Encapsulation of the Fullerene Derivative [6,6]-Phenyl-C61-Butyric

Jul 1, 2009 - Instituto de Investigación en Comunicación Óptica, Universidad Autónoma de San Luis Potosí, Alvaro Obregon 64, San Luis Potosí 78000, ...
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J. Phys. Chem. C 2009, 113, 13677–13682

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Encapsulation of the Fullerene Derivative [6,6]-Phenyl-C61-Butyric Acid Methyl Ester inside Micellar Structures Jose´ M. Na´poles-Duarte,† Roma´n Lo´pez-Sandoval,‡ Andrei Yu. Gorbatchev,† Marisol Reyes-Reyes,*,† and David L. Carroll§ ´ ptica, UniVersidad Auto´noma de San Luis Potosı´, AlVaro Instituto de InVestigacio´n en Comunicacio´n O Obregon 64, San Luis Potosı´ 78000, Mexico, AdVanced Materials Department, IPICYT, Camino a la Presa San Jose´ 2055, Col. Lomas 4a. Seccio´n, San Luis Potosı´ 78216, Mexico, and Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest UniVersity, Winston-Salem, North Carolina 27109 ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: June 7, 2009

The solubilization and micellar encapsulation of fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been investigated by the use of the amphiphilic block copolymer polystyrene-blockpoly[ethylene oxide] (PS-b-PEO). Aqueous solutions were formed, starting from a codissolution of PCBM and PS-b-PEO in a mixture of chlorobenzene and ethanol, inducing micellization by replacing the common solvent chlorobenzene with water, using evaporation at 65 °C. The control of the experimental process has permitted us to obtain only two kinds of micellar morphologies encapsulating PCBM: small spheres whose diameters increase as a function of the PCBM concentration, from approximately 60 to 120 nm, and wormlike micelles with diameters smaller than 60 nm and lengths ranging from a few hundred nanometers to several micrometers. The optical properties reflect the expected encapsulation of PCBM. Furthermore, we show that the optical properties of the confined PCBM molecules can be modified by varying the quantity of fullerenic molecules in the micellar structures, which is due to the PCBM packing, as shown by our density functional theory (DFT) calculations. 1. Introduction The design of well-defined structures at the nano and mesoscale through the use of self-assembly is one of the most exciting challenges in modern materials science, with great potential in the development of new fields such as nanoelectronics, nanophotonics, and nanomedicine. Basically, selfassembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent forces.1 This is attained by the additive contribution of a number of weak noncovalent forces, such as hydrogen bonds, van der Waals forces, and Coulomb and hydrophobic interactions that, under the appropriate conditions, produce ordered aggregates with similar structural characteristics.2-4 Several methods have been developed to achieve this goal, and often, they are simple and nonexpensive techniques that can provide results in a relatively short period of time.5,6 Amphiphilic self-assembly is one of the most common routes for large scale production of self-assembled structures, forming well-organized clusters of amphiphilic compounds such as lipids, soaps, and other surfactants. Block copolymers are polymer molecules consisting of two or more chemically distinct chains covalently bound at their ends.7 When they have amphiphilic properties, these can be classified as macromolecular surfactants, which have material advantages not associated with the conventional low molecular weight amphiphiles. It is known that the amphiphilic block copolymer aggregates in selective solvents, forming micelles.8 Thereby, amphiphilic block copolymers have been used in the formation of micelles with a * To whom correspondence should be addressed. E-mail: reyesm@ cactus.iico.uaslp.mx. † Universidad Auto´noma de San Luis Potosı´. ‡ IPICYT. § Wake Forest University.

variety of geometries over the last decades.8-18 The micelle morphologies, which have been obtained by varying the copolymer composition and (or) solvent dependent parameters, include, but are not limited to, spherical, cylindrical, wormlike, helical, bilayered, vesicle-like, and others. Furthermore, block copolymer micelles have been used to encapsulate a variety of materials with different purposes such as solubility enhancement, drug delivery, nanoparticle formation and dispersion, and nanolithography, among others.19-23 Fullerenes (C60, C70) and carbon nanotubes have been solubilized and encapsulated within amphiphilic block copolymer micelles, forming aqueous solutions,24-28 but no studies with fullerenes derivates, particularly PCBM, have been reported. Jenekhe et al. reported the encapsulation of fullerene molecules in spherical micelles, while different micelle morphologies have been formed by using poly[phenylquinoline]block-polystyrene copolymer and mixtures of selective and common solvents.24,25 Later, Mountrichas et al. reported a method of producing spherical micelles by using poly[ethylene oxide]-block-polystyrene copolymer encapsulating fullerene C60, with a size distribution that is dependent on the fullerene concentration.26 In most of these works, the use of polystyrene (PS) as the core block for encapsulating fullerenes has been common, due to the affinity between fullerenes and PS, which is reflected in the small value of the Flory-Huggins parameter for C60 and PS.24,25 Indeed, the Hansen solubility parameters (HSP) have been determined for C60, and a large number of good solvents and compatible polymers, including PS, was compiled in a database.29 The reported values for HSP are indicative of an essentially nonpolar material. Furthermore, it was observed that polymers with functional groups (e.g., aromatic rings) have solubilities similar to those found for the best solvents of C60.29

10.1021/jp903731d CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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Figure 1. Chemical structure of PS-b-PEO and PCBM.

Microdomains, where a large number of fullerene molecules are condensed, could be of importance for the development of emerging technologies.30-33 The intermolecular interactions between fullerene molecules result in several effects that are otherwise not observed. These effects allow for tuning of the electronic properties, which, in low dimensional systems, enhances their applications for a wide set of devices.30,34 Recently, our group showed that the formation of well-defined PCBM structures improves the efficiency of organic photovoltaic devices (OPVDs).35,36 PCBM is currently the most used fullerene derivative in OPVDs, forming blends with conductive polymers.33,35-37 As for the case of C60, PS appears to be a compatible polymer for PCBM, since it has been used to embed PCBM molecules in a PS matrix in several works.32-34,38 The aim of this work is to explore the potential of selfassembled polystyrene-block-poly[ethylene oxide] copolymer (PS-b-PEO) micelles to solubilize and encapsulate PCBM, forming aqueous solutions. One interest in block-copolymer micelles is that they may provide a new case study for the properties of PCBM in nanoscale domains. Note that the reported experimental techniques for inducing micellization and the C60 and C70 encapsulation use a common solvent for dissolving the involved material and a selective solvent for the encapsulation of the fullerenic molecules.24,25 In this work, we show that, in the case of PCBM, it is necessary to incorporate ethanol in the initial solution of chlorobenzene (CB) with PSb-PEO and PCBM, and heat this solution at 65 °C. In addition, we study the effect of PCBM inclusion in the formation of micelles by varying the content of these fullerenic molecules. Transmission electron microscopy (TEM) was used for studying their morphological characteristics, and their corresponding optical properties were determined by absorbance and photoluminescence measurements. Moreover, our density functional theory (DFT) calculations using SIESTA39,40 support the suggestion that the change in the optical properties is due to the PCBM packing, in qualitative agreement with our experimental results. 2. Experimental Section 2.1. Materials and Sample Preparation. The solvents used were CB, ethanol, and distilled water, without further purification. The fullerene derivative PCBM (99% purity) was purchased from American Dye Source, Inc., and block copolymer PS-b-PEO (MnPS ) 12200, MwPS ) 12900, MnPS-b-PEO ) 23900, polydispersity index ) 1.05) was acquired from Polymer Source, Inc. Both were used as received. The copolymer is soluble in nonpolar solvents such as tetrahydrofuran, toluene, benzene, and dioxane, and we have found that it is also soluble in CB, forming a transparent solution at a proportion of 100 mg/mL at room temperature. Schematic representations of PS-b-PEO and PCBM are shown in Figure 1. PCBM and PS-b-PEO were initially diluted with CB in separate vials. The solutions were stirred for at least 20 min (via a magnetic stirrer at a medium stirring rate). From these two solutions in CB, the required quantities

Figure 2. Photograph of micelle solutions with different fullerene concentrations. From left to right, samples with 0, 5, 10, 50, and 80 wt % PCBM with respect to PS. The images are from diluted samples in 4 mL of water.

of PS-b-PEO and PCBM were taken. The initial solutions were formed with 100 µL of the solution of PS-b-PEO, containing 3 mg of the copolymer (1.02 mg of PS), and 10 µL of the PCBM solution with a quantity of fullerene equal to 5, 10, 50, and 80 wt % PCBM with respect to the polystyrene (PS) block. For all of the samples, ethanol was added to set the volume level of the resulting PCBM/PS-b-PEO solution to 300 µL. The mixtures were stirred for 15 min at room temperature and then for 30 min at 65 °C. After obtaining a clear magenta solution, 100 µL of distilled water was incorporated by depositing 10 µL drops every 5 min upon stirring, keeping the temperature fixed at 65 °C. Complete evaporation of the solvents (ethanol and CB) was achieved by continuously adding distilled water (see complementary section). The final solution was homogeneously opaque, with a brown intensity that changes with the PCBM concentration (Figure 2). This experimental process has permitted us to control the morphology of the micelles encapsulating PCBM, partially filled (or filled) spherical and worm-like micelles. The encapsulation of fullerenes or fullerene derivates in worm-like micelles has not been reported yet. The experimental procedure was tested several times, giving in all cases the same result. The PCBM pristine films were obtained by dissolving the fullerene in CB (10 mg/mL). Several drops of the solution were then deposited on a glass substrate (1 cm × 1 cm), and the CB was allowed to evaporate at room temperature, forming a thick film (∼5 µm). 2.2. Equipment and Experiments. The morphology of the solutions was examined by bright-field transmission electron microscopy (BF-TEM). The samples were prepared by depositing one drop of the suspension on a carbon-coated grid, with the excess being removed by capillary tubes. The grids were then dried at room temperature and atmospheric pressure and stored for at least one night before examination in the TEM. BF-TEM images were recorded on a JEOL JEM-1230 transmission electron microscope operated at 100 kV. Films (about 5 µm thick) were prepared for photoluminescence (PL) measurements by drying the micellar PS-b-PEO/PCBM solutions on glass slides at room temperature. For the PL measurements, a 514 nm Ar-ion laser with an incident energy of 20 mW was used as the excitation source. UV-visible absorbance spectra of the solutions were measured on a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. These measurements were carried out in a 10 mm path quartz cuvette at room temperature. 3. Results and Discussion 3.1. Micelle Morphology. Homogeneous and opaque aqueous solutions were obtained at the end of the preparation process. These solutions were diluted in 4 mL of water without having a further effect on their structure and micellar morphology. Therefore, this is our first observation indicating that the PCBM

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Figure 3. TEM images of samples: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 50 wt %, and (e) 80 wt % PCBM with respect to PS. Note that sample (e) contains spherical micelles of larger size than samples (b) and (c). Samples with low (b) or no (a) PCBM show also spherical micelles but with a lower radius. Large length worm-like micelles (up to several micrometers) were found in all samples.

molecules were encapsulated inside the micellar structures. Note that pristine PCBM is not soluble and forms agglomerates in water. On the other hand, Figure 2 shows that the color of the solutions at high PCBM concentration tends to be brown, and the solution becomes transparent when we decrease the quantity of PCBM. In agreement with other reported fullerene (C60) experiments, the low PCBM concentration solutions (0, 5, and 10 wt %) show no precipitation, while the high PCBM concentration solutions (50 and 80 wt %) show agglomeration and precipitation of a brownish material (presumably large micelles).26 In general, we successfully obtained two main types of micellar morphology, spherical and worm-like, as shown in Figure 3. It was observed that if residual CB remains in the final solution, we also obtain intermediate irregular micellar morphologies (see complementary section). On the basis of the TEM images, we observe that there is no dramatic change in the morphology of the spherical and wormlike micelles when we incorporate PCBM. Nevertheless, the color of the micellar solution is determined by the PCBM concentration (Figure 2). Moreover, the PCBM, which is possibly encapsulated inside the micellar structures, would be soluble in water, which is not the case of pristine PCBM. No agglomerations of PCBM structures were observed, indicating that they were within the micelles, due to the hydrophobicity of PCBM. The spherical micelles show a radius that increases with fullerene concentration, while no significant alteration was observed in the worm-like micellar structures, which present a radius that is predominantly less than 60 nm and a length that ranges from a few hundred nanometers to several micrometers. However, the possibility that some worm-like micelles are encapsulating fullerenic molecules in high PCBM concentration is suggested from TEM images (not shown), which show bulges in the core of these micelles, as occurs for the spherical micelles. In addition, we observe differences in contrast due to the presence of material in the core of these worm-like micelles. Another possibility is that the PCBM molecules are trapped in

Figure 4. Diameter distribution of samples (a) 10 wt %, (b) 50 wt %, and (c) 80 wt % PCBM, for populations of 300 spherical micelles. The counting was performed from the BF-TEM images of the samples. The mean values were 57, 87, and 124 nm for samples 10, 50, and 80 wt %, respectively.

the worm-like micelles at low quantities such that they cannot be resolved very well by TEM. BF-TEM images of samples with 80 wt % (Figure 3e) and 50 wt % (Figure 3d) PCBM with respect to PS show spherical micelles larger than those observed for samples with low weight percentages of PCBM (Figure 3a-c). A statistical measurement of the observed diameters from TEM images results in a mean value of about 57 nm for the low PCBM percentage, 10 wt % PCBM (and similar values for the 0 and 5 wt % samples), while, for the 50 and 80 wt % PCBM, the mean values are approximately 87 and 124 nm, respectively (Figure 4). These measurements were carried out on each recognizably spherical object; in some cases, the heads of the worms were included, but only if they were nearly spherical in shape. 3.2. Absorbance. The effect of the PCBM packing inside the micellar structures was studied by optical absorbance spectroscopy. The optical spectra of the encapsulated PCBM in the micelles diluted in water were compared with PCBM molecules diluted in CB. Figure 5 shows the normalized UV-visible absorption spectra obtained for solutions of PCBM molecules and micellar structures with 10, 50, and 80 wt % PCBM with respect to the PS block weight. The sample of 5 wt % PCBM is not included because it has a low intensity and is not suitable for comparison. Note that the PS-b-PEO solution presents negligible absorption in this spectral region. There is an appreciable increase in the absorbance measurements in the visible region when the PCBM concentration is raised (Figure 5), which has been attributed to intermolecular interactions between the fullerenes, due to the closely packed structures.34

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Figure 5. Normalized optical absorbance spectra of solutions of micelles with 5, 10, 50, and 80 wt % PCBM diluted in water and PCBM molecules diluted in chlorobenzene. Note that the intensity in the 400-600 nm region increases as a function of the PCBM inside micellar structures.

Mountrichas et al. have reported a similar behavior for PS-bPEO micelles encapsulating C60.26 On the other hand, it has been observed that the characteristic emission peak of pristine PCBM (at 330 nm) is more evident with increasing fullerene concentration, but a red shift in the peak position and/or a large broadening of this peak has not been observed, as we show in the case of micellar structures encapsulating PCBM. In addition, the 330 nm peak is broadened and slightly red-shifted when the PCBM concentration is increased up to 50 wt %. Furthermore, in the visible region, 350-750 nm, the spectrum of micellar structures encapsulating PCBM shows more absorption than that of the pristine PCBM solution. The increase in the absorption spectrum was reported by Cook et al. when they compared the absorption measurements from films of PCBM diluted in PS to those of pristine PCBM films.34 This heightened optical absorption has been assigned to the intermolecular interactions between the fullerenic molecules, due to a close packing.34 Thus, our results show that the absorption measurements of the micellar samples with high PCBM percentages resemble those obtained for the pristine PCBM film, where the molecules are expected to be highly packed, whereas the micellar samples with low PCBM percentages are similar to the case of the film of PCBM diluted in the PS matrix, where the molecules are more dispersed and consequently less packed. Figure 6 shows the results obtained from theoretical calculations for the optical absorption in terms of the absorption coefficient of isolated molecules C60 and PCBM, and highly packed simple cubic and hexagonal PCBM solids. The DFT calculations were performed using the SIESTA package.39,40 The molecules were originally fully relaxed,41 and the optical properties were calculated using SIESTA, with a scissor operator of 0.8 eV and a broadening of 0.1 eV for incident nonpolarized light with a fixed propagation wavevector. The optical properties of C60 have been extensively studied by many, and the spectrum features found in our calculations are in good agreement with those presented in other reports.42-44 Absorption peaks at 2.7, 3.6, 4.7, and 5.6 eV are well recognized for solid C60, both experimentally and theoretically. The lowest energy peaks have been assigned to transitions between the bands near the valence and conduction bands.42-44 On the other hand, on the basis of our calculations for the closely packed structures of PCBM (simple cubic and hexagonal), the visible region shows an appreciable increase in the absorption spectrum in comparison with the absorbance spectrum of a PCBM molecule (Figure 6); similar experimental results have been reported for C60 films.42-44 This is an expected result because PCBM molecules roughly

Figure 6. Comparison of the calculated absorption coefficient for C60 and PCBM isolated molecules, and for simple cubic and hexagonal highly packed PCBM crystals. The insets show the magnification for the curves of C60 and PCBM molecules. Note the scale for the insets in the vertical axis.

Figure 7. PL spectra of pristine PCBM film and micellar structures with 5, 10, 50, and 80 wt % PCBM with respect to PS. Samples were measured at room temperature and atmospheric pressure. It is clearly visible that the emission peaks from the films of PCBM encapsulated in micelles depend on PCBM concentration.

preserve the electronic properties of C60 molecules. In addition, a clear difference between the absorption features in the 400-600 nm region of the PCBM and C60 molecules is observed (inset of Figure 6). Their intensities are evidently an effect of the proximity of the PCBM molecules, and these intensities are similar regardless of the crystalline structure, because the volume of the unit cell is similar in both of these highly packed systems. The simulation results help us to understand in a qualitative way the features observed in the visible region of the absorbance spectra of our samples and support the idea of interacting PCBM clusters in the core of the PS-b-PEO micelles. The deviations from the experimental results are due to the fact that the optical properties of PCBM are described by excitons, which cannot be described by a DFT calculation. Nevertheless, we are only interested in the qualitative behavior of the optical properties as a function of the PCBM packing. 3.3. Photoluminescence. Photoluminescence spectra of a film of pristine PCBM and from films of PCBM/PS-b-PEO micellar structures with PCBM weight percentages of 5, 10, 50, and 80 with respect to the PS block weight are shown in Figure 7. The PL signal from the pristine PCBM film shows an emission at ∼745 nm, consistent with previous observations.45 On the other hand, the films of micellar structures without PCBM show a negligible photoluminescence spectrum; therefore, the observed response in Figure 7 is due entirely to the

Encapsulation of PCBM inside Micellar Structures PCBM molecules. A visible PL signal and peaks are observed, even for low PCBM percentages encapsulated inside micellar structures. Note that an annealing treatment of the films is not necessary in order to obtain curves similar to those reported by Hoppe and co-workers for films of MDMO-PVV:PCBM blends.45 Therefore, our results suggest that the PCBM packing consists of crystalline domains and, taking into account the observations of this group, we could attribute the increase in the PL intensity to the radiative recombination of excitons occurring in the PCBM nanoclusters. Moreover, while the first emission peak for 5 and 10 wt % PCBM occurs at approximately 734 nm, this shifts to approximately 747 nm for the samples with 50 and 80 wt % PCBM. It is straightforward to identify the peak near 745 nm with the S1 f S0 transition by comparison with C60, because the energy levels of PCBM molecules in the neighborhood of the HOMO-LUMO gap are slightly perturbed with respect to C60 due to symmetry breaking.41 This peak is followed by a vibronic structure similar to that observed for C60 films, suggesting that the relaxation processes for excited states may be quite similar for both cases. The red shift observed for the high percentage samples is related to the increase of the orbital overlap between neighboring molecules due to the packing as well as the formation of “energy bands” due to the increase in PCBM cluster size. Therefore, it is expected that the PCBM molecules in micelles with low PCBM percentages preserve their “molecular character”. Note that the PL spectra have not been normalized nor corrected for difference in sample absorption at the excitation wavelength because we are not interested in the change of PL intensities in the different samples. In fact, the corrections due to the sample absorption will only modify the intensities of the PL spectra, but this will not change the peak positions. Figure 8 shows the PL spectra of micellar structures obtained from samples with PCBM weight percentages of 10 and 80 and a PCBM pristine film at 45 K and a pressure of 10-2 Torr (similar results are obtained for other samples with different weight percentages of PCBM). At low temperatures, the existence of three peaks becomes evident, as is shown in Figure 8a and b. The emission peaks of the 80 wt % sample are redshifted with respect to the 10 wt % sample by at least 20 nm for the first peak and approximately 40 nm for the second peak (see Figure 8c). The peak positions of the PL spectra of the PCBM pristine film and the 80 wt % micellar sample do not show remarkable differences; this indicates that, in both configurations, the PCBM cluster sizes are very similar. For the 10 wt % sample, the difference in energy between the first and second peaks (from left to right) is about 0.15 eV, whereas the difference between the second and third peaks is about 0.21 eV (Figure 8a). In the case of the 80 wt % sample, the difference in energy between the first and second peaks is about 0.19 eV, whereas that of the second and third peaks is about 0.17 eV (Figure 8b). These energy differences are on the order of vibronic transitions, which has been reported by several groups for C60 films synthesized by vacuum sublimation of microcrystalline C60 powder.46,47 Another notable feature is that the PL signal in the range 600-700 nm is quenched when we decrease the temperature; this feature is due to the heating of the micellar structure, which is avoided by decreasing temperatures. 4. Conclusions In summary, micellar structures were formed using amphiphilic block-copolymer PS-b-PEO by the slow incorporation of water, as a selective block for polyethylene oxide (PEO), and the elimination of the common solvent CB at a temperature

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Figure 8. PL spectra at 45 K and 10-2 Torr under a 514 nm laser excitation for samples at (a) 10 wt % and (b) 80 wt % PCBM. (c) The comparison between both signals shows a red shift for the high PCBM concentration.

of 65 °C. As a result of this process, two kinds of micellar morphologies were obtained: worm-like and spherical. When PCBM molecules are incorporated in the micelle formation, the morphology of the micelles is preserved, but their sizes as well as their optical properties are modified. Therefore, these properties can be controlled by the PCBM concentration. Three aspects can be stated: First, the increase in the mean value of the radius with PCBM concentration is a consequence of PCBM encapsulation. Second, an appreciable increase is observed in the absorbance measurements in the visible region when the PCBM concentration is raised. The latter has been assigned to intermolecular interactions between the fullerenes, due to a closely packed arrangement of the molecules. This is supported by our DFT calculations using crystalline arrays of PCBM molecules in closely packed structures, compared with the isolated molecules of C60 and PCBM. Third, PL measurements are presented where a red shift is observed for the highest PCBM percentage inside the micellar samples, which is associated with the intermolecular interactions in the crystalline PCBM domains. Furthermore, three peaks are observed, which are associated with vibronic states similar to those observed for films of C60. Therefore, we have demonstrated that the vibrational properties are modulated by the confinement of the molecules in an inert host copolymer by a rather simple synthesis technique. Moreover, the possibility that worm-like micelles encapsulate fullerenes or fullerene derivatives is shown, which, to our knowledge, has

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not been reported. Finally, we aim to make possible in the future the use of these structures as low dimensional templates for the formation of PCBM domains for the development of new devices. Acknowledgment. We thank Claudia G. Elias-Alfaro for technical assistance and Dr. Francisco Castro-Roman for helpful discussions. This work was supported at UASLP by SEP-PROMEP through Grant No. 103.5/07/2574 and by CONACYT through Grant No. J48897-Y and for a scholarship (J.M.N.-D.). Supporting Information Available: Photograph of capillary tubes filled with a worked solution at successive stages of the synthesis process and TEM image for the sample prepared with a solution containing CB. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312–1319. (2) Nagarajan, R.; Ganesh, K. Macromolecules 1989, 22, 4312–4325. (3) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934–2969. (4) Smith, W. F. Nat. Nanotechnol. 2007, 2, 77–78. (5) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. Engl. 2006, 45, 38–68. (6) Whitesides, G. M.; Boncheva, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4769–4744. (7) Bahadur, P. Curr. Sci. 2001, 80, 1002–1007. (8) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923–7927. (9) Zhang, L.; Yu, K.; Eisenberg, A. Science 1996, 272, 1777–1779. (10) Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1397–1407. (11) Jain, S.; Bates, F. S. Science 2003, 300, 460–464. (12) Webber, S. E. J. Phys. Chem. B 1998, 102, 2618–2626. (13) Yelamanchili, R. S.; Walther, A.; Mu¨ller, A. H. E.; Breu, J. Chem. Commun. 2008, 489–491. (14) Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6359–6361. (15) Bhargava, P.; Zheng, J. X.; Li, P.; Quirk, R. P.; Harris, F. W.; Cheng, S. Z. D. Macromolecules 2006, 39, 4880–4888. (16) Bhargava, P.; Tu, Y.; Zheng, J. X.; Xiong, H.; Quirk, R. P.; Cheng, S. Z. D. J. Am. Chem. Soc. 2007, 129, 1113–1121. (17) He, Y.; Li, Z.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745–2750. (18) He, Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666–12667. (19) Vandermeulen, G.; Rouxhet, L.; Arien, A.; Brewster, M. E.; Pre´at, V. Int. J. Pharm. 2006, 309, 234–240. (20) Yeh, S. W.; Wei, K. H.; Sun, Y. S.; Jeng, U. S.; Liang, K. S. Macromolecules 2003, 36, 7903–7907. (21) Bhaviripudi, S.; Reina, A.; Qi, J.; Kong, J.; Belcher, A. M. Nanotechnology 2006, 17, 5080–5086.

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