Langmuir 2006, 22, 3081-3088
3081
Stable Superhydrophobic and Lipophobic Conjugated Polymers Films Mael Nicolas,* Fre´de´ric Guittard, and Serge Ge´ribaldi Laboratoire de Chimie des Mate´ riaux Organiques et Me´ talliques, EA 3155, UniVersite´ de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France ReceiVed NoVember 12, 2005. In Final Form: February 3, 2006 With a view to developing repellent materials combining both low surface energy and rough structure, original semi-fluorinated polythiophenes have been chemically and electrochemically synthesized and characterized by cyclic voltammetry, GPC, and UV-visible measurements. Polymer films have been deposited onto different substrates by drop casting a soluble polymer fraction on glass plate or by electrodeposition on ITO plate. Surface properties and particularly water and oil repellencies have been investigated by goniometry and correlated with the surface morphology and structure observed by SEM. The incorporation of fluorocarbon chains in the rigid polythiophene backbone yields very low surface free energy materials. Moreover, the way of deposition has a huge influence on the quality and performance of the film surface properties. Electroformed polymers, due to rough surfaces, exhibit great superhydrophobic and lipophobic properties together with exceptional time stability.
Introduction
* To whom correspondence should be addressed. E-mail: nicolasm@ unice.fr.
structure or rough morphology can be easily prepared by electrochemical deposition.5 The main approach for the chemical factor is the introduction of perfluorocarbon chain in the polymer backbone. Indeed, polymers containing fluorine have been extensively used for the modification of surface to confer upon them repellency toward both polar and apolar liquids (e.g., water and oil). They present the advantage to combine at the same time a low friction coefficient, low surface energy, chemical inertness, and thermal resistance, which make them useful for a variety of applications.6 However, fluorinated polymers are usually quite susceptible to rapid rearrangement when the polymer surface is contacted with water, and high contact angles observed on dry surfaces can quickly decrease at wet state. This is contradictory with a long-term use in polar environment.7 Thus, combining the “fluorine” approach with the electrochemical deposition surely represents an interesting strategy to affect both the chemical and geometrical aspects in order to get super-liquid repellency and may resolve the problem of surface reconstruction from polar media thanks to the organization of conjugated polymer chains onto the electrode during the electropolymerization process. Some thiophenes have been functionalized by fluorinated groups, such as 3-(fluoromethyl)thiophene, 3-(difluoromethyl)thiophene, and 3-(trifluoromethyl)thiophene (a);8 perfluoroalkyl alkyl thiophenes (b);9-12 perfluoroalkyl methoxy thiophenes
(1) (a) Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986. (b) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. ReV. 1988, 88, 183. (c) Gorman, C. B.; Grubbs, R. H. In Conjugated Polymers: The NoVel Science and Technology of Conducting and Nonlinear Optically ActiVe Materials; Bredas, J. L., Silbey, R., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; pp 1-48. (d) Feast, W. J.; Tsibouklis, J.; Pouwer, K. L.; Groenendaal, L.; Meijer, E. W. Polymer 1996, 37 (22), 5017. (e) Handbook of AdVanced Electronic and Photonic Materials and DeVices; Nalwa, H. S., Ed.; Academic Press: San Diego, CA, 2001. (2) (a) Czerwinski, A.; Cunningham, D.; Amer, A.; Schrader, J. R.; Zimmer, H.; Van Pham, C.; Mark Jr, H. B.; Pons, S. J. Electrochem. Soc. 1987, 134 (5), 1158. (b) Hoshino, S.; Yoshida, M.; Uemura, S.; Kodzasa, T.; Takada, N.; Kamata, T.; Yase, K. J. Appl. Phys. 2004, 5088. (3) (a) Boutrois, J. P.; Jolly, R.; Petrescu, C. Synth. Met. 1997, 85, 1405. (b) Kousik, G.; Pitchumani, S.; Renganathan, N. G. Prog. Org. Coat. 2001, 43, 286. (c) Tallman, D. E.; Spinks, G.; Dominis, A.; Wallace, G. G. J. Solid State Electrochem. 2002, 6, 73. (4) (a) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12 (9), 2125. (b) Tsujii, K. In Surface ActiVity: Principles, Phenomena and Applications; Tanaka, T., Ed.; Acadamic Press: New York, 1998; pp 52-54. (c) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Colloid Interface Sci. 1998, 208, 287. (d) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41 (7), 1221.
(5) (a) Handbook of conjugated polymers; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1996. (b) Zhang, Z.; Qu, L.; Shi, G. J. Mater. Chem. 2003, 13 (12), 2858. (c) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453. (6) (a) Thomas, R. R. In Fluoropolymers 2; Hougham, G., Cassidy, P. E., Johns, K., Davidson, T., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; Chapter 4. (b) Banks, R. E.; Tatlow, J. C. In Organofluorine Chemistry; Banks, R. E., Smart, B. E., Tatlow, J. G., Eds.; Plenum Press: New York, 1994; Chapter 1. (7) (a) Maekawa, T.; Kamata, S.; Matsuo, M. J. Fluorine Chem. 1991, 54 (1-3), 84. (b) Takahashi, S.; Kasemura, T.; Asano, K. Polymer 1997, 38 (9), 2107. (c) Morita, M.; Ogisu, H.; Kubo, M. J. Appl. Polym. Sci. 1999, 73, 1741. (d) Xiang, M.; Li, X.; Ober, C. K.; Char, K.; Genzer, J.; Sivaniah, E.; Kramer, E. J.; Fischer, A. Macromolecules 2000, 33, 6106. (8) Ritter, S. K.; Noftle, R. E.; Ward, A. E. Chem. Mater. 1993, 5, 752. (9) Bu¨chner, W.; Garreau, R.; Lemaire, M.; Roncali, J.; Garnier, F. J. Electroanal. Chem. 1990, 277, 355. (10) Bu¨chner, W.; Garreau, R.; Roncali, J.; Lemaire, M. J. Fluorine Chem. 1992, 59, 301. (11) Robitaille, L.; Leclerc, M. Macromolecules 1994, 27, 1847. (12) Hong, X.; Tyson, J. C.; Middlecoff, J. S.; Collard, D. M. Macromolecules 1999, 32, 4232.
Throughout the last few decades, extensive work has been devoted to conjugated polymers (polythiophene, polypyrrole, polyaniline, polyfluorene, etc.) and oligomers, both theoretically and experimentally.1 These materials have applications in many kinds of optoelectronic devices such as light-emitting diodes (LEDs), field-effect transistors (FETs), solar cells, electrochromic displays, nonlinear optic, owing to their high conductivity and environmental stability. These conducting materials and the efficiency of the devices are markedly influenced by the environmental exposure, particularly moisture.2 Thus, it appears that it is of great interest to obtain stable liquid repellent conducting polymers. Moreover, coating surfaces with these materials may also be useful for corrosion protection, antistatics, and conductive textiles.3 The wettability of a solid surface by a liquid is a characteristic property of materials and is determined by two factors: the chemical composition and the geometrical microstructure of the surface.4 A combination of these two parameters should be useful to obtain super repellent materials, which represents an important topic in daily life and in industry. Concerning the geometric factor, it is known that conductive polymer films with a fractal
10.1021/la053055t CCC: $33.50 © 2006 American Chemical Society Published on Web 03/07/2006
3082 Langmuir, Vol. 22, No. 7, 2006 Chart 1. Some Semifluorinated Thiophenes.
Chart 2. Chemical Structures of the Synthesized Monomers.
(c);11,13 perfluoroalkyl ethyldimethylsilyl thiophenes (d)14 (Chart 1) and their reactivity toward chemical and electrochemical polymerization tested. However, few works were devoted to the surface properties of the corresponding polymers, and only the poly(perfluoroalkyl alkyl thiophenes) were investigated by goniometry with dodecane to account for their lipophobicity.12 Contact angles from 40 to 80° were collected, depending on the fluorinated chain length but found to be smaller than the 87° value observed for Teflon. We report here an easy but original processing to get very stable superhydrophobic and lipophobic properties of conductive polymers by electropolymerization of perfluoroalkyl ethylpropanoate thiophenes 1a-c and 2a-c (Chart 2). The influence of deposition methods (electrodeposition against classical coating) on the polymer surface properties is discussed just as the role of the perfluorocarbon chain compared to the hydrocarbon homologue 1d and 2d. Experimental Section Synthesis of Monomers. All organic reactions were carried out under nitrogen. 1H, 13C, and 19F NMR spectra were recorded in CDCl3 solution with a Bruker AC 200 spectrometer. Melting points were determined with a Bu¨chi 510 melting point apparatus. Refractive indexes were measured at 22 ( 1 °C with a 2WA ABBE refractometer. Silica gel (230-400 mesh, 40-63 µm) for column chromatography was purchased from Merck. Mass spectra were measured with a Thermo Quest Automass III spectrometer equipped with an electron ionization source (70 eV) and a quadripole analyzer, fitted with a TraceGC gas chromatograph. Typically, in a 50 mL three-necked flask under nitrogen, 4,4,5,5,6,6,7,7,7-nonafluoroheptanoic acid, 4,4,5,5,6,6,7,7,8,8,9,9,9tridecafluorononanoic acid, 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11heptadecafluoroundecanoic acid or nonanoic acid (5 mmol), N,N′dicyclohexylcarbodiimide (DCC, 5 mmol, 1.032 g), and (dimethylamino)pyridine (DMAP, 0.5 mmol, 56 mg) were mixed in CH2Cl2 (13) El Kassmi, A.; Bu¨chner, W.; Fache, F.; Lemaire, M. J. Electroanal. Chem. 1992, 326, 357. (14) Thobie-Gautier, C.; Guy, A.; Gorgues, A.; Jubault, M.; Roncali, J. AdV. Mater. 1993, 5 (9), 637.
Nicolas et al. (20-25 mL) and the reaction mixture was stirred for 2-3 h. Afterward, 3-thienylmethanol or 2-(3-thienyl)ethanol (5 mmol) was added and stirring was continued overnight. After filtration and evaporation of the solvant, the crude product was purified by column chromatography (eluent: dichloromethane/hexane 1:1). 1a: (Thiophen-3-yl)methyl-4,4,5,5,6,6,7,7,7-nonafluoroheptanoate. Yield: 75.8%; colorless oil, ηD ) 1.4091; δH (CDCl3): 2.35-2.51 (m, 2 H, CH2-CF2), 2.61-2.71 (m, 2 H, CH2-CH2-CF2), 5.17 (s, 2 H, CH2-O), 7.10 (dd, 1 H, HTh, 3JHH ) 4.2 Hz, 4JHH ) 2.0 Hz), 7.32-7.35 (m, 2 H, HTh); δC (CDCl3): 25.88 (t, CH2-CH2-CF2, 3J 2 CF ) 4.1 Hz), 26.78 (t, CH2-CH2-CF2, JCF ) 22.3 Hz), 62.30 (CH2-O), 125.22, 126.86 and 127.98 (CHTh), 136.60 (CqTh), 171.33 (C)O); δF (CDCl3): -126.50 (2 F, CF2CF3), -124.94 (2 F, CH2CF2-CF2), -115.52 (2 F, CH2-CF2), -81.45 (3 F, CF3); MS (EI, 70 eV): 388 (M+.). 1b: (Thiophen-3-yl)methyl-4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononanoate. Yield: 86.2%; white solid, mp: 24-25 °C; δH (CDCl3): 2.35-2.52 (m, 2 H, CH2-CF2), 2.60-2.71 (m, 2 H, CH2CH2-CF2), 5.17 (s, 2 H, CH2-O), 7.09 (dd, 1 H, HTh, 3JHH ) 4.2 Hz, 4JHH ) 2.1 Hz), 7.32-7.35 (m, 2 H, HTh); δC (CDCl3): 25.89 (t, CH2-CH2-CF2, 3JCF ) 4.1 Hz), 26.87 (t, CH2-CH2-CF2, 2JCF ) 22.3 Hz), 62.29 (CH2-O), 125.20, 126.83 and 127.98 (CHTh), 136.60 (CqTh), 171.32 (C)O); δF (CDCl3): -126.67 (2 F, CF2CF3), -124.04 (2 F, CH2-CF2-CF2), from -123.40 to -122.42 (4 F, CH2-CF2-CF2-(CF2)2), -115.30 (2 F, CH2-CF2), -81.27 (3 F, CF3); MS (EI, 70 eV): 488 (M+.). 1c: (Thiophen-3-yl)methyl-4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate. Yield: 77.5%; white solid, mp: 37-38 °C; δH (CDCl3): 2.34-2.52 (m, 2 H, CH2-CF2), 2.61-2.71 (m, 2 H, CH2-CH2-CF2), 5.17 (s, 2 H, CH2-O), 7.09 (dd, 1 H, HTh, 3JHH ) 4.2 Hz, 4JHH ) 2.0 Hz), 7.31-7.35 (m, 2 H, HTh); δC (CDCl3): 25.90 (t, CH2-CH2-CF2, 3JCF ) 4.1 Hz), 26.91 (t, CH2CH2-CF2, 2JCF ) 22.5 Hz), 62.29 (CH2-O), 125.17, 126.82 and 127.97 (CHTh), 136.61 (CqTh), 171.31 (C)O); δF (CDCl3): -126.58 (2 F, CF2CF3), -123.96 (2 F, CH2-CF2-CF2), from -123.19 to -122.35 (8 F, CH2-CF2-CF2-(CF2)4), -115.29 (2 F, CH2-CF2), -81.22 (3 F, CF3); MS (EI, 70 eV): 588 (M+.). 1d: (Thiophen-3-yl)methyl nonanoate. Yield: 84.0%; colorless oil, ηD ) 1.4893; δH (CDCl3): 0.72 (t, 3 H, CH3, 3JHH ) 6.4 Hz), 1.06-1.17 (m, 10 H, (CH2)5), 1.43-1.51 (m, 2 H, O-CO-CH2CH2), 2.17 (t, 2 H, O-CO-CH2-CH2, 3JHH ) 7.5 Hz), 4.96 (s, 2 H, CH2-O), 6.92 (dd, 1 H, HTh, 3JHH ) 4.6 Hz, 4JHH ) 1.6 Hz), 7.10-7.17 (m, 2 H, HTh); δC (CDCl3): 14.52 (CH3), 23.06, 25.38, 29.53, 29.63 and 32.22 ((CH2)6), 34.73 (CO-CH2), 61.52 (CH2-O), 124.57, 126.59 and 127.97 (CHTh), 137.42 (CqTh), 174.06 (C)O). 2a: 2-(Thiophen-3-yl)ethyl-4,4,5,5,6,6,7,7,7-nonafluoroheptanoate. Yield: 85.9%; white solid, mp: 27-29 °C; δH (CDCl3): 2.31-2.49 (m, 2 H, CH2-CH2-CF2), 2.57-2.67 (m, 2 H, CH2CH2-CF2), 2.99 (t, 2 H, CH2-CH2-O, 3JHH ) 6.9 Hz), 4.34 (t, 2 H, CH2-CH2-O, 3JHH ) 6.9 Hz), 6.97 (dd, 1 H, HTh, 3JHH ) 4.9 Hz, 4JHH ) 1.3 Hz), 7.03 (dd, 1 H, HTh, 4JHH ) 2.9 Hz, 4JHH ) 1.3 Hz), 7.29 (dd, 1 H, HTh, 3JHH ) 4.9 Hz, 4JHH ) 2.9 Hz); δC (CDCl3): 25.86 (t, CH2-CH2-CF2, 3JCF ) 4.1 Hz), 26.77 (t, CH2-CH2-CF2, 2J ) 22.1 Hz), 29.87 (CH -CH -O), 65.38 (CH -CH -O), 122.08, CF 2 2 2 2 126.20 and 128.53 (CHTh), 138.05 (CqTh), 171.43 (C)O); δF (CDCl3): -126.50 (2 F, CF2CF3), -124.94 (2 F, CH2-CF2-CF2), -115.52 (2 F, CH2-CF2), -81.45 (3 F, CF3); MS (EI, 70 eV): 402 (M+.). 2b: 2-(Thiophen-3-yl)ethyl-4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononanoate. Yield: 71.9%; white solid, mp: 48-49 °C; δH (CDCl3): 2.32-2.51 (m, 2 H, CH2-CH2-CF2), 2.56-2.68 (m, 2 H, CH2-CH2-CF2), 2.99 (t, 2 H, CH2-CH2-O, 3JHH ) 6.9 Hz), 4.34 (t, 2 H, CH2-CH2-O, 3JHH ) 6.9 Hz), 6.97 (dd, 1 H, HTh, 3JHH ) 4.9 Hz, 4JHH ) 1.1 Hz), 7.03 (dd, 1 H, HTh, 4JHH ) 3.0 Hz, 4JHH ) 1.1 Hz), 7.29 (dd, 1 H, HTh, 3JHH ) 4.9 Hz, 4JHH ) 3.0 Hz); δC (CDCl3): 25.87 (t, CH2-CH2-CF2, 3JCF ) 4.1 Hz), 26.87 (t, CH2CH2-CF2, 2JCF ) 22.1 Hz), 29.87 (CH2-CH2-O), 65.36 (CH2CH2-O), 122.07, 126.19 and 128.51 (CHTh), 138.05 (CqTh), 171.42 (C)O); δF (CDCl3): -126.66 (2 F, CF2CF3), -124.08 (2 F, CH2CF2-CF2), from -123.48 to -122.36 (4 F, CH2-CF2-CF2-(CF2)2), -115.33 (2 F, CH2-CF2), -81.27 (3 F, CF3), MS (EI, 70 eV); 502 (M+.).
Stable Superhydrophobic Films 2c: 2-(Thiophen-3-yl)ethyl-4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate. Yield: 80.2%; white solid, mp: 68-69 °C; δH (CDCl3): 2.31-2.49 (m, 2 H, CH2-CH2-CF2), 2.572.67 (m, 2 H, CH2-CH2-CF2), 2.99 (t, 2 H, CH2-CH2-O, 3JHH ) 6.9 Hz), 4.34 (t, 2 H, CH2-CH2-O, 3JHH ) 6.9 Hz), 6.97 (dd, 1 H, HTh, 3JHH ) 4.9 Hz, 4JHH ) 1.1 Hz), 7.03 (dd, 1 H, HTh, 4JHH ) 3.0 Hz, 4JHH ) 1.1 Hz), 7.28 (dd, 1 H, HTh, 3JHH ) 4.9 Hz, 4JHH ) 3.0 Hz); δC (CDCl3): 25.91 (t, CH2-CH2-CF2, 3JCF ) 4.1 Hz), 26.92 (t, CH2-CH2-CF2, 2JCF ) 22.0 Hz), 29.87 (CH2-CH2-O), 65.36 (CH2-CH2-O), 122.05, 126.17 and 128.50 (CHTh), 138.03 (CqTh), 171.42 (C)O); δF (CDCl3): -126.59 (2 F, CF2CF3), -123.99 (2 F, CH2-CF2-CF2), from -123.20 to -122.35 (8 F, CH2-CF2CF2-(CF2)4), -115.33 (2 F, CH2-CF2), -81.24 (3 F, CF3); MS (EI, 70 eV): 602 (M+.). 2d: 2-(Thiophen-3-yl)ethyl nonanoate. Yield: 84.9%; colorless oil, ηD ) 1.4875; δH (CDCl3): 0.73 (t, 3 H, CH3, 3JHH ) 6.4 Hz), 1.07-1.18 (m, 10 H, (CH2)5), 1.41-1.48 (m, 2 H, O-CO-CH2CH2), 2.14 (t, 2 H, O-CO-CH2-CH2, 3JHH ) 7.5 Hz), 2.81 (t, 2 H, CH2-CH2-O, 3JHH ) 7.0 Hz), 4.13 (t, 2 H, CH2-CH2-O, 3JHH ) 7.0 Hz), 6.82 (dd, 1 H, HTh, 3JHH ) 5.0 Hz, 4JHH ) 1.3 Hz), 6.96 (dd, 1 H, HTh, 4JHH ) 3.0 Hz, 4JHH ) 1.3 Hz), 7.11 (dd, 1 H, HTh, 3J 4 HH ) 5.0 Hz, JHH ) 3.0 Hz); δC (CDCl3): 14.53 (CH3), 23.08, 25.39, 29.55, 29.66 and 32.24 ((CH2)6), 34.77 (CO-CH2), 30.06 (CH2-CH2-O), 64.46 (CH2-CH2-O), 121.93, 125.97 and 128.66 (CHTh), 138.53 (CqTh), 174.20 (C)O). Polymerization, Characterization of Polymers, and Film Preparation. FeCl3 (5 mmol, 811 mg) was added to a solution of monomer 1a-d or 2a-d (1 mmol) in 20 mL of dry chloroform. The reaction mixture was stirred overnight, and 100 mL of methanol was added to precipitate the polymer. After filtration, the dark-blue polymer was washed continuously with methanol in a Soxhlet apparatus for 24 h (during while the polymer is reduced) and the soluble fraction obtained by extraction with chloroform for supplementary 24 h. Gel permeation chromatography (GPC) of the polymers was carried out using an Agilent 1100 liquid chromatograph equipped with a PL gel 5 Mixed D column, an Agilent G1362A refractive index detector and an Agilent G1315A UV detector. THF (HPLC grade) was used as the eluent at a flow rate of 1 mL‚min-1. Molecular weights and molecular weight distributions were calculated on the basis of monodispersed polystyrene standards. UV-vis spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer. The polythiophene films were coated onto glass by drop casting a solution of poly-2a-d in chloroform or in forane 113 (1,1,2trichloro-1,2,2-trifluoroethane) (5 g‚L-1). Tetra-n-butylammonium hexafluorophosphate Bu4NPF6 from Fluka (puriss., electrochemical grade) was used as received. Acetonitrile (anhydrous analytical grade) from Fluka was used without further purification and stored under dry argon. All electrolytic solutions were degassed with argon prior to use and all experiments were carried out at room temperature. Cyclic voltammetry experiments were performed with an Autolab PGSTAT 30 potentiostat from Eco Chemie B. V. equipped with General Purpose Electrochemical System GPES software (version 4.9 for windows). The working electrode was either a platinum disk (area ) 7.1 mm2) or an ITO plate (area ) a few cm2) and the counter electrode a glassy carbon rod. All potentials were relative to the system Ag/AgCl reference electrode (E′ ) 0.14 V/ENH). The poly-2b and poly-2c films were potentiostatically electroformed on ITO with an anodic limit close to the irreversible peak of the monomer 2b or 2c, under the following conditions: applied voltage, 2.10 V; monomer concentration, 20 mM; reaction time, 300 s. The films were washed with acetonitrile to eliminate reactants. All films were finally dried for a few days at room temperature under reduced pressure. Surface Characterization Techniques. Contact angles measurements were performed using the sessile drop method on a Kru¨ss DSA10 contact angle goniometer. The angles reported here were the averages of at least five measurements. Reproducibility was within 3°. They were recorded at 22 ( 1 °C using distilled water, diiodomethane, and hexadecane as wetting liquids. Water and
Langmuir, Vol. 22, No. 7, 2006 3083 Scheme 1. Synthesis of Monomers.
hexadecane contact angles were taken as the index of the hydrophobicity and lipophobicity, respectively. Surface energy γ, and its dispersive and polar components, denoted as γd and γp, respectively, were evaluated using the Owens, Wendt, Fowkes concept.15 The advancing and receding contact angles for water were determined by sessile drop experiments which were analyzed by axisymmetric drop shape analysis-profile (ADSA-P).16 Scanning electron microscopy (SEM) of the films was carried out with a JEOL 6700F microscope, after covering them with a thin layer of sputtered carbon obtained by thermal evaporation. Electrodeposited films were directly observed.
Results and Discussion Synthesis of Monomers. With a view to investigating the effect of fluorinated chains on the surface properties and selfassembly of conjugated polymers, a series of new thiophenes, containing a flexible connector between the rigid fluorocarbon chain and the polymerizable heterocycle, were synthesized by coupling commercially available 2-(3-thienyl)ethanol or 3-thienylmethanol with the desired 3-perfluoroalkylpropanoic acid, using N,N′-dicyclohexylcarbodiimide (DCC) as a carboxy group activated reagent and 4-(dimethylamino)pyridine (DMAP) as a catalyst17 (Scheme 1). Corresponding alkyl substituted monomers were also synthesized for comparison. 3-Perfluoroalkylpropanoic acids were obtained by carboxylation of the corresponding organomagnesium compound of 2-perfluoroalkylethyl iodide.18 Polymerization and Polymer Characterization. Fluorinated polythiophene films were prepared either by oxidative polymerization of the corresponding monomers and then deposited on solid substrates by drop-casting a soluble fraction of chemically made polymer or by electrodeposition (Scheme 2). The monomers 1a-d and 2a-d were polymerized using iron chloride (FeCl3) as an oxidant,19 in dry chloroform, at room temperature, for 24 h. Afterward, the soluble fraction were collected by Soxhlet extraction and characterized. (15) (a) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538. (b) Owens, D. K.; Webdt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (16) Kwok, D. Y.; Gietzelt, T.; Grundke, K.; Jacobasch, H.-J.; Neumann, A. W. Langmuir 1997, 13 (10), 2880. (17) Han, S.-Y.; Kim, Y.-A. Tetrahedron 2004, 60, 2447. (18) Jouani, A. M.; Szo¨nyi, F.; Cambon, A. J. Fluorine Chem. 1992, 56, 85. (19) Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoshino, K. Chem. Express 1986, 1, 635.
3084 Langmuir, Vol. 22, No. 7, 2006
Nicolas et al. Table 2. Optical Data for Monomers and Polymers
Scheme 2. Polymer Synthesis: (a) Chemical Polymerization and (b) Electropolymerization.
monomera
polymerb
compound
λmax (nm)
(L‚mol ‚cm )
λmax (nm)
2a 2b 2c 2d
235 235 235 232
6200 6200 5800 6200
401 359 353 422
a
-1
-1
5 × 10-5 M in CH2Cl2. b Soluble fraction in CHCl3. Table 3. Voltammetric Data for Monomers and Polymers
Table 1. Molecular Weights of Soluble Fluorinated Polythiophenes polymer
soluble fraction yield
Mw (g‚mol-1)
PDa
Xnb
2a 2b 2c 2d
100% 86% 51% 100%
21 900 15 700 8600 31 000
2.3 1.9 1.4 3.8
55 31 14 116
a
Polydispersity index. b Average degree of polymerization.
Figure 1. UV-visible spectra of polymers in chloroform: (a) poly2a (60 mg‚L-1), (b) poly-2b (120 mg‚L-1), (c) poly-2c (280 mg‚L-1), and (d) poly-2d (50 mg‚L-1).
Polymerization reaction was unefficient for the monomers 1a-d that contains only one methylene spacer between the heterocycle and the oxygen. They gave rise, in low yields (2040%), to brown insoluble solids. This can be explained by the inductive electron-withdrawing character of the oxygen atom that decreases the electronic density of the thiophene ring, as supported by the electrochemical results. Introducing another methylene group solved this problem. Monomers 2a-d furnished, in high yields (80-90%), classical powdery black solids. After Soxhlet extraction with chloroform, soluble fractions were analyzed by GPC and UV-visible spectrometry. Increasing the length of the fluorocarbon chain led to a decrease of solubility (100, 86, and 51% respectively for n ) 4, 6, and 8) as well as shorter conjugated segments (average degree of polymerization Xn ) 55, 31, and 14 respectively for n ) 4, 6, and 8; Table 1). It must be pointed out that all samples were few polydispersed. The corresponding alkylated polymer poly-2d afforded longer conjugated segments (Xn ) 116) due to a better solubility of oligomeric and polymeric species during polymerization. Polymers were red in their solid state and yellow in solution, due to solvatochromic effect. The latter absorb visible light in the violet-indigo-blue region (about 400-500 nm; Figure 1), which renders the substance yellow since the human eye discerns the complementary absorbed color. Optical band gaps of these polymers (Eg ) 2.3-2.5 eV) were calculated from their absorption
compound
Epa monomer (V)a
E°′ polymer (V)b
1a 1b 1c 1d 2a 2b 2c 2d
2.39 2.38 2.35 2.35 2.25 2.15 2.14 2.19
1.51 1.49 1.47 1.50 1.34 1.24 1.33 1.27
a Anodic peak potential relative to the compound at 5 × 10-2 M for 1 and 2 × 10-2 M for 2, V ) 100 mV‚s-1. b Average of anodic and cathodic peak potentials corresponding to p doping-undoping process.
edge and represent typical values for polythiophenes.20 No substituent effect was observed for the monomer or for the polymer when observing the optical data. The bathochromic and hyperchromic effects observed for poly-2a and poly-2d (λmax ) 401 and 422 nm) compared to poly-2b and poly-2c (λmax ) 359 and 353 nm) were essentially due to higher conjugation length and better solubility. To summarize, soluble fluorinated conjugated polymers with different fluorocarbon chain length were obtained by chemical oxidation of the corresponding monomers. They can be easily handled and deposited on surfaces for analysis. Compared to other semifluoroalkyl-substituted thiophenes that exhibit poor solubilities and restrict the choice of the solvents for electrochemical polymerization (use of propylene carbonate, nitrobenzene, etc.) and need to be more concentrated (0.1 M) to polymerize,9,11,21 monomers 1a-c and 2a-c were soluble in most common organic solvents, particularly in acetonitrile, even with a perfluorooctyl chain (n ) 8) and polymerize at low concentration (a few mM). Cyclic voltammetric characterization of the five-membered heterocycles in CH3CN, Bu4NPF6 10-1 M revealed an irreversible oxidation peak Epa in the range 2.10-2.40 V at V ) 100 mV‚s-1 (Table 3). The incorporation of fluorinated chains did not really have influence on the thiophene potential peak but the place of the oxygen in the connector did. Indeed, monomers 1a-d were oxidized at higher potential than monomers 2a-d because of the electron-withdrawing effect of the oxygen that decreases the electronic density of thiophene and increases the reactivity of the corresponding radicals which can thus undergo rapid reactions with the solvent or anions to form soluble products rather than to electropolymerize.22 It has been demonstrated that the introduction of two methylene groups was necessary to neutralize this effect.23 Thiophene derivatives were anodically electropolymerized at 20 mM in thoroughly dried acetonitrile containing Bu4NPF6 10-1 M as the supporting electrolyte. The PF6- doped films were (20) Roncali, J. Chem. ReV. 1997, 97, 173. (21) Middlecoff, J. S.; Collard, D. M. Synth. Met. 1997, 84, 221. (22) (a) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76. (b) Roncali, J. Chem. ReV. 1992, 92, 711. (23) (a) Roncali, J.; Garreau, R.; Delabouglise, D.; Garnier, F.; Lemaire, M. Synth. Met. 1989, 28, C341. (b) Lemaire, M.; Garreau, R.; Roncali, J.; Delabouglise, J.; Korri, H.; Garnier, F. New J. Chem. 1989, 13, 863.
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Figure 2. Successive cyclic voltammograms of (a) 1b (5 × 10-2 M, final electropolymerization charge: Qs ) 660 mC‚cm-2) and (b) 2b (2 × 10-2 M, Qs ) 520 mC‚cm-2). Electrochemical response (10 first cycles) of the electroformed polymers: (c) poly-1b and (d) poly-2b. Electrolyte medium: CH3CN, Bu4NPF6 10-1 M, V ) 100 mV‚s-1.
potentiodynamically electroformed with an anodic limit close to the irreversible oxidation peak of the monomer. For instance, successive cyclic voltammograms of 1b (Figure 2a) or 2b (Figure 2b) showed the emergence of new redox systems attributable to the doping-undoping process of the electroformed polymers. Monomers 1a-d need to be more concentrated (50 mM) to give rise to the formation of a polymer film on the working electrode. Following their electrosynthesis, the polymer-coated electrodes were examined in monomer-free acetonitrile solution. Poly-2a-d exhibited stable (toward electrochemical cycling) reversible redox system at about 1.30 V corresponding to the p doping-undoping process (Figure 2d), whereas poly-1a-d showed a very unstable system at about 1.50 V (Figure 2c). The electronic effect of oxygen could explain the low polymer yield (due to formation of large amount of soluble products), the low conductivities, and the poor electrochemical properties. Incorporation of a second methylene group led to materials with higher conductivities and longer conjugated segments.23 A linear relationship between the anodic peak current Ipa and the potential scan rate V was obtained, as expected for the surfaceimmobilized redox species.24 The peak-to-peak separation ∆Ep associated with oxidation and reduction of the polymer was in the range 100-500 mV depending on film thickness. Indeed, ∆Ep continues to increase upon repeated potential cycling as more polymer is deposited and presents a barrier to diffusion of counteranion in and out of the film. This is all the more true with a longer fluorocarbon chain. The doping level deduced from the cyclic voltammetry curves was in the range 0.15-0.30 depending on the film thickness, excepted for poly-1a, poly-1d, and poly-2a that exhibited low values (0.02-0.06) due to a lack of adhesion of the polymer onto the platinum electrode surface and a partial solubility. (24) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods, Fundamental and Applications; Wiley: New York, 1980.
No n doping-undoping system was observed for any electroformed polymer. On examining the chemical and electrochemical polymerization results, it clearly appeared that poly-2a-d were good candidates for surface properties analysis. They exhibited classical behavior similar to those of polythiophenes and contained varying amount of fluorine. Emphasis was so laid on the water and oil repellent properties of these polymers as well as the influence of the method of deposition (drop casting or electrodeposition) on their surface properties. Surface Properties. The surface properties of poly-2a-d were investigated in their neutral stable reduced state. Indeed, when exposed to air, chemical reduction was taking place, as indicated by a color change from blue-green (oxidized state) to red-orange (reduced state). Polymers were deposited on substrates either from slow evaporation of a neutral polymer solution onto the desired substrate, glass for example, or by electroformation on indium tin oxide (ITO) plate followed by electrochemical reduction. Chemically formed polymer samples were coated on glass plates by drop casting a solution of poly-2a-d in chloroform (5 g‚L-1) or in forane 113 (5 g‚L-1). By applying this method, the control of the macroscopic homogeneity and the film thickness was rather intricate and some defects such as cracks and bubbles were observed. When comparing contact angle and surface energy values for the polymers prepared from chloroform solution (Table 4), it clearly appears that the introduction of fluorocarbon chain on the 3 position of thiophene leads to a decrease of the surface free energy as well as an increase of contact angles. It is worth noting that the length of the fluorinated chain does not really interfere with the results, as it could have been expected. Measures were made at different positions in the coated substrate to be aware of the accuracy and the reproducibility of the results as well as the homogeneity of the deposit.
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Table 4. Contact Angles Data of Fluorinated Polythiophenes method of preparation
polymer
θwater (°)
θdiiodomethane (°)
θhexadecane (°)
γ (mN/m)
γd(mN/m)
γp(mN/m)
from chloroform solution
2a 2b 2c 2d 2a 2b 2c 2a 2b 2c
112 105 108 80 130 131 132
94 92 96 59 114 113 109
77 76 78 33 97 102 96
11.4 13.2 11.8 32.0 5.0 4.6 5.7
10.2 10.5 9.5 24.2 4.8 4.4 5.7
1.2 2.7 2.3 7.8 0.2 0.2 0
146 153
137 137
129 135
from forane solution electrodeposition
The use of fluorinated solvent such as forane 113 was dictated by a better solubility of fluorinated compounds in fluorinated solvents and also by a better organization and orientation of the fluorinated chains at the polymer-air interface, induced by the interaction with the solvent molecules during evaporation. It is important to bear in mind that the history of every polymer or material plays a role on its properties. The change of only one condition can modify drastically one or several properties. Indeed, that is the case here. Surface properties were greatly improved by the use of forane 113 instead of chloroform and very low surface energies (4-6 mN‚m-1) were calculated. Nevertheless, if oil repellency was quite good and stable (no change of θhexadecane with time), a decrease of the contact angle with water was observed for all of the chemically prepared fluorinated polymers in short time (about 20-40% in 10 min; Figure 4) indicating some interactions occurred between polymer surface and water drop. This is evidence for a surface rearrangement of fluorinated side chains. To check this statement and to describe more precisely the hydrophobicity of these surfaces, both advancing and receding contact angles for water, and the difference between them (or hysteresis), must be considered. Measurements made on chemically formed poly(fluorinated thiophene) surfaces showed a high hysteresis value (upper than 70°). In this case, constant receding values were not
Figure 3. Photographs of water drops (2 µL) on a poly-2c (a) drop cast from a chloroform solution, (b) drop cast from a forane solution, and (c) electrodeposited.
Figure 4. Evolution of water contact angle with time for ([) electrodeposited poly-2c; (9) electrodeposited poly-2b; (]) poly2c (from forane solution); (0) poly-2b (from forane solution); (4) poly-2a (from forane solution); ([) poly-2c (from chloroform solution); (9) poly-2b (from chloroform solution); (2) poly-2a (from chloroform solution).
obtained and an initial receding contact angle value is given. For instance, the advancing and receding contact angles values were respectively 105° and 32° for poly-2c coated from chloroform solution. The time dependent receding angle and the high hysteresis as well as the short time instability of the static angle suggest that the surface changes as a result of the interaction with water. The reasons of this phenomenon can be reorganization of the surface, penetration of the liquid, heterogeneity, etc.25,26 It is well-known that more homogeneous conjugated polymers can be obtained by electrodeposition on a conducting electrode (Pt, Au, glassy carbon, ITO). Due to the lack of adhesion of poly-2a onto ITO plate, only poly-2b and poly-2c could be analyzed. Electropolymerization was carried out at a fixed potential (i.e., potentiostatically) and the control of the electropolymerization charge Qs permit us to obtain films with relatively constant thickness. Indeed, the quantity of polymer deposited on the electrode is function of the charge consumed during the film electroformation.1a Results obtained were remarkable, in terms of performance and stability. Contact angles with water, diiodomethane, and hexadecane were exceptionally high (Table 4). Surface energies could not have been calculated from these values because they found to be out of the limits of the Owens, Wendt, and Fowkes method. Results were slightly higher with a perfluorooctyl chain (n ) 8), compared to a perfluorohexyl chain (n ) 6), and adhesion was also better. The contact angle with water over 150° defines a super-hydrophobic surface. Moreover, whereas a decrease was observed with chemically formed films, no loss at all was noticed with electroformed polymer, in short time, but also during several months. The electrodeposited poly(fluorinated thiophene) surfaces exhibit very small contact angle hysteresis (lower than 10°) with the result that water droplets roll easily. Advancing and receding contact angles values were 151° and 145° for electrodeposited poly-2c. Thus, these surfaces can be defined as “ultrahydrophobic”, i.e., exhibiting both large water contact angle and small hysteresis.27,28 Arising from their low surface energy, very good oil repellency was also observed with contact angles of about 130° and more. It is worth noting that these values are extremely high since there are few lipophobic materials having contact angle higher than 100°.27,29 Finally, the surface modification by electrodeposited fluorinated polythiophenes appears as an excellent method to reach very stable super-hydrophobic and lipophobic surfaces. (25) Bongiovanni, R.; Malucelle, G.; Priola, A. J. Colloid Interface Sci. 1995, 171, 283. (26) Takahashi, S.; Kasemura, T.; Asano, K. Polymer 1997, 38, 2107. (27) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J.; MacCarthy, T. J. Langmuir 1999, 15 (10), 3395. (28) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782. (29) (a) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int Ed. Engl. 1997, 36, 1011. (b) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21 (8), 3235.
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Figure 5. SEM images of the surface of poly-2c (a) deposited on glass plate by evaporation of polymer chloroform solution, (b) deposited on glass plate by evaporation of polymer forane solution, (c) and (d) electrodeposited on ITO plate (Eimp ) 2.1 V, Qs ) 500 mC‚cm-2).
Scanning electron microscopy (SEM) was used to examine the morphology and the structural characteristics of the modified surfaces. The SEM micrographs of chemically and electrochemically prepared poly-2c were shown in Figure 5. The polymers deposited by drop casting (Figure 5, panels a and b) were composed of conglomerates of different microscopic size and not very regular whereas the electrodeposited polymer presented a better organization and the well-known cauliflowerlike appearance of electrochemically obtained polythiophene30 (Figure 5, panels c and d). Spherical submicroscopic structures were clearly seen and the film had a rough microstructure, whereas the drop cast films were much smoother. Deposition techniques employed here are totally different: the polymerization with a chemical oxidant such as FeCl3 takes place in solution and only the soluble fraction of the polymer is deposited on surface; the disposition and arrangement of the polymer chains mainly occurred during evaporation of the solvent; the electroformation of a polymer on a conductive electrode results from a slow polymer growth on nucleation sites randomly distributed on the conductive underlayer. This nucleation and growth mechanism during anodic electropolymerization is at the roots of the surface roughness that contributed undoubtedly to the higher contact angles noticed for electrodeposited polymers. The reaction conditions for the electrochemical polymerization (electrochemical method: po(30) Ugalde, L.; Bernede, J. C.; Del Valle, M. A.; Diaz, F. R.; Leray, P. J. Appl. Polym. Sci. 2002, 84, 1799.
tentiostatic or potentiodynamic deposition, monomer concentration, electrolytic medium, time, working electrode) govern the nucleation and growth mechanism of polythiophenes, influence the surface structure and could be optimized. Nevertheless, no modification was observed on the contact angles data when varying the film thickness (from a few mC‚cm-2 to a few C‚cm-2), i.e., when modifying time of electrodeposition.
Conclusion A series of low surface energy fluorinated conjugated polymers were prepared by oxidative polymerization and characterized. Incorporation of perfluoroalkyl moieties into a polythiophene backbone reduced the surface energy significantly compared to the alkyl analogue due to a high percentage of fluorine at the outermost layer of the polymer. The length of the fluorocarbon chain (from perfluorobutyl to perfluorooctyl) did not have a real influence on the surface properties of the materials but varying the method of deposition (i.e., drop casting or electrodeposition) modified them drastically. On one hand, electrodeposited fluorinated polythiophene films exhibited excellent superhydrophobic and lipophobic properties with exceptional time stability since no loss of properties or degradation were observed during several months. On the other hand, drop cast films were less stable and homogeneous. This is correlated with a better organization and orientation of the fluorocarbon chains at the polymer-air interface. SEM images highlighted the fact that the
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electrodeposited films were more structured and presented higher roughness. Finally, controlling the chemical composition (use of fluorinated chains) as well as the surface structure (by electrodeposition) leads to new materials with both excellent and stable water and oil repellency.
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Fluorinated grafted conductive polymers can also represent in a short-term prospect a nonconventional tool to achieve strong bioactive surface properties and works are currently underway. LA053055T
