Fabrication and Superhydrophobic Behavior of Fluorinated

Oct 4, 2010 - (3) Mittal, K. L. Polymer Surface Modification: Relevance to Adhesion; VSP BV: The Netherlands,1996. (4) Denes, F. S.; Manolache, S. Pro...
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Fabrication and Superhydrophobic Behavior of Fluorinated Microspheres A. Mu~noz-Bonilla,§ A. Bousquet,† E. Ibarboure,† E. Papon,† C. Labrugere,‡ and J. Rodrı´ guez-Hernandez*,†,§ †

Laboratoire de Chimie des Polym eres Organiques (LCPO), CNRS, Universit e Bordeaux I, ENSCPB. 16, Avenue Pey Berland, 33607 Pessac-Cedex, France, ‡Institut de Chimie de la Mati ere Condens ee de Bordeaux (ICMCB-CNRS), 87, Av. Albert Schweitzer, 33600 Pessac, France, and §Instituto de Ciencia y Tecnologı´a de Polı´meros (ICTP-CSIC), C/Juan de la Cierva n°3, 28006 Madrid, Spain Received July 7, 2010. Revised Manuscript Received August 20, 2010 We describe the preparation of fluorinated microspheres by precipitation polymerization and their use to fabricate superhydrophobic surfaces. For that purpose, two different approaches have been employed. In the first approach, a fluorinated monomer (either 4-fluorostyrene or 2,3,4,5,6-pentafluorostyrene) was added to the initial mixture of monomers constituted by styrene (S) and divinylbenzene (DVB). The second approach is based on the encapsulation of a block copolymer, polystyrene-b-poly(2,3,4,5,6-pentafluorostyrene), during the polymerization of the monomers (S and DVB), thus enabling the formation of particles with perfluorinated chains instead of single functional groups at the interface. Both approaches led to narrow polydisperse particles with fluoro-functional groups at the interface as demonstrated by scanning electron microscopy (SEM), infrared (IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Surface array of particles obtained by simple solvent casting presented superhydrophobic behavior with contact angles of water droplets of ca. 160-165°.

Introduction Both surface chemical composition and interfacial micro- and nanostructure play a key role on the final surface properties. As a consequence, different approaches have been reported to control either the surface chemistry (polar, nonpolar groups)1,2 or the structure (nano- or micrometer scale).3 The design of polymer surfaces with either suitable chemical composition or surface topography has been extensively studied. As a result, a variety of methodologies are available to modify the surface chemical composition including chemical and physical treatments such as plasma,4 surface grafting,5 metal coating,6 or surface segregation.7,8 Equally, surface texture has been controlled by using lithographic techniques,9 imprinting methods,10 self-assembly to reach micro- and nanopatterned surfaces,11 or “breath figures” formation.12 Whereas the modification of either chemical composition or surface microstructure has a limited influence on the surface behavior,13 the combination of both aspects can significantly *To whom correspondence should be addressed. Fax: (34) 91 564 48 53. Telephone: (34) 91 258 76 23. E-mail: [email protected]. (1) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1, 725–740. (2) Yarbrough, J. C.; Rolland, J. P.; DeSimone, J. M.; Callow, M. E.; Finlay, J. A.; Callow, J. A. Macromolecules 2006, 39, 2521–2528. (3) Mittal, K. L. Polymer Surface Modification: Relevance to Adhesion; VSP BV: The Netherlands,1996. (4) Denes, F. S.; Manolache, S. Prog. Polym. Sci. 2004, 29, 815–885. (5) Jordan, R., Ed. Advances in Polymer Science; Springer: Berlin, Heidelberg, New York, 2006; Vol. 197 and 198. (6) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Nature 1998, 393, 146–149. (7) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2942–2956. (8) Senshu, K.; Yamashita, S.; Mori, H.; Ito, M.; Hirao, A.; Nakahama, S. Langmuir 1999, 15, 1754–1762. (9) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823–1848. (10) Dusseiller, M. R.; Schlaepfer, D.; Koch, M.; Kroschewski, R.; Textor, M. Biomaterials 2005, 26, 5917–5925. (11) Matsen, M. W. Curr. Opin. Colloid Interface Sci. 1998, 3(1), 40–47. (12) Stenzel, M. H. Aust. J. Chem. 2002, 55, 239–243. (13) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652.

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improve the surface properties of a material in a wide variety of applications including wettability, biocompatibility, or adhesion. In particular, the preparation of superhydrophobic surfaces, that is, surfaces with high water contact angle (above 150°) and small contact angle hysteresis have aroused much interest because of their relevance in self-cleaning, antifogging, antifouling, or water repellency.14-16 The preparation of superhydrophobic surfaces requires chemical modification that lowers the surface energy and control of the topography of the hydrophobic surface, for instance, by increasing the surface roughness.17-26 Examples reported in which the roughness has been artificially modified include the fabrication of hierarchical structures using different approaches such as electrodeposition, nanowire arrays, and colloidal systems.27,28 Structured surfaces formed of spherical particles have attracted great attention due to their potential in applications not only in the preparation of superhydrophobic surfaces, but also as photonic crystals and gas sensing materials (14) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377– 1380. (15) Blossey, R. Nat. Mater. 2003, 2, 457–460. (16) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800–802. (17) Coulson, S. R.; Woodward, I.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. J. Phys. Chem. B 2000, 104, 8836–8840. € (18) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395–3399. (19) Veeramasuneni, S.; Drelich, J.; Miller, J. D.; Yamauchi, G. Prog. Org. Coat. 1997, 31, 265–270. (20) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321–4323. (21) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125–2127. (22) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. B 1996, 100, 19512–19517. (23) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 743–744. (24) Richard, D.; Quere, D. Europhys. Lett. 1999, 48, 286–291. (25) Hozumi, A.; Takai, O. Thin Solid Films 1998, 334, 54–59. (26) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (27) Chong, M. A. S.; Zheng, B.; Gao, H.; Tan, L. K. Appl. Phys. Lett. 2006, 89, 233104. (28) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929–1932.

Published on Web 10/04/2010

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Figure 1. Approaches employed for the synthesis of particles containing fluoro moieties at the interface: (1) using either 4-fluorostyrene (A) or 2,3,4,5,6-pentafluorostyrene (B) as comonomer; (2) by encapsulation of a copolymer polystyrene-b-poly(2,3,4,5,6-pentafuorostyrene). Table 1. Characteristics of the Particles Obtained by Precipitation Copolymerization of S, DVB and FS, 5FS or the Block Copolymer PS-b-P5FSa sample

monomers involved

relative amount of monomers (wt %)

targeted % of fluorinated styrene vs styrene

1 DVB/S 45/55 2 DVB/S/FS 45/49.5/5.5 10 3 DVB/S/FS 45/44/11 20 4 DVB/S/FS 45/27.5/27.5 50 5 DVB/FS 45/55 100 6 DVB/S/5FS 45/49.5/5.5 10 7 DVB/S/5FS 45/44/11 20 8 DVB/S/5FS 45/27.5/27.5 50 9 DVB/5FS 45/55 100 10 DVB/S 45/55 11 DVB/S 45/55 12 DVB/S 45/55 a ACN, acetonitrile; Tol, toluene; FS, 4-fluorostyrene; 5FS: 2,3,4,5,6-pentafluorostyrene.

among others.29 Previous works on structured fluorinated interfaces prepared from ordered arrays of particles required the postmodification of the surface chemistry independently of the strategy employed to obtain the close packed lattice: nanosphere lithography30-32 or layer by layer deposition.33-35 For instance, Hsieh et al. fabricated superhydrophobic silica nanosphere arrays by using an assembly technique to prepare well-ordered arrangements and then them covered with a fluoro-containing mixture of perfluoroalkyl methacrylic copolymer.36 An alternative to a postmodification consists of the deposition of hydrophobic (29) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028–1032. (30) gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894–897. (31) Zhang, J.; Xue, L.; Han, Y. Langmuir 2005, 21, 5–8. (32) Shiu, J. Y.; Kuo, C. W.; Mou, C. Y. Chem. Mater. 2004, 16, 561–564. (33) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J Phys.Chem B 2005, 109 (44), 20773–20778. (34) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349–1353. (35) Han, J. T.; Jang, Y.; Lee, D. Y.; Park, J. H.; Song, S. H.; Ban, D. Y. J. Mater. Chem. 2005, 15, 3089–3092. (36) Hsieh, C. T.; Chen, W.-Y.; Wu, F.-L.; Shen, Y.-S. In Superhydrophobic Surfaces; Carre, A., Mittal, K. L., Eds.; VSP: Leiden, Boston, 2009; pp 285-295.

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block copolymer added (wt %)

reaction solvent

P5FS21-b-PS31 (5%) P5FS21-b-PS31 (10%) P5FS21-b-PS31 (20%)

ACN/Tol (95/5) ACN ACN ACN ACN ACN ACN ACN ACN ACN/Tol (95/5) ACN/Tol (95/5) ACN/Tol (95/5)

materials to produce rough textures. Although this approach has been extensively employed by using sol-gel processes,37 by growing 3D networks of fibers created by electrospinning,38-41 or with plasma polymerization,25-42 to the best of our knowledge, only one example has been recently reported in which the authors describe the use of fluorinated polyphosphazene based microspheres. In this case, the structure of the microspheres has an invariable composition dictated by the stoichiometry 1:1 of the polycondensation reaction employed.43 In this Article, we prepared fluorinated microspheres with two main features: First, the preparation of the particles was carried (37) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626–5631. (38) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338–4341. (39) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 43, 5210–5213. (40) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrickh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549–5554. (41) Singh, A.; Steely, L.; Allcock, H. R. Langmuir 2005, 21, 11604–11607. (42) Matsumoto, Y.; Ishida, M. Sens. Actuators, A 2000, 83, 179–185. (43) Wei, W.; Huang, X.; Zhang, P.; Tang, X. Chem. Commun. 2010, 46, 487– 489.

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Article cryodistilled. Azobisisobutyronitrile (AIBN, 98%) as the initiator was recrystallized from methanol before use. All other solvents were used as received unless otherwise specified. Characterization. 1H NMR spectra of the diblock copolymers were recorded at room temperature on a Bruker Avance 400 MHz spectrometer using the residual proton resonance of the deuterated solvent as internal standard. Average molar masses and molar mass distributions were determined by size exclusion chromatography (SEC) using a Varian 9001 pump with both a refractive index detector (Varian RI-4) and a UV detector (Spectrum Studies UV 150). Calibration was obtained using narrowly distributed PS standards and tetrahydrofuran (THF) as the mobile phase at a flow rate of 0.5 mL min-1.

Synthesis. a. Synthesis of the Diblock Copolymer: P5FSb-PS. a.1. Synthesis of Poly(2,3,4,5,6-pentafluorostyrene) (P5FS). 2,3,4,5,6-Pentafluorostyrene (5FS) was polymerized in bulk using 1-bromoethylbenzene as initiator and CuBr/PMDETA as catalyst with the following stoichiometry [monomer]/ [initiator]/[ligand]/[CuBr] = 50:1:1:1. The Schlenk tube was charged with 3 mL (22.1 mmol) of S5F, 0.09 mL (0.45 mmol) of PMDETA, 0.0630 g (0.45 mmol) of CuBr, and 0.06 mL (0.45 mmol) of initiator. The reaction mixture was degassed during three freeze-thaw cycles and then immersed into a thermostatically controlled oil bath at 90 °C under stirring. After 60 min, the reaction mixture was diluted in THF and passed over neutral alumina column to remove the catalyst. The solution was concentrated under reduced pressure; the mixture, poured into ethanol, was precipitated and dried under vacuum (MnNMR = 4100 g/mol, Mw/MnSEC = 1.07).

a.2. Synthesis of Poly(2,3,4,5,6-pentafluorostyrene)-bpolystyrene (P5FS-b-PS) Diblock Copolymer. The diblock copolymer was prepared via ATRP of styrene from P5FS-Br used as macroinitiator. A total of 1.1 g of P5FS-Br (MnNMR = 4100 g/mol, 0.26 mmol) was placed in a Shlenk tube and dissolved in 1 mL of acetone and 3 mL (26.2 mmol) of styrene. Then 0.05 mL (0.26 mmol) of PMDETA and 0.038 g (0.26 mmol) of CuBr were added. The mixture was degassed three times by freezing and thawing and subsequently heated at 90 °C. After 240 min, the polymerization was stopped and purified over alumina column. Finally, the block copolymer was obtained by precipitation into ethanol and dried under vacuum (MnNMR = 7300 g/mol, Mw/ MnSEC=1.09). b. Preparation of the Particles. The preparation of the particles was carried out by precipitation polymerization based on the following procedures (targeted cross-linking density of 40%):

b.1. Particles Prepared by Copolymerization with FS (the Procedure to Obtain Particles by Copolymerization with 5FS is Identical), Ratio S/SF of (90:10). The ratio (S þ SF) to DVB Figure 2. (i-iv) SEM images of the microspheres obtained by precipitation copolymerization of S, DVB, and FS while varying the amount of S and FS: (i) sample 2, (ii) sample 3, (iii) sample 4, and (iv) sample 5. (v) Illustrative histogram of sample 5. The calculated polydispersity for the particles varied between 1.07 and 1.1.

out by polymerization precipitation which is an easy and straightforward technique to obtain narrow particle size distribution and in the absence of any stabilizing agent. Second, the particles include fluorinated moieties within the structure introduced by copolymerization of different fluoro-containing monomers. This method simplifies previous approaches, and, as will be explained, the amount of fluorinated monomers included can be controlled as well.

Methods Chemicals. Styrene (S) and divinylbenzene (DVB) were purchased from Sigma-Aldrich. Styrene, 4-fluorostyrene (FS), and 2,3,4,5,6-pentafluorostyrene (5FS) were dried under CaH2 and Langmuir 2010, 26(22), 16775–16781

was maintained constant during all the experiments (55%/45%). Thus, for this particular experiment, 0.79 mL of DVB, 0.71 mL of S, 0.082 mL of SF, and 0.72 mg of AIBN were added to 30 mL of acetonitrile. Upon addition of these quantities, the solution was transparent. The mixture was heated up to 85 °C in order to initiate the polymerization and was maintained for 4 h. Then, the particles were filtered and extensively washed with acetonitrile and THF.

b.2. Particles Prepared by Encapsulation of P5FS-b-PS Diblock Copolymer during Precipitation. Amounts of 0.79 mL of S, 0.79 mL of DVB, and 72 mg of AIBN were dissolved in 30 mL of an acetonitrile/toluene (v/v 95%/5%) mixture. In a separate step, the diblock copolymer, P5FS-b-PS (5, 10, and 20% weight related to the total amount of S/DVB) was dissolved in the same solvent mixture and added to the previous solution. The polymerizations were carried out at a constant temperature of 85 °C during 4 h. After the polymerization was accomplished, the soluble polymer was separated from the insoluble fraction by vacuum filtration. The insoluble microspheres were extensively washed with THF. DOI: 10.1021/la102686y

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Figure 3. FTIR spectra of the microspheres prepared by precipitation polymerization: F1 (reference), F28, F27, F20, and F21.

Scanning Electron Microscopy (SEM). The morphological characterization of the functionalized microspheres was carried out with a scanning electron microscope (JEOL JSM-5200 scanning microscope). The particles were dropped onto a sample holder, placed under vacuum at room temperature, and goldcoated prior to examination. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded with a 220i-XL ESCALAB instrument from VG. The particles, supported on indium, were put under ultrahigh vacuum (UHV) to reach the 10-8 Pa range. The nonmonochromatized Mg X-ray source was used at 100 W, as well as a flood gun to compensate for the nonconductive samples. The spectra were calibrated in relation to the C1s binding energy (284.6 eV), which was applied as an internal standard. Fitting of the high-resolution spectra was provided through the AVANTAGE program from VG. Fourier Transform Infrared (FTIR) Spectroscopy (Transmission Mode). Spectra were taken in KBr pellets containing dispersed particles with concentration 1:100 at room temperature. The IR spectra were recorded at 20 ( 1 °C in the spectral range of 650-4000 cm-1 using a Perkin-Elmer Spectrum One spectrometer. Contact Angle Measurements. Water contact angles were measured using a KSV Theta goniometer. The volume of the droplets was controlled to be about 3 μL.

Results The preparation of hydrophobic polystyrene microspheres with fluoro-functional groups was achieved by two different strategies depicted in Figure 1. The first strategy involves the copolymerization of styrene and divinylbenzene with a third monomer, either 4-fluorostyrene (FS) or 2,3,4,5,6-pentafluorostyrene (5FS). 16778 DOI: 10.1021/la102686y

The targeted cross-linking density was 40%, and thus, the weight percent of divinylbenzene and styrene in the feed was 55:45. In the second approach, recently developed by our group,44,45 a block copolymer is introduced within the initial mixture of monomers (S/DVB). In order to favor the encapsulation, the block copolymer employed, PS-b-P5FS, contains a polystyrene block similar in nature to the monomers employed for the precipitation polymerization, that is, styrene and divinylbenzene. During the copolymerization step, the diblock copolymer is encapsulated within the particle structure most probably due to the affinity of the polystyrene block with the styrene and divinylbenzene units that constitute the particle. The second block, in this case, poly(2,3,4,5,6-pentafluorostyrene), directs, upon interfacial reorientation, the fluoro-functional groups to the interface. Whereas the polymerization parameters temperature (85 °C) and time (4 h) were maintained constant during all the polymerizations, the relative amount of mono- and pentafluorostyrene to styrene was varied: 0, 10, 20, 50, and 100%. The characteristics of the particles prepared by this approach are summarized in Table 1. Copolymerization of Styrene/Divinylbenzene (S/DVB) and Either Fluorostyrene (FS) or Pentafluorostyrene (5FS). The first approach to obtain fluorinated microspheres consisted of the copolymerization of divinylbenzene with styrene and a third comonomer, either FS or 5FS. Whereas the amount of (44) Bousquet, A.; Perrier-Cornet, R.; Ibarboure, E.; Papon, E.; Labrugere, C.; Heroguez, V.; Rodrı´ guez-Hernandez, J. Macromolecules 2007, 40, 9549–9554. (45) Bousquet, A.; Perrier-Cornet, R.; Ibarboure, E.; Papon, E.; Labrugere, C.; Heroguez, V.; Rodrı´ guez-Hernandez, J. Biomacromolecules 2008, 9, 1811–1817.

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Figure 4. FTIR spectra of the microspheres prepared by precipitation polymerization: F1 (reference), F28, F27, F20, and F21.

Figure 5. Comparison of the relative intensity of the bands C(Ar)-F to C(Ar)-H as a function of the amount of fluorinated monomer introduced in the feed: (a) series obtained with FS as comonomer and (b) series obtained using 5FS as comonomer.

divinylbenzene was maintained constant for all the experiments (55%), the amount of S (45 wt % initially) was successively substituted by either FS or 5FS with the objective to modify the amount of fluorinated functional groups within the particles. Therefore, the amount of FS and 5FS was increased from 5 to 45 wt % in the initial mixture, and thus, the amount of styrene accordingly reduced from 45 to 0%. The shape and average diameter of the particles have been analyzed in all cases by SEM. Figure 2 shows the images of monodisperse particles obtained from the comonomer FS. Independently of the comonomer employed FS or 5FS, the particles prepared from initial mixtures with up to 27.5% of either FS or 5FS are narrowly polydisperse. A further increase of the amount Langmuir 2010, 26(22), 16775–16781

Figure 6. SEM images of the particles obtained by copolymerization of S and DVB with diblock copolymer PS-b-P5FS in the initial feed: 5% (i), 10% (ii), and 20% (iii).

of the fluorinated monomer results in a slight increase of the polydispersity. Nevertheless, in all cases, particles with average sizes between 2 and 3 μm were obtained. The particles were further characterized by FTIR to obtain additional information on the chemical composition. Figures 3 and 4 depict the FTIR spectra for the particles prepared using FS and 5FS, respectively. In both cases, the signals corresponding to the C-F bands from FS or 5FS (for instance, at ∼1500 cm-1) increase rather linearly with the composition. On the opposite, those related to styrene (i.e., C-H at 701 cm-1) tend to decrease as a consequence of the feed composition employed. Moreover, DOI: 10.1021/la102686y

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Figure 7. Illustrative FTIR spectra of the reference particle prepared by copolymerization of S/DVB (F1) and the particle obtained by encapsulation of the diblock copolymer PS21-b-P5FS31 (the initial feed contains 20% block copolymer).

the C-F bands from the particles containing 5FS are more intense than the same bands from particles containing FS for an equal percentage of monomer in the feed composition (Figure 5). Thus, by using this approach, monodisperse particles with a controlled/ targeted amount of fluorinated moieties (controlled by the type of monomer and its relative amount in the initial feed) can be prepared by either modifying the amount of fluor-functional groups within the monomer or varying the monomer composition in the feed. Copolymerization of Styrene/Divinylbenzene (S/DVB) and Encapsulation of Polystyrene-b-pentafluorostyrene (PS-b-P5FS). As an alternative to the previously described approach, we attempted to obtain fluorinated microspheres by encapsulation of a diblock copolymer, that is, PS-b-P5FS during the precipitation polymerization step. This strategy has been already employed by our group to obtain particles with other functionalities including amine or carboxylic groups.44,45 Encapsulation of the diblock copolymer would lead, upon annealing, to particles with peripheral fluorinated moieties. In this case, instead of a single functional group, the particle surface is covered by a perfluorinated segment. Since the diblock copolymer has been prepared by controlled polymerization techniques, the block chain lengths and thus the composition of the block copolymer can be easily modified. In addition, annealing of the particles can reversibly lead to significant changes on the surface chemical composition. The preparation of S/DVB particles and encapsulation of the diblock copolymers produces narrow particle size distributions as well. Figure 6 shows the SEM micrographs of the particles obtained using different amounts of diblock copolymer in the feed. In spite of the variable amount of block copolymer, by using this approach, particles with sizes ranging from 2.5 to 4 μm were obtained with relatively narrow distribution. Nevertheless, the encapsulation of the block copolymer is directly related with the amount introduced in the feed. In Figure 7 are depicted the FTIR spectra of the particles using a 5%, 10%, and 20% block copolymer. A spectrum of particles prepared by copolymerization of S and DVB without any additional block copolymer has been added as reference. As observed in the inset of Figure 7, the characteristic peak of C(Ar)-F that can be seen at 1510 cm-1 due to the presence of diblock copolymer increases with the amount of diblock copolymer introduced in the feed. Thus, the amount of 16780 DOI: 10.1021/la102686y

Figure 8. XPS survey spectrum for samples 6, 8, and 9. The increasing amount of 5FS in the initial feed leads to a surface with a surface composition varying between 4.8% for sample 6 and 27% in 5FS in sample 9.

diblock copolymer encapsulated within the particles is directly related to the composition of the initial mixture. Since the final properties of a material rely not only on their bulk properties but also on the surface, the chemical composition of the surface was analyzed as well. Whereas the FTIR analysis gives information of the composition of the particles combining surface and bulk, XPS analysis of the particles provides further information exclusively on the surface composition. In Figure 8 Langmuir 2010, 26(22), 16775–16781

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films measured varied from 150 ( 2° for sample 1 to 165 ( 2.1° for sample 5. Sample 2 exhibits a contact angle of 150 ( 1.8°, sample 3 of 154 ( 1.6°, and sample 4 of 163 ( 1.5°. A similar tendency was found for particles prepared with pentafluorostyrene (samples 6-9), but no significant differences between both series were observed. Moreover, in the case of samples 5 and 9, independently of the roughness of the films, similar contact angles were obtained for all cases above 163°-165°. These values are above those found recently by Han et al.46 They fabricated surfaces with polystyrene microspheres and obtained contact angle values of around 148°. In addition, since the contact angle for a flat surface47 containing F-moieties has been measured between 100 and 120°, this clearly indicates the influence of both chemistry and roughness on the wettability of the films.

Conclusions Figure 9. Top: SEM images of the films ordered (a) and disordered (b). Bottom: (c,d) Illustrative images of the water droplet on the superhydrophobic surfaces.

are depicted the XPS spectra obtained for samples 6, 8, and 9. As expected, an increasing amount of fluorinated monomer in the polymerization feed leads to particles with an increasing amount of F not only in bulk (as observed by using FTIR) but also at the surface. Therefore, precipitation polymerization provides an appropriate way to modulate and finely tune the interfacial properties in contrast to other methodologies including postmodification approaches in which the amount of peripheral functional groups can be hardly controlled. Superhydrophobic Behavior of Microsphere Arrays. Arrays of microspheres on surfaces have already been studied to improve the hydrophobicity of surfaces.29-35 In those cases, the hydrophilic particles were disposed in an ordered array on surfaces and covered with hydrophobic polymers. Herein, we prepared films in which the hydrophobic microspheres are disposed in a fairly ordered monolayer (Figure 9a) fashion and a rather disordered multilayer, thus with an enhanced roughness prepared by solvent casting (Figure 9b). The contact angle of the (46) Xue, L.; Li, J.; Fu, J.; Han, Y. Colloids Surf., A 2009, 338, 15–19. (47) Bong, B. S.; Han, J. H.; Kim, S. T.; Cho, Y. J.; Park, M. S.; Dolukhhanyan, T.; Sung, C. Thin Solid Films 1999, 351, 274–278.

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We prepared fluorinated microspheres by using two different approaches: The first approach consists of the copolymerization of styrene and divinylbenzene with a third fluorinated monomer (either fluoro- or pentafluorostyrene). The second strategy is based on the encapsulation of a diblock copolymer within the particle during the precipitation polymerization step. Both strategies lead to rather monodisperse particles with sizes ranging from 2 to 3 μm. The strategies employed offer important advantages over previous methodologies: the amount of fluorinated monomer units within the particle can be controlled by the initial mixture of the feed. The particles are hydrophobic and, when disposed on a surface, exhibit supehydrophobicity by combination of the chemical composition of the particles and the roughness provoked by the spherical shape of the particles. We observed that size distribution and the fashion in which the particles are distributed at the surface did not produce significant changes in the contact angle. Hence, the functionality and the shape of the particles appear to be the major parameters to obtain superhydrophobic surfaces. Supporting Information Available: 1H NMR spectra of poly(2,3,4,5,6-pentafluorostyrene) and poly(2,3,4,5,6-pentafluorostyrene)-b-polystyrene; GPC traces of the PS-b-P5F5 block copolymer. This material is available free of charge via the Internet at http://pubs.acs.org.

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