pubs.acs.org/Langmuir © 2010 American Chemical Society
Hydrocarbon versus Fluorocarbon in the Electrodeposition of Superhydrophobic Polymer Films Thierry Darmanin, Elisabeth Taffin de Givenchy, Sonia Amigoni, and Frederic Guittard* Universit e de Nice - Sophia Antipolis, Laboratoire de Chimie des Mat eriaux Organiques et M etalliques, Equipe Chimie Organique aux Interfaces, Parc Valrose, 06108 Nice Cedex 2, France Received August 19, 2010. Revised Manuscript Received September 10, 2010 To elaborate on superhydrophobic surfaces, we report the electrochemical synthesis, surface morphology, and wettability of hydrocarbon conductive polymer films obtained by the electrodeposition of polythiophene, poly(3,4ethylenedioxythiophene) (i.e., PEDOT), and poly(3,4-ethylenedioxypyrrole) (i.e., PEDOP) derivatives. Highly hydrophobic films were obtained from n-C14H29 and n-C8H17 chains in the cases of polythiophenes and PEDOP, respectively. By contrast, superhydrophobic films were formed by the deposition of PEDOT substituted with n-C10H21 chains (PEDOT-methyl undecanoate): static contact angle ≈ 160.6°, hysteresis ≈ 2°, and sliding angle ≈ 3°. Their surface properties were compared to those of previously reported fluorinated analogues. The water-repellent properties of PEDOT-methyl undecanoate were similar to the best surface properties obtained with fluorinated monomers. Even if the main approach for the chemical factor to build up superhydrophobic surfaces is via a coating of a fluorinated compound, this work confirms that the formation of fractal surfaces is able to achieve super-anti-wetting properties within a hydrocarbon series (less expensive with a favorable ecotoxic approach), and it opens a new path to bioinspired surfaces.
Introduction Because of their various wetting properties, the elaboration of superhydrophobic surfaces (a water contact angle of >150° and a low roll-off angle)1 has attracted the attention of the scientific and industrial communities because of potential applications in various domains such as self-cleaning windows, water-proof textiles, and antisnow and antifog surfaces.2-5 On the basis of two wetting concepts, the Wenzel and Cassie-Baxter theories,6-8 the biomimetism of superhydrophobic natural surfaces9-17 was possible by combining surface structuration (micro and/or nano) with a hydrophobic substance. If the choice of hydrophobic substance is relatively limited (hydrocarbon, silicon, or fluorocarbon), many strategies for structuring surfaces were employed, for example, *Corresponding author. E-mail:
[email protected]. (1) (a) Gao, L. C.; McCarthy, T. J. Langmuir 2008, 24, 9183–9188. (b) Gao, L. C.; McCarthy, T. J. Langmuir 2009, 25, 14105–14115. (2) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193–202. (3) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350–1368. (4) (a) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621–633. (b) Xia, F.; Jiang, L. Adv. Mater. 2008, 20, 2842–2858. (5) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224–240. (6) Wenzel, R. N. Ing. Eng. Chem. 1936, 28, 988–994. (7) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (8) Baxter, S.; Cassie, A. B. D. J. Text. Ind. 1945, 36, T67–90. (9) Wagner, T.; Neinhuis, C.; Barthlott, W. Acta Zool. 1996, 77, 213–225. (10) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (11) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667–677. (12) Parker, A. R.; Lawrence, C. R. Nature 2001, 414, 33–34. (13) Gao, X.; Jiang, L. Nature 2004, 432, 36. (14) Lee, W.; Jin, M.-K.; Yoo, W.-C.; Lee, J.-K. Langmuir 2004, 20, 7665– 7669. (15) Shirtcliffe, N. J.; Pyatt, F. B.; Newton, M. I.; McHale, G. J. Plant Physiol. 2006, 163, 1193–1197. (16) Gu, Z.-Z.; Wei, H.-M.; Zhang, R.-Q.; Han, G.-Z.; Pan, C.; Zhang, H.; Tian, X.-J.; Chen, Z.-M. Appl. Phys. Lett. 2005, 86, 201915. (17) Zheng, Y.; Gao, X.; Jiang, L. Soft Matter 2007, 3, 178–182. € (18) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (19) Yeh, K.-Y.; Chen, L.-J. Langmuir 2008, 24, 245–251. (20) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103.
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lithographic methods (photolithography,18,19 electron-beam lithography,20 soft lithography,21,22 nanoimprint lithography,23,24 and nanosphere lithography25), polymer fiber electrospinning,26,27 templating with porous membranes,28 acid treatment,29 and the layer-by-layer growth of particles or polymers.30,31 Electrochemical methods are able to reach superhydrophobicity very quickly and with high reproducibility.32-37 Among them, the electrochemical polymerization or electrodeposition of conductive polymers is able to control the surface growth and morphology using many parameters.38-42 Moreover, the possibility to (21) Liu, B.; He, Y.; Fan, Y.; Wang, X. Macromol. Rapid Commun. 2006, 27, 1859–1864. (22) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978–8981. (23) Lee, S.-M.; Kwon, T. H. J. Micromech. Microeng. 2007, 17, 687–692. (24) Pozzato, A.; Dal Zilio, S.; Fois, G.; Vendramin, D.; Mistura, G.; Belotti, M.; Chen, Y.; Natali, M. Microelectron. Eng. 2006, 83, 884–888. (25) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 561– 564. (26) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338–4341. (27) Lim, J.-M.; Yi, G.-R.; Moon, J. H.; Heo, C.-J.; Yang, S.-M. Langmuir 2007, 23, 7981–7989. (28) (a) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221–1223. (b) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978–8981. (29) Taffin de Givenchy, E.; Amigoni, S.; Martin, C.; Andrada, G.; Caillier, L.; Geribaldi, S.; Guittard, F. Langmuir 2009, 25, 6448–6453. (30) (a) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713–4716. Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064–3065. (31) Amigoni, S.; Taffin de Givenchy, E.; Dufay, M.; Guittard, F. Langmuir 2009, 25, 11073–11077. (32) (a) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. 1997, 36, 1011–1012. (b) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Colloid Interface Sci. 1998, 208, 287–294. (33) Jiang, Y.; Wang, Z.; Yu, X.; Shi, F.; Xu, H.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986–1990. (34) Yu, X.; Wang, Z.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483–4486. (35) Li, Y.; Shi, G. J. Phys. Chem. B 2005, 109, 23787–23793. (36) Wang, S.; Feng, L.; Liu, H.; Sun, T.; Zhang, X.; Jiang, L.; Zhu, D. ChemPhysChem 2005, 6, 1475–1478. (37) Li, M.; Zhai, J.; Liu, H.; Song, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2003, 107, 9954–9957.
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Article Scheme 1. (A) Synthesized Hydrocarbon Monomers and (B) Previously Reported Fluorinated Analogues
introduce hydrophobic groups consisting of fluorinated or hydrocarbon substitutents into the monomer structure leads to a onepot method for the elaboration of superhydrophobic films. Tsujii’s group reported the possibility of obtaining a superhydrophobic surface by the electrochemical polymerization of a hydrocarbon monomer, 1-n-octadecylpyrrole.38 Other works have dealt with the introduction of fluorinated substitutents, allowing superoleophobic or superhydrophobic properties to be obtained. Thus, electrodeposited fluorinated polythiophenes39 [poly(3,4-ethylenedioxythiophenes) or PEDOT],40 polypyrroles41 [poly(3,4-ethylenedioxypyrroles) or PEDOP], and poly(3,4-propylenedioxypyrroles) [ProDOP42] allowed us to obtain surfaces with various antiwetting properties. The interest in using fluorinated compounds is the formation of both superhydrophobic and also oleophobic or superoleophobic surfaces. Recently, superoleophobic surfaces were obtained by the electrodeposition of fluorinated PEDOP.42 However, the use of fluorinated compounds is not necessary to reach superhydrophobic surfaces as observed in nature10-17 or reported in the literature.38,43 Moreover, the use of fluorinated alkyl substances (FASs) such as perfluorinated carboxylates or perfluorinated sulfonates has environmental consequences.44-46 Recent studies in a variety of wildlife, even in polar bears and humans, revealed their persistence and bioaccumulation potential. In the case of perfluorinated acids, the bioaccumulation depends on the perfluoroalkyl chain length.47 Thus, to avoid the use of bioaccumulative substances, hydrocarbon chains should be used for the elaboration of superhydrophobic surfaces, which is a key challenge for new bioinspired materials. (38) (a) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453–3456. (b) Yan, H.; Kurogi, K.; Tsujii, K. Colloids Surf., A 2007, 292, 27–31. (c) Kurogi, K.; Yan, H.; Mayama, H.; Tsujii, K. J. Colloid Interfaces Sci. 2007, 312, 156–163. (d) Chiba, K.; Kurogi, K.; Monde, K.; Hashimoto, M.; Yoshida, M.; Mayama, H.; Tsujii, K. Colloids Surf., A 2010, 354, 234–239. (39) Nicolas, M.; Guittard, F.; Geribaldi, S. Langmuir 2006, 22, 3081–3088. (40) (a) Darmanin, T.; Nicolas, M.; Guittard, F. Langmuir 2008, 24, 9739–9746. (b) Darmanin, T.; Nicolas, M.; Guittard, F. Phys. Chem. Chem. Phys. 2008, 10, 4322– 4326. (41) Darmanin, T.; Guittard, F. Langmuir 2009, 25, 5463–5466. (42) (a) Darmanin, T.; Guittard, F. J. Am. Chem. Soc. 2009, 131, 7928–7933. (b) Darmanin, T.; Guittard, F. J. Mater. Chem. 2009, 19, 7130–7136. (c) Zenerino, A.; Darmanin, T.; Taffin de Givenchy, E.; Amigoni, S.; Guittard, F. Langmuir 2010, 26, 13545–13549. (43) (a) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712–2718. (b) Liu, B.; Lange, F. L. Langmuir 2010, 24, 3637– 3640. (c) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750–5754. (44) Houde, M.; Martin, J. W.; Letcher, R. J.; Solomon, K. R.; Muir, D. C. G. Environ. Sci. Technol. 2006, 40, 3463–3473. (45) Giesy, J. P.; Kannan, K. Environ. Sci. Technol. 2001, 35, 1339–1342. (46) Kudo, N.; Kawashima, Y. J. Toxicol. Sci. 2003, 28, 49–57. (47) Conder, J. M.; Hoke, R. A.; de Wolf, W.; Russell, M. H.; Buck, R. C. Environ. Sci. Technol. 2008, 42, 995–1003.
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Here, we report the surface properties of electrodeposited hydrocarbon polythiophenes (PEDOT and PEDOP) with various alkyl chain lengths, as shown in Scheme 1A. Their surface wettability and morphology will be examined and compared with those of previously reported fluorinated analogues (Scheme 1B).
Experimental Section Monomer Characterization. The monomer retention time (rt) was obtained using a 5890 series II gas chromatography from Hewlett-Packard (capillary column HP5, 30 m, 0.32 mm; heating program, 60-250 °C at 10 °C/min). The melting points were determined with a Perkin-Elmer Jade DSC using the following protocol: heating from 30 to 100 °C, cooling until -20 °C, and then heating to 100 °C (scan rate 10 °C/min). The melting points were recorded during the second heating. 1H and 13C NMR spectra were obtained in CDCl3 with a Bruker W-200 MHz spectrometer. The monomer mass spectra were obtained by electron ionization at 70 eV with a Thermofischer Corp. Thermo TRACEGC fitted with an Automass III Multi spectrometer. Infrared spectra in KBr were obtained using a Perkin-Elmer Spectrum 100 FT-IR spectrometer. Monomer Synthesis. 2-(3-Thienyl)ethanol, thiodiglycolic acid, and iminodiacetic acid were purchased from Sigma-Aldrich. (2,3-Dihydrothieno[3,4-b]-1,4-dioxin-2-yl)methanol was obtained in 6 steps from thiodiglycolic acid, and 2-(2,3-dihydro-[1,4]dioxino[2,3-c]pyrrol-6-yl)ethanol was obtained in 10 steps from iminodiacetic acid using established procedures.42,48 The various linear hydrocarbon chains were introduced using the following procedure (Scheme 2): EDC (1.0 g, 5.2 mmol) and 4-dimethylaminopyrridine (50 mg, 0.4 mmol) were added to a solution of the corresponding aliphatic acid (5.2 mmol) in dichloromethane. After the solution was stirred for 30 min at room temperature, the corresponding alcohol (5.2 mmol) was added. After a day, the solvent was removed and the crude mixture was purified by column chromatography (silica gel; eluent: dichloromethane) to yield the products. The characterization details are given for ThH4, ETH4, and EPH4; the characterization of the other monomers is described in the Supporting Information. (ThH4): 2-(Thiophen-3-yl)ethyl Heptanoate. Yield, 42%; r.t., 13.0 min; colorless liquid. 1H NMR δH(200 MHz, CDCl3): 7.24 (dd, 3JHH = 4.9 Hz, 4JHH = 2.9 Hz, 1H), 7.00 (m, 1H), 6.94 (dd, 3JHH = 4.9 Hz, 4JHH = 1.2 Hz, 1H), 4.26 (t, 3JHH = 6.9 Hz, 2H), 2.94 (t, 3JHH = 6.9 Hz, 2H), 2.27 (t, 3JHH = 7.5 Hz, 2H), 1.57 (m, 2H), 1.25 (m, 6H), 0,85 (t, 3JHH = 6.5 Hz, 3H). 13C NMR δC(50 MHz, CDCl3): 173.79, 138.08, 128.22, 125.54, 121.49, 64.02, 34.32, 31.43, 29.60, 28.77, 24.89, 22.45, 14.01. FTIR (main vibrations): ν = 2956, 2928, 2858, 1735 (OCdO), 1466, 1168. MS(70 eV, m/z (%)): 110 (100) [C6H6Sþ•], 97 (11) [C5H5Sþ]. (48) Lima, A.; Schottland, P.; Sadki, S.; Chevrot, C. Synth. Met. 1998, 93, 33–41.
DOI: 10.1021/la103310m
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Darmanin et al. Scheme 2. Synthesis Route to Monomers and Polymers
(ETH4): (2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl Heptanoate. Yield, 61%; r.t., 17.1 min; colorless liquid. 1H NMR
δH(200 MHz, CDCl3): 6.36 (d, 4JHH = 3.8 Hz, 1H), 6.34 (d, 4 JHH = 3.8 Hz, 1H), 4.36 (m, 2H), 4.31 (m, 1H), 4.22 (dd, 2JHH = 11.6 Hz, 3JHH = 2.2 Hz, 1H), 4.03 (dd, 2JHH = 11.6 Hz, 3JHH = 6.8 Hz,1H), 2.35 (t, 3JHH = 7.5 Hz, 2H), 1.62 (m, 2H), 1.29 (m, 6H), 0,88 (t, 3JHH = 6.5 Hz, 3H). 13C NMR δC(50 MHz, CDCl3): 173.35, 141.17, 141.04, 100.00, 99.88, 71.48, 65.60, 62.02, 33.98, 31.36, 28.71, 24.77, 22.41, 13.96. FTIR (main vibrations): ν = 2928, 2860, 1738 (OCdO), 1486, 1181. MS(70 eV, m/z (%)): 284 (42) [Mþ], 154 (100) [C7H6O2Sþ•], 113 (63) [C7H13Oþ].
(EPH4): 2-(2,3-Dihydro-[1,4]dioxino[2,3-c]pyrrol-6-yl)ethyl Heptanoate. Yield, 85%; r.t., 17.2 min; colorless liquid. 1H NMR
δH(CDCl3): 6.08 (s, 2H), 4.23 (t, 3JHH = 5.4 Hz, 2H), 4.17 (s, 4H), 3.89 (t, 3JHH = 5.4 Hz, 2H), 2.31 (t, 3JHH = 7.5 Hz, 2H), 1.60 (m, 2H), 1.28 (m, 6H), 0.88 (t, 3JHH = 6.7 Hz, 2H). 13C NMR δC(50 MHz, CDCl3): 173.49, 132.28, 101.40, 65.77, 63.82, 48.87, 34.15, 31.40, 28.74, 24.78, 22.44, 13.99. FTIR (main vibrations): ν = 2944, 2922, 2856, 1736 (OCdO), 1552, 1164. MS(70 eV, m/z (%)): 281 (2) [Mþ], 151 (100) [C8H9NO2þ•], 138 (44) [C7H8NO2þ]. Monomer Electropolymerization Study. For each electrochemical experiment, two glass electrochemical cells were connected to an Eco Chemie B. V. Autolab PGSTAT 30 potentiostat (GPES software). Ten milliliters of an anhydrous acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) was put inside the two cells and degassed with argon. After 10 min, the monomer was added to one of the cells. The concentration of the monomers was 0.02 M for the thiophene derivatives and 0.01 M for the EDOT and EDOP derivatives. For the electropolymerization study, the cell containing the monomer was connected to the potentiostat using a 7.1 mm2 platinum disk working electrode, a glassy carbon counter electrode, and an SCE reference electrode. The monomer oxidation potential was determined by cyclic voltammetry: 1 scan from 0 to 3 V versus SCE. Then, to study the polymerization ability, 10 scans were performed until a potential close to the monomer oxidation potential was obtained. To study the stability of the polymer film, the electrodes (the polymer film is present on the working electrode) were washed and placed in the solution without monomer and another 10 scans were performed by cyclic voltammetry. Polymer Film Characterization. For polymer characterization, gold plates from Neyco (deposition of 1500 A˚ of Cr þ Au on silicon wafers) were used as the working electrodes and polymer films were electrodeposited by chronoamperometry using a potential slightly lower than the monomer oxidation potential. The mean static contact angle (CA) measurements (average of five measurements) were performed using a Kr€ uss DSA-10 contact angle goniometer. Water droplets (2 μL each, γL = 72.0 mN/m) were deposited on the surfaces for these analyses. The dynamic contact 17598 DOI: 10.1021/la103310m
Figure 1. (A) Cyclic voltammogram of ThH10 (0.02 M) on a Pt electrode recorded in 0.1 M Bu4NPF6/CH3CN and (B) cyclic voltammogram of polyThH10 in a solution without monomer (10 scans). angle measurements were performed using the tilted-drop method. The sliding angles were determined by depositing a 6 μL water droplet on the surface and inclining it until the droplet rolled off the surface. Just before the water droplet rolled off the surface, the hysteresis (advanced contact angle CAwater,adv - receding contact angle CAwater,rec) was determined, with the inclination deforming the droplet. SEM images were obtained with a JEOL 6700F microscope.
Results and Discussion Study of Thiophene Derivatives (ThHn). The monomer oxidation potential slightly decreased with the increase in the Langmuir 2010, 26(22), 17596–17602
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Article Table 1. Wettability Data for the Electrodeposited Polymersa dynamic contact angles of water (deg) static contact angles of water (deg)
advancing
receding
hysteresis
ThH10 131.8 ( 1.1 sticking behavior 156.1 ( 1.2 sticking behavior ThH12 146.6 ( 1.5 sticking behavior ThF6 153.0 ( 1.3 154.2 148.2 ThF8 97.4 ( 2.1 sticking behavior ETH4 133.5 ( 2.9 sticking behavior ETH6 160.6 ( 0.9 161.8 160.4 ETH8 159.7 ( 1.2 160.0 158.7 ETF4 159.2 ( 1.2 159.6 158.4 ETF6 157.1 ( 1.4 158.1 155.9 ETF8 120.0 ( 3.1 sticking behavior EPH4 154.2 ( 2.0 sticking behavior EPH6 156.8 ( 1.4 sticking behavior EPH8 159.3 ( 0.9 161.9 156.4 EPF4 160.0 ( 0.7 160.5 158.5 EPF6 161.1 ( 1.0 162.0 159.8 EPF8 a Static and dynamic contact angles were measured with 2 and 6 μL water droplets respectively.
alkyl chain length (from 2.30 V for ThH4 to 2.04 V for ThH12). This effect was due to the electron-donating effect of the alkyl chains, which favored the electrochemical polymerization.49 A study of the electrochemical polymerization ability was performed by multiple scans (cyclic voltammetry). Using ThH4 and ThH6 as monomers, no peaks, corresponding to the polymer oxidation and reduction were observed because of the high solubility of the corresponding polymer: the electroformed polymers were present in solution but not on the working electrode. With ThH8, a peak was observed but its intensity was very low and very unstable. Thus, ThH4, ThH6, and ThH8 cannot be used to form stable electrodeposited polymer films, but an improvement has been detected by increasing the alkyl chain length. Indeed, alkyl chains were often used in the literature to produce soluble conducting polymers, which is not desired in the electrodeposition of polymer films. By increasing the alkyl chain length by two and four methylene units (ThH10 and ThH12, respectively), relatively stable polymer films were obtained, as shown in Figure 1 for ThH10, even if the polymers were not completely insoluble in the solvent. Indeed, as shown in Figure 1B, the intensity and, as a consequence, the amount of polymer on the surface decreased significantly after each scan in a solution without monomer. To explore the surface properties, the polymers were electrodeposited on gold plates by an imposed potential (Eimp = 1.86 V versus SCE) and using deposition charges (Qs) of 500 mC/cm2. Indeed, the study of the influence of Qs on the surface properties (cf. ESI) showed that the best antiwetting properties are independent of Qs for Qs>200 mC 3 cm-2. Only ThH10 and ThH12 could form polymer films, and fractions of soluble polymer were observed during the electrodeposition of all of the thiophene derivatives. The mean static contact angles were determined using 2 μL droplets of water. PolyThH10 and polyThH12 were hydrophobic and very hydrophobic with static CAwater values of 131.8 and 156.1°, respectively (Table 1). However, when water droplets were deposited on surfaces and the surfaces were inclined, water droplet stuck to them. This particular surface property, known as a very adhesive surface (both a high water contact angle and a large contact angle hysteresis), has also been observed on the (49) (a) Waltman, R. J.; Bargon, J. Tetrahedron 1984, 40, 3963–3970. (b) Waltman, R. J.; Diaz, A. F.; Bargon, J. J. Electrochem. Soc. 1984, 131, 1452–1456. (c) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76–95. (d) Ritter, S. K.; Noftle, R. E.; Ward, A. E. Chem. Mater. 1993, 5, 752–754.
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sliding angle
6.0
25 mC/cm2). Indeed, whatever the value of Qs, polyETH4 films were not structured and nanostructuration was present in polyETH8 films. For polyETH6, the variation of the water contact angle with Qs was very important, and highly hydrophobic films (CAwater = 156.2°, no sliding angle) were obtained for Qs = 100 mC 3 cm-2. SEM images revealed that the porosity increased with Qs from 25 to 100 mC 3 cm-2 and then decreased from 100 to 300 mC 3 cm-2 to form relatively smooth surfaces (cf. ESI), which is in agreement with the wettability measurements. Study of 3,4-Ethylenedioxypyrrole Derivatives (EPHn). The monomer oxidation potentials of EPH4, EPH6, and EPH8 were about 0.94 V versus SCE. The effect of the alkyl chain on monomer polymerization was insignificant. The cyclic voltammetry curves of these three monomers are represented in Figure 6. These curves displayed a well-defined reversible doping/dedoping process with two peaks during the forward and backward scans. We previously demonstrated, by comparing the electrochemical curves of fluorinated EDOP and ProDOP derivatives,42a that the presence of the second peak, which corresponds to bipolaronic forms, may have an influence on the surface morphology (presence of surface nanoporosity) and, as a consequence, on the surface wettability. The bipolaronic forms are present only if π-stacking interactions are possible (i.e., if the substitutents are not too flexible). Following Figure 5, the length of the alkyl chain had a very strong influence on the presence of these forms. If we compare the intensity of the second peak with that of the first one, then these forms were most likely to be present using EPH6 DOI: 10.1021/la103310m
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(n-octyl chain) whereas there were almost absent with EPH8 (n-decyl chain). However, the decrease in the alkyl chain length also induced a great increase in the polymer solubility, decreasing the intensity of the peaks from EPH8 to EPH4. After deposition on gold plates via Eimp = 0.82 V versus SCE and using Qs ≈ 225 mC 3 cm-2, which is necessary to obtain the best surface properties, the wettability study revealed the high hydrophobicity of polyEPH6 (CAwater = 154.2°) and polyEPH8 (CAwater = 156.8°) but polyEPH4 was only hydrophobic (CAwater = 120.0°). Water droplets could not roll off these substrates via inclination, even with a tilt angle of 90°. These static contact angles were lower than those of previously reported polyEPFn (Table 1).42 The exceptional properties of polyEPFn were due to the presence of surface nanoporosity. SEM images showed the microstructuration of polyEPH6 and polyEPH8 films with quite similar morphology (Figure 7), but the surface structures are smaller in polyEPH6 films. However, the surface morphology of polyEPH4 is similar to the surface morphology of polyETH6. The wettability differences observed for these three surfaces are due to the differences in surface morphology and intrinsic hydrophobicity as a function of the alkyl chain length. Even if the surfaces obtained with EPH4 were not superhydrophobic, this is the first time that structured films were electrodeposited from monomers containing extremely short alkyl chains (n-hexyl). No surface nanoporosity was observed in these films, in comparison with that observed in polyEPFn,42 which explains their lower liquid-repellency properties.
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Conclusions Here, we have succeeded in the elaboration of superhydrophobic films by the electropolymerization of hydrocarbon monomers (thiophene, EDOT, and EDOP derivatives), and their surface properties were compared to those of previously reported fluorinated analogues. Highly hydrophobic surfaces were obtained by the electrodeposition of hydrocarbon polythiophenes substituted with n-C14H29 chains (ThH12, static contact angle = 156.1°, no sliding angle) and PEDOP substituted with n-C8H17 and n-C10H21 (EPH6 and EPH8, static contact angle = 154-157°, no sliding angle). In the case of hydrocarbon PEDOT, superhydrophobic films were obtained for n-C10H21 (ETH8, static contact angle = 160.6°, hysteresis ≈ 2°, and sliding angle ≈ 3°). To date, more than 700 papers justifying the use of a highly fluorinated moiety to obtain superhydrophobic properties have been published. Here, the water-repellency properties of polyETH8 are comparable to the best properties obtained with fluorinated polymers. We demonstrate that the molecular design of hydrocarbon monomers is enough to obtain stable super-water-repellent surfaces, which is the key result in the elaboration of less-expensive biomimetic materials with a favorable ecotoxic approach. Supporting Information Available: Monomer characterization, SEM images, and influence of the deposition charge. This material is available free of charge via the Internet at http://pubs.acs.org.
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