Synthesis and Properties of Perfluorinated ... - ACS Publications

Jul 3, 2008 - Thierry Darmanin,* Mael Nicolas,* and Frédéric Guittard*. Institut de ..... G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (d...
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Synthesis and Properties of Perfluorinated Conjugated Polymers Based on Polyethylenedioxythiophene, Polypyrrole, and Polyfluorene. Toward Surfaces with Special Wettabilities Thierry Darmanin,* Mael Nicolas,* and Frédéric Guittard* Institut de Chimie de Nice, FR 3037, 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 April 3, 2008. ReVised Manuscript ReceiVed May 8, 2008 The electrochemical deposition of organic materials is a convenient and straightforward method that affords rough films in mild conditions. The presence of fluorinated chains covalently attached on the polymer backbone allows the control of the second criterion which plays a role on the wetting properties of the surface, that is, the chemical composition. By modification of the nature of the polymer, films with different surface energies were obtained. Thus, original semifluorinated polypyrrole (PPy-RFn), polyfluorene (PFl-RFn), and polyethylenedioxythiophene (PEDOTRFn) have been chemically and electrochemically synthesized and characterized. On one hand, the chemical polymerization affords highly fluorinated soluble polymers. Soluble PFl-RFn exhibits blue fluorescence in solution while soluble PEDOT-RFn presents optical properties similar to those of PEDOT. Consequently, they represent interesting candidates for optical devices (OLEDs for PFl-RFn, electrochromic materials for PEDOT-RFn). On the other hand, surface properties have been investigated on the electroformed polymers by goniometry and microscopy. Fluorinated surfaces of electrodeposited polypyrrole, like polythiophene, give birth to high hydrophobic and oleophobic surfaces, while the use of polyethylenedioxythiophene as the polymer increases sufficiently the surface energy to get combined superhydrophobicity and superoleophilicity. The influence of the chemical composition is discussed through the comparison of the wetting properties of polyethylenedioxythiophene and semifluorinated polythiophene and polyethylenedioxythiophene.

Introduction The incorporation of fluorine and perfluorocarbon units into polymers has been extensively studied due to its advantage to combine at the same time a low friction coefficient, rigidity, low surface energy, hydrophobicity, self-organization, chemical inertness, and thermal resistance, which make them useful for a wide variety of applications.1 This has been achieved by different ways for various polymers including poly(methyl methacrylate), polyamide, polystyrene, polyethylene, polybutadiene, silica polymers, etc. The combination of the unique characteristics of fluorine and the specific properties of conjugated polymers2 has also led to the development of new materials with interesting properties from both scientific and industrial viewpoints. Indeed, most of the properties of conducting polymers and the efficiency of the devices in which they are engaged are sensitive to oxygen and moisture. It is expected that such fluorinated conducting polymers enhance their environmental stability, provided that their intrinsic electronic and optical properties be conserved. For instance, conducting polymers, such as polythiophene,3 poly* Corresponding authors. (T.D.) E-mail: [email protected]. Tel: ++33 49207 6158. (M.N.) E-mail: [email protected]. (F.G.) E-mail: [email protected]. Tel: ++33 49207 6159. (1) (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. (2) (a) Handbook of Conducting Polymers; Skotheim, T. A., Eds.; Marcel Dekker: New York, 1986. (b) Handbook of AdVanced Electronic and Photonic Materials and DeVices; Nalwa, H. S., Eds.; Academic Press: San Diego, CA, 2001. (3) (a) Bu¨chner, W.; Garreau, R.; Lemaire, M.; Roncali, J.; Garnier, F. J. Electroanal. Chem. 1990, 277, 355. (b) Ritter, S. K.; Noftle, R. E.; Ward, A. E. Chem. Mater. 1993, 5, 752. (c) Robitaille, L.; Leclerc, M. Macromolecules 1994, 27, 1847. (d) Hong, X.; Tyson, J. C.; Middlecoff, J. S.; Collard, D. M. Macromolecules 1999, 32, 4232. (e) Ganapathy, H. S.; Kim, J. S.; Jin, S.-H.; Gal, Y.-S.; Lim, K. T. Synth. Met. 2006, 156, 70.

Chart 1. Fluorinated Electroformed Polythiophenes

p-phenylenevinylene,4 polyfluorene,5 polyethylenedioxythiophene,6 etc., have been functionalized by fluorine or perfluoroalkyl groups and some of the electrical, electrochemical, optical, thermal, and surface properties of the fluorinated polymers were analyzed in solution or in their solid state. Nevertheless, the surface analysis and properties are not always investigated or incomplete. Recently, our group focused on the wetting properties of these materials, and as a result, stable superhydrophobic and lipophobic surfaces were obtained by electrodeposition of fluorinated thiophenes (Chart 1),7 combining thus the two parameters that govern the wettability of a surface, that is the chemical (4) Krebs, F. C.; Jensen, T. J. Fluorine Chem. 2003, 120, 77. (5) Kameshima, H.; Nemoto, N.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3143. (6) (a) Schwendeman, I.; Gaupp, C. L.; Hancock, J. M.; Groenendaal, L. B.; Reynolds, J. R. AdV. Funct. Mater. 2003, 13, 541. (b) Zuo, L.; Qing, F.-L.; Meng, W.-D.; Huang, X.; Zhang, S.; Wu, Q. J. Fluorine Chem. 2004, 125, 1441. (7) (a) Nicolas, M.; Guittard, F.; Ge´ribaldi, S. Angew. Chem., Int. Ed. 2006, 45, 2251. (b) Nicolas, M.; Guittard, F.; Ge´ribaldi, S. Langmuir 2006, 22, 3081. (c) Nicolas, M.; Guittard, F.; Ge´ribaldi, S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 4707.

10.1021/la801040z CCC: $40.75  2008 American Chemical Society Published on Web 07/03/2008

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Scheme 1. Synthesis of the Fluorinated Monomers and Polymers

composition (use of perfluorocarbon chain) and the geometrical microstructure (rough morphology obtained by electropolymerization). Here, we report the synthesis of fluorinated monomers derived from ethylenedioxythiophene (EDOT), pyrrole (Py), and fluorene (Fl), their chemical and electrochemical polymerization, and some spectroscopic and surface properties of the resulting polymers, in solution and/or in their solid state. While electrodeposited fluorinated polypyrrole films exhibited excellent highly hydrophobic and lipophobic properties, the covalent binding of low free energy fluorinated chains on the EDOT monomer before polymerization lowers the surface tension of the electroformed polymers sufficiently to give rise to superhydrophobic behavior together with superoleophilicity. It promises to be an interesting candidate for the separation of water and oil.

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

Figure 1. UV-visible and fluorescence spectra of PFl-RFn.

were determined with a Bu¨chi 510 melting point apparatus. 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 quadrupole analyzer, fitted with a TraceGC gas chromatograph. Typically, in a three-neck flask under argon, 4,4,5,5,6,6,7,7,7nonafluoroheptanoic acid, 4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononanoic acid, or 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoic acid (5 mmol), N,N′-dicyclohexylcarbodiimine (DCC, 5 mmol), and 4-dimethylaminopyridine (DMAP, 0.5 mmol) were mixed in dichloromethane (15 mL), and the reaction mixture was stirred for 1 h. Afterward, (2,3-dihydro-thieno[3,4-b][1,4]dioxin2-yl)-methanol, 1H-pyrrole-1-ethanol, or 9-fluorenemethanol (5 mmol) was added, and stirring was continued for 24 h. After filtration and evaporation of the solvent, the crude product was purified by column chromatography (eluent, ether/cyclohexane 1:1 for EDOTRFn; dichloromethane/cyclohexane 3:7 for Fl-RFn; ether for PyRFn). EDOT-RF4: (2,3-Dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl 4,4,5,5,6,6,7,7,7-Nonafluoroheptanoate. Yield 74%; colorless liquid. δH(CDCl3): 6.37 (d, 1 H, HTh, 4JHH ) 3.8 Hz), 6.35 (d, 1 H,

Varying Surface Properties with Perfluorinated Polymers

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Table 1. Optical Data for Monomers and Polymers monomera

polymerb

compound

λmax (nm)

 (L · mol-1 · cm-1)

λmax (nm)

Eg (eV)

EDOT EDOT-RF4 EDOT-RF6 EDOT-RF8 Py-RF4 Py-RF6 Py-RF8 Fl-RF4 Fl-RF6 Fl-RF8

259 257 257 257 229 229 229 267 267 267

10130 7300 7160 7160 3460 4240 3660 19680 19840 19600

370 450 372 357

2.17 1.97 2.15 2.22

361 350 349

3.09 3.15 3.18

a

5 × 10-5 M in CH2Cl2.

b

Soluble fraction in CHCl3.

Table 2. Voltammetric Data for Monomers and Polymers compound

Epa monomer (V)a

E°′ polymer (V)b

EDOT-RF4 EDOT-RF6 EDOT-RF8 Py-RF4 Py-RF6 Py-RF8 Fl-RF4 Fl-RF6 Fl-RF8

1.79 1.77 1.72 1.58 1.62 1.65 2.39 2.30 2.43

0.38 0.51 0.46 0.95 1.03 1.03

a anodic peak potential relative to the compound at 2.10-2 M, V ) 100 mV.s-1 b average of anodic and cathodic peak potentials corresponding to p doping-undoping process.

HTh, 4JHH ) 3.8 Hz), 4.39 (m, 1 H, CH2-CH(O)CH2-O-CdO), 4.37 (m, 2 H, CH2-CH(O)CH2-O-CdO), 4.23 (dd, 1 H, CH2-CH(O)CH2-O-CdO, gemJ ) 11.8 Hz, 3JHH ) 2.0 Hz), 4.05 (dd, 1 H, CH2-CH(O)CH2-O-CdO, gemJ ) 11.8 Hz, 3JHH ) 6.6 Hz), 2.69 (t, 2 H, CH2-CH2-CF2, 3JHH ) 7.2 Hz), 2.48 (tt, 2 H, CH2-CF2, 3JHF ) 18.8 Hz,3JHH ) 7.2 Hz). δC(CDCl3): 170.73 (CdO), 141.08 (CqTh), 140.83 (CqTh), 100.15 (CHTh), 100.07 (CHTh), 71.23 (CH2-CH(O)CH2-O-CdO), 65.41 CH2-CH(O)CH2-O-CdO), 63.01 (CH2-CH(O)CH2-O-CdO), 26.33 (t, CH2-CF2, 2JCF ) 22.4 Hz), 25.27 (t, CH2-CH2-CF2, 3JCF ) 4.2 Hz). δF(CDCl3): -126.48 (m, 2 F, CF2-CF3), -124.88 (m, 2 F, CF2-CF2-CH2), -115.43 (m, 2 F, CF2-CH2, -81.86 (tt, 3 F, CF3, 3JFF ) 10.3 Hz, 4J ) 3.2 Hz). FT-IR (main vibrations): 3118 ν(CH), 2967 ν(CH), FF 2946 ν(CH), 2860 ν(CH), 1741 ν(CdO), 1490 ν(CdC)ring, 1454 δ(CH), 1380 ν(C-C)ring, 1220 ν(C-CO-O), 1182 ν(CdC)ring, 1130 ν(CF), 1090 ν(C-O-CH2-CH2-O-C), 934 ν(C-S)ring, 840 ν(C-S)ring, 768 γ(CH), 696 δ (C-S-C). MS (EI, 70 eV): 446 (M+). Py-RF4:2-(1H-pyrrol-1-yl)ethyl4,4,5,5,6,6,7,7,7-nonafluoroheptanoate. Yield 71%; colorless liquid. δH(CDCl3): 6.67 (t, 2 H, HPy, 3J 3 HH ) 2.1 Hz), 6.18 (t, 2 H, HPy, JHH ) 2.1 Hz), 4.38 (t, 2 H, 3 CH2-CH2-O, JHH ) 5.4 Hz), 4.15 (t, 2 H, CH2-CH2-O, 3JHH ) 5.4 Hz), 2.64 (t, 2 H, CH2-CH2-CF2,3JHH ) 7.3 Hz), 2.43 (tt, 2 H, CH2-CH2-CF2, 3JHF ) 19.1 Hz,3JHH ) 7.3 Hz). δC(CDCl3): 170.69 (CdO), 120.78 and 108.75 (CPy), 64.58 (CH2-O), 48.04 (CH2-CH2-O), 26.31 (t, CH2-CF2, 2JCF ) 22.2 Hz), 25.33 (t, CH2-CH2-CF2, 3JCF ) 40.3 Hz). δF(CDCl3): -126.51 (m, 2 F, CF2-CF3), -124.94 (m, 2 F, CF2-CF2-CF3), -115.49 (m, 2 F, CF2-CH2), -81.50 (tt, 3 F, CF3, 3JFF ) 10.4 Hz, 4JFF ) 3.2 Hz). FT-IR (main vibrations): 2940 ν(CH), 2862 ν(CH), 1747 ν(CdO), 1501 ν(CdC)ring, 1218 ν(C-CO-O), 1176 ν(CdC)ring, 1132 (CF). MS (EI, 70 eV): 385 (M+). Fl-RF4:(9H-Fluoren-9-yl)methyl4,4,5,5,6,6,7,7,7-Nonafluoroheptanoate. Yield 64%; white solid, mp 57-58 °C. δH(CDCl3): 7.78 (d, 2 H, H4,5, 3JHH ) 6.9 Hz,4JHH ) 0.8 Hz), 7.58 (dd, 2 H, H1,8, 3J ) 7.3 Hz, 4JHH ) 0.8 Hz), 7.43 (td, 2 H, H3,6, 3JHH ) 7.3 Hz, 4JHH ) 0.8 Hz), 7.33 (td, 2 H, H2,7, 3JHH ) 7.3 Hz, 4JHH ) 1.3 Hz), 4.48 (d, 2 H, CH2-O, 3JHH ) 6.8 Hz), 4.23 (t, 1 H, H9, 3JHH ) 6.8 Hz), 2.69 (t, 2 H, CH2-CH2-CF2, 3JHH ) 7.4), 2.42 (tt, 2 H, CH2-CH2-CF2, 3JHF ) 19.3 Hz,3JHH ) 7.4 Hz). δC(CDCl3): 170.99

(CdO), 143.46 (CqFl), 141.33 (CqFl), 127.90 (C2,7), 127.14 (C3,6), 124.87 (C1,8), 120.09 (C4,5), 66.89 (CH2-O), 46.71 (C9), 26.34 (t, CH2-CF2, 2JCF ) 22.1 Hz), 25.46 (t, CH2-CH2-CF2, 3JCF ) 4.0 Hz). δF(CDCl3): -126.47 (m, 2 F, CF2-CF3), -124.91 (m, 2 F, CF2-CF2-CF3), -115.48 (m, 2 F, CF2-CH2), -81.44 (tt, 3 F, CF3, 3JFF ) 10.4 Hz, 4JFF ) 3.2 Hz). FT-IR (main vibrations): 3064 ν(CH), 2942 ν(CH), 1732 ν(CdO), 1448 ν(CdC)ring, 1226 ν(C-CO-O), 1194 ν(CdC)ring, 1132 ν(CF). MS (EI, 70 eV): 470 (M+). Oxidative Polymerization with Iron Chloride. FeCl3 (5 mmol, 811 mg) was added to a solution of monomer (1 mmol) in 20 mL of dry chloroform at room temperature. The reaction mixture was stirred 24 h (EDOT-RFn and Py-RFn) or 72 h (Fl-RFn) and 100 mL of methanol was added to precipitate the polymer. After filtration, the polymer was washed continuously with methanol in a Soxhlet apparatus for 24 h and the soluble fraction obtained by extraction with chloroform for supplementary 24 h. Electropolymerization. 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 indium tin oxide (ITO) or Au plate (area was a few square centimeters) and the counter electrode was a glassy carbon rod. All potentials were relative to the system Ag/AgCl reference electrode (E°′ ) 0.14 V/ENH). The polymer films were potentiostatically or potentiodynamically electroformed on ITO and Au with an anodic limit close to the irreversible peak of the monomer. The films were washed with acetonitrile to eliminate reactants. All films were finally dried for a few days at room temperature under reduced pressure. Spectroscopic Data. UV-visible and fluorescence spectra were recorded respectively with a Perkin-Elmer lambda 35 and a PerkinElmer LS 45 spectrometer. Surface Characterization. Contact angle measurements were performed on the electroformed polymer films 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. They were recorded at 22 ( 1 °C using distilled water, diiodomethane, and hexadecane as wetting liquids. Water and hexadecane contact angles were taken as the index of the hydrophobicity and lipophobicity, respectively. The surface energy γ and its dispersive and polar components γd and γp, respectively, were obtained by using the Owens-Wendt-Fowkes concept.8 Scanning electron microscopy (SEM) of the films was carried out with a JEOL 6700F microscope. Electrodeposited films were directly observed without covering them with a thin layer of sputtered carbon.

Results and Discussion Synthesis of Monomers. The monomers were synthesized in good yields by a coupling reaction according previously described procedures9 using N,N′-dicyclohexylcarbodiimide (DCC) as an activate reagent for the carboxylic acid and 4-(dimethylamino)pyridine (DMAP) as a catalyst (Scheme 1). The yields were slightly lower with the heptadecafluoroundecanoic acid due to the lower solubility of the latter in dichloromethane. 3-Perfluoroalkylpropanoic acids were obtained by carboxylation of the corresponding organomagnesium compound of 2-perfluoroalkylethyl iodide.10 The intermediate (2,3-dihydrothieno[3,4b][1,4]dioxin-2-yl)methanol was obtained in six steps from thiodiglycolic acid11 with an overall yield of 34% and 2-(1Hpyrrol-1-yl)ethanol by action of the pyrrolyl potassium salt on (8) (a) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538. (b) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (9) Han, S.-Y.; Kim, Y.-A. Tetrahedron 2004, 60, 2447.

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Figure 2. (a) Successive cyclic voltamogramms of EDOT-RF6 (20 mM, final electropolymerization charge Qs ) 640 mC · cm-2). (b) Electrochemical response of the electroformed polymer. Electrolyte: Bu4NPF6 10-1 M, CH3CN, V ) 20 mV · s-1. Platinum working electrode.

2-chloroethanol.12 9-Fluorenemethanol was commercially available. In these monomers, the perfluorocarbon unit is separated from the polymerizable entity via a flexible connector in order to prevent the electronic effect of the fluorine substitution to have deleterious influence on the polymerization and on the resulting properties of the polymers. Polymerization and Characterization. The derivatives of ethylenedioxythiophene (EDOT), pyrrole (Py), and fluorene (Fl) were first polymerized using iron trichloride as an oxidant in dry chloroform to afford the corresponding polymers as powders. The polymerization reaction was efficient for the fluorinated EDOT and Py (>70%, 24 h) while it reveals lower yields (10-40%, 72 h) for the Fl monomers. Soluble fractions were obtained by a Soxhlet extraction with chloroform and analyzed by UV-visible spectroscopy. The fluorinated polypyrrole PPyRFn were found to be insoluble in chloroform (soluble fraction inferior to 5%). For PFl-RFn, the soluble fraction was in the range 45-85%. Polyfluorenes display a rich and attractive variety of optical and electronic properties and have emerged as the most promising (10) Jouani, A. M.; Szo¨nyi, F.; Cambon, A. J. Fluorine Chem. 1992, 56, 85. (11) Ste´phan, O.; Schottland, P.; Le Gall, P.-Y.; Chevrot, C.; Mariet, C.; Carrier, M. J. Electroanal. Chem. 1998, 443, 217. (12) Bidan, G. Tetrahedron Lett. 1985, 26, 735.

class of active, emissive materials for deep blue OLEDs.13 Indeed, they contain a rigid biphenyl unit which leads to a large band gap with efficient emission. Moreover, the facile substitution at the remote C9 position provides the possibility of improving the processability of polymers. For instance, the presence of fluorinated segments should add thermal stability and chemical and moisture resistance. And above all, it can decrease, thanks to the general behavior of fluorinated molecules known as “the fluorophobic effect”, the interchain interactions that are responsible for the decrease of the color stability of the light emission in the OLEDs fabricated with PFl. Figure 1 shows the UV-visible and fluorescence spectra of the PFl-RFn in solution. They absorb in the near-UV region, with a maximum, which is the π-π* transition for the conjugated polymer backbone, at around 350 nm. The bathochromic effect observed for PFl-RF4 compared to PFl-RF6 and 8 was certainly due to higher conjugation length. All the PFl-RFn showed blue light emission spectra (Figure 1) with a maximum at 409 nm whatever the size of the fluorinated chain, with a pronounced shoulder at 394 and 431 nm for PFl-RF8 and PFl-RF4, respectively. Optical band gaps of these polymers were (13) (a) Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2867. (b) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477.

Varying Surface Properties with Perfluorinated Polymers

Figure 3. SEM image of a PPy-RF8 film electrodeposited on Au plate (Qs ) 200 mC · cm-2). Insets: camera images of water and hexadecane droplets on the film; magnified view.

Figure 4. SEM image of a PEDOT-RF8 film electrodeposited on Au plate (Qs ) 200 mC · cm-2). Insets: camera images of water and hexadecane droplets on the film; magnified view.

calculated from their absorption edge (Table 1) and represent typical values for polyfluorenes. For PEDOT-RFn, the soluble fraction was 25-30% for n ) 4 and 6 while it decreases to 10% for n ) 8. As already observed for fluorinated polythiophenes, increasing the length of the fluorocarbon segment leads to a decrease of solubility.7b Among conducting polymers, PEDOT is probably the most successful for commercial applications and particularly electrochromic devices.14 PEDOT and PEDOT-RFn were orange in solution and present a broad absorption in the violet-blue-green region with a maximum at 360-370 nm (Table 1) and a shoulder at around 450 nm, except for the PEDOT-RF4 that absorbs in a much broader region with a maximum at 450 nm and a shoulder at 360 nm. It results a smaller band gap compared to its analogues. Longer conjugated segments due to a better solubility of oligomeric and polymeric species during polymerization are undoubtedly responsible for that. Finally, soluble highly fluorinated polyfluorene and poyethlenedioxythiophene, which can be easily handled and deposited on different kinds of solid substrates, are easily obtained by chemical oxidation of the corresponding fluorinated monomer. Despite the presence of the rigid perfluorocarbon segment, they conserve their interesting optical properties (low bandgap for

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PEDOT and blue emission for PFl) together with the specific properties brought by the fluorinated chains, which make them useful as promising candidates in electrooptical devices. Electropolymerization. Another huge advantage concerning conjugated polymers consists in the possibility to obtain structured films deposited directly onto a solid electrode, in one step, by the electropolymerization technique. Compared to other semifluoroalkyl-substituted monomers that exhibit poor solubility and restrict the choice of the solvent for the electropolymerization,3 all the monomers are soluble in common organic solvents thanks to the presence of the flexible connector. Thus, the electrochemical study was performed in a classical electrolytic medium, that is, acetonitrile with 0.1 M of tetrabutylammonium hexafluorophosphate Bu4PF6 as electrolyte salt. Cyclic voltammetry characterization of the monomers revealed an irreversible oxidation peak at around 1.7 V for EDOT-RFn, 1.6 V for Py-RFn, and 2.4 V for Fl-RFn, versus Ag/Ag+ (Table 2). The influence of substitution is almost inexistent. For instance, nonsubstituted EDOT is oxidized at 1.74 V against 1.70, 1.77, and 1.72 V for the fluorinated analogues with n ) 4, 6, and 8, respectively. Due to a higher conjugation, fluorene monomers have high oxidation potentials and do not give rise to polymeric electroactive films onto the electrode. Indeed, the difficulty to electropolymerize monomers of high oxidation potentials has generally been attributed to 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.15 Unlike fluorene, PEDOT and PPy films were potentiodynamically and/ or potentiostatically deposited onto the conductive electrode with an anodic limit close to the irreversible oxidation peak of the monomer. For instance, the successive voltammograms of EDOTRF6 (Figure 2a) showed the emergence and the constant increase of a new redox system, corresponding to the p-doping/undoping process of the electrofomed polymer, which indicates a homogeneous growth of the electroactive film on the electrode. Its electrochemical response (Figure 2b) displays a well-defined and reversible process at 0.51 V (∆E p ) 130 mV at 20 mV · s-1) together with another oxidation step at 0.28 V whose cathodic component is badly resolved. The same trend is observed for EDOT-RFn. The doping level deduced electrochemically was roughly equivalent (0.25-0.30 positive charges per monomer unit) for PEDOT-RFn films. Furthermore, a linear relationship between the anodic peak current Ipa and the potential scan rate V was obtained, as expected, for the surface-immobilized redox species.16 Similarly, Py-RFn gave rise to the formation of polymer films on the working electrode; the electron conductivity of the electroformed films was slightly lowered as proved by a larger peak-to-peak separation. The data are gathered in Table 2. Finally, films of PEDOT-RFn and PPy-RFn were prepared at fixed potentials on ITO and Au electrodes (area ) a few square centimeters) and the surface properties of these electrodeposited films were investigated by goniometry analysis and microscopic observations. Surface Properties. The wettability of a solid surface is generally governed by both the chemical composition and the morphology of the surface.17 Among the various methods to (14) Groenendall, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481. (15) (a) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76. (b) Roncali, J. Chem. ReV. 1992, 92, 711. (16) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods, Fundamental and Applications; Wiley: New York, 1980. (17) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (c) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999.

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Figure 5. SEM images of (a) PEDOT, (b) PEDOT-RF4, (c) PEDOT-RF6, and (d) PEDOT-RF8 films electrodeposited on ITO plate (Qs ) 200 mC · cm-2).

create superwetting and/or antiwetting surfaces,18 the electrodeposition of organic19 or inorganic20 materials represents an effective method that requires mild conditions and allows the control of the surface structure. Concerning the electropolymerization technique, the thickness of the film is one parameter, among many (nature of the electrolytic medium, electrode, electrochemical method, concentration of the monomer, etc.), that influences the surface properties of the film. Thin films are compact and smooth in morphology while roughness is enhanced by increasing thickness of the film.21 Combining low free surface (18) (a) Feng, X.; Jiang, L. AdV. Mater. 2006, 18, 3063. (b) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193. (c) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (19) (a) Zhang, Z.; Qu, L.; Shi, G. J. Mater. Chem. 2003, 13, 2858. (b) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453. (c) Zhu, Y.; Zhang, J.; Zheng, Y.; Huang, Z.; Feng, L.; Jiang, L. AdV. Funct. Mater. 2006, 16, 568. (20) (a) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (b) Wang, S.; Feng, L.; Liu, H.; Sun, T.; Zhang, X.; Jiang, L.; Zhu, D. ChemPhysChem. 2005, 6, 1475. (c) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (d) Tian, Y.; Liu, H.; Deng, Z. Chem. Mater. 2006, 18, 5820. (e) Larmour, I. A.; Bell, S. E. J.; Saunders, G. C. Angew. Chem., Int. Ed. 2007, 46, 1710. (21) (a) Tourillon, G.; Garnier, F. J. Polym. Sci.: Polym. Phys. Ed. 1984, 22, 33. (b) Del Valle, M. A.; Cury, P.; Schrebler, R. Electrochim. Acta 2002, 48, 397. (c) Ulgade, L.; Bernede, J. C.; Del Valle, M. A.; Diaz, F. R.; Leray, P. J. Appl. Polym. Sci. 2002, 84, 1799.

energy with such rough surfaces electrochemically obtained can provide superhydrophobic surfaces. For instance, super-waterrepellent films of conducting polymers such poly(alkylpyrrole)22 or poly(fluorinated thiophene)7 (see PTh-RFn in Chart 1) have been recently achieved by electropolymerization. For the following surface analysis, PPy-RFn, PEDOT, and PEDOT-RFn films have been electrodeposited on ITO and Au plates conserving the same experimental conditions (electrolytic medium, acetonitrile, Bu4NPF6 0.1 M; monomer concentration, 20 mM; electrodeposition at fixed potential, Qs ) 200 mC · cm-2). SEM micrographs of electroformed PPy-RF8 on Au are shown in Figure 3. It reveals a dense and rough packed assembly of polymer microstructures with morphology like bowls and cups. These microstructures are themselves composed of submicrometric particles (cf. magnified view in Figure 3) with diameter of hundreds nanometers. The fine-scale and coarse-scale roughness is favorable for mimicking the lotus-leaf effect for repelling water. Indeed, these films showed high hydrophobicity with a contact angle of 142° with water. Arising from their low surface energy, very good oil repellency was also observed with contact angles of about 110° and more. The length of the perfluorocarbon (22) Yan, H.; Kurogi, K.; Mayama, H.; Tsujii, K. Angew. Chem., Int. Ed. 2005, 44, 3453.

Varying Surface Properties with Perfluorinated Polymers

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Figure 6. Evolution of water contact angle θwater with the electropolymerization charge Qs for polymer films electrodeposited on a Au plate.

chain has no influence on these properties and the same results are observed, even with a short perfluorobutyl segment. It is worth noting there are few lipophobic materials having contact angles higher than 100°. On the basis of the recent work devoted to the obtaining of PPy microtubes or needles, using sulfonate salt or sulfonic acid as electrolytes,23 a plan is made to test different electrolytic media to enhance liquid repellency and to achieve superhydrophobicity together with superoleophobicity, also called as superamphiphobicity. Indeed, in the field of surfaces with special wettabilities, combining any of two properties among superhydrophobicity, superoleophobicity, superhydrophilicity, and superoleophilicity has emerged as a challenge from both academic and industrial viewpoints. Simple and low-cost methods are needed to construct these artificial functional surfaces. Especially, combining superhydrophobicity with superoleophilicity appears as the most interesting and important challenge for future applications since it should permit effective separation of oil and water without any additional chemical agents. To our knowledge, reports on both superhydrophobic and superoleophilic surfaces are rare.24 Typically, a low surface energy material, intrinsically hydrophobic and oleophilic, is converted into superhydrophobic and superoleophilic by an appropriate surface design that increases roughness. The same result should be obtained by adequately lowering the surface energy of a rough hydrophilic and oleophilic material. Among conducting polymers, PEDOT demonstrates a unique wettable behavior compared to polythiophene, polypyrrole, or polyaniline films, thanks to the presence of the polar ethylenedioxy ring. Indeed, electrodeposited films of PEDOT shows a rough surface (see for instance an electrodeposited film of PEDOT on ITO in Figure 5a) and are wettable by liquids with a contact angle with water and oil respectively of about 30-40°, depending on the nature of the electrode, and 0°. It thus appears as a potential matrix for water (23) Qu, L.; Shi, G.; Yuan, J.; Han, G.; Chen, F. J. Electroanal. Chem. 2004, 561, 149. (24) (a) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 2012. (b) Zhang, J.; Huang, W.; Han, Y. Macromol. Rapid Commun. 2006, 27, 804. (c) Tang, K.; Yu, J.; Zhao, Y.; Liu, Y.; Wang, X.; Xu, R. J. Mater. Chem. 2006, 16, 1741. (d) Wang, S.; Song, Y.; Jiang, L. Nanotechnology 2007, 18, 015103/1–015103/5. (e) Yang, Y.; Nakazawa, M.; Suzuki, M.; Shirai, H.; Hanabusa, K. J. Mater. Chem. 2007, 17, 2936.

and oil separation provided that its surface energy (γ ) 60-66 mN · m-1) is lowered with low free energy substituent, while keeping roughness. This can be achieved by covalently attaching a fluorinated chain on the EDOT monomer before polymerization. The presence of low surface free energy perfluorinated fragment in the PEDOT-RFn films adds superhydrophobicity while keeping the superoleophilic property. Contact angle values with water were found to be 155° and 156° for thick PEDOT-RFn films (Qs ) 200 mC · cm-2) electroformed respectively on ITO and Au plates, whatever the length of the perfluorocarbon chain. On the other hand, a drop of hexadecane (2 µL) spreads quickly on the surface of the film and penetrates it within 3 s, as for PEDOT deposit. The fluorinated surfaces present low surface free energy (γ ) 13-16 mN · m-1) with a low polar component (γp ) 3-4 mN · m-1) and a moderate dispersive one (γd ) 10-12 mN · m-1). Moreover, compared to PEDOT, their morphology remains quite similar (Figure 5), proving the importance of the chemical factor to switch from hydrophilicity (PEDOT) to superhydrophobicity (PEDOT-RFn). The SEM image of a PEDOT-RF8 film on Au (Figure 4) or ITO (Figure 5d) reveals an extended rough surface composed of spherical submicroscopic structures (cf. magnified view in Figure 4) of varied diameters. This morphology is similar to that obtained for superhydrophobic and oleophobic fluorinated polythiophenes (PTh-RFn) surfaces,7 proving the necessity of the ethylenedioxy ring for increasing the surface energy of the electrodeposited polymers surfaces, compared to these analogous polythiophenes (surface energy were so low that they could not have been calculated). It thus appears that the difference observed between these two kinds of surfaces (PTh-RFn vs PEDOT-RFn) is a question of chemistry rather than morphology. It is noteworthy that the submicroscopic polymers particles can slightly differ in terms of sizes, forms, and assemblings in function of the nature of the electrode and the monomer EDOT or EDOT-RFn used but the electrodeposited films conserved their rough behavior and the wetting properties remain the same whatever the electrode utilized (Au or ITO) and the length of the perfluorinated chain (RFn with n ) 4, 6, 8). To get further insight and information, supplementary goniometry studies have been performed by varying the electropo-

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lymerization charge (Qs). This parameter is one of the most important in determining the surface properties of an electrodeposited polymer. Indeed, due to the particular nucleation and growth mechanism during electropolymerization21 which follows first a two-dimensional growth followed by a three-dimensional one, thin films are smooth in morphology whereas roughness and disorder increase with the thickness of the film. Water contact angles of 60° and 120-140° were observed for thin films (Q ∼ 20 mC · cm-2) of PEDOT and PEDOT-RFn, respectively (Figure 6). When Qs is increased, and so on roughness, hydrophilicity is enhanced for PEDOT films while hydrophobicity increases for PEDOT-RFn films until a plateau is reached. This observation can be explained by the two classical models (Wenzel17a and Cassie-Baxter17b) describing the contact angle for a liquid droplet on a rough solid surface, which indicate the surface roughness enhances both the hydrophilicity of hydrophilic surfaces (θ < 90°) and the hydrophobicity of hydrophobic ones (θ > 90°).

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the chemical composition. By modification of the nature of the polymer, films with different surface energy were obtained. Fluorinated surfaces of electrodeposited polythiophene and polypyrrole give birth to high or superhydrophobic surfaces together with high oleophobic behavior, while the use of polyethylenedioxythiophene as the polymer increases sufficiently the surface energy to get combined superhydrophobicity and superoleophilicity. It could represent an interesting candidate for industrial application in water and oil separation and work is currently underway in this direction. Our efforts are also focused now on one of the most employed material that is stainless steel as the working electrode and electrodeposition of polymers will be realized onto substrates with various forms (flat or curved plate, mesh, rod, wire, etc.) and sizes. We are also willing to improve the wetting behavior of these polymers by changing the electrolytic medium and particularly by using sulfonate salts or boron trifluoride diethyl etherate, BF3 · OEt2 (BFEE).

Conclusions In summary, electrodeposited surfaces of fluorinated conducting polymers with special wettabilities can be easily obtained on various supports (Pt, Au, ITO) by electropolymerization. Electrochemistry represents a convenient method that occurs in mild conditions and permits the obtaining of rough structures. The presence of fluorinated chains covalently attached on the polymer backbone allows the control of the second criterion which plays a role on the wetting properties of the surface, i.e.,

Supporting Information Available: Characterization data for EDOT-RF6, EDOT-RF8, Py-RF6, Py-RF8, Fl-RF6, and Fl-RF8. This information is available free of charge via the Internet at http://pubs.acs.org. Note Added after ASAP Publication. This article was released ASAP on July 3, 2008. Frédéric Guittard was added as an author, and the correct version was posted on August 7, 2008. LA801040Z