Superhydrophobic Aluminum Surfaces by Deposition of Micelles of

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Superhydrophobic Aluminum Surfaces by Deposition of Micelles of Fluorinated Block Copolymers )

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Simon Desbief,*,†,‡ Bruno Grignard,§ Christophe Detrembleur,§ Romain Rioboo, Alexandre Vaillant, David Seveno, Michel Voue, Joel De Coninck, Alain M. Jonas,^ Christine Jer^ome,§ Pascal Damman,‡ and Roberto Lazzaroni†

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† Service de Chimie des Mat eriaux Nouveaux (SCMN), Centre d’Innovation et de Recherche en Mat eriaux Polym eres (CIRMAP), Universit e de Mons, 20 Place du Parc, 7000, Mons, Belgium, ‡Laboratoire Interfaces et Fluides Complexes (InFlux), Centre d’Innovation et de Recherche en Mat eriaux Polym eres (CIRMAP), Universit e de Mons, 20 Place du Parc, 7000, Mons, Belgium, §Centre d’Etude et de Recherche sur les Macromol ecules (CERM), Universit e de Li ege, B6 Sart-Tilman, 4000 Li ege, Belgium, Laboratoire de Physique des Surfaces et des Interfaces (LPSI), Universit e de Mons, 20 Place du Parc, 7000, Mons, Belgium, and ^ Institute of Condensed Matter and Nanosciences, Universit e catholique de Louvain, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium

Received July 14, 2009. Revised Manuscript Received August 28, 2009 Superhydrophobic surfaces are generated by chemisorption on aluminum substrates of fluorinated block copolymers synthesized by reversible addition-fragmentation chain transfer in supercritical carbon dioxide. In an appropriate solvent, those block copolymers can form micelles with a fluorinated corona, which are grafted on the aluminum substrate thanks to the presence of carboxylic acid groups in the corona. Water contact angle and drop impact analysis were used to characterize the wettability of the films at the macroscale, and atomic force microscopy measurements provided morphological information at the micro- and nanoscale. The simple solvent casting of the polymer solution on a hydroxylated aluminum surface results in a coating with multiscale roughness, which is fully superhydrophobic over areas up to 4 cm2.

Introduction For over a decade, superhydrophobic surfaces have been extensively studied in the prospect of applications such as selfcleaning and anticorrosion.1-3 Numerous ways exist to generate superhydrophobic surfaces, for example, by solvent casting of polymers,4 electrodeposition,5-7 layer-by-layer deposition,8-10 colloidal assemblies,11-13 chemical vapor deposition,14,15 *Corresponding author. Tel þ 32 65 37 38 68; fax: þ 32 65 37 38 61; e-mail: [email protected]. (1) Schmit, D. L.; Coburn, C. E.; Benjamin, M. D. Nature 1994, 368, 39. (2) Wang, R.; Hashimoto, K.; Fujishima, A. Nature 1997, 388, 431. (3) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350. (4) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (5) Shirtcliffe, N. J.; McHale, G.; Newton, M. L.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929. (6) McHale, G.; Shirtcliffe, N. J.; Aqil, S.; Perry, C. C.; Newton, M. I. Phys. Rev. Lett. 2004, 93, 36102. (7) Wu, X. F.; Shi, G. Q. J. Phys. Chem. B 2006, 110, 11247. (8) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J. Phys.Chem. B 2005, 109, 20773. (9) Jiang, W. H.; Wang, G. J.; He, Y. N.; An, Y. L.; Wang, X. G.; Song, Y. L.; Jiang, L. Chem. J. Chin. Univ. (Chinese) 2005, 26, 1360. (10) Soeno, T.; Inokuchi, K.; Shiratori, S. Appl. Surf. Sci. 2004, 237, 543. (11) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A. Langmuir 2005, 21, 7299. (12) Li, J.; Fu, J.; Cong, Y.; Wu, Y.; Xue, L. J.; Han, Y. C. Appl. Surf. Sci. 2006, 252, 2229. (13) Zhang, G.; Wang, D. Y.; Gu, Z. Z.; Mohwald, H. Langmuir 2005, 21, 9143. (14) Wu, Y.; Sugimura, H.; Inoue, Y.; Takai, O. Chem. Vap. Deposition 2002, 8, 47. (15) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; L.; Zhu, D. Angew. Chem. 2001, 113, 1793. (16) Qu, M. N.; Zhang, B. W.; Song, S. Y.; Chen, L.; Zhang, J. Y.; Cao, X. P. Adv. Funct. Mater. 2007, 17, 593. (17) Wang, S. T.; Feng, L.; Jiang, L. Adv. Mater. 2006, 18, 767. (18) Pan, Q. M.; Jin, H. Z.; Wang, H. B. Nanotechnology 2007, 18, 355605. (19) Yabu, H.; Takebayashi, M.; Tanake, M.; Shimomura, M. Langmuir 2005, 21, 3235.

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dip-coating and/or self-assembly,16-19 and chemical grafting.20-24 Most of those methods cannot be easily scaled up as they require very strict conditions of preparation, and low adhesion coatings are often obtained. As observed in nature, e.g., lotus or rice leaves, which are known as self-cleaning surfaces, the superhydrophobicity effect is mainly driven by the surface free energy of the material and its surface roughness.25-31 The wettability of a solid hydrophobic surface is known to decrease when the ratio of the real surface area to the projected surface area (i.e., the Wenzel roughness) increases, as well as when the solid surface free energy decreases. In this work, the Wenzel roughness is determined from atomic force microscopy (AFM) and optical profilometry images. It should be noted that features above 3 mm and below 10 nm are not taken into account in these estimates of the Wenzel roughness measurements. When the roughness amplitude is enhanced, superhydrophobic behavior is more likely to appear.32 There is experimental (20) Callies, M.; Quere, D. Soft Matter 2005, 1, 55. (21) Li, X.; Reindhout, D.; Crego-Calama, M. Chem. Soc. Rev 2007, 36, 1350. (22) Ma, M. L.; Hille, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193. (23) Blossey, R. Nat. Mater. 2003, 2, 301. (24) Nakajima, A.; Hashimoto, K.; Watanabe, T. Mon. Chem. 2001, 132, 31. (25) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Lium, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (26) Jinxin, Y.; Pihui, P.; Xiufang, W.; Dafeng, Z.; Mengyi, X.; Jiang, C.; Zhuoru, Y. Appl. Surf. Sci. 2009, 255, 3507. (27) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (28) Woodward, I.; Roucoules, W. C. E. V.; Badyal, J. P. S. Langmuir 2007, 23, 8212. (29) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (30) Shang, H. M.; Wang, Y.; Limmer, S. J.; Chou, T. P.; Takahashi, K.; Cao, G. Z. Thin Solid Films 2005, 472, 37. (31) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357. (32) Quere, D. Rep. Prog. Phys. 2005, 68, 2495.

Published on Web 09/18/2009

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evidence that a Wenzel roughness as low as 1.232 and 1.333,34 can enable superhydrophobic behavior on surfaces with periodic patterns. However, no lower bound of the Wenzel roughness is presently known for random-like or multiscale type of roughness. Thus the tuning of surface properties, both in terms of roughness and surface free energy, is primordial to obtain superhydrophobic surfaces. In particular, there are two possible ways to increase the surface roughness: the first one is to increase the roughness without affecting the substrate, for example by incorporation of nanoparticles in the coating.35 The second approach is to modify the substrate in order to obtain a higher roughness, for example by plasma treatment.36,37 In order to lower the surface free energy on flat surfaces, a large variety of polymer coatings have been tested, such as acrylates, fluorinated acrylic polymers, fluoropolyethers, and siloxanes.38-44 Among those, acrylates and siloxanes have proved to be more suitable and are widely applied nowadays, but they still do not satisfy all the demands for an ideal protective coating. Partially fluorinated and perfluoropolymers were also investigated, since the introduction of fluorine atoms substantially reduces the surface energy of the polymer film.45-48 However, even perfluoroethers with the lowest surface energy do not yield water contact angles exceeding 120.49 In this article, we describe a novel approach to obtain a superhydrophobic effect on hydroxylated aluminum surfaces by chemically grafting micelles of a fluorinated block copolymer synthesized by reversible addition-fragmentation chain transfer (RAFT) in supercritical carbon dioxide (scCO2). The aluminum substrate is chemically modified to have an appropriate roughness, and hydroxylated, and then the polymer is deposited by solvent casting from a micellar solution. Thus the surface roughness is enhanced both by the surface modification (at the microscale) and by the polymer deposition (at the nanoscale). The use of scCO2 as a green polymerization medium for the synthesis by controlled radical polymerization (CRP) of perfluorinated polymers with well-defined characteristics is a promising alternative to the use of potentially toxic and expensive (33) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (34) He, B.; Patankar, N.; Lee, J. Langmuir 2003, 19, 4999. (35) Tserepi, A. D.; Vlachopoulou, M.-E.; Gogolides, E. Nanotechnology 2006, 17, 3977. (36) Coulson, S.; Woodward, I.; Badyal, J.; Brewer, S.; Willis, C. J. Phys. Chem. B 2000, 104, 8836. (37) Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Panayiotou, C. Langmuir 2008, 24, 11225. (38) Tsakalof, A.; Manoudis, P.; Karapanagiotis, I.; Chryssoulakis, I.; Panayiotou, C. J. Cult. Heritage 2007, 8, 69. (39) Alessandrini, G.; Aglietto, M.; Castelvetro, V.; Ciardelli, F.; Peruzzi, R.; Toniolo, L. J. Appl. Polym. Sci. 2000, 76, 962. (40) Toniolo, L.; Poli, T.; Castelvetro, V.; Manariti, A.; Chiantore, O.; Lazzari, M. J. Cult. Herit. 2002, 3, 309. (41) Suzuki, H.; Takeishi, M.; Narisawa, I. J. Appl. Polym. Sci. 2000, 78, 1955. (42) Castelvetro, V.; Aglietto, M.; Ciardelli, F.; Chiantore, O.; Lazzari, M.; Toniolo, L. J. Coat. Technol. 2002, 74, 57. (43) Ciardelli, F.; Aglietto, M.; di Mirabelo, M.; Passaglia, E.; Giancristoforo, S.; Castelvetro, V.; Ruggeri, G. Prog. Org. Coat. 1997, 32, 43. (44) Puterman, M.; Jansen, B.; Kober, H. J. Appl. Polym. Sci. 1996, 59, 1237. (45) Poli, T.; Toniolo, L.; Chiantore, O. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 347. (46) Malshe, V. C.; Sangaj Nivedita, S. Prog. Org. Coat. 2005, 53, 207. (47) Ciardelli, F.; Aglietto, M.; Castelvetro, V.; Chiantore, O.; Toniolo, L. Macromol. Symp. 2000, 152, 211. (48) Delucchi, M.; Turri, S.; Barbucci, A.; Bassi, M.; Novelli, S.; Cerisola, G. J. Polym. Sci. Part B 2002, 40, 52. (49) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (50) Villarroya, S.; Zhou, J.; Duxbury, C. J.; Howdle, S. M. Macromolecules 2006, 39, 633. (51) McHugh, M.; Krukonis, V. Supercritical Fluid Extraction;Principles and Practice; Butterworths: Boston, 1986. (52) Tuminello, W.; Dee, G.; McHugh, M. Macromolecules 1995, 28, 1506.

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fluorinated solvents. It is well-known that noncrystalline fluoropolymers50-53 and silicones54 are the only classes of polymers that show good solubility in this particular medium. Recently the use of scCO2 for polymer synthesis by CRP, i.e., atom transfer radical polymerization (ATRP),55-60 nitroxide mediated polymerization (NMP),61-65 and RAFT66-74 has gained increasing interest in academic laboratories. In the field of fluoropolymer synthesis by CRP in scCO2, Desimone et al. were the first to report on the synthesis of poly(1,1-dihydroperfluorooctyl methacrylate) (PFOMA) and poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) by copper catalyzed homogeneous ATRP using a CO2-soluble fluoroalkyl-substituted ligand, i.e., 4,40 -di(tridecafluoro-1,1,2,2,3, 3-hehahydrononyl)-2,20 -bipyridine (dRf6bpy).55 Later, Howdle et al. described the synthesis of PFOMA using CuBr ligated to bipyridine in the presence of caprolactone as cosolvent,56 as well as PFOMA-b-poly(ε-caprolactone) (PCL) diblock copolymers by combination of ATRP and ring-opening polymerization in a one-pot process.57 Recently, Detrembleur et al. reported on the successful synthesis of CO2-philic poly(2,2,2-trifluoroethyl methacrylate) (PFMA) either by homogeneous or supported ATRP using copper bromide as a catalyst ligated to highly CO2soluble perfluorinated polymeric ligands bearing tetraethyldiethylenetriamine (TEDETA) pendant groups.60 Finally, Lacroix-Desmazes et al. reported on the controlled synthesis of PFOA by RAFT using various chain transfer agents,66,67 i.e., benzyl dithiobenzoate, tert-butyl dithiobenzoate, and 1-(ethoxycarbonyl)-ethyl dithiobenzoate. Our approach in this work is to design and synthesize diblock copolymers composed of blocks with different solubility properties, so that micelles can be generated by the use of a selective solvent for one block. Nanoscale roughness can then be obtained (53) Desimone, J.; Guan, Z. Science 1992, 257, 945. (54) Hoefing, T.; Beckman, E. J. Supercrit. Fluids 1992, 6, 165. (55) Xia, J.; Johnson, T.; Gaynor, S. G.; Matyjaszewski, K.; DeSimone, J. Macromolecules 1999, 32, 4802. (56) Villarroya, S.; Zhou, J.; Duxbury, C. J.; Heise, A.; Howdle, S. M. Macromolecules 2006, 39, 633. (57) Villarroya, S.; Zhou, J.; Thurecht, K. J.; Howdle, S. M. Macromolecules 2006, 36, 9080. (58) Grignard, B.; Jerome, C.; Calberg, C.; Jerome, R.; Wang, W.; Howdle, S. M. Macromolecules 2008, 41, 8575. (59) Grignard, B.; Jerome, C.; Calberg, C.; Jerome, R.; Wang, W.; Howdle, S. M. Chem. Commun. 2008, 3, 314. (60) Grignard, B.; Calberg, C.; Jerome, C.; Wang, W.; Howdle, S. M.; Detrembleur, C. Chem. Commun. 2008, 44, 5803. (61) Odell, P. G.; Hamer, G. K. Polym. Mater. Sci. Eng. 1996, 74, 404. (62) McHale, R.; Aldabbagh, F.; Zetterlund, P. B.; Okubo, M. Macromol. Chem. Phys. 2007, 208, 1813. (63) McHale, R.; Aldabbagh, F.; Zetterlund, P. B.; Minami, H.; Okubo, M. Macromolecules 2006, 39, 6853. (64) Aldabbagh, F.; Zetterlund, P. B.; Okubo, M. Macromolecules 2008, 41, 2732. (65) McHale, R.; Aldabbagh, F.; Zetterlund, P. B.; Okubo, M. Macromol. Rapid Commun. 2006, 27, 1465. (66) Ribaut, T.; Lacroix-Desmazes, P.; Oberdisse, J.; Fournel, B.; Sarrade, S. Polym. Prepr. 2008, 49, 329. (67) Lacroix-Desmazes, P.; Ma, Z.; DeSimone, J. M.; Boutevin, B. RAFT Polymerization and Supercritical Carbon Dioxide; International Society for the Advancement of Supercritical Fluids, Ed; Proceedings of the 6th International Symposium on Supercritical Fluids; Versailles, France, April 28-30,2003; Institut National Polytechnique de Lorraine: Vandoeuvre, France, 2003; Vol. 2; p 1405. (68) Arita, T.; Beuermann, S.; Buback, M.; Vana, P. Macromol. Mater Eng. 2005, 290, 283. (69) Gregory, A. M.; Thurecht, K. J.; Howdle, S. M. Macromolecules 2008, 41, 1215. (70) Howdle, S. M.; Thurecht, K. J.; Wang, W.; Gregory, A. M. PCT Int. Appl. WO 2008009997, A1 20080124, 2008. (71) Thurecht, K. J.; Gregory, A. M.; Wang, W.; Howdle, S. M. Macromolecules 2008, 40, 2965. (72) Lee, H.; Terry, E.; Zong, M.; Arrowsmith, N.; Perrier, S.; Thurecht, K. J.; Howdle, S. M. J. Am. Chem. Soc. 2008, 130, 12242. (73) Thurecht, K. J.; Gregory, A. M.; Villarroya, S.; Zhou, J.; Heise, A.; Howdle, S .M. Chem. Commun. 2006, 42, 4383. (74) Zong, M.; Thurecht, K. J.; Howdle, S. M. Chem. Commun. 2008, 45, 5942.

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Figure 1. Top: Structure of P(FDA-co-AA)-b-PAN block copolymers; bottom: molecular weight of the blocks (g/mol).

upon deposition of the micelles on the substrate. Those copolymers are made of a polyacrylonitrile (PAN) block (marked as “N” in Figure 1), which is expected to form a rigid core in the micelles, and a fluorinated block (marked as “F” in Figure 1), which is expected to form the corona of the micelles and to induce the superhydrophobic effect in micellar deposits by reducing the surface tension. Those copolymers were designed for also having carboxylic functions in the fluorinated block, in order to permit the grafting of the chains to hydroxyl groups present on the aluminum surface. On the basis of this general structure, four copolymers were synthesized by varying the molecular weight of the constituting blocks, which is expected to modify the micelle size and the amount of fluorine at the micelle surface and therefore influence the nanoscale roughness and wettability of the coating. Figure 1 also contains a table describing each block in each polymer in terms of molecular weight. The deposition procedure described in the Experimental Section leads to the formation of superhydrophobic surfaces over large areas (up to 2  2 cm2) with a good reproducibility and can easily be scaled up to larger areas. The morphology at the microand nanoscale is determined by AFM and optical profilometry measurements; those two techniques are complementary in terms of the length-scale that is probed (500 nm to 100 μm for the AFM and 100 μm up to several millimeters for profilometry). Contact angle measurements and drop impact analysis are used to characterize the wettability properties of the deposited films.

Experimental Section Materials. Acrylic acid (AA, Aldrich), 1H,1H,2H,2H-heptadecafluorodecylacrylate (FDA, Aldrich), acrylonitrile (AN, Aldrich), azobisisobutyronitrile (AIBN, Aldrich) and trifluorotoluene (TFT, Aldrich) were used as received. S-1-dodecyl-S-(R,R0 -dimethyl-R00 -acetic acid)trithiocarbonate was prepared according to the procedure described by Shea et al.75 Characterization Techniques. 1H NMR spectra were recorded in CDCl3 at 400 MHz in the FT mode with a Bruker AN 400 apparatus at 25 C. Polymer

Synthesis.

Synthesis of Poly(heptadecafluorodecyl acrylate-co-acrylic acid) (P(FDA-co-AA)) Copolymers. In a typical experiment

(Table 1, entry 1), 1.2576 g of S-1-dodecyl-S-(R,R0 -dimethyl-R00 acetic acid)trithiocarbonate (3.4  10-3 mol), 0.0405 g of AIBN (2.39  10-4 mol), and 0.9686 g of AA (1.345  10-2 mol) were introduced in a 35 mL high-pressure cell and deoxygenated by a (75) Lai, J. T.; Filla, D.; Shea, R. Macromolecules 2002, 35, 6754.

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CO2 flux for 5 min. Then, 10 mL of deoxygenated FDA (3.14  10-2 mol) was added under a CO2 flux with the help of a syringe before equilibrating the high-pressure cell at 80 C and 300 bar for 18 h. After polymerization, the pressure was slowly released, and the polymer was collected as a yellow powder that was purified by two cycles of solubilization in TFT/precipitation in methanol. The polymer was finally dried at 60 C under vacuum for 24 h, and the monomer conversion was determined gravimetrically (94%). The purified copolymer was then analyzed by 1H NMR (250 MHz, 298 K, 50/50 v/v CFC113/CDCl3 mixture) δ (ppm): 5.12 (1H, CH-S-C(S)-S- chain end), 4.55 (2H, CH2-O-C(O)- of FDA), 3.53 (2H, C11H23-CH2-S-C(S)-S- chain end), 2.65 and 2.85 (1H, CH-C(O)-O- of FDA and AA, respectively), 2.2 < δ < 1.7 (2H, -CH2-CH-C(O)-O- of P(FDA-co-AA)). 1.5 (20H, CH3-C10H20-CH2-S-C(S)-S- of the chain end), 1.3 (6H, CH3-C-CH3 of the chain end) and 1.05 (3H, CH3-C10H20-CH2-S-C(S)-S- of the chain end). The Mn was calculated by comparison of the relative intensities of the peak characteristic of the methylene protons of the chain end (CH2-S-C(S)-S, δ = 3.56 ppm) and the peaks corresponding the CH group (δ = 2.64 ppm) of AA and the methylene protons (CH2-O-C(O), δ = 4.52 ppm) as well as the CH groups (CH-C(O)-O, δ = 2.45 ppm) of FDA, whereas the composition was deduced from the relative intensities of the CH groups (δ = 2.64 ppm) of AA and the methylene protons as well as the CH groups of FDA (CH2-O-C(O), δ = 4.52 ppm and CH-C(O)-O, δ = 2.45 ppm, respectively): Mn,RMN = 6600 g/mol; [FDA]/[AA] in the copolymer = 85/15.

Synthesis of P(FDA-co-AA)-b-PAN Diblock Copolymers. In a typical experiment, 4.155 g of P(FDA-co-AA) (Mn = 6600 g/mol, FDA/AA exp = 85/15) were introduced in a 35 mL highpressure cell in the presence of 0.0140 g of AIBN (8.28  10-5 mol). The high-pressure cell was then flushed with CO2 for 5 min before adding 10 mL of AN (0.157 mol, expected Mn for the PAN block = 13000 g/mol). The high pressure cell was then equilibrated at 300 bar and 80 C for 18 h. After polymerization, the pressure was slowly released, and the diblock copolymer was collected as a powder. Residual AN was removed under vacuum before characterization by 1H NMR spectroscopy and size exclusion chromatography (SEC) in dimethylformamide (DMF) (Table 2, entry 1 (CP1): Mn,app, SEC = 43000 g/mol, Mw/Mn = 1.19). The purified copolymer was then analyzed by 1H NMR (250 MHz, 298 K, 50/50 v/v CFC113/DMF mixture) δ (ppm): 5.12 (1H, CH-S-C(S)-S- chain end), 4.55 (2H, CH2-O-C(O)- of FDA), 3.53 (2H, C11H23-CH2-S-C(S)-S- chain end), 3.42 (1H, CH-CN of PAN) 2.65 and 2.85 (1H, CH-C(O)-Oof FDA and AA, respectively), 2.2 < δ < 1.7 (2H, -CH2CH-C(O)-O- of P(FDA-co-AA) and PAN block). 1.5 (20H, CH3-C10H20-CH2-S-C(S)-S- of the chain end), 1.3 (6H, CH3-C-CH3 of the chain end) and 1.05 (3H, CH3-C10H20-CH2-S-C(S)-S- of the chain end). Mn was calculated by 1H NMR spectroscopy (250 MHz, 298 K, 50/50 v/v CFC113/DMF mixture) by comparison between the relative intensities of the methyl end group of the polymer (CH3, δ = 0.95 ppm) and the CH group of AN (CH-CN, δ = 3.42 ppm): Mn, NMR = 13000 g/mol. The results are summarized in Table 2. Substrate Cleaning. The substrates are made of aluminum plates prepared by the aerospace company SONACA (Belgium), and were either used as received or after a cleaning process. This cleaning process consists in a degreasing in isopropanol and acetone (1/1, Aldrich), followed by a basic attack in NaOH (100 mg/mL, Aldrich) for 45 min, and an acidic attack in HNO3 (100 mg/mL, Aldrich) for a few seconds. After this cleaning process, the surface became white and 10 times rougher than the original substrate (see Figure 2 and Table 3). To favor the formation of hydroxyl groups on the surface, the substrate is finally treated in a UV-ozone oven for 45 min. Film Preparation. 50 mg/mL solutions of the copolymers were prepared either in DMF (Aldrich) or in TFT (Aldrich). They were then heated up to 70 C for half an hour (samples denoted DOI: 10.1021/la902565y

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Desbief et al. Table 1. Experimental Data for the Synthesis of P(FDA-co-AA) copolymers by RAFT in scCO2

entry

FDA/CTA

conv (%)a

AA/CTA

FDA/AA theor

FDA/AA expb

Mn theor (g/mol)c

Mn exp (g/mol)b

1 2 3

9.5 4.5 94 70/30 85/15 4900 6600 18 8 91 70/30 88/12 9000 9700 27 12 95 70/30 89/11 14000 16000 a Gravimetrically determined. b Estimated by 1H NMR spectroscopy in a 50/50 v/v CFC113/CDCl3 mixture. c Mn theor = weight of FDA/nCTA  FDA conversion þ weight of AA/nCTA  AA conversion.

Table 2. P(FDA-co-AA)-b-PAN Diblock Copolymers Prepared in scCO2 by RAFT FDA-co-AA block entry

FDA/AA exp

PAN block

Mn exp (g/mol)

a

Mn theor (g/mol)b

conv (%)

Mn exp (g/mol)c

PDI

CP1 85/15 6600 96 12500 13000 1.19 CP2 85/15 6600 82 19000 18000 1.24 CP3 89/11 16000 95 10000 14000 1.17 CP4 89/11 16000 93 20000 26000 1.32 a Gravimetrically determined. b Mn theor = weight of AN/n macroinitiator  AN conversion. c Determined by 1H NMR spectroscopy in a 50/50 v/v CFC113/DMF mixture.

Figure 2. 20  20 μm2 AFM height images of aluminum substrates before (A) and after (B) the cleaning process. Table 3. Wenzel and RMS Roughness (RQ) Measured on AFM Imagesa surfaces aluminum as received

aluminum coated with CP4 cleaned

after deposition

washed

Wenzel roughness 1.02 ( 0.025 1.07 ( 0.04 1.25 ( 0.03 1.12 ( 0.02 1  1 μm2 8 nm 21 nm 30 nm 23 nm RQ 36 nm 370 nm 620 nm 450 nm 20  20 μm2 a The RQ roughness is reported for 1  1 μm2 and 20  20 μm2 images. Since the value does not vary significantly with the image size, the Wenzel roughness is the mean value obtained on all images recorded (from 1  1μm2 to 50  50 μm2).

“-f”) or 24 h (samples denoted “-24h”), in order to favor the solubilization of the block copolymers and obtain homogeneous solutions. On hundred microliters of solution was then solvent cast on a 2  2 cm2 sheet of cleaned aluminum, and left to evaporate overnight under a solvent-saturated atmosphere in order to obtain a slow evaporation of the solvent. AFM Measurements. AFM measurements were performed using a Multimode microscope equipped with a Nanoscope V controller from Veeco Instruments, Inc. All images were taken under ambient conditions, in tapping mode AFM, with microfabricated Si cantilevers with a resonance frequency in the 150-300 kHz range and a spring constant of 30 N 3 m-1. Different areas of each sample were analyzed to check the uniformity of the 2060 DOI: 10.1021/la902565y

surface. Images were usually recorded with a maximum resolution of 4096  4096 pixels2 and a scanning frequency of 0.2 Hz, due to the high roughness of the analyzed samples. All images presented have been treated (flatten) and analyzed (e.g., area roughness measurements) using the Nanoscope software. Drop Impact and Contact Angle Analysis. Drop impact experiments were performed to quantify the reboundness of MilliQ water drops on the copolymer surfaces. Drops of different diameter (D) were produced using a syringe pump and various needles of different diameters. Changing the height of the fall enabled various impact speeds (V). A Phantom V5.2 high-speed camera has been used to record drop impact sequences. Depending on the impact velocity, we varied the image size to be able to Langmuir 2010, 26(3), 2057–2067

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visualize all phenomena. The maximum possible frame rate was varied between 2800 to 3000 images per second. To test the reproducibility, three experiments were performed for each drop size and resulted in the same outcome. As a result of the bouncing of the drop, one single experiment resulted in various cases of impact speeds (and sometimes also various drop sizes when breakup occurs) as for each rebound the velocity is decreasing. Due to the impact pressure during the impact, the drop is strongly deformed, and its shape after the first rebound is no longer spherical. Thus drop impact velocities were analyzed and calculated by the position of the center of mass of the drop from two consecutive images just before contact with the solid surface. The impact velocity varied between 0.006 and 1.49 m/s, while the drop diameter varied between 0.22 and 4.62 mm. Besides drop impact experiments, advancing and receding static contact angles were measured using the sliding method49,76 where a drop is slid over the surface over a relatively long distance (at least 5 mm) while contact angles are recorded. The advantage of the sliding method is that it enables a statistical analysis of the superhydrophobicity of the surface in terms of localization over it. A Kr€ uss DSA100 contact angle analyzer and water drops of 15 μL were used for those measurements. A frame rate of 12.5 images per second was used, while the sliding velocity was kept as low as possible, i.e., 0.06 mm/s. The analysis was performed using a spherical approximation of the drop shape from side views near the solid surface. Intersection between the spherical fit and the solid surface avoided the difficulty to actually detect the contact line for such high contact angles. The drop size for these measurements was always smaller than the capillary length, justifying the use of the spherical cap approximation. The topography was measured at various magnifications using a Wyko NT1100 optical profilometer.

Results and Discussion Polymer Synthesis. P(FDA-co-AA)-b-PAN diblock copolymers were prepared by RAFT in a two-step process (see Supporting Information), using a carboxylic acid-functional control transfer agent (CTA), i.e., S-1-dodecyl-S-(R,R0 -dimethyl-R00 acetic acid)trithiocarbonate,75 which is known for controlling the polymerization of acrylates,75,77 styrene,75 AA,78 AN,78 and isoprene.79 In a first step, CO2-soluble P(FDA-co-AA) random copolymers were prepared using CTA as the controlling agent in the presence of AIBN in scCO2 at 80 C and 300 bar. After 16 h, the pressure was slowly released, and the polymer was collected as a yellow powder. The total monomer conversion, determined gravimetrically, was high (>90%), and the experimental molecular weight as well as the FDA/AA molar composition were determined by 1H NMR spectroscopy (see Experimental Section and Table 1). For a constant theoretical FDA/AA molar ratio, the experimental molecular weight was in good agreement with the theoretical value. We observe that the amount of FDA in the copolymer is higher compared to the FDA/AA molar ratio in the monomer feed, probably in line with the difference of reactivity ratio, in favor of FDA (let us note that values of reactivity ratio for the copolymerization of FDA and AA are not reported in the literature). The results are summarized in Table 1. In a second step, the polymerization of AN was conducted at 300 bar and 80 C from P(FDA-co-AA) as the first block, in the (76) Rioboo, R.; Voue, M.; Vaillant, A.; Seveno, D.; Conti, J.; Bondar, A. I.; Ivanov, D. A.; De Coninck, J. Langmuir 2008, 24, 9508. (77) Grignard, B.; Jerome, C.; Calberg, C.; Detrembleur, C.; Jerome, R. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1499. (78) Aqil, A.; Detrembleur, C.; Gilbert, B.; Jerome, R.; Jerome, C. Chem. Mater. 2007, 19, 2150. (79) Germack, D. S.; Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 4100.

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Figure 3. (A,B) 250  500 nm2 height and 500  500 nm2 phase AFM images of CP1 deposits on aluminum (from TFT solutions). (C,D) 500  500 nm2 phase and 250  500 nm2 height AFM images of CP4 deposits on aluminum (from TFT solutions).

presence of a small amount of AIBN. After 24 h of polymerization, the cell was cooled to room temperature, the pressure was slowly released, and the final P(FDA-co-AA)-b-PAN diblock copolymer was collected as a powder consisting of ill-stabilized micrometric particles. The polymer was then dried in vacuum overnight at 40 C in order to remove the residual monomer. The AN conversion, determined gravimetrically, was high; the molecular weight of the PAN sequence, estimated by 1H NMR spectroscopy using a CFC113/DMF mixture (see Experimental Section), was in good agreement with the theoretical value, and the polydispersity of the copolymers was low (1.15 < Mw/Mn < 1.35). The results are summarized in Table 2. Morphological Study. Aluminum Substrates. Before the characterization of the polymer deposits, a morphological analysis of the substrate is required. Figure 2 shows 20  20 μm2 AFM height images of the aluminum substrate before and after the cleaning process. The substrate “as received” (Figure 2A) shows lamination patterns resulting from the fabrication process. The cleaning process deeply modifies the surface morphology, probably as a result of the etching of the surface layers: the lamination patterns are no longer visible at the micrometer scale, where corrosion pits are observed (Figure 2B). Let us note, however, that the largest lamination patterns are still visible with the naked eye. The treatment also strongly enhances the surface roughness (Table 3): the initial substrate presents root-mean square (rms) roughness (RQ) values of 8 and 36 nm for 1  1 μm2 and 20  20 μm2 areas, respectively. These values increase to 21 and 370 nm after the cleaning process. Table 3 also shows that the Wenzel roughness measured from AFM images increases from 1.02 for the aluminum substrate “as received” to 1.07 after the cleaning process. Copolymer Deposits. Solutions of the copolymers (50 mg/mL in DMF or TFT) were solvent cast onto sheets of cleaned aluminum and left to evaporate overnight under a solventsaturated atmosphere. The following data correspond to deposits generated from the matured solutions (“-24h”). Sliding tests49 were performed manually on surfaces coated with the different copolymers deposited from DMF or TFT. We observe that the deposits obtained from TFT present better sliding behavior with respect to deposits prepared from DMF. This can be explained by the fact that DMF and TFT cannot fully solubilize the copolymers, which is expected to lead to the formation of micelles. DMF can only solubilize the PAN block of the copolymers, leading to the formation of micelles with a PAN corona and a fluorinated DOI: 10.1021/la902565y

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Figure 4. Morphology of CP4 deposits after washing. (A) 0.9  1.2 mm2 optical profilometry image. The Wenzel roughness is 1.10. (B) 30  18 μm2 AFM height image. (C,D) 500  500 nm2 AFM height and phase images.

core. In contrast, TFT is a good solvent for the P(FDA-co-AA) part of the copolymers, which is expected to lead to the formation of micelles with a fluorinated corona and a PAN core. AFM images in Figure 3 show that the deposits from TFT solutions are indeed made of the assembly of small, round-shaped objects, i.e., micelles, with a mean diameter of 17 nm for CP1 (Figure 3A,B) and 32 nm for CP4 (Figure 3C,D). As a result, the outer surface of the deposits prepared from TFT is expected to be rich in fluorinated groups, which is consistent with the best sliding behavior and the lower wettability. Despite the good sliding behavior, these deposits are not fully grafted on the surface, as reflected by a partial loss of material when performing wettability measurements. In order to remove the excess material from the surface, we washed the samples in TFT heated up to 60 C and gently agitated them for 10 min. This simple procedure resolubilizes all the material that is not strongly adhered to the substrate. The remaining copolymer deposit is most probably grafted on the surface via chemical bounds between carboxylic acid groups present in the micelle corona and hydroxyl groups on the aluminum surface. The formation of such bounds between polymer chains bearing pendant carboxylic groups and hydroxylated aluminum surfaces has been demonstrated on model systems by XPS and FTIR measurements.80 We believe that the same type of chemical process is taking place here (our deposits are still too thick to allow for proper XPS or FTIR characterization of the interface). The strong adhesion between the polymer deposit and the aluminum surface is further substantiated by the fact that the (80) Alexander, M. R.; Payan, S.; Duc, T. M. Surf. Interface Anal. 1998, 26, 961.

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Figure 5. One-dimensional power spectral densities of the CP4 surface, and fit to a Palasantzas function to obtain ÆP1(s)æ.

sliding behavior is preserved, even after mechanical scratching of the coated surfaces. Those stable coatings can then be submitted to wettability measurements and drop impact analysis (see next section). From those results, we focused this study on the two “best” copolymers in terms of lower wettability: copolymer1 (CP1) and copolymer4 (CP4) deposited from TFT and washed. The fact that the CP4 and CP1 copolymers give the best results, Langmuir 2010, 26(3), 2057–2067

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Figure 6. (A) Evolution of the rms roughness of the CP4 surface versus sampling length. (B) Water contact angle as a function of droplet size, predicted from the power spectrum of the sample surface, ÆP1(s)æ, and assuming 120 for the contact angle of water on the chemically identical flat surface. (C) Ratio between the developed and projected areas (Wenzel roughness) versus sampling size.

compared to CP2 and CP3, may be related to the composition: the molecular weight ratio between P(FDA-co-AA) and PAN is nearly equal to 1:2 for CP1 and CP4, while it is 1:3 and 1:1 for CP2 and CP3, respectively. This 1:2 value probably represents the best combination for micelle formation, grafting to the aluminum surface, and micelle aggregation. Figure 4A shows a 0.9  1.2 mm2 optical profilometer image of a CP4 deposit after washing. Large-scale features (e.g., scratches and pits) are visible, and the surface is very rough: the mean roughness (RA) is equal to 3 μm, the rms roughness (RQ) is 3.93 μm, while the difference between the lowest and the highest points on the image (RZ) is 36 μm. Very large structures are present on the surface. Figure 4B shows a 30  18 μm2 AFM height image of the same deposit, confirming the presence of a rough surface with microscale structures. Indeed, the rms roughness after the film washing process (450 nm for 20  20 μm2 images) remains higher than the roughness of the cleaned aluminum substrate (370 nm for 20  20 μm2 images). Moreover, the Wenzel roughness remains higher than 1.1. At the nanoscale (Figure 4C and D) the micellar morphology is maintained, and the size of the micelles is unchanged after the washing procedure (compare Figure 3C,D and Figure 4C,D). Spectral Analysis of the Surface Morphology. The bidimenR sional power spectral densities |~ z(sB)|2 = (1/Ao)| Aoz(rB) exp(2πjsB 3 B r )drB|2 of the images were computed as described previously,81 where z(rB) is the height of the surface at the location designated by B r and A0 is the average macroscopic area of the image. The bidimensional spectra were averaged circularly to R B(s,φ)|2dφ, thereafter called the oneobtain P1(s) = 2π 0 (1/2π)|z dimensional power spectral densities (Figure 5). A good agreement is found between the data from AFM and optical profilometry, within experimental accuracy. The individual P1(s)’s were combined together into the average power spectral density of the sample, ÆP1(s)æ. An ad hoc expression describing the roughness (81) Bollinne, C.; Cuenot, S.; Nysten, B.; Jonas, A. M. Eur. Phys. J. E 2003, 12, 289.

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spectrum of a self-affine fractal surface, ÆP1(s)æ = σ2ξ2(1 þ (2πsξ)2/2h)-(1þh), was fit to the data (Figure 5), where σ = 9.54 μm is the saturation value of the roughness when measured over surfaces of increasing areas, ξ = 31 μm is the in-plane correlation length, and h = 1 is the roughness exponent (or Hurst exponent) related to the local fractal dimension of the surface D = 3 - h.82 ξ and h control how far a point can move on the surface before losing memory of its initial height. In addition, h is related to the appearance of the surface, a small h being associated to very jagged surfaces, whereas h close to 1 is typical for surfaces displaying smooth hills and valleys.83,84 It is remarkable that a single self-affine law is sufficient to fit the roughness spectrum over about 5 orders of magnitude of reciprocal distances s, despite the fact that the roughness at small frequencies arises from the lamination patterns of the aluminum substrate, whereas high frequencies are dominated by the morphology resulting from the adsorption of the block copolymer micelles. The knowledge of ÆP1(s)æ allows us to obtain the rms roughness, Æw(R)æ, versus the size of the probed surface, R, by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z sc 2πsÆP 1 ðsÞædsæ ÆwðRÞæ ¼ Æ

ð1Þ

1=R

where 1/sc is close to a molecular dimension. Here, we have taken 1/sc = 1/4 nm-1, corresponding to the upper bound of our experimental s-range. The evolution of the rms roughness with sampling length is given in Figure 6A. The rms roughness progressively saturates above 100 μm to a value of a few micrometers, which is compatible with the presence of scratches and pits on the aluminum surface separated by distances of up to ∼100 μm. (82) Palasantzas, G. Phys. Rev. B 1993, 48, 14472. (83) Krim, J.; Indekeu, J. O. Phys. Rev. E 1993, 48, 1576. (84) Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. Phys. Rev. B 1988, 38, 2297.

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We now turn to the consideration of Wenzel’s theory, which relates the contact angle of a liquid on a rough surface, θr, to the value measured on a chemically equivalent flat surface, θf, by cos θr ¼

ÆAæ cos θf A0

ð2Þ

which assumes that the liquid effectively fills the cavities of the surface. It is possible to compute the ratio between the average real (developed) surface, ÆAæ, and the macroscopic (projected) surface, A0, from the knowledge of ÆP1(s)æ. Indeed,  Z  Z qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1þjrzðrBÞj2 drBæ ≈ 1þ ÆjrzðrBÞj2 æ drB ÆAæ ¼ Æ 2 A0 A0

ð3Þ

where r designates the gradient operator. Using Parseval’s theorem,85 Z

Z 2

ð2πsÞ2 Æj~zðsBÞj2 ædsB

ÆjrzðrBÞj ædrB ¼ A0 Z ¼ A0

ð2πsÞ3 ÆP 1 ðsÞæds

ð4Þ

From eqs 1 and 4, one finally obtains ÆAæ 1 ¼ 1þ A0 2

Z ð2πsÞ3 ÆP 1 ðsÞæds

ð5Þ

When using eq 5 to compute ÆAæ/Ao (the Wenzel roughness) in eq 2, the integration in eq 5 should be performed between 1/Rmax, where Rmax is the length scale associated to the area probed by the liquid droplet, and an upper bound sc, which was selected as before. The plot of ÆAæ/Ao versus the experimental length scale Rmax is presented in Figure 6C. The developed area is seen to be larger than the projected area by an amount that increases with Rmax, up to a saturation value close to 1.3. As a result, the contact angle θr is predicted to increase slightly as shown in Figure 6B, where we have assumed a contact angle θf = 120 for the flat surface, corresponding to water on perfluorinated materials. From the power spectral density of the surface measured over a wide length scale, the increase of contact angle for the surface can be predicted by Wenzel’s theory. The relatively moderate roughness results in a limited predicted increase of contact angle, well below the values usually associated with superhydrophobic surfaces. There is a couple of possible reasons for this. First, we note that, assuming that superhydrophobicity begins when θr is larger than 135, a value of ÆAæ/Ao of 1.41 is needed if θf = 120, and 1.35 if θf = 130. This last value is not very different from the experimental Wenzel roughness (which is an underestimation of the real value), showing that the surfaces are not that far from being able to switch to a Cassie state, based on this analysis. Second, the AFM tip does not sample the roughness at spatial frequencies larger that the reciprocal radius of the tip. Hence, the measured developed surface is lower than the real one; however, it is unlikely to be strongly different. A somewhat related argument is that the roughness may be re-entrant, which would not be detected by AFM. This is not expected for the large-scale roughness due to scratches in the aluminum; however, for the nanoscale roughness, which results from the adsorption of (85) Braslau, A.; Pershan, P. S.; Swislow, G.; Ocko, B. M.; Als-Nielsen, J. Phys. Rev. A 1988, 38, 2457.

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Figure 7. Time evolution of an impact on a CP4 surface. Water drop of 2.3 mm diameter impacted at 0.85 m/s. Rebound and then break-up of the drop is occurring.

Figure 8. Outcome of the drop impact events on CP4-f deposits.

spherical micelles, a cauliflower-type re-entrant roughness is much more likely. This could be the main factor contributing to increasing the developed surface: surfaces with a re-entrant surface curvature were shown recently to exhibit superior Langmuir 2010, 26(3), 2057–2067

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Figure 9. Advancing and receding static contact angles taken from sliding experiments for both CP4-f (top) and CP4-24 h (bottom) surfaces in both directions of sliding. Insets are distributions of the receding static contact angle all along the sliding experiments; this enables one to quantify the homogeneity of the surface in terms of superhydrophobicity.

superlipophobic properties.86 Detecting this morphology would require measuring the sample morphology with a high-resolution scanning electron microscope, which does not suffer from tipdilation effects. Nevertheless, Yoshimitsu et al. found drops in a Cassie-Baxter state with a Wenzel roughness of 1.2.33 Therefore the experimental observations of the present study suggest that the Cassie-Baxter model and its extension87 should be applied. But, since we are unable to measure the part of the liquid in contact with the surface and the one suspended (as for any nonperiodic type of roughness), any quantitative comparison is impossible. Wettability Study. Accurate wettability measurements can be used to determine whether the deposited drop is in the Cassie-Baxter or the Wenzel state. In particular, rebound tests allow an easy differenciation of the two states.88 Thus, impact as well as wettability measurements by the sliding method on the CP1 and CP4 deposits on aluminum were performed. From previous results,49,88-91 it is known that the outcome of a drop impact on hydrophobic and superhydrophobic surfaces is a function of the drop size and velocity. Drop impact on hydrophobic surfaces may result in a rebound, a receding break-up, a partial rebound, or a deposition, depending on the drop impact parameters and the (86) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618. (87) De Coninck, J.; Miracle-Sole, S.; Ruiz J. Phys. Rev. E 2002, 65, 36139. (88) Rioboo, R.; Voue, M.; Vaillant, A.; De Coninck, J. Langmuir 2008, 24, 24074. (89) Rioboo, R.; Marengo, M.; Tropea, C. Atomization Sprays 2001, 11, 155. (90) Rioboo, R. Impact de gouttes sur surfaces solides et seches. Ph.D. Thesis, Universite Pierre et Marie Curie (Paris VI), France, Paris, February 2001. (91) Reyssat, M.; Pepin, A.; Marty, F.; Chen, Y.; Quere, D. Europhys. Lett. 2006, 74, 306.

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surface characteristics.89 It is thus of crucial importance to test the reboundness at various impact speeds and drop sizes. In this study, two outcomes will be tested: rebound or deposition, where, for the first case, all the liquid of the drop just bounces off the solid surface, while in the second case it stays attached to the solid surface. In some cases, even with a complete rebound, the impact pressure is strong enough to break the drop during the impact process. In particular, a satellite droplet often emerges from big drop impacts. Two different mechanisms can explain this ejection: the central jet break-up89 due to strong receding motion, and pinch-off due to capillary waves and/or the high spreading of liquid on solid.92,93 Figure 7 shows a typical time sequence of an impact and rebound on a superhydrophobic surface consisting of a CP4 coating on aluminum surface. Similar results are found on washed CP1 and CP4 surfaces, prepared from either the “-f” or the “-24h” solutions. In such a case, the receding motion is strong enough to provoke rebound but also subsequent break-up of the drop. Figure 8 presents the outcome of the impact (rebound or deposition) on CP4 surfaces as a function of the impact velocity (between 0.006 and 0.85 m/s) and drop diameter (between 0.22 and 4.62 mm). As the drop impacts, impact pressure is transformed in deformation of the liquid surface, and a liquid lamella is formed and spreads radially mainly governed by inertia.94 After reaching a maximum diameter, the liquid recedes,95 so that the (92) (93) 5068. (94) (95) 1411.

Thoroddsen, S. T.; Takehara, K. Phys. Fluids 2000, 12, 1265. Rioboo, R.; Ad~ao, M. H.; Voue, M.; De Coninck, J. J. Mater. Sci. 2006, 41, Rioboo, R.; Marengo, M.; Tropea, C. Exp. Fluids 2002, 33, 112. Roisman, I. V.; Rioboo, R.; Tropea, C. Proc. R. Soc. London A 2002, 2022,

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Table 4. Percentage of Superhydrophobic Zones and Peak Values of the Distribution of the Receding Static Contact Angles during Sliding Events along Two Directions of Slidinga surfaces washed CP1

washed CP4

percentage superhydrophobicity 61.3 45.1 126 129 peak value of θrec ((3) rebound upon drop impact difficult on some location a Qualitative information on reboundness is also provided.

contact line is passing from an advancing motion to a receding motion. The energy lost during this change of motion is minimized by the so-called “hysteresis energy”.88 The strength of the receding motion is a function of the receding static contact angle, the liquid viscosity, and the dimensionless maximum spreading diameter.94,95 If the energy remaining in the liquid and the receding static contact angle are high enough, rebound may occur.88 It is possible to correlate the impacting parameters to the hysteresis to predict rebound or not; the correlation relates the hysteresis and the Weber number, We (defined as We = FDV2/σ, where F and σ are the liquid density and surface tension, respectively), which corresponds to the ratio between the inertia forces and the surface forces.88 The range of Weber number used is between 0.001 and 42. Figure 8 shows that, at similar impact speed and drop diameter, both outcomes are possible (rebound and deposition). This is due to the fact that the surface is not homogeneous in terms of roughness, which results in different local wettability characteristics, particularly in terms of receding contact angle, which is the key parameter for the receding motion and thus reboundness.76,90 The lowest critical Weber number88 to get rebound is a function of the wettability characteristics of the impacting zone. Thus, if locally the receding static contact angle is too low, rebound will not occur. Both types of CP4 deposits (“-f” and “-24h”) present fewer zones or areas, which result in a high critical Weber number (in other words, poorer superhydrophobic behavior), compared to CP1 deposits. When the solubilization time in TFT is longer (24 h versus 30 min), the deposit show slightly better results in terms of the critical Weber number. It appears that all surfaces have a nonhomogeneous roughness as the aluminum substrates present lamination patterns that are even visible at the macroscopic scale (Figure 4A). These multiscale roughness surfaces thus present irregular properties of wettability and, in particular, of superhydrophobicity. Results from advancing and receding static contact angles measurements made from sliding experiments show that, on some points of the surface, the superhydrophobicity is lost as the contact line is pinned, and the drop detaches from the needle to stay on the moving surface instead of sliding over it. We consider that a surface presenting pinning of a water drop cannot be qualified as “superhydrophobic”. We choose to consider areas that are superhydrophobic and areas that are not as a function of the sliding possibility or lack thereof. We remarked that the areas that present pinning resulted in contact angles below 135. Such zones were considered as “non-superhydrophobic”, while the zones that present a good sliding behavior and an angle above 135 were considered “superhydrophobic”. Figure 9 presents static contact angles and the distribution of the receding contact angles for CP4 deposits. Results are provided in both directions of sliding (parallel and perpendicular to the main grooves of the surfaces). While both surfaces present the same peak value in the distribution of the receding static contact angle, it appears clearly that 2066 DOI: 10.1021/la902565y

^

84.7 0.36 144 132 difficult on some location

24 h ^

98.5 87.4 156 156 very easy

)

^

fresh )

24 h )

sliding direction versus lamination patterns

)

fresh

^

100 100 156 156 always very easy

CP4-24 h is more superhydrophobic than CP4-f since 100% of its surface is superhydrophobic in both directions, while, for the CP4-f, the value is between 87% and 98% (Table 4). This result indicates that measuring the receding static contact angle only on one location of the surface is not reliable unless it is proved that the surface is homogeneous. Moreover, for a non-homogeneous surface, taking the peak value is not enough, and a distribution is necessary to estimate the possibility of pinning of the liquid. It must be noted that measuring the critical Weber number for CP424 h deposits is very difficult, as even at very low impact speeds the drop is still bouncing. In such a case, it is almost impossible to detect the transition between deposition and rebound. Table 4 presents the results for the CP1 and CP4 deposits for both directions: parallel and perpendicular to the lamination patterns. Drop impact event results roughly follow the quantitative results of the sliding: the CP4 deposits are better than the CP1 deposits, indicating that the chain length of the fluorinated block is of importance. Similarly, it seems that longer solubilization time allows the formation of better deposits, probably as a result of a more complete micelle formation process. Finally, sliding against the natural grooves of the surface is more difficult than in the perpendicular direction. This can be explained by the relatively high depth of these grooves.

Conclusions Novel fluorinated P(FDA-co-AA)-b-PAN diblock copolymers, synthesized by RAFT in scCO2, have been successfully used for imparting superhydrophobicity to aluminum surfaces. For that purpose, copolymer micelles made of PAN core and a fluorinated corona were prepared in TFT and solvent casted onto the surface. The deposition process results in a multiscale structuration of the surface, with a very rough morphology of the aluminum at large scale, combined with a micellar (∼32 nm diameter for CP4 deposits) structure of the copolymer deposits at the nanoscale. The AA functions of the copolymer most probably favor the anchoring of the micelles to the aluminum surface, and consequently improve the adhesion of the coating after washing with a solvent for the copolymer. The advantage of these copolymers is that the films can be annealed up to 150 C, and, as the PAN, which constitute the core of the micelles, is infusible, the three-dimensional (3D) structure is not modified, and so the roughness and the superhydrophobic properties are preserved. The surface Wenzel roughness directly measured on AFM and profilometry images is very low for superhydrophobic surfaces, as its value is close to 1.1. From the spectral analysis of images, the predicted value of the Wenzel roughness saturates at 1.3, still below usual values for superhydrophobic surfaces. It thus seems that we underestimate the real developed surface by missing features smaller than the AFM tip radius for example. Those results demonstrate the importance of the chemical nature of the coating, as the cleaned aluminum and the washed deposits present Langmuir 2010, 26(3), 2057–2067

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the same value of the Wenzel roughness (derived from AFM images), and the cleaned aluminum is not superhydrophobic while the surface with the fluorinated coating is. Superhydrophobicity has been tested via the sliding method and water drop impacts. The sliding method proved to be more reliable to get a quantitative estimation of the superhydrophobic zones over a given area than the drop impact method. Considering the large range of drop sizes and impact speeds that result in rebound, it can nevertheless be said that the surfaces coated by these fluorinated copolymers present superhydrophobic characteristics.

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Acknowledgment. This work has been supported by Region Wallonne (CORRONET Project), the Science Policy Office of the Belgian Federal Government (PAI 6/27) and FNRS-FRFC. C.D. is “Senior Research Associate” from the National Fund for Scientific Research (FRS-FNRS), Belgium. Supporting Information Available: Scheme describing the synthesis of the copolymers and procedure for the estimation of the molecular weight from NMR measurements. This material is available free of charge via the Internet at http:// pubs.acs.org.

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