Formation of Fractal-Like Structures Driven by Carbon Nanotubes

By evaporation of the dispersions obtained, polymeric fractal patterns from the degradation products of the MF core were formed onto silicon wafers...
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2007, 111, 331-334 Published on Web 12/21/2006

Formation of Fractal-Like Structures Driven by Carbon Nanotubes-Based Collapsed Hollow Capsules Vero´ nica Salgueirin˜ o-Maceira,† Cristina E. Hoppe,*,† and Miguel A. Correa-Duarte*,‡ Departamento de Quı´mica Fı´sica, UniVersidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain and Departamento de Quı´mica Fı´sica, UniVersidade de Vigo, 36310 Vigo, Spain ReceiVed: October 25, 2006; In Final Form: NoVember 29, 2006

Carbon nanotubes (CNTs) based hollow capsules were obtained by degradation under acidic conditions of core-shell nanocomposites build up upon adsorption of multilayers of CNTs (shell) onto melamineformaldehide (MF) spheres (core). By evaporation of the dispersions obtained, polymeric fractal patterns from the degradation products of the MF core were formed onto silicon wafers. The proposed mechanism for the formation of these structures is based on the role of the capsules as arrangements of heterogeneities that facilitate the dewetting of the liquid polymeric films.

Introduction A significant challenge faced in material synthesis and device fabrication is the development of methods to organize interconnecting and patterned assemblies of individual components at all dimensions including nanoscales and macroscales.1 Several routes in the case of nanostructures have been developed and multiply branched, three-dimensional (3D) hierarchical nanostructures based on the one-dimensional (1D) structures have been fabricated.2 However, despite some degree of success in aligning and patterning nanowires into novel meso-scale structures, such as dandelions, snowflakes, nanoflowers,3 and tubular metal-polymer assemblies,4 attempts for the spontaneous assembling of polymeric units into novel mesostructures have been only met with limited success. Hierarchical nanostructures have large surfaces areas and allow for heterostructures; thus, they can be applied in photovoltaics and multifunctional nanoelectronics. Furthermore, the large number of identical chemical units in this enhanced surface area can be designed for various purposes, including sensing, catalysis, or biochemical activity. This potential comes from lots of identical molecules being present at the same time and place, permitting the incorporation of functional molecules either covalently or noncovalently attached in such a way that biologically active substances could then be liberated by chemical or biological methods.5 Hence, the development of simple and low cost methods for the patterning of thin polymer films on a nano and microscopic scale length is currently a topic of major interest.6. On the other hand, since the first discovery of carbon nanotubes (CNTs) in 1991, great efforts have been invested in the research of related nanoscale structures.7 In materials science, CNTs have long been employed in various fields including composites, electrochemical, field emission and nanoscale electronic devices, ultrastrong materials,8 and sensors.9 Ad* Corresponding author: E-mail: (M.A.C.-D.) [email protected]; (C.E.H.) [email protected]. † Universidade de Santiago de Compostela. ‡ Universidade de Vigo.

10.1021/jp0669963 CCC: $37.00

ditionally, in recent times they also were used in biological environments as scaffolds for cells or in bacteria electroporation,10 or as 1D templates for particle deposition.11,12 Multiwall carbon nanotubes (MWNTs) also have been assembled onto flat films8 and on colloidal templates13 using the well-known polyelectrolyte-assisted layer-by-layer (LbL) self-assembly technique,14 producing dense multilayers of CNTs. Melamine formaldehyde (MF) particles coated with these dense multilayers of negatively charged CNTs have been used in this study, demonstrating a facile wet-chemical synthesis method to yield polymer fractal-like structures assembled onto silicon substrates. The proposed route is based on spherical core-shell units as sources of polymer, consisting of a core formed by an acid-soluble polymer network of MF and an insoluble shell made of multilayers of CNTs. The CNTs shells obtained after the acid-mediated dissolution process are very promising as hollow capsules, as the porosity of their walls can be tuned as increasing the number of deposited layers or the ionic strength and because of their unique CNTs-based mechanical properties as opposed to all other capsules. A different and original use for these structures is analyzed here, based on the ability of the hollow CNTs capsules deposited onto a silicon subtrate to induce the formation of polymer fractal patterns by favoring the process of dewetting of liquid films. Fractal-like morphologies obtained as evaporating the solvent of the droplets formed during the dewetting process are analyzed and a possible mechanism for the formation of these structures is proposed. Experimental Section Functionalized CNTs. Carbon nanotubes were oxidized by means of the following procedure: 10 mg of MWNTs were sonicated for 2 h in 10 mL of a mixture of H2SO4/HNO3 (3:1); then the sample was washed first with a dilute NaOH aqueous solution and then three times with water by centrifugation/ redispersion cycles. Finally, the MWNTs were dispersed in water, obtaining a stable dispersion of oxidized MWNTs with © 2007 American Chemical Society

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Figure 1. SEM images of a MF sphere coated with five layers of CNTs (a) and a collapsed hollow capsule of CNTs after the dissolution of the core (b).

carboxylic groups on the nanotube ends and side-walls, providing a negative surface charge. Previous works have shown by X-ray photoelectron spectroscopy (XPS) analysis that the oxygen content for oxidized single wall carbon nanotubes is between 9 and 14 atom %. CNTs-Coated MF particles. Polyelectrolyte-coated MF particles were prepared by diluting 50 µL of a suspension of MF spheres (3 wt. %) with water up to 1 mL, adding 1 mL of poly(sodium 4-styrenesulfonate) (PSS) solution (1 mg/mL, containing 0.5 M NaCl), waiting for 20 min to allow PSS adsorption. PSS excess was removed by three repeated centrifugation/wash cycles. Poly(diallyldimethyl ammonium chloride) (PDDA) (1 mg/mL containing 0.5 M NaCl) then was deposited onto the coated MF particles in a similar fashion and using the same conditions, forming the outermost layer. CNTs/ PDDA multilayer coatings were deposited onto the MF spheres as follows: 15 µL of the (PDDA/PSS)-coated MF particles (2 wt. %) were dispersed in 4 mL of a 0.2 M NaCl solution, and then 4 mL of CNTs dispersion (0.5 mg/mL) was added. An adsorption time of 30 min then was allowed, and excess of CNTs was removed by five repeated centrifugation (800 g, 10 min) wash cycles. PDDA (1 mg/mL solution containing 0.5 M NaCl, adsorption time of 30 min) was subsequently deposited and the excess was removed from the supernatant after centrifugation (1200 g, 10 min). Additional CNTs/PDDA multilayers were deposited as described above. Hollow Capsules and Fractal-Like Structures. Hollow capsules were prepared by exposing 0.5 mL of MF spheres coated with 5 bilayers (CNTs/PDDA) to 1 mL of HCl solution (1 mL, pH ≈ 1) for 20 min. A droplet of this dispersion was evaporated at room temperature onto a silicon wafer. Results and Discussion Synthesis of Core-Shell Nanocomposites. The method used for the assembly of CNTs onto spherical melamine resin (MF) particles (350 nm diameter; see experimental details) is based on the formation of polyelectrolyte multilayers by sequential adsorption of oppositely charged polymers, relying on the mutual electrostatic attraction between the charged species.8,13,14 Since initially CNTs do not carry a surface charge, they were previously modified by chemical oxidation using sulfuric and nitric acid, leading to the formation of carboxylic groups that are responsible for the presence of negative charges on their surface.15,16 Figure 1a shows a representative scanning electron microscopy (SEM) image of the core-shell nanocomposites obtained upon adsorption of multilayers of CNTs (shell) onto MF spheres (core). When CNTs were deposited onto the MF particles, they associated forming a compact shell surrounding the polymer

Letters particles. In this case, five layers of CNTs were deposited onto the polymeric particles, alternating the CNTs with PDDA to ensure the electrostatic interactions between the positively charged polyelectrotyte and the negatively charged CNTs. Formation of Hollow CNTs Capsules. By treatment of the nanocomposites with hydrochloric acid, the weakly crosslinked MF core was dissolved to produce hollow capsules with walls constituted by a net of CNTs and polyelectrolytes. Melamine resin microparticles are a network of melamine and formaldehyde, cross-linked by ether linkages and methylene bridges. The dissolution of the core is based on the acid hydrolysis of the ether linkages which produces oligo- and polymeric units typically characterized by sizes in the order of 3.0 nm.17 The formed oligomers are expelled readily as they permeate through the multilayers of CNTs/PDDA, forming hollow capsules. In systems in which the walls are formed only by polyelectrolytes, the typical time for the complete decomposition of the core at pH ) 1.1 is about 20 s.18 In our case, the porosity of the wall depends on the structure and thickness of the shell, which also can be tuned adjusting the experimental conditions in solution (ionic strength, pH, etc), changing therefore the permeability. Since a more compact wall is expected because of the presence of the five bilayers of CNTs/PDDA, a much longer time was used for the acid treatment to guarantee the complete dissolution of the core. Once this step was completed, the system was formed by hollow CNTs/PDDA capsules dispersed in a solution of oligo- and polymeric units and some residual sodium chloride arising from the initial CNTs and polyelectrolytes assembly step. In Figure 1b, a representative-collapsed capsule deposited onto the silicon substrate can be observed. The diameter was slightly larger than that of the core-shell precursors (∼350 nm), which is attributed to the swelling produced during the core dissolution process. It is known that the sudden increase of the concentration of degradation products inside the capsules produces a large difference in the osmotic pressure respect to the bulk and acts as a driving force for the swelling of the shell.19 Fractal Patterning. Most of the fractal modes of growth are caused by nonequilibrium phenomena due to interfacial instabilities dominated by surface tension.20 Colloidal aggregation1 and dendritic crystal growth21 are some of the commonly encountered micro and macroscopic structural forms. Fractal topographical polymeric patterns also have been found during the dewetting of thin polymer films.22 This dewetting process occurs for any liquid on a nonwettable surface and has been recently recognized as an alternative technique for the microscopic patterning of polymers that would avoid the use of templates.6a,b The process is initiated by the formation of holes in smooth films that grow to form almost regular structures of droplets.23 The analysis of the dewetting of thin polymer films have been normally carried out onto spun cast samples prepared on nearly homogeneous substrates. On the contrary, the study of dewetting during evaporation is scarce. However, some previous studies have shown that the process is characterized by the initial formation of holes produced as in ordinary dewetting, but that the boundary of the rapidly growing holes breaks up into fingering patterns that resemble nonequilibrium crystallization and Hele-Shaw patterns.22 Fractal patterns were observed when a droplet of the CNTs/ PDDA capsules dispersion obtained after acid treatment was allowed to dry at room temperature on the silicon substrate (Figure 2). As can be seen, the polymer arising from the dissolution process of the core-shell particles was distributed over the silicon surface forming fractal-like patterns over areas of about 50 × 50 µm2. The diameter of the branches ranges

Letters

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Figure 2. Polymeric droplets deposited onto the silicon wafer forming fractal-like aggregates. White cubic aggregates correspond to NaCl crystals.

between 300 and 400 nm becoming slender after multiple branching or even just small aggregates. Ultimately, the size of these aggregates decreased to approximately 100 nm in diameter. A more detailed examination of the patterns reveals that the droplets are arranged forming the sides of quasihexagonal polygons (indicated in Figure 2). This morphology has been characterized as typical of the late stages of dewetting of thin liquid films,23 which draws attention to this proposed mechanism as a potential responsible for the formation of the obtained structures. The polymer droplets are formed by small subunits coming from the dissolution process of the polymeric core. As was stated above, the mean size of these subunits was reported to be about 3 nm for the degradation at pH 1.1. When the dissolution process occurs in confined media as in this case, the acid hydrolysis is expected to be a complex function of the pH. In fact, in the case of MF particles coated with polyelectrolyte multilayers, a strong effect of the pH value on the size of the polymeric units has been reported.18 On the other hand, at lower pH values (0.50.8) the hydrolysis products remain active and easily can form supramolecular structures due to hydrogen bonding as was observed for naked particles by incubation of the decomposition products at room temperature. The oligomers from these naked particles self-aggregate into complexes with an average size of 160 nm. If the pH is too low, the acid-catalyzed repolymerization occurs in parallel with hydrolysis and results in the formation of C-N-C linkages.18 In the CNTs-coated MF particles case, because of the optimal initial pH value the dissolution of the core is expected to be fast providing smaller units that can diffuse through the CNTs net. However, the incubation of these oligomers onto the substrate during evaporation of the solvent at increasingly more acidic conditions can therefore favor the repolymerization process and drive the system to the formation of bigger polymeric units. To determine the influence of the hollow capsules in the formation of the fractal patterns, a control experiment without CNTs was carried out using naked MF particles in conditions similar to that used for the formation of the capsules. Polymeric films with a much more continuous morphology were observed, indicating that the carbon-based structures play an essential role in the formation of the fractal patterns (data not shown). It is known that dewetting can be facilitated by the presence of defects such as dust particles or surface heterogeneities.24 It seems feasible, therefore, that the collapsed capsules, randomly deposited onto the silicon wafer, may act as an arrangement of chemical and morphological heterogeneities that during the evaporation process induce the heterogeneous nucleation of holes facilitating the dewetting process. The almost circular arrangement of these heterogeneities would be responsible for

Figure 3. Schematic illustration of the followed process: Initial MF core particles (a) are coated with a multilayered shell of CNTs and polyelectrolytes using the LbL self-assembly protocol (b). The dissolution of the polymeric core through the CNTs shell at low pH produces hollow capsules with walls formed by CNTs and polyelectrolytes (c). During the evaporation, fractal-like structures are produced by dewetting of the film facilitated by the heterogeneities of the CNTs shell (d).

the radial appearance of the distribution of the fractal patterns observed on large areas of the substrates. A summary of the general strategy adopted for the formation of fractal patterns is sketched in Figure 3. Polymeric fractal patterns have been previously observed in a similar system using fullerenes.25 Given that the CNTs multilayers were formed based on electrostatic interactions reinforced by adding NaCl to the solution during the self-assembly process, the simultaneous crystallization of sodium chloride following the polymeric pattern also can be observed for samples deposited onto TEM grids at short times after the initiated dissolution process (Figure 4) as well as in the final obtained structures (Figure 2). In the first case, only a low amount of the MF content was degraded and can be observed as polymer islands outside the composite. On the contrary, the formation of extended polymer patterns and NaCl crystals is clearly observed for the final stages of the process in which the formation of cubic crystals at the ends of some branches also can be distinguished. The patterning process described here presents potential applications in the synthesis of highly active large-surface coatings. For example, to include biologically active moleculesfunctionalized polymers as the core material to be dissolved. Evaporation would allow the guiding of these molecules to pattern a large surface area. A similar approach also could be used to direct the crystallization of different inorganic substances in fractal patterns by including these materials in the capsules dispersion.

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Figure 4. TEM picture showing the dissolution of the polymeric core of the core-shell spheres of melamine resin particles coated with CNTs and the formation of NaCl nanocrystals. Polymeric fractal-like structures, formed by the degradation products of melamine formaldehyde (MF) spheres, were obtained by means of a dewetting process using carbon nanotubes (CNTs)-based collapsed capsules as an arrangement of heterogeneities onto silicon wafers. The hollow CNTs-based capsules were produced by degradation under acidic conditions of core-shell nanocomposites build up upon adsorption of multilayers of CNTs (shell) onto melamine-formaldehide (MF) spheres (core).

On the other hand, it is worth noting that although the common procedure for the dissolution of the MF cores is based on the acid treatment at low pH values, these particles also are decomposable in organic solvents as N,N-dimethylformamide (DMF) or dimethylsulfoxide (DMSO). Previous work on the dissolution of MF particles in DMF/H2O (1:9 v/v) has demonstrated that under this treatment the obtained degradation units are much bigger than the oligomers obtained by acid dissolution,18 extending therefore the versatility of the present strategy by modifying the nature of the building blocks. Moreover, as has been demonstrated, different polymeric particles can be coated by CNTs,26 which permits the generalization of the method to different polymeric patterns. Conclusion Hollow CNTs-based capsules were obtained by degradation under acidic conditions of core-shell nanocomposites build up upon adsorption of multilayers of CNTs (shell) onto MF spheres (core). By evaporation of the dispersions obtained, polymeric fractal patterns formed by the degradation products of the MF core and NaCl crystals from the LbL self-assembly process were directly obtained. A potential mechanism for the formation of these structures is proposed based on the role of the capsules as arrangements of heterogeneities that facilitate the dewetting of the liquid polymeric films. Acknowledgment. V.S.-M. and M.A.C.-D. thank the financial support from the Isidro Parga Pondal Program (Xunta de Galicia Regional Government, Spain). C.E.H. gratefully acknowledges European Commission for IIF Marie Curie fellowship under Project 021689 (ANaPhaSeS). References and Notes (1) (a) Lee, S.-W.; Lee, S. K.; Belcher, A. M. AdV. Mater. 2003, 15, 689-692. (b) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237-240. (c) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115-2117. (d) Salgueirin˜o-Maceira, V.; Correa-Duarte, M. A.; Hucht, A.; Farle, M. J. Magn. Magn. Mater. 2006, 303, 163-166.

Letters (2) (a) Hu, J.; Bando, Y.; Zhan, J.; Yuan, X.; Sekiguchi, T.; Golberg, D. AdV. Mater. 2005, 17, 971-975. (b) May, S. J.; Zheng, J. G.; Wessels, B. W.; Lauhon, L. J. AdV. Mater. 2005, 17, 598-602. (c) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380-384 (d) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190-195. (e) Wang, Y.; Tang, Z.; Liang, X; Liz-Marza´n, L. M.; Kotov, N. A. Nano Lett. 2004, 4, 225-231. (3) Zhou, X.; Chen, S.; Zhang, D.; Guo, X.; Ding, W.; Chen, Y. Langmuir 2006, 22, 1383-1387. (4) (a) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124-8125. (b) Lu, Q.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2004, 126, 54-55. (c) Chen, A.; Peng, X.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 1964-1965. (d) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348-351. (5) Cloninger, M. J. Curr. Opin. Chem. Biol. 2002, 6, 742-748. (6) (a) Higgins, A. M.; Jones, R. A. L. Nature 2000, 404, 476-479. (b) Lu, G.; Li, W.; Yao, J.; Zhang, G.; Yang, B.; Shen, J. AdV. Mater. 2002, 14, 1049-1053. (c) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2003, 44, 2379-2384. (7) Iijima, S. Nature 1991, 354, 56-58. (8) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190-194. (9) (a) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (b) Baughman, R. H. Science 2000, 290, 1310-1311. (c) An, K. H.; Kim, W. S.; Park, Y. S.; Moon, J.-M.; Bae, D. J.; Lim, S. C.; Lee, Y. S.; Lee, Y. H. AdV. Func. Mater. 2001, 11, 387-392. (d) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340-1344. (e) de Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179-1180. (f) Sugie, H.; Tanemura, M.; Filip, V.; Iwata, K.; Takahashi, K.; Okuyama, F. Appl. Phys. Lett. 2001, 78, 2578-2588. (g) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker: C. Science 2001, 294, 1317-1320. (h) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker: C. Nature 1999, 402, 273-274. (i) Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94-97. (j) Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature 1996, 384, 147-150. (k) Kim, P.; Lieber, C. M. Science 1999, 286, 2148-2150. (10) (a) Correa-Duarte, M. A.; Wagner, N.; Rojas-Chapana, J.; Morsczseck, C.; Thie, M.; Giersig, M. Nano Lett. 2004, 4, 2233-2236. (b) RojasChapana, J.; Correa-Duarte, M. A.; Ren, Z.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 985-988. (11) (a) Correa-Duarte, M. A.; Liz-Marza´n, L. M. J. Mater. Chem. 2006, 16, 22-25. (b) Correa-Duarte, M. A.; Sobal, N.; Liz-Marza´n, L. M.; Giersig, M. AdV. Mater. 2004, 16, 2179-2184. (c) Correa-Duarte, M. A.; Pe´rezJuste, J.; Sa´nchez-Iglesias, A.; Giersig, M.; Liz-Marza´n, L. M. Angew. Chem., Int. Ed. 2005, 44, 4375-4378. (12) (a) Correa-Duarte, M. A.; Grzelczak, M.; Salgueirino-Maceira, V.; Giersig, M.; Liz-Marzan, L. M.; Farle, M.; Sierazdki, K.; Diaz, R. J. Phys. Chem. B 2005, 109, 19060-19063. (b) Grzelczak, M.; Correa-Duarte, M. A.; Salgueirin˜o-Maceira, V.; Giersig, M.; Diaz, R.; Liz-Marza´n, L. M. AdV. Mater. 2006, 18, 415-420. (13) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Liz-Marza´n, L. M. Chem. Mater. 2005, 17, 3268-3272. (14) (a) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (b) Decher, G. Science 1997, 277, 1232-1237. (c) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111-1114. (15) Yu, R.; Chen, L.; Liu, Q.; Lin, J.; Tan, K.-L.; Ng, S. C.; Chan, H. S. O.; Xu, G.-Q.; Hor, T. S. A. Chem. Mater. 1998, 10, 718-722. (16) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275-277. (17) Dong, W.; Ferri, J. K.; Adalsteinsson, T.; Scho¨nhoff, M.; Sukhorukov, G. B.; Mo¨hwald, H. Chem. Mater. 2005, 17, 2603-2611. (18) Gao, C.; Moya, S.; Lichtenfeld, H.; Casoli, A.; Fiedler, H.; Donath, E.; Mo¨hwald, H. Macromol. Mater. Eng. 2001, 286, 355-361. (19) Gao, C.; Moya, S.; Donath, E.; Mo¨hwald, H. Macrom. Chem. Phys. 2002, 203, 953-960. (20) Bishkek, T. Fractal Growth Phenomenon, 2nd ed.; World Scientific: Singapore, Singapore, 1992. (21) Spengler, J. F.; Coakley, W. T. Langmuir 2003, 19, 3635-3642. (22) Gu, X.; Raghavan, D.; Douglas, J. F.; Karim, A. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2825-2832. (23) Reiter, G. Phys. ReV. Lett. 1992, 68, 75-78. (24) (a) Seeman, R.; Herminghaus, S.; Jacobs, K. J. Phys.: Condens. Matter. 2001, 13, 4925-4938. (b) Stange, T. G.; Evans, D. F.; Hendrickson, W. A. Langmuir 1997, 13, 4459-4465. (25) Tan, C. H.; Ravi, P.; Dai, S.; Tam, K. C. Langmuir 2004, 20, 99019904. (26) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Salgueirin˜o-Maceira, V. Small 2006, 2, 220-224.