Fabrication of superhydrophobic-PDMS-surfaces-by-acidic-treatment-and-perfluorinated-monolayers-Lang

Fabrication of superhydrophobic-PDMS-surfaces-by-acidic-treatment-and-perfluorinated-monolayers-Lang

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Unformatted text preview: pubs.acs.org/Langmuir © 2009 American Chemical Society Fabrication of Superhydrophobic PDMS Surfaces by Combining Acidic Treatment and Perfluorinated Monolayers Elisabeth Patricia Taffin de Givenchy, Sonia Amigoni, Cdric Martin, Guillaume Andrada, e Laurent Caillier, Serge Gribaldi, and Frdric Guittard* e ee    Universite de Nice-Sophia Antipolis, Laboratoire de Chimie des Materiaux Organiques et Metalliques (CMOM), Institut de Chimie de Nice, Equipe Chimie Organique aux Interfaces, Parc Valrose, 06108 Nice Cedex 2, France Downloaded by UNIV OF FLORIDA on September 15, 2009 | http://pubs.acs.org Publication Date (Web): April 9, 2009 | doi: 10.1021/la900064m Received January 7, 2009. Revised Manuscript Received March 5, 2009 In this paper, polydimethylsiloxane (PDMS) with a superhydrophobic surface was generated by the combination of an acid corrosion followed by the covalent grafting of a highly fluorinated monolayer. The acid corrosion was performed with H2SO4 or HF, and the more effective was concentrated H2SO4. The resulting surface had a contact angle with water of 135°. All the acid-treated samples were then functionalized by the covalent grafting of triethoxyaminopropylsilane followed by the reaction with semifluorinated acid chlorides, via the formation of an amide bond, or directly by a commercially available highly fluorinated silane, 1H,1H,2H,2H-perfluorodecyltriethoxysilane, to afford superhydrophobic surfaces (contact angle with water exceeding 160°). The introduction of an amide function in the fluorinated monolayer afforded the best water repellency properties probably due to the organization induced by Hbonding between the surface grafted molecules. Introduction In recent years, superhydrophobic surfaces have been the subject of a lot of academic studies, supposing perfect control of the roughness as well as the chemical nature of the surface.1-5 In particular, increasing water repellence properties is becoming more and more important for many biological applications such as prevention of contamination and biocompatibility. Polydimethylsiloxane (PDMS) is one of the most important highperformance polymers and can be used in many conventional technologies, such as sealing, adhesive, and coatings, but it also has great potential for applications in biomaterial science (i.e., the design of microfluidic devices,6 contact lenses,7 implants,8 tissue engineering, and so forth)9 because of its chemical inertness, thermal stability, and biocompatibility.10 Therefore, several approaches have been developed in the literature to increase polydimethylsiloxane hydrophobicity. In most cases, the intrinsic hydrophobicity of silicone supports is exploited and the superhydrophobicity is reached by creating a geometrical nanostructure at the surface with sophisticated mechanical processes like nanotexturation induced by plasma *To whom correspondence should be addressed. Tel.: +33 (0)4 92 07 6159; fax: +33 (0)4 92 07 6156. E-mail: Frederic.Guittard@unice.fr. (1) Darmanin, T.; Guittard, F. Chem. Commun. 2009, . (2) Campbell, J. L.; Breedon, M.; Latham, K.; Kalantar-zadeh, K. Langmuir 2008, 24, 5091. (3) Zheng, Y.; Wynne, K. J. Langmuir 2007, 23, 11964. (4) Sun, C.; Zhao, X.-W.; Han, Y.-H; Gu, Z.-Z. Thin Solid Films 2008, 516, 4059. :: (5) Jarn, M.; Brieler, F. J.; Kuemmel, M.; Grosso, D.; Lindn, M. Chem. Mater. e 2008, 20, 1476. (6) Hu, S.; Ren, X.; Bachmann, M.; Sims, C. E.; Li, G. P.; Allbritton, N. Anal. Chem. 2003, 74, 4117. :: :: (7) Volcker, N.; Klee, D.; Hocker, H.; Langefeld, S. Mater. Med. 2001, 12, 111. (8) Lee, S. D.; Hsiue, G. H.; Kao, C. Y.; Chang, P. C. T. Biomaterials 1996, 17, 587. (9) Roth, J.; Albrecht, V.; Nitschke, M.; Bellmann, C.; Simon, F.; Zschoche, S.; Michel, S.; Luhmann, C.; Grundke, K.; Voit, B. Langmuir 2008, 24, 12603. (10) Park, J. H.; Park, K. D.; Bae, Y. H. Biomaterials 1999, 20, 943. (11) Tserepi, A. D.; Vlachopoulou, M. E.; Gogolides, E. Nanotechnology 2006, 17, 3977. (12) Yan, Y. H.; Chan-Park, M. B.; Yue, C. Y. Langmuir 2005, 21, 8905. 6448 DOI: 10.1021/la900064m etching [SF6 or CF4 plasmas]11-13 or soft lithographic imprinting and micropattern replications.14-19 Recently, Qian et al. have reported a method where the nanostructure formation is induced by the surface-initiated polymerization itself.20 It is also possible to use a more rudimentary way of surface structuring, but it is then necessary to modify the surface chemistry also.21,22 For application purposes the generation of controlled roughness is still of great interest. Therefore, the work presented here aims at creating superhydrophobic surfaces of PDMS by forming surface roughness with an acid corrosion followed by chemical grafting of a highly fluorinated monomolecular layer on the surface, according to the general synthetic procedure described in Scheme 1. Two strategies were investigated to fluorinate the PDMS surfaces: A “direct” strategy (surfaces 2a-c) in which a highly fluorinated silane (1H,1H,2H,2H-perfluorodecyltriethoxysilane) reacts directly with the silanol functions previously created on the PDMS surface with an oxygen plasma treatment. An “indirect” strategy (surfaces 4a-c) where, in a first step, the silanols react with 3-aminopropyltrimethoxysilane in order to generate primary amine functions onto the surface (surfaces 3a-c) and then, in a second step, the highly fluorinated monolayer is obtained by reaction of these amine groups with 3-perfluorooctylpropanoic (13) Manca, M.; Cortese, B.; Viola, I.; Arico, A. S.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 1833. (14) Liu, B.; He, Y.; Fan, Y.; Wang, X. Macromol. Rapid Commun. 2006, 27, 1859. (15) Chen, Y.; He, B.; Lee, J.; Patankar, N. A. J. Colloid Interface Sci. 2005, 281, 458. (16) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712. (17) Jin, M.; Feng, X.; Xi, J.; Zhai, J.; Cho, K.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805. (18) Holgerson, P.; Sutherland, D. S.; Kasemo, B.; Chakarov, D. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 51. (19) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F. Langmuir 1994, 10, 4610. (20) Qian, T.; Li, Y.; Wu, Y.; Zheng, B.; Ma, H. Macromolecules 2008, 41, 6641. (21) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (22) Tsougeni, K.; Tserepi, A.; Boulousis, G.; Constantoudis, V.; Gogolides, E. Plasma Proc. Polym. 2007, 4, 398. Published on Web 4/9/2009 Langmuir 2009, 25(11), 6448–6453 de Givenchy et al. Article Downloaded by UNIV OF FLORIDA on September 15, 2009 | http://pubs.acs.org Publication Date (Web): April 9, 2009 | doi: 10.1021/la900064m Scheme 1. General synthetic scheme: (1) O2 plasma activation then perfluorodecyltriethoxysilane, EtOH, acetic acid; (2) O2 plasma activation then N,N-dimethylaminopropyltriethoxysilane, EtOH, acetic acid (3) 3-perfluorooctylpropanoic acid chloride, AcOEt. acid chloride. This second way needed more steps but allowed us to introduce amide functions, which are known to be efficient in contributing to the stability of self-assembled monolayers.23-25 2. Chemical Functionalization of the 1a-c PDMS Plates. PDMS plates (2 cm2) are cleaned by an overnight washing with ethanol in a Soxhlet apparatus to remove surface contaminants and residual low molecular weight species. After one night of drying at 65 °C, acidic corrosion is performed by spreading the solution of concentrated acid onto one side of the sample. The acidic treatment is performed until the maximum contact angle with water is obtained before the alteration of the PDMS sample. The optimized contact times appear to be 2 min with H2SO4 (97 wt %) and 10 min with HF (40 wt %). Finally, the PDMS plates are carefully washed in pure water. After one night of drying in a 65 °C oven, the PDMS plates 1a-c are activated with an O2 plasma [time, 90 s; power, 70 W; pressure, 180 Torrs]. The plates are then rapidly dipped in a silane solution previously hydrolyzed for 30 min (0.5 mL of 3aminopropyltrimethoxysilane or 1H,1H,2H,2H-perfluorodecyltriethoxysilane in 5 mL of EtOH and 3 drops of acetic acid). The plates are immersed during 10 min, then cured in a 120 °C oven for 3 h, and washed three times in EtOH under sonication for 15 min to afford surfaces 2a-c and 3a-c. ToF-SIMS positive ions analysis. Reference PDMS plate 1a, peaks [m/z]: 28 [Si+], 43 [CH3Si+], 73 [(CH3)3Si+], 147 [C5H15OSi(], 207 [C5H15O3Si+]. Samples 2a and 2b, peaks 2 3 [m/z]: 31 [CF+], 69 [CF+], 100 [C2F+], 119 [C2F+], 131 3 4 5 [C3F+], 147 [C5H15OSi(], 169 [C6F+]. 5 2 8 Kaiser test: the intermediary surfaces 3a-c were characterized by coloration with the Kaiser test,27,28 which confirms the presence of the amine functions onto the material. Three solutions are prepared: solution A is an ethanolic solution (10 mL) of phenol (40 mg); for solution B, KCN (1.32 mg) is diluted in 100 mL of a water/pyridine mixture (2/98); solution C is an ethanolic solution (10 mL) of ninhydrine (500 mg). For the test, the samples are dipped into a flask containing 0.2 mL of each solution. Sample turns violet if primary amine functions are present on the surfaces. 3. Amide Formation on PDMS Plates 3a-c. The aminefunctionalized plates (3a-c) are dipped into 5 mL of a solution of 3-perfluorooctylpropanoic acid chloride (0.53 mmol) in ethyl acetate for 8 h at room temperature. The plates 4a-c obtained are washed twice with ethyl acetate under sonication for 15 min. (23) (24) (25) (26) 1615. (27) Karousis, N.; Ali-Boucetta, H.; Kostarelos, K.; Tagmatarchis, N. Mater. Sci. Eng., B 2008, 152, 8. (28) Sarin, V. K.; Kent, S. B. H.; Tam, J. P.; Merrifield, R. B. Anal. Biochem. 1981, 117, 147. Experimental Section Materials. Solvents, 1H,1H,2H,2H-perfluorodecyltriethoxysilane, 3-aminopropyltrimethoxysilane, concentrated acids, potassium cyanide (KCN), and ninhydrine were available from Sigma/Aldrich and were of the highest purity. 3-Perfluorooctylpropanoic acid chloride was synthesized according to a previously described procedure.26 H2SO4 (97 wt %), HF (40 wt %), and KCN are extremely corrosive and/or toxic products and must be used carefully according to their related safety data sheets. PDMS films of about 1 mm in thickness were supplied by Goodfellow Inc., Cambridge, U.K., as platinum-catalyzed heatcured silicone. O2 plasma activation is realized in BSET EQ NT-1 plasma oven from Digit-Concept SA. Procedures. 1. Acidic Corrosion of PDMS Plates. Malysheva, L.; Onipko, A.; Liedberg, B. J. Phys. Chem. A 2008, 112, 1683. Boal, A. K.; Rotello, V. M. Langmuir 2000, 16, 9527. Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319. Massi, L.; Guittard, F.; Levy, R.; Gribaldi, S. Eur. J. Med. Chem. 2009, 44, e Langmuir 2009, 25(11), 6448–6453 DOI: 10.1021/la900064m 6449 Article de Givenchy et al. ToF-SIMS positive ions analysis results. Samples 4a-c, peaks [m/z]: 31 [CF+], 69 [CF+], 100 [C2F+], 119 [C2F+], 131 [C3F+], 3 4 5 5 147 [C5H15OSi(], 475 [C11H4F17O+]. 2 Table 1. Relationship between Contact Angle with Water and Roughness for 1a, 1b, and 1c PDMS Surfaces θ (° ) Surface Characterization. 1. Contact Angle and Surface Energy Determination. Static contact angles and the Downloaded by UNIV OF FLORIDA on September 15, 2009 | http://pubs.acs.org Publication Date (Web): April 9, 2009 | doi: 10.1021/la900064m deducted surface free energy of polymer samples were performed using the sessile drop method on a motorized syringe mechanism :: equipped Kruss DSA 10 contact angles goniometer interfaced with image-capture software. They were recorded at (25 ( 1) °C. Measurements of static contact angle were made with deionized water, diiodomethane, and hexadecane taking an average of five 1 μL drops with each type of reference liquids. Reproducibility was within (3°. Surface energy γ, and its dispersive and polar components, noted γd and γp, respectively, were evaluated using the Owens, Wendt, Rabelt, and Kaeble concept.29 2. Surface Roughness. A VEECO noncontacting laser profilometer (WYKO NT1100) was used to assess the three-dimensional surface roughness of the ground surfaces at nanometer resolution in terms of arithmetic mean roughness (Ra). Scanning electron microscopy (SEM) observations were carried out with a JEOL 6700F microscope. 3. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). In ToF-SIMS, a pulsed, focused, energetic ion beam bombards a surface, leading to interactions that cause the emission of positive and negative secondary ions.30 ToF-SIMS experiments were performed on a Physical Electronics model TRIFT 2 equipped with a 15 keV “69Ga+” primary ion source. Secondary emitted ions are accelerated at 3 keV. The pulsed ion beam was raster scanned across a surface area of 200 μm  200 μm. The mass range was from 1 to 3500 amu, and the mass resolution [M/dm] is 1500 and the time of primary ions pulse is about 3 ns for a repetition frequency of 5.5 kHz. Results and Discussion 1. Acidic Corrosion. The reference surface, coded 1a, is a crude PDMS sample washed overnight with ethanol in a Soxhlet apparatus. Samples 1b and 1c result from the acidic treatment with sulfuric acid H2SO4 and fluorhydric acid HF, respectively (Scheme 1). In order to preserve the whole material integrity, only one side of the PDMS sample is submitted to acidic exposure by the deposition of the desired concentrated aqueous solution during the optimized time of 2 or 10 min (for H2SO4 or HF, respectively) followed by an extensive washing with water. Surface modification is shown by the evolution of static contact angles with probe liquids and calculation of the corresponding surface free energies. Measurements were made at different positions on the corroded substrates to verify the accuracy and the reproducibility of the treatments as well as the homogeneity of the modified surfaces. Surface free energies of the substrates were deducted from the contact angles with the eqs 1 and 2 using the Owens, Wendt, Rabelt, and Kaeble concept. According to this approach, the total surface energy of a solid, γs, can be resolved into dispersion and polar components.29 0qffiffiffiffiffi1 0qffiffiffiffiffi1 d qffiffiffiffiffi qffiffiffiffiffi γp C γl C l pB dB 1 þ cos θ ¼ γs @ A þ 2 γs @ A γl γl γs ¼ γd þ γp s s ð1Þ ð2Þ where θ is the contact angle, γl is the surface free energy of the liquid, and γd and γp are its dispersion and polar components l l (29) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (30) Benninghoven, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 1023. 6450 DOI: 10.1021/la900064m water CH2I2 nC16H34 γs (mN/m)a Ra (nm) 115 73 32 23.0 90 130 110 30 16.4 7100 127 103 45 15.5 560 a Surface energy calculated with Owens, Wendt, Rabelt, and Kaeble concept,29 using water diiodomethane and hexadecane as probe liquids. 1a 1b 1c respectively; γs is the solid free energy, and γd and γp are s s its components. If the contact angles formed on a solid surface with three liquids (for which the surface tension and the dispersion and polar components are known) are measured, the surface free energy of the solid can be calculated via eq 1. The surface tension, dispersion, and polar components of probe liquids (water, hexadecane, and diiodomethane) were employed for calculation of the surface energy. Contact angle measurements and surface free energies calculated for surfaces 1a-c are summarized in Table 1. The original PDMS surface 1a exhibits contact angle with water θ = 115° and free surface energy γs = 23.0 mN/m. The acidic corrosion with H2SO4 (surface 1b) or HF (surface 1c) leads, in both cases, to an important increase of surface hydrophobicity as the measured contact angles with water are, respectively, 130° and 127°. These values can be considered in the same range because the contact angles are measured with an uncertainty of (3°. The increase in hydrophobicity can also be traduced by a reduction of the free surface energies calculated: γs = 16.4 mN/m for 1b and γs = 15.5 mN/m for 1c. These results can be correlated with the formation of a significant roughness on the PDMS surface, as it is well-known that the enhancement of chemical hydrophobicity is due to significant micro- or nanotexturing.31,32 Surface roughness is measured by noncontacting laser profilometry. Table 1 summarizes the arithmetic mean roughness (Ra), measured for surfaces 1a-c, which is the integral of the absolute value of the roughness profile height over the evaluation length. A significant roughness is obtained with H2SO4 97 wt % (surface 1b: Ra = 7100 nm) and with HF 40 wt % (surface 1c: Ra = 560 nm) as compared to the original smooth PDMS sample (surface 1a: Ra = 90 nm). It is noteworthy that the impact of the roughness on the wettability reaches a maximum in that case, as the water contact angles are very similar for both surfaces 1b and 1c, while the arithmetic mean roughness is twelve times higher for surface 1b. SEM images obtained from surfaces 1b (H2SO4 corroded) and 1c (HF corroded) are shown in Figure 1. The comparison of their morphologies highlights a nanoroughness for surface 1b, which is not present for surface 1c. As PDMS samples are known to have a varying surface state,33 the evolution of the contact angles with water are tested after one month, and no modification is observed for both hydrophobic samples. 2. Surface Chemical Hydrophobization. Two pathways are explored: (i) the direct grafting of a highly fluorinated silane (surfaces 2a-c) and (ii) a two-step process where the PDMS supports are first functionalized with 3-aminopropyltrimethoxysilane (surfaces 3a-c) and then allowed to react with a semifluorinated acid chloride (surfaces 4a-c). (31) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (32) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (33) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. Langmuir 2009, 25(11), 6448–6453 de Givenchy et al. Article Downloaded by UNIV OF FLORIDA on September 15, 2009 | http://pubs.acs.org Publication Date (Web): April 9, 2009 | doi: 10.1021/la900064m Figure 2. Colorimetric revelation of the amine: 1a is the reference sample; 3a is the aminated PDMS; 3a + Kaiser test is the aminated sample treated with the Kaiser test. Figure 1. SEM photographs of (a) H2SO4 corroded surface 1b, (b) HF corroded surface 1c. i. One-Step Process (Direct Hydrophobization). Surface modification of PDMS with silanes is quite well documented34,35 and necessitates several steps: the first one is an activation by oxygen plasma pretreatment that generates hydroxyl groups at the surface of the material. At the same time, the silane is hydrolyzed in an acidic alcohol solution and then reacted with the activated surface for a defined time. After several steps of washing, the plates are dried in a 120 °C oven for several hours to favor the curing of the silanes on the surface. Two-Step Process. In this process, the protocol is adjusted on the reference PDMS plate 1a, which is first functionalized with 3-aminopropyltrimethoxysilane to give surface 3a. The primary amine function could be further easily modified using classical organic chemistry reactions. The intermediary surface 3a is characterized by coloration with the Kaiser test,27,28 which is a useful tool to qualify the presence of primary amino terminal groups as shown in Figure 2. Moreover, an immediate change in contact angle with water illustrates the effectiveness of the surface chemical modification (θ = 115° for 1a against θ = 90° for 3a). Formation of the amide spacer on surface 3a is performed by the reaction with 3-perfluorooctylpropanoic acid chloride according to the classical procedure detailed in the Experimental Section. The amide functions are detected by IRTF spectroscopy with, in particular, the presence of a carboxyl band at 1624 cm-1 (see Figure 3).These chemical sequences are then applied to the rough surfaces 1b and 1c readily after the acidic treatment to afford samples 4b and 4c. (34) Wu, D.; Zhao, B.; Dai, Z.; Qin, J.; Lin, B. Lab Chip 2006, 6, 942. (35) Sharma, V.; Dhayal, M.; Shivaprasad, S. M.; Jain, S. C. Vacuum 2007, 81, 1094. Langmuir 2009, 25(11), 6448–6453 Figure 3. IR-ATR spectrum of surfaces PDMS surface 1a and amide functionalized surface 4a. ToF-SIMS experiments are performed on all highly fluorinated surfaces. In all cases, the presence of characteristic fragments from the fluorinated tails, CF+ (31 m/z), CF+ 3 (69 m/z), C2F+ (100 m/z), C2F+ (119 m/z), and C3F+ 4 5 5 (131 m/z) attests the effectiveness of the processes. Figure 4 shows the ToF-SIMS spectra obtained for the highly fluorinated samples 4b (H2SO4 treatment) and 4c (HF treatment) compared with the spectrum of the crude PDMS reference surface 1a. Among the characteristic peaks, the 147 m/z [C5H15OSi+] 2 detected in all spectra is noticeable of PDMS substrate. In Figure 4B (sample 4b) and 4C (sample 4c), the 475 m/z [C11H4F17O+] is characteristic of the fragmentation of the amide function. Relative height of peaks 131 m/z [C3F+] and 5 147 m/z [C11H4F17O+] in spectra in Figure 4B,C highlights the higher density of grafted fluorinated molecules on sample 4b compared to sample 4c. This is probably due to the more important specific surface of the H2SO4 corroded sample 4b induced by its higher roughness. 3. Hydrophobicity Performances. Contact angles with water were measured for all surface treatments, and the data obtained for final fluorinated surfaces 2a-c and 4a-c compared with the reference PDMS surfaces 1a-c are collected in Figure 5. These results clearly show that roughness induced by acidic treatment has a direct impact on the hydrophobicity of the PDMS surfaces (water contact angles go from 115° for untreated surface 1a to 127° for 1b and 130° for 1c), but is also able to enhance the hydrophobicity of later highly fluorinated monolayers. Indeed, with the HF corroded surfaces 2c and 4c, the contact angles with water seem to reach a plateau, as in both cases the measured values cannot exceed 127°. In contrast, in the case of the H2SO4 corroded surfaces, with roughness distinctly higher (see Table 1), the grafting of a highly fluorinated DOI: 10.1021/la900064m 6451 Downloaded by UNIV OF FLORIDA on September 15, 2009 | http://pubs.acs.org Publication Date (Web): April 9, 2009 | doi: 10.1021/la900064m Article de Givenchy et al. Figure 4. Positive ions ToF-SIMS spectra of (A) reference PDMS surface 1a; (B) highly fluorinated surface after H2SO4 treatment 4b; (C) highly fluorinated surface after HF treatment 4c. Figure 5. Left: evolution of water contact angle θ as a function of the surface treatments. * Surface too hydrophobic to measure water contact angle θ. ** As-received PDMS after several cycles of ethanol washing. Right: video frame (frame rate of 25 fps) of attempt to measure water contact angle on surface 4b. t = 0.00 s to t = 2.12 s: attempt to deposit the water droplet with a syringe. t = 2.12 s to t = 2.24 s: as soon as the drop is free, it rolls off the surface (see the full video in ESI). monolayer induced highly hydrophobic surfaces (θ > 145°). This effect can be explained by two independently developed models: on one hand, Wenzel31 stated that surface roughness enhances the real surface area of hydrophobic material and thus increases the measured hydrophobicity. On the other hand, Cassie32 proposed a model where an air layer is trapped between the rough surface and the drop leading to a superhydrophobic surface, because drops sits partially on the surface and partially on air (see Figure 6). For the two models, θ*, the apparent angle on the rough surface, is introduced and given as a function of θ, the contact angle on a corresponding flat surface. In the Wenzel theory, the relation between θ* and θ is given by eq 3: cosðθÃÞ ¼ r cosðθÞ 6452 DOI: 10.1021/la900064m ð3Þ where r is the ratio between the rough surface and the flat surface (r > 1). With r > 1, eq 3 gives |cos(θ)| > |cos(θ*)|. Thus a natural hydrophobic surface (θ > 90°) will become more hydrophobic, while a hydrophilic surface (θ < 90°) will become more hydrophilic. In the Cassie approach, because of the air trapped between the surface and the liquid, the measured contact angle is the average between the contact angle value on the surface and the contact angle value on air. The relation between θ* and θ is given by eq 4: cosðθÃÞ ¼ -1 þ ΦSL ð1 þ cosðθÞÞ ð4Þ where ΦSL is the fraction of solid in contact with the liquid. In terms of superhydrophobic performances, if contact angles in both states are comparable, a Wenzel drop interacts with many surface defects, and the drop (even if the contact angle is more Langmuir 2009, 25(11), 6448–6453 de Givenchy et al. Downloaded by UNIV OF FLORIDA on September 15, 2009 | http://pubs.acs.org Publication Date (Web): April 9, 2009 | doi: 10.1021/la900064m Figure 6. Schematic representation of a drop on a rough surface according to Wenzel and Cassie’s theories. than 160°) adheres to the substrate. The behavior of surface 4b detailed in the video frame captures (Figure 5) and the corresponding film (electronic Supporting Information) shows a nonsticking water drop during our contact angle experiments. This drop rolling on the superhydrophobic surface is characteristic of a Cassie scenario.36,37 It shows the drastic impact of the monolayer organization on a nanorough surface, as surfaces 4b and 4c present the same scale roughness (H2SO4 induced), but 4b, in which monolayer cohesion is achieved by H bonds, is the only surface to present this superhydrophobic quality. Surface stability is verified after one month, and no perceptible modification of the surface chemistry is detected either by IR-ATR or with water contact angle measurements. Conclusions In this paper, a strategy to generate a superhydrophobic PDMS surface has been reported; it is first based on the creation of (36) Lafuma, A.; Qur, D. Nat. Mater. 2003, 2, 457. ee (37) Manoudis, P. N.; Karapanagiotis, I.; Tsakalof, A.; Zuburtikudis, I.; Panayiotou, C. Langmuir 2008, 24, 11225. Langmuir 2009, 25(11), 6448–6453 Article surface roughness by means of acid corrosion with sulfuric or hydrofluoric acids. In a second step, the chemical grafting of the semifluorinated agent is achieved by means of silane derivatives in order to create a highly fluorinated monomolecular layer on the surface. Two pathways are tested: the direct grafting of a highly fluorinated silane or a two-step process where the PDMS supports are first functionalized with 3-aminopropyltrimethoxysilane and then allowed to react with a semifluorinated acid chloride, providing to the surface monolayer a structured amide function. Concentrated H2SO4 acid treatment is the more effective, and the resulting surface shows a contact angle with water of 130° and a nanometric scale roughness. Superhydrophobic performance with a Cassie state is obtained after the covalent grafting of semifluorinated amides in two steps. This work leads to a performing nanorough surface with only one monolayer of covalently grafted fluorinated active chemicals. Its covalent anchorage to the surface ensures the toughness of the coating. Acknowledgment. We acknowledge Dr. Montmitonnet and Dr. Repoux (Ecole des Mines de Paris - Sophia Antipolis) for ToF SIMS measurements. Supporting Information Available: Video showing the contact angle measurement with water for surface 4b is provided. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la900064m 6453 ...
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