was found. The main problem, as already mentioned, is due to the fact that the polymer’s SLD is low and is expected to be matched with the solvent having about 17% D2O, not 70% D2O, as shown by our experimental results (Figures 5 and 6). To address this issue, we consider a particular study by Richter and co-workers35 dealing with spherical micelles of a diblock copolymer consisting of a hydrophobic block of poly(ethylene-propylene) PEP and a hydrophilic PEO block. This copolymer forms spherical micelles in water, the structure of which was studied by SANS. Richter and co-workers modified the SLD of the PEO chain by considering water molecules tightly bound to the polymer such that their density is somewhat higher than that of bulk water. The volume of a PEO chain, VPEO, is thus modified by including the hydration shell that contains water at a slightly higher density (dsb) than bulk water (ds). The hydration shell is considered to have a volume of RVPEO, where R is a proportionality factor. Richter and coworkers35 calculated the SLD of the PEO chain, Fchain, modified by the presence of the hydration layer as Fchain ¼ FPEO þ f Fs

Figure 8. Schematic drawing of SWCNT/polymer (model B) with a length, L, of 5000 A˚.

still below its CMT. The models to be considered in fitting the scattering patterns should be compatible with the cryo-TEM images, which demonstrate isolated, thin, rodlike CNTs. We started from the models of block copolymer micelles presented by Pederson and co-workers’31-34 that have been widely used over the last years and tried to fit a few models to the results: (A) a cylindrical core-shell model;26 (B) a cylindrical core-adsorbed chains model, which is Dror et al.’s14 modification of the cylindrical block copolymer micelle model by Pedersen;34 and (C) a cylindrical core-shell-chains model, which is a modified combination of models A and B. We first discuss the analysis of the data at six contrasts measured for CNT dispersions with F108. Model A, the simple core-shell model, did not fit the experimental data even without logical constraints on its parameters, consisting of core and shell radii, polymer content in the shell, and SLD of the polymer chains (which without constraint may account for tightly bound water as discussed further below). Proposed model B describes a cylindrical core formed by a thin bundle of SWCNTs, which is decorated by loosely adsorbed polymer chains14 as depicted schematically in Figure 8. In this crude model, we do not make a distinction between PEO and PPO units. The equations describing the form (31) Pedersen, J. S.; Gerstenberg, M. C. Macromolecules 1996, 29, 1363. (32) Pedersen, J. S. Adv. Colloids Interface Sci. 1997, 70, 171. (33) Pedersen, J. S.; Svaneborg, C.; Almdal, K.; Hamley, I. W.; Young, R. N. Macromolecules 2003, 36, 416. (34) Pedersen, J. S. J. Appl. Crystallogr. 2000, 33, 637.

Langmuir 2011, 27(2), 751–759

ð3Þ

where FPEO and Fs are the PEO and the solvent SLDs, respectively, and f is a correction factor given as ! dsb -1 ð4Þ f ¼ R ds In their work, it was found that a correction factor of f = 0.25 is needed, which is consistent with an increase in water density of about 6% in the hydration shell. In our model, the correction factor f may have some dependence on the solvent composition because the water density in the hydration shell may be somewhat dependent on the isotopic content and there may even be a slight isotopic selectivity between the water isotopes, which are adsorbed on the PEO chains. The correction factor may be modified as ! dsb Fbs 0 -1 ð5Þ f ¼ R ds Fs where (Fsb/Fs) accounts for the slight change between the SLDs of the hydration shell (Fsb) and bulk water (Fs). Fitting the experimental SANS data of the CNT/polymer hybrids at different solvent contrasts to the structural model B described above was done using the parameters describing the adsorption of water molecules onto the polymer chains, which are compatible with the correction factor suggested by Richter and colleagues.35 To obtain a good fit of the experimental data, we used the modified correction factor, f 0 , and had to allow a variation of 4-11% in the values of Fsb/Fs for the different contrasts, except at 40% D2O for which an unreasonable variation was needed.36 Using the properties of the adsorbed water layer (35) Poppe, A.; Willner, L.; Allgaier, J.; Stellbrink, J.; Richter, D. Macromolecules 1997, 30, 7462. (36) This would imply, for example, a change in the isotopic content of the hydration shell from 40 to 20% D2O.

DOI: 10.1021/la103096n

755

Article

Granite et al. Table 2. Fitted Parameters of Core-Chains Model B for SWCNT/Pluronic Dispersionsa parameter

SWCNT/F108

SWCNT/F127

20 ( 1 20 ( 1 Rcore (radius of the nanotubes core, A˚) 140 ( 10 120 ( 10 Rg (polymer radius of gyration, A˚) 11 ( 0.5 14 ( 0.5 Nagg (number of PEO chains per nanotube length of 100 A˚) 1.5
1014 ( 0.5
1014 2.1
1014 ( 0.5
1014 n (density number, hybrids/cm3) a Fcore = 4.9
10-6 A˚-2, the SLD of the core nanotube, was taken according to Zhou.24 Vchain and Fchain were calculated to be 23.85
103 A˚3 and 5.28
10-7 A˚-2, respectively, for Pluronic F108 and 21.13
103 A˚3 and 5.00
10-7 A˚-2, respectively, for Pluronic F127, using a bulk density of 1.01 g/cm3 for both EO and PO following Mortensen.37 The SLDs of the solvents, Fs (A˚-2), are 6.34
10-6, 5.65
10-6, 4.96
10-6, 4.27
10-6, 3.58
10-6, and 2.2
10-6 at solvent compositions (wt % D2O) of 100, 90, 80, 70, 60, and 40, respectively.

Figure 9. Fitting of SWCNT/F108 dispersions for model B of the contrasts: 100, 90, 80, 70, 60, and 40% D2O.

discussed above, the other structural parameters of the model are obtained for both polymers, as presented in Table 2. These parameters are identical for all six contrasts of the SWCNT/ F108 hybrid and for all four contrasts of the SWCNT/F127 hybrid. Fitting the shape of the curves can now provide structural information on the CNT/polymer hybrid, as given in Table 2. The data fitting also provides the number density of hybrids in the dispersion (n). The original model of Pedersen34 and its modification by Dror et al.14 use a chain-core exclusion factor, d, which was also used in the present case (Supporting Information) with a value of d = 0.8. Figure 9 shows the experimental scattering patterns with the fitted model B curves for the six contrasts of the SWCNT/F108 hybrid. Figure 10 shows the experimental scattering patterns with the fitted model B curves for the four contrasts of the SWCNT/ F127 hybrid. In both cases, a reasonable fit is achieved for all contrasts. 756 DOI: 10.1021/la103096n

After obtaining the model fit described above, we can now analyze the physical and structural meaning of the fitted parameters given in Table 2. As shown schematically in Figure 8, a thin rodlike core of bundled nanotubes (e.g., four nanotubes of 10 A˚ radius) is covered with a shell of loosely adsorbed polymer chains having a coiled conformation and extending into the surrounding solvent. Its radius of gyration (Rg) is about 140 A˚ for Pluronic F108, which is quite large for this polymer, indicating significant extension. As mentioned above, this model does not distinguish between EO and PO units, although the latter are deemed to be adsorbed onto the CNT surface, being less hydrophilic. The high extension of PEO chains is in accord with the computer simulation of Pluronic chains adsorbed onto carbon nanotubes as reported by Yerushalmi-Rozen and co-workers.19 The full hybrid thus has a large cross-sectional radius that explains why the q-1 power law, typical of long, thin units such as nanotubes, does not appear in the experimental scattering patterns except under the Langmuir 2011, 27(2), 751–759

Granite et al.

Article

Figure 10. Fitting of SWCNT/F127 dispersions for model B of the contrasts: 100, 80, 70, and 40% D2O.

core wrapped with a shell of PPO chains, which as a result of their more hydrophobic nature are drawn to the nanotubes. This PPO shell is assumed to contain water, as was shown for the core of micelles of this type of polymer (above their CMT).38 Note that the measurement temperature is below the polymer’s CMT and its aggregation is due only to the presence of the CNTs.20 It is thus reasonable to assume the hydration of the adsorbed PPO. Because Nagg represents the number of polymer chains attached to the cylindrical core per unit length, there are 2Nagg PEO segment blocks, which are accounted for in eq 6 below

Figure 11. Schematic cross-sectional structure of the CNT/poly-

Fhybrid ðqÞ ¼ F

mer (model C).

core - shell ðqÞ þ 2Nagg ðΔβÞchain

2

Fchain ðqÞ þ 2 3 2Nagg ðΔβÞchain Score - shell - chains ðqÞ

ð6Þ

2

þ 2Nagg ð2Nagg - 1ÞðΔβÞchain Schain - chain ðqÞ polymer matching conditions. Also, the existence of a polymer coating on the nanotube bundles was demonstrated by the cryoTEM images, especially by its response to electron irradiation. From the number density of hybrids in the dispersion evaluated by the model (1.5
1014 particles/cm3), it is possible to estimate the weight fraction of SWCNT, which is about 30% of its initial value in the as-prepared dispersion (0.5% w/w). This indicates significant mass loss during centrifugation that is necessary for the removal of impurities such as the catalyst and amorphous carbon. The parameters obtained by fitting the data of the SWCNT/F127 dispersions are comparable regarding the slight difference in block lengths. As mentioned above, model B does not distinguish between PEO and PPO units. Proposed model C (core-shell-chains) alleviates this restriction by considering the CNT/polymer hybrid to be a cylindrical core formed by a thin bundle of one or a few SWCNTs wrapped with a thin hydrated PPO shell from which the PEO chains emanate into the surrounding solvent, as shown schematically in Figure 11. In this model, we take into account the different SLDs of PPO and PEO blocks. Equations 6-14 describe the form factor of the SWCNT/ polymer hybrid (Fhybrid(q)) for model C, a SWCNT thin bundle (37) Mortensen, K. Polym. Adv. Technol. 2001, 12, 2.

Langmuir 2011, 27(2), 751–759

where the core-shell form factor,F F

core - shell ðqÞ

Ampcore - shell ðqÞ ¼

core-shell

(q), is given by

¼ Ampcore - shell 2 FL ðqÞ Vcore ðΔFÞcore

ð7aÞ

2J1 ðqRcore Þ qRcore

! 2J1 ðqRshell Þ 2J1 ðqRcore Þ ð7bÞ þ Vshell ðΔFÞshell - Vcore ðΔFÞshell qRshell qRcore ðΔFÞcore ¼ ðFcore - Fs Þ

ð8Þ

ðΔFÞshell ¼ ðFshell - Fs Þ

ð9Þ

Fshell ¼ φPPO in shell FPPO þ ð1 - φPPO in shell ÞFs

ð10Þ

L (= 5000 A˚) is an arbitrarily chosen unit length; Vcore and Vshell are the volumes (A˚3) of the nanotube core and the PPO shell (38) Goldmints, I.; Holzwarth, J. F.; Smith, K. A.; Hatton, T. A. Langmuir 1997, 13, 6130.

DOI: 10.1021/la103096n

757

Article

Granite et al. Table 3. Fitted Values of the Core-Shell-Chains Model for SWCNT/Pluronic Dispersionsa parameter

SWCNT/F108

SWCNT/F127

16 ( 1 18 ( 1 Rcore (radius of nanotubes core, A˚) 160 ( 10 130 ( 10 Rg (polymer radius of gyration, A˚) 9 ( 0.5 11.5 ( 0.5 Nagg (number of PEO chains per nanotube length of 100 A˚) 0.4 ( 0.02 0.48 ( 0.02 ΦPPO in shell (volume fraction of PPO in the shell) 2
1014 ( 0.2
1014 2.2
1014 ( 0.2
1014 n (density number, hybrids/cm3) a Fcore = 4.9
10-6 A˚-2 was taken according to Zhou.24 Calculations were made using VPPO and VPEO as 4.77
103 and 9.54
103 A˚3, respectively, for Pluronic F108 and as 6.67
103 and 7.23
103 A˚3, respectively, for Pluronic F127 using a bulk density of 1.01 g/cm3 for both EO and PO following Mortensen.37 FPPO and FPEO were calculated to be 3.47
10-7 and 5.72
10-7 A˚-2 for PPO and PEO, respectively. The SLDs of the solvents, Fs (A˚-2), are 6.34
10-6, 5.65
10-6, 4.96
10-6, 4.27
10-6, 3.58
10-6, and 2.2
10-6 at solvent compositions (wt % D2O) of 100, 90, 80, 70, 60, and 40, respectively.

Figure 12. Fitting of SWCNT/F108 dispersions for model C of the contrasts: 100, 90, 80, 70, 60, and 40% D2O.

(of length L), respectively; and Rcore and Rshell are the nanotube core and the PPO shell radii (A˚), respectively. Fcore, FPPO, and Fs are the scattering length densities (SLDs) (A˚-2) of the nanotube core, the PPO blocks, and bulk water, respectively; φPPO in shell is the volume fraction of PPO in the shell; J0 and J1 are the zeroth- and first-order Bessel functions, respectively; and Fchain is the form factor of the polymer chain and is given in the Supporting Information. ðΔβÞchain ¼ Vchain ðFchain - Fs Þ

ð11Þ

Fchain ¼ FPEO þ f 0 Fs ðf 0 defined in eq 5Þ

ð12Þ

where Vchain and FPEO are the PEO volume (A˚3) and SLD (A˚-2), respectively. The cross term between the nanotubes/PPO core-shell 758 DOI: 10.1021/la103096n

and the PEO chains, Score-shell-chains, and the cross term between every two chains, Schain-chain, are given in eqs 13 and 14, respectively, Score - shell - chains ðqÞ ¼ ψðqRg ÞJ0 ½qðRcore - shell þ dRg ÞFL ðqÞAmpcore - shell ðqÞ

ð13Þ

Schain - chain ðqÞ ¼ ψ2 ðqRg ÞJ0 2 ½qðRcore - shell þ dRg ÞFL ðqÞ ð14Þ where Rcore-shell is the nanotube/PPO core-shell radius (A˚). The scattering amplitude of the PEO chains, ψ(qRg), and the form factor of an infinitely thin rod of length L, FL, are given in the Supporting Information. Langmuir 2011, 27(2), 751–759

Granite et al.

Article

The final scattering cross section due to the dispersed CNT/ polymer hybrids at a number density of n hybrids per unit volume (hybrids/cm3) is given by 2 dΣ ðcm - 1 Þ ¼ n10 - 16 Fhybrid ðA˚ Þ dΩ

ð15Þ

To fit the experimental data to this model, we also had to implement the idea presented by Richter35 of a dense water layer around the PEO chains with the modified correction factor, f 0 , as described for model B in eq 5. The parameters to be fitted in model C are similar to these of model B except for the addition of the PPO volume fraction in the shell around the nanotube (φPPO in shell). The set of fitted parameters for hybrid SWCNT/ F108 shown in Table 3 indicates that there are about 9 polymer chains per nanotube length of 100 A˚ and an area of 850 A˚2 per PEO chain at the PPO/PEO interface. It also implies significant PPO hydration of about 60% solvent by volume. The values obtained for the number density of nanotube/polymer hybrids represent a significant mass loss of about 75% relative to the initial CNT content of 0.5% (w/w), which is due to the centrifugation process. Figure 12 shows the model fit to the experimental data of CNT/F108 dispersions. The fit of the data for F127 dispersions is given in the Supporting Information.

4. Conclusions This research was aimed at characterizing the nanostructure and interactions between SWCNT and amphiphilic block copolymers Pluronic F108 and F127. The SWCNT/polymer dispersions were centrifuged in order to remove extraneous materials such as catalyst particles and amorphous carbon, resulting in homogeneous stable inklike dispersions at a significant loss of solid mass. Cryo-TEM images revealed very small isolated bundles of carbon nanotubes, the diameters of which ranged from 1 to 5 nm. The tubes appeared to be straight and longer than the observed images, approximately 500 nm long. Controlled exposure to electron irradiation revealed the bare nanotubes within the bundle, indicating that the radiationsensitive polymer that covered them had decomposed in that process. SANS experiments were conducted at different solvent contrasts. The observed scattering patterns differed in shape as well as in intensity at the different contrasts suggesting a hybrid entity. A match point contrast was found, at which the low scattering intensity exhibited almost a q-1 power law for the whole q range, an indication of a good dispersion of rodlike entities. This power law indicates that the surrounding polymer is masked at that solvent composition, revealing a long, thin SWCNT bundle. Two

Langmuir 2011, 27(2), 751–759

models were suggested. The first is a core-chains model of a cylindrical core formed by a thin bundle of SWCNTs that is decorated by loosely adsorbed polymer chains, where no distinction between PEO and PPO units was made. The second is a modified core-shell-chains model that differentiates between the SLDs of PEO and PPO. This model describes a hybrid entity consisting of three layers. The first is a thin, core rodlike bundle of nanotubes, and the second is a PPO shell adsorbed onto the nanotube core, which may be significantly hydrated. The third consists of PEO chains that loosely emanate from the core-shell rod structure, with a rather large radius of gyration. Both models were found to fit to the experimental data reasonably well. Although the match point of the Pluronic PEO chains, according to its calculated SLD, should be around 17% D2O, it was found in our experiments to be closer to 70% D2O. As a result of these findings, some issues regarding PEO-water interactions needed to be considered. The experimental data could be fit only when it was taken into account that every PEO chain is hydrated by a water layer that has a somewhat higher density than bulk water. Furthermore, partial selectivity between the water isotopes, which were adsorbed onto the PEO chains, is indicated. The values of the SWCNT rodlike core and the packing formation of the PPO shell imply that there are about 9 polymer chains per nanotube length of 100 A˚ and an area of about 850 A˚2 per PEO chain at the PPO-water interface. Further research is required into the issues raised regarding the water adsorbed onto the PEO chain. Acknowledgment. We thank Dr. Y. Dror, Prof. R. YerushalmiRozen, and Dr. R. Shvartzman-Cohen for many fruitful discussions and Dr. E. Kesselman, B. Shdemati, and J. Schmidt for their generous help with cryo-TEM. This research was supported by the Israel Science Foundation (grant no. 191/07). This work is based in part on experiments performed at the J€ ulich Centre for Neutron Science JCNS, Forschungszentrum J€ulich, Germany. This project has been supported by the European Commission under the Seventh Framework Program through the key action “Strengthening the European Research Area, Research Infrastructures” (contract no. 226507 (NMI3)). Supporting Information Available: TEM and HR-SEM images of raw AP-grade SWCNT and a cryo-TEM image of the SWCNT/F108 dispersion in 70% D2O. Model B in the analysis of SANS patterns from CNT/Pluronic hybrids at different contrasts. SWCNT/F127 data fittings for the coreshell-chains model (model C). This material is available free of charge via the Internet at http://pubs.acs.org.5

DOI: 10.1021/la103096n

759