Diverse Access to Artificial Superhydrophobic Surfaces Using Block

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Diverse Access to Artificial Superhydrophobic Surfaces Using Block Copolymers Joong Tark Han, Xurong Xu, and Kilwon Cho* Department of Chemical Engineering, Polymer Research Institute, Pohang University of Science and Technology, Pohang, 790-784, Korea Received April 19, 2005. In Final Form: June 1, 2005 The creation of an artificial superhydrophobic surface with micro- and nanostructures has been demonstrated using a block copolymer micelle solution and silica nanoparticles. The unique technique of a nanoparticle-supported micelle stabilization together with changes in the solvent power guarantees the precise morphology control of certain block copolymer-mediated surfaces. The approaches presented here provide a new strategy for the fabrication of a wettability-controlled organic-inorganic hybrid or organic coatings.

The wettability of a solid surface is an important aspect governed by the chemical composition and the geometric microstructure of the surface; the wettability can be decreased by creating a local geometry with a large geometric area relative to the projected area.1 Nature provides a unique example in the lotus flower, in which the leaves utilize superhydrophobicity as the basis of a mechanism to control the surface morphology for the protection and self-cleaning of that surface.2 Inspired by this natural superhydrophobicity, similar mimetic surfaces having water contact angles greater than 150° have attracted considerable interest over the past few years for both fundamental research and practical applications. Several methods such as the sol-gel process,3 the solidification of alkylketene dimers,4 the generation of fibrillar structures on surfaces,5 the creation of rough surfaces covered with low surface energy molecules,6 the phase * To whom correspondence should be addressed. E-mail: kwcho@ postech.ac.kr. Fax: +82-54-279-8269. Tel: +82-54-279-2270. (1) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (c) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (d) Xu, Y.; Fan, W. H.; Li, Z. H.; Wu, D.; Sun, Y. H. Appl. Opt. 2003, 42, 1. (e) Lafuma, A.; Que´re´, D. Nature Materials 2003, 2, 457. (f) Marmur, A. Langmuir 2004, 20, 3517. (2) (a) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (b) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (c) Blossey, R. Nat. Mater. 2003, 2, 301. (d) Gao, X, Jiang, L. Nature 2004, 432, 36. (e) Cheng, Y.-T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 14101. (f) Fu¨rstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 242, 339. (3) (a) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. J. Am. Chem. Soc. 2004, 126, 4796. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (c) Satoh, K.; Nakazumi, H. J. Sol-Gel Sci. Technol. 2003, 27, 327. (d) Roig, A.; Molins, E.; Rodrı´guez, E.; Marı´nez, S.; Moreno-Man˜as, M. Vallribera, A. Chem. Commun. 2004, 2316. (4) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (5) (a) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (b) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (c) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (d) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (6) (a) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. Adv. Mater. 2004, 16, 302. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929. (c) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 2012. (d) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (e) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235. (f) Shang, H. M.; Wang, Y.; Limmer, S. J.; Chou, T. P.; Takahashi, K.; Cao, G. Z. Thin Solid Films 2005, 472, 37.

separation of certain polymers in mixed solvent systems,7 and layer-by-layer process,8 among others,9 have been adopted in attempts to fabricate superhydrophobic surfaces. Here, we describe three diverse strategies for fabricating artificial superhydrophobic surfaces: micellization of a specific block copolymer, nanoparticle-assisted micelle stabilization, and simple hydrophobization. Recently, Xie et al.7b reported that the apparent water contact angle on structured poly(methyl methacrylate) surfaces can be increased if the substrate is prepared from a micelle solution of a polypropylene-b-poly(methyl methacrylate) block copolymer. Our approach introduces a new strategy for fabricating both artificial superhydrophobic organic/ inorganic nanocomposite surfaces, and organic surfaces, mediated with a suitable block copolymer and silica nanoparticles. Atom transfer radical polymerization (ATRP) has been used extensively to synthesize functional block copolymers.10,11 Since the propagating chain ends are generally believed to be radicals, ATRP is much more tolerant of monomer functionality than ionic polymerizations. In this study, poly(tert-butyl acrylate)-block-poly(dimethylsiloxane)-block-poly(tert-butyl acrylate) (PtBA-b-PDMS-bPtBA) triblock copolymer (Mn ) 56 kg/mol; 20K:16K:20K and Mw/Mn ) 1.16) was synthesized by ATRP using difunctional PDMS as a macroinitiator.12 It is well-known that one of the most important and useful properties of block copolymers is their ability to form micelles in selective solvents, depending on the concentration of the copolymer.13 Above the critical micelle concentration, (7) (a) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (b) Xie, Q.; Fan, G.; Zhao, N.; Guo, X.; Xu, J.; Dong, J.; Zhang, L.; Zhang, Y.; Han, C. C. Adv. Mater. 2004, 16, 1830. (8) (a) Zhai, L.; Cebeci, F. C¸ .; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (b) Soeno, T.; Inokuchi, K.; Shiratori, S. Appl. Surf. Sci. 2004, 237, 543. (c) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782. (d) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713. (9) (a) Li, M.; Zhai, J.; Liu, H.; Song, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2003, 107, 9954. (b) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (c) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (d) Nakajima, A.; Abe, K.; Hashimoto, K.; Watanabe, T. Thin Solid Films 2000, 376, 140. (e) Lu, X.; Zhang, C.; Han Y. Macromol. Rapid Commun. 2004, 25, 1606. (f) Lu, X.; Zhang, J.; Zhang, C.; Han Y. Macromol. Rapid Commun. 2005, 26, 637. (10) Sawamoto, M.; Kitano, K.; Kamigaito, M. Macromolecules 1995, 28, 1721. (11) Wang, J. S.; Matyjaszewski, K. Macromolecules 1995, 28, 7901. (12) (a) Han, J. T.; Cho, K. Macromolecules 2003, 36, 8902. (b) Miller, P. J.; Matyjaszewski, K. Macromolecules 1999, 32, 8760. (13) Hamley, I. W. The physics of block copolymers; Oxford: New York, 1998.

10.1021/la051042+ CCC: $30.25 © 2005 American Chemical Society Published on Web 06/21/2005

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Figure 1. SEM images of various PtBA-b-PDMS-b-PtBA surfaces prepared from different micelle solution concentrations, pictured with images (inset) of the corresponding water contact angles. (a) Block copolymer film deposited on the glass substrate from a methanol/block copolymer micelle solution (0.005 g/mL). (b) Side view of stacked block copolymer spheres pictured in panel a. (c) Interconnected micellar block copolymer film prepared from a micelle solution (0.01 g/mL). d) Surface morphology of a film cast from a block copolymer micelle solution (0.01 g/mL) adding with a further 80 µL water. Inset images indicate the water contact angle of a 3 mm water drop on the corresponding film surface.

monomolecular micelles form as a result of the length of the molecular chains, whereas at higher concentrations, multimolecular block copolymer micelles are generally formed. In the present study, the PtBA-b-PDMS-b-PtBA triblock copolymer synthesized by ATRP was subsequently dissolved in methanol (the selective solvent of the PtBA blocks), resulting in the formation of a micelle solution. Each micelle comprised an insoluble PDMS core surrounded by a soluble PtBA shell. Using this micelle solution, we attempted to control the surface roughness of the resulting block copolymer film via two different methods: (1) changing the solvent power and (2) nanosilica-mediated stabilization of the block copolymer micelle structure during film formation. First, block copolymer films were obtained by casting the PtBA-b-PDMS-b-PtBA block copolymer micelle solution onto clean glass substrates at room temperature. Figure 1 shows the typical scanning electron microscopy (SEM) images, obtained for micelles deposited on glass substrates at different concentrations, together with optical micrographs (inset) indicating the corresponding water contact angle of the film surface. It is noticeable that well-stacked block copolymer sphere (ca. 700 nm) films formed at low concentration (0.005 g/mL) (Figure 1a,b), whereas at high concentration (0.01 g/mL), the resulting block copolymer film comprises an interconnected granular structure composed of aggregated micelles (Figure 1c). In the latter case, the surface roughness is insufficient for superhydrophobicity purposes, because the granule size is over 3 µm. However, the addition of a small amount of water to the block copolymer micelle solution results in the formation of a multi-level rough surface, simply by changing the solvent power of methanol. This results in the formation of a nanostructured block

copolymer surface, as shown in Figure 1d. Decreasing the solvent ratio of methanol, on the other hand, causes the PtBA chains to shrink, leading to the formation of a dense layer around the PDMS core, which may inhibit the interpenetration of the PtBA chains, resulting in the formation of significantly smaller, interconnected micelles than those obtained in the absence of water (Figure 1c). Methanol is used in this study as the selective solvent associated with the PtBA blocks and as a nonsolvent for the PDMS blocks. In this respect, the surface of the asformed micelle films is composed of PtBA polymer chains. The flat PtBA film spin coated on flat substrates has a water contact angle of only about 80°. However, the rough surface of PtBA-b-PDMS-b-PtBA triblock copolymer, prepared from a block copolymer micelle solution in methanol, has higher water repellency than a flat surface, as reflected by the corresponding water contact angles (ca. 3 mm drop size) in Figure 1, panels a (136.0 ( 2.2°), c (145.0 ( 1.7°), and d (163.0 ( 1.8°). The water contact angle can increase so much on nonwetted surfaces explained in Cassie and Baxter’s model1b even though the water contact angle of the surface itself is below 90°. Therefore, it is speculated that the reason for the contact angle increase in our system could be due to the small size of the pores situated between the block copolymer spheres. Recently, Abdelsalam et al.14 also indicated that the apparent water contact angle is likely to increase on structured gold surfaces formed by electrodeposition through monolayer templates of closepacked uniform submicrometer spheres, even though the gold itself is hydrophilic (water CA ) 70°). (14) Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Langmuir 2005, 21, 1753.

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Figure 2. Scheme representing two novel approaches for the formation of a superhydrophobic surface, mediated by a nanoparticle-stabilized block copolymer micelle solution. (a) Nanoparticle-stabilized block copolymer micelle solution formed on a glass substrate. (b) Formation of a nanosilica-coated block copolymer sphere film on a glass substrate. (c) Fluorination route (I): the chemical vapor deposition resulting in an embossed block copolymer (BC) micelle/nanosilica film. (d) Fluorination route (II): dipping in perfluorosilane/n-hexane solution, followed by subsequent washing with n-hexane, resulting in the formation of a block copolymer micelle-inverse nanosilica film.

In a second approach, it was possible to obtain two kinds of superhydrophobic surfaces from a PtBA-b-PDMS-bPtBA micelle solution stabilized by hydrophilic silica nanoparticles (size ) ca. 11 nm), and by hydrophobization of the surface, as shown in Figure 2. It is well-known that finely divided insoluble solid particles constitute an important class of emulsifying agents.15 It is necessary that the size of the particles should be much smaller than the size of the emulsion droplets for the particles to be properly located around the droplets. At the interface, these particles usually form rigid structures that can sterically inhibit the coalescence of emulsion droplets. If charged, particles at the interface can also impart a degree of an electrostatic repulsion, which will further enhance the stability of the emulsion. Usually, hydrophilic particles stabilize oil-in-water emulsions, whereas hydrophobic particles stabilize water-in-oil emulsions. In our study, hydrophilic nanosilica particles (ca. 11 nm) were used to stabilize the block copolymer micelle at high concentration, as shown in Figure 2a. As can be seen in Figure 3a, in the absence of nanosilica particles, the block copolymer micelles aggregate together at high concentration (0.01 g/mL), resulting in an interconnected granular structure with larger micelles than observed at low concentration (0.005 g/mL), due to the severe interdiffusion of PtBA chains. However, it is elucidated from the SEM images in Figure 3, panels b and c, that the addition of nanosilica particles to the block copolymer micelle solution inhibits severe aggregation of (15) (a) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (b) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 157, 244. (c) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.

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Figure 3. SEM images showing the effect of nanosilica particles on the stabilization of block copolymer micelles during a film formation. (a) Block copolymer film cast from a 0.01 g/mL micelle solution. (b) Block copolymer/nanosilica film produced from micelle solution containing 0.52 mg nanosilica. (c) Separated block copolymer micelles coated with nanosilica particles cast from micelle solution containing 1.3 mg nanosilica. Inset images in panels b and c show the corresponding high magnification images of the nanosilica-coated block copolymer micelles.

the micelles during film casting, even in highly concentrated solutions (0.01 g/mL methanol solution), resulting in the formation of stabilized block copolymer micelle/ nanosilica films. In Figure 3c, the high magnification image (inset) shows that the block copolymer spheres are in fact coated with nanosilica particles. Therefore, this surface possesses a micro- and nanoscale binary structure similar to the surface of the lotus leaf, where every microscale papilla on the leaf surface is covered by nanoscale papillae.2 Since these nanosilica particles have hydroxyl groups on their surface, it is possible to further functionalize the as-formed films by reaction with organosilane compounds. Therefore, we adopted two methods of surface fluorination; chemical vapor deposition (CVD) Route (I), and solution dipping Route (II) (see Figure 2). In the first method, the surface of Figure 3c was fluorinated with 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (PFS) vapor. The resulting advancing water contact angle (ca. 3 mm drop size) was measured at over 170°, whereas the corresponding sliding angle was determined to be less than 2°. In the second method, block copolymer/nanosilica films prepared from a 0.01 g/mL methanol solution (Figure 3c) were dipped in a 20 mM PFS/n-hexane solution for 30 min, followed by copious washing with excess n-hexane. Interestingly, this dipping approach resulted in the

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Figure 4. SEM and optical images of a block copolymer micelle-inverse nanosilica film, formed after treatment of the film in Figure 3c with PFS/n-hexane solution, and subsequent washing in n-hexane. (a) Low magnification image. (b) High magnification image of panel a showing the trace of block copolymer spheres (dotted lines) and the multi-leveled structure composed of only nanosilica particles. (c) Optical image of a water droplet on the BC micelle-inverse nanosilica film.

formation of block copolymer micelle-inverse films composed of only nanosilica particles on the surface, in which traces of block copolymer micelles could be distinguished by SEM (Figure 4b). It was revealed by energy dispersive analysis with X-ray spectroscopy (EDAX) that the exposed nanosilica particles of these inverse nanosilica films are coated with a semi-fluorinated layer (Supporting Information Figure S1c). The absence of block copolymer micelles on the surface was further corroborated by EDAX analysis, in which the atomic carbon ratio is very small (Supporting Information Figure S1c) compared to that obtained for the nanosilica-stabilized block copolymer micelle film (Supporting Information Figure S1b). This inverse image suggests that the block copolymer spheres coated with a fluorinated nanosilica layer were forced to the surface during the washing stage with n-hexane, resulting in the formation of an interconnected nanosilica film with a polymerized PFS surface layer, as shown in Figure 2d. The advancing water contact angle (ca. 3 mm drop size) on this nanosilica film was extremely high (over 170°), whereas the sliding angles of water drops on the same surface were found to be less than 2°, which suggests a very small contact angle hysteresis with essentially no pinning of the water droplet. Figure 4c shows an optical image of a water drop formed on the inverse nanosilica film. On this artificial superhydrophobic surface, water droplets roll freely off the surface due to the high water contact angle and very small sliding angle. This lotus leaflike character remained even after prolonged exposure to a high humidity environment (at least one month). To summarize, in an effort to realize the formation of artificial superhydrophobic surfaces, we have presented

three methods for controlling the morphology of organic and organic-inorganic nanocomposite films, mediated by block copolymer micelle solutions. The change in solvent power of a selective solvent, the nanoparticle-stabilized micellization technique, and a simple surface treatment have all been presented as suitable methods with which to control the morphology of a block copolymer-mediated surface. Using this approach, stable superhydrophobic surfaces with high contact angles and low contact angle hysteresis have been successfully developed. Our approach provides a new strategy for mimicking the superhydrophobicity associated with lotus leaves, having a multilevel structure formed from block copolymer micelle solutions, and nanoparticle-stabilized micelle solutions. Acknowledgment. This work was supported by a grant (Code No. 05K1501-01010) from “Center for Nanostructured Materials Technology” under “21st Century Frontier R&D Programs” and the National Research Laboratory Program of the Ministry of Science and Technology, the Regional R&D Cluster Project designated by the Ministry of Science and Technology & the Ministry of Commerce, Industry, and Energy, and BK21 program of Ministry of Education & Human Resources Development, Korea. Supporting Information Available: Experimental details of the synthesis of block copolymer and the preparation and characterization of superhydrophobic surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. LA051042+