pubs.acs.org/Langmuir © 2009 American Chemical Society
Fluorophobic Effect for Building up the Surface Morphology of Electrodeposited Substituted Conductive Polymers Thierry Darmanin and Frederic Guittard* Universit e de Nice Sophia-Antipolis, Laboratoire de Chimie des Mat eriaux Organiques et M etalliques, EA 3155, Institut de Chimie de Nice, Equipe Chimie Organique aux Interfaces, Parc Valrose, 06108 Nice Cedex 2, France Received April 4, 2009. Revised Manuscript Received April 19, 2009 During the past decade, several works display the electrochemical polymerization of fluorinated monomers as a highly efficient method (one-pot method, mild conditions, various morphologies) to obtain superhydrophobic or superoleophobic surfaces. Here, we point out the fluorinated tails not only are useful to increase both hydrophobicity and oleophobicity but also are involved in the formation of surface structurations: two necessary conditions for liquid dewetting. To support this assertion, a series of fluorinated pyrrole derivatives and their hydrocarbon homologues were synthesized and electrochemically deposited in the same conditions. Whereas hydrocarbon pyrroles give rise to smooth films, structured films are achieved from fluorinated pyrroles. The abundance of the surface structurations depends on the length of the fluorinated tail.
Electrochemical polymerization of hydrophobic monomers1-4 and more precisely highly fluorinated monomers2-4 is a very competitive method for the elaboration of superhydrophobic or superoleophobic surfaces with various surface morphologies. To understand the exact role of the perfluorinated tail on the morphology of electrodeposited conductive polymers, a series of fluorinated monomers and their hydrocarbon homologues were synthesized and electropolymerized in the same conditions. In this paper, the influence of the fluorophobic effect on the surface morphology construction is pointed out. With the intention of producing superhydrophobic surfaces,5-10 the use of hydrophobic material and the formation of surface roughness are two required conditions. Indeed, two theories (Wenzel11 and Cassie-Baxter12,13 theories) are often used to predict the wettability of rough surfaces. Usually, the used hydrophobic materials are silicone, hydrocarbon, or fluorinated compounds. The necessary roughness can be reached by numerous methods such as lithography,14-17 particle or layer-by*Corresponding author. E-mail:
[email protected]. (1) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453–3456. (2) Darmanin, T.; Guittard, F. Chem. Commun. 2009, 2210–2211. (3) (a) Darmanin, T.; Nicolas, M.; Guittard, F. Phys. Chem. Chem. Phys. 2008, 10, 4322–4326. (b) Darmanin, T.; Nicolas, M.; Guittard, F. Langmuir 2008, 24, 9739–9746. (4) (a) Nicolas, M.; Guittard, F.; Geribaldi, S. Angew. Chem., Int. Ed. 2006, 45, 2251–2254. (b) Nicolas, M.; Guittard, F.; Geribaldi, S. Langmuir 2006, 22, 3081– 3088. (5) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667–677. (6) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (7) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857–1860. (8) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350–1368. (9) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193–202. (10) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224–240. (11) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (12) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc 1944, 40, 546–551. (13) Baxter, S.; Cassie, A. B. D. J. Text. Inst. 1945, 36, T67–90. :: (14) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (15) Yeh, K.-Y.; Chen, L.-J. Langmuir 2008, 24, 245–251. (16) Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. Adv. Mater. 2007, 19, 2213–2217. (17) Lee, S.-M.; Kwon, T. H. Nanotechnology 2006, 17, 3189–3196. (18) Nakajima, A.; Saiki, C.; Hashimoto, K.; Watanabe, T. J. Mater. Sci. Lett. 2001, 20, 1975–1977.
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layer assembly,18-21 physical and chemical treatments,22-25 electrospinning,26,27 or electrochemical polymerization.1-4 It is worthy to note that, in many of them, two steps are required: First, surface roughness is constructed with the desired morphology. Second, a post treatment is necessary with highly fluorinated materials. However, Jiang et al. reported a one-step process to elaborate a superhydrophobic surface by immersing a copper plate into a solution of fatty acids resulting in the formation of flowerlike clusters.25 The electrochemical polymerization of monomers substituted with hydrophobic groups is also a method of choice to obtain superhydrophobic surfaces in one step: fast method, mild conditions, and tunable surface wettability. Previously, we demonstrated that surfaces with different microstructures can be obtained by electrochemical polymerization of fluorinated monomers.3,4 In these studies, numerous structured surfaces were developed using various polymerizable cores (3,4-alkylenedioxypyrrole,2 pyrrole,3 3,4-ethylenedioxythiophene,3 and thiophene4) and various fluorinated tail lengths. However, the exact involvement of the fluorinated tail on the elaboration of surface morphology has never been highlighted. Here, to determine the influence of the fluorine in the surface morphology of electrodeposited conductive polymers, the synthesis and characterization of original fluorinated pyrroles and their hydrocarbon homologues (Scheme 1) are reported. The surface morphology and wettability of the electrodeposited conductive
(19) Xie, Q.; Fan, G.; Zhao, N.; Guo, X.; Xu, J.; Dong, J.; Zhang, L.; Zhang, Y.; Han, C. C. Adv. Mater. 2004, 16, 1830–1833. (20) Ji, J.; Fu, J.; Shen, J. Adv. Mater. 2006, 18, 1441–1444. (21) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713–4716. (22) Jin, M.; Feng, X.; Xi, J.; Zhai, J.; Cho, K.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805–1809. (23) Kim, S. H.; Kim, J. H.; Kang, B. K.; Uhm, H. S. Langmuir 2005, 21, 12213– 12217. (24) Cao, M.; Song, X.; Zhai, J.; Wang, J.; Wang, Y. J. Phys. Chem. B 2006, 110, 13072–13075. (25) Wang, S.; Feng, L.; Jiang, L. Adv. Mater. 2006, 18, 767–770. (26) Zhu, Y.; Zhang, J.; Zheng, Y.; Huang, Z.; Feng, L.; Jiang, L. Adv. Funct. Mater. 2006, 16, 568–574. (27) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742–9748.
Published on Web 4/24/2009
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Letter Scheme 1. Synthesis Route of the Hydrocarbon and Fluorinated Pyrroles and Corresponding Polypyrroles
polymer films are evaluated by contact angle measurements and scanning electron microscopy and compared. The general synthesis of the monomers is represented in Scheme 1. The synthesis of 2-(1H-pyrrol-3-yl)acetic acid was performed from pyrrole in four steps and following the general pathway described by Lemaire and co-workers.28 This synthesis route involves the protection of the nitrogen by a tosyl group, the Friedel-Crafts acylation at the 3-position, the thallium transposition of the carbonyl by a Willgerodt-Kindler reaction, and the deprotection of nitrogen and acid groups in alkaline solution. Afterward, the hydrocarbon or highly fluorinated monomers (coded, respectively, H4-8 and F4-8 as illustrated in Scheme 1) were obtained, by an esterification reaction in acetonitrile, between 2-(1H-pyrrol-3-yl)acetic acid and a semifluorinated alcohol or its hydrocarbon homologue with N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC) and 4-dimethylaminopyridine (DMAP) as coupling agents. The products were obtained in 26-45% isolated yields (Scheme 1). The detailed synthesis of the monomers is given in the Supporting Information. The electropolymerization of the monomers (0.01 M) was studied in anhydrous acetonitrile, with tetrabutylammonium hexafluorophosphate (Bu4NPF6) (0.1 M), and by cyclic voltammetry using a platinum disk working electrode. The monomer oxidation potentials were determined by a single potential scan between 0 and 2 V vs SCE. The oxidation potential is about the same for all the studied monomers (1.21-1.26 V vs SCE). Thus, the electron-withdrawing and electron-donating effects of the fluorinated tails and hydrocarbon tails are inhibited by the methylene spacer and the ester connector. The electrochemical polymerization of the monomers was studied by consecutive cyclic voltammetry from a potential lower than the polymer reduction potential and until a potential close to the monomer oxidation potential, denoted EP,opt. The apparition of the reversible process before the monomer oxidation potential is attributed to the polymer oxidation (doping) and reduction (dedoping). The increase of the intensity of the oxidation and the reduction peaks after each scan is the evidence of successive deposition of polymer (Figure 1). The oxidation and reduction potentials of the polymer were determined in a monomer-free solution. The detailed potentials are given in the Supporting Information. The half-wave potentials E1/2 of the semifluorinated polypyrroles are 300500 mV higher than that of nonsubstituted polypyrrole. The half-wave potential of the fluorinated polypyrroles is also higher than those of the polypyrroles containing shorter perfluoroalkyl tails (2,2,3,3,4,4,4-heptafluorobutyl 2-(1H-pyrrol-3-yl)acetate and 2,2,2-trifluoroethyl 2-(1H-pyrrol-3-yl)acetate) reported by Lemaire and his co-workers.28c As a result, the half-wave (28) (a) Ho-Hoang, A.; Fache, F.; Lemaire, M. Synth. Commun. 1996, 26, 1289– 1304. (b) Ho-Hoang, A.; Fache, F.; Boiteux, G.; Lemaire, M. Synth. Met. 1994, 62, 277–280. (c) Ho-Hoang, A.; Schulz, E.; Fache, F.; Boiteux, G.; Lemaire, M. J. Mater. Chem. 1996, 6, 1107–1112.
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Figure 1. Cyclic voltammogram of the monomer F6 (0.01 M) on Pt electrode recorded in 0.1 M Bu4NPF6/CH3CN (10 scans).
Figure 2. Wettability of electrodeposited polymers on gold (salt, Bu4NPF6; Qs ≈ 150 mC/cm2).
potential of the fluorinated polypyrroles seems to increase with the fluorinated tail lengthening. On the contrary, the half-wave potentials of hydrocarbon polypyrroles are lower than those of fluorinated polypyrroles and are almost independent of the hydrocarbon tail length. To study the surface properties (wettabiliy and morphology), the films were deposited by chronoamperometry (imposed potential E = EP,opt) with a deposition charge Qs ≈ 150 mC/cm2 and on gold plate. The wettability of the surfaces was determined by measuring contact angles (CA) of three probe liquids: water (γL = 72.8 mN/m), diiodomethane (γL = 50.0 mN/m) and hexadecane (γL = 27.6 mN/m). The hydrophobicity and the oleophobicity were explored, respectively, with water and hexadecane. The measured contact angles are represented in Figure 2. The hydrocarbon polymers are more hydrophobic (87° < CAwater < 92°) than nonsubstituted polypyrrole (CAwater = 54°), used as reference, and are superoleophilic. Indeed, the hexadecane droplet spreads in few seconds. As compared to the Langmuir 2009, 25(10), 5463–5466
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reference, the presence of the hydrocarbon tails increases the CA of water by about 35-40°. However, despite the hydrocarbon tail lengthening, the CA values are almost the same (asymptotic value) for the three hydrocarbon synthesized polymers. In contrast, the fluorinated polymers are very hydrophobic with CAwater = 136° for PolyF8 and almost oleophobic. The CA values increase with
Figure 3. SEM image of electrodeposited (A) PolyH8, (B) PolyF4, (C) PolyF6, and (D) PolyF8. Scale bar represents 1 μm; magnification is 10 000; salt, Bu4NPF6; Qs ≈ 150 mC/cm2.
the fluorinated tail length. The replacement of the hydrogen by fluorine increases the CAwater between 20° and 45° following the fluorinated tail length and increase with the F/H ratio. In order to find an explanation, scanning electron microscopy (SEM) experiments were carried out over a wide range of surfaces. SEM images of the hydrocarbon polymers PolyH4-8 exhibit the same morphology for all the three polymers and that the surfaces are not structured, as shown in Figure 3A for PolyH8. In contrast, the fluorinated tails lengthening in the PolyF4-8 increases the surface structuration from flat (PolyF4, Figure 3B) to cauliflower morphology (PolyF8, Figure 3D). The surfaces consist of spherical structures, and the abundance of these structures increases with the fluorinated tail length. In this example, the replacement of hydrogen atoms by fluorine atoms induces a surface morphology during the electrochemical polymerization. This result is in contradiction with the results previously reported by Tsujii et al.1 for hydrocarbon polypyrrole, but their surface properties can be explained by different experimental conditions, the nature of the salt, for example, or by the use of a very long hydrocarbon tail (n-octadecyl). The high incompatibility of the fluorinated tails generates microsegregation during the electrochemical formation of the polymers which is propitious to the nucleation of surface morphology like these spherical microstructures. When a potential is applied beetween the working electrode and the counter electrode (E = EP,opt), the monomer oxidizes to form its radical cation as shown in Figure 4. However, due to the
Figure 4. Schematic representation of electropolymerization of the highly fluorinated monomers. Langmuir 2009, 25(10), 5463–5466
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fluorinated tail, the radical cation is amphiphilic. The incompatibility of the fluorinated tails with the pyrrole moieties should preferentially lead to cisoid couplings, as already observed in hydrocarbon monomer,29 but with fluorinated tails the effect should be more important.30 Thus, the intrinsic properties of fluorinated tails (high incompatibility, rigidity, helical conformation)31 should have a consequence on the polymer conformation and therefore on the three-dimensional arrangement of the polymer as observed by SEM. The introduction of a fluorinated chain seems to have a double effect: the increase in hydrophobicity and oleophobicity and the creation of surface morphologies. This is the first time that the influence of the fluorinated tail on the surface morphology is shown. To confirm this observation, other hydrocarbon and fluorinated polymer were electrodeposited in the same conditions, as the 3,4-ethylenedioxythiophene derivatives described in the Supporting Information. Here, SEM images also show fluorinated poly(3,4-ethylenedioxythiophene) films are very structured but not their hydrocarbon homologues, which confirms again the double effect induced by fluorinated tails. Obviously, in the near future, other hydrocarbon and (29) Lemaire, M.; Garreau, R. New J. Chem. 1987, 11, 703–707. (30) Taffin de Givenchy, E.; Amigoni, S.; Martin, C.; Andrada, G.; Caillier, L.; Geribaldi, S.; Guittard, F. Langmuir, published online April 9, http://dx.doi.org/. (31) Kirsch, P. Modern fluoroorganic chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2004; p 8. (32) Fornasieri, G.; Guittard, F.; Geribaldi, S. Liq. Cryst. 2003, 30, 663–669. (33) Guittard, F.; Geribaldi, S. Anisotropic Organic Materials -Approaches to polar order; Glaser, R., Kaszynski, P., Eds.; ACS Symposium Series 798; American Chemical Society: Washington, DC, 2001; pp 180-194. (34) Tschierske, C. J. Mater. Chem. 1998, 8, 1485–1508.
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fluorinated monomers (thiophene, 3,4-ethylenedioxypyrolle derivatives, for example) will have to be electrochemically deposited in the same conditions to give a universal conclusion. In conclusion, if fluorine containing compounds have been described in the past for the elaboration of nonconventional liquid crystals or vesicles from microsegregation and self-organization,32-34 we have shown, in this work, that the fluorine contained in electropolymerizable monomers can also contribute to induce surface morphology. Indeed, in the same polymerization conditions, highly fluorinated pyrroles give rise to spherical microstructures, whereas hydrocarbon pyrroles give almost smooth films. In this example, the fluorinated tails not only increase the chemical hydrophobicity but also are involved in the formation of surface roughness. For the first time, the effect of fluorinated tails on the surface morphology of electrodeposited polymers is highlighted. In this example, we point out the increase of the fluorophobic effect can play a dual effect in order to modify, in a one-pot process, the two parameters necessary to the formation of antiwetting surfaces. Acknowledgment. T.D. thanks the French Ministry for a research grant. Supporting Information Available: Synthesis and characterization of the monomers, electrochemical data, and SEM images of hydrocarbon and fluorinated poly(3,4-ethylenedioxythiophene) films. This material is available free of charge via the Internet at http://pubs.acs.org.
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