Electrospinning of a Functional Perfluorinated Block Copolymer as a

Dec 9, 2010 - Micro- and nanostructuration of the diblock copolymer coating was obtained by electrospinning which proved to be an easy and cheap elect...
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Electrospinning of a Functional Perfluorinated Block Copolymer as a Powerful Route for Imparting Superhydrophobicity and Corrosion Resistance to Aluminum Substrates Bruno Grignard,† Alexandre Vaillant,‡ Joel de Coninck,‡ Marcel Piens,§ Alain M. Jonas, Christophe Detrembleur,† and Christine Jerome*,† †

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Center for Research and Education on Macromolecules (CERM), University of Li ege, Sart Tilman, Bat. B6a, 4000 Li ege, Belgium, ‡Laboratory for Physics of Surfaces and Interfaces (LPSI), University of Mons, Parc Initialis, 7000 Mons, Belgium, §Coating Research Institute (CoRI), Avenue P.Holoffe 21, 1342 Limelette, Belgium, and Institute of Condensed Matter and Nanosciences, Bio- & Soft Matter, Universit e catholique de Louvain, Croix du Sud 1 box 4, 1348 Louvain-la-Neuve, Belgium Received July 14, 2010. Revised Manuscript Received October 4, 2010

Superhydrophobic aluminum surfaces with excellent corrosion resistance were successfully prepared by electrospinning of a novel fluorinated diblock copolymer solution. Micro- and nanostructuration of the diblock copolymer coating was obtained by electrospinning which proved to be an easy and cheap electrospinning technology to fabricate superhydrophobic coating. The diblock copolymer is made of poly(heptadecafluorodecylacrylate-co-acrylic acid) (PFDA-co-AA) random copolymer as the first block and polyacrylonitrile (PAN) as the second one. The fluorinated block promotes hydrophobicity to the surface by reducing the surface tension, while its carboxylic acid functions anchor the polymer film onto the aluminum surface after annealing at 130 °C. The PAN block of this copolymer insures the stability of the structuration of the surface during annealing, thanks to the infusible character of PAN. It is also demonstrated that the so-formed superhydrophobic coating shows good adhesion to aluminum surfaces, resulting in excellent corrosion resistance.

Introduction Superhydrophobic surfaces, characterized by high surface roughness, very low hysteresis, and low surface free energy,1 are of special interest in both the academic and industrial fields due to their antisticking2-6 and self-cleaning properties.7-10 Template and lithographic approaches,11-13 etching or plasma treatments,14-17 *To whom correspondence should be addressed. E-mail: [email protected]. (1) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350–1368. (2) Blossey, R. Nat. Mater. 2003, 2, 301. (3) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (4) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (5) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (6) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405. (7) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (8) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (9) Saito, H.; Takai, K.; Takazawa, H.; Yamauchi, G. Mater. Sci. Res. Int. 1997, 3, 216. (10) Kako, T.; Nakajima, A.; Irie, H.; Kato, Z.; Uematsu, K.; Watanabe, T.; Hashimoto, K. J. Mater. Sci. 2004, 39, 547. (11) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (12) Callies, M.; Chen, Y.; Marty, F.; Pepin, A.; Quere, D. Microelectron. Eng. 2005, 78-79, 100. (13) Abdelsalam, M.; Bartlett, P.; Kelf, T.; Baumberg, J. Langmuir 2005, 21, 1753. (14) Teshima, K.; Sugimura, H.; Inoue, Y.; Takai, O.; Takano, A. Appl. Surf. Sci. 2005, 244, 619. (15) Song, X.; Zhai, J.; Wang, Y.; Jiang, L. J. Phys. Chem. B 2005, 109, 4048. (16) Qian, B.; Shen, Z. Langmuir 2005, 21, 9007. (17) Guo, Z.; Zhou, F.; Hao, J.; Liu, W. J. Am. Chem. Soc. 2005, 127, 15670. (18) Li, M.; Zhai, J.; Liu, H.; Song, Y. L.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2003, 107, 9954. (19) Zhao, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Langmuir 2005, 21, 4713. (20) Shi, F.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2005, 17, 1005. (21) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K.; Rutledge, G. Macromolecules 2005, 38, 9742. (22) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 5659.

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electrochemical18-20 and chemical bath or vapor deposition,21-24 layer by layer25,26 or micelle deposition,27 sol-gel method,28-35 and electrospinning21,36-45 are examples of techniques that were (23) Honoso, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458. (24) Riboo, R.; Voue, M.; Vaillant, A.; Seveno, D.; Conti, J.; Bondar, A. I.; Ivanov, D. A.; De Coninck, J. Langmuir 2008, 24, 9508. (25) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J. Phys. Chem. B 2005, 109, 20773. (26) Soeno, T.; Inokuchi, K.; Shiratori, S. Appl. Surf. Sci. 2004, 237, 543. (27) Desbief, S.; Grignard, B.; Detrembleur, C.; Rioboo, R.; Vaillant, A.; Seveno, D.; Voue, M.; De Coninck, J.; Jerome, C.; Damman, P.; Lazzaroni, R. Langmuir 2009, 26(3), 2057. (28) Jung, D.; Park, I. J.; Choi, Y.; Lee, S.; Park, H.; Ruhe, J. Langmuir 2002, 18, 6133. (29) Li, X.; Cao, Z.; Liu, F.; Zhang, Z.; Dang, H. Chem. Lett. 2006, 35, 94. (30) Mahltig, B.; Bottcher, H. J. Sol-Gel Sci. Technol. 2003, 27, 43. (31) Makita, K.; Akamatsu, Y.; Yamazaki, S.; Kai, Y.; Abe, Y. J. Ceram. Soc. Jpn. 1997, 105, 1012. (32) Nakagawa, T.; Soga, M. J. Non-Cryst. Solids 1999, 260, 167. (33) Rao, A.; Kulkarni, M.; Amalnerkar, D.; Seth, T. J. Non-Cryst. Solids 2003, 330, 187. (34) Roig, A.; Molins, E.; Rodriguez, E.; Martinez, S.; Moreno-Manas, M.; Vallribera, A. Chem. Commun. 2004, 2316. (35) Shirtcliffe, N.; McHale, G.; Newton, M.; Perry, C. Langmuir 2003, 19, 5626. (36) Acatay, K.; S-ims-ek, E.; Ow-Yang, C.; Menceloglu, Y. Angew. Chem., Int. Ed. 2004, 43, 5210. (37) Singh, A.; Steely, L.; Allcock, H.; Harry Langmuir 2005, 21, 11604. (38) Ma, M.; Hill, R.; Lowery, J.; Fridrikh, S.; Rutledge, G. Langmuir 2005, 21, 5549–5554. (39) Zhu, Y.; Zhang, J.; Zheng, Y.; Huang, Z.; Feng, L.; Jiang, L. Adv. Func. Mater. 2006, 16, 568. (40) Lim, J.-M.; Yi, G.-R; Moon, J.; Heo, C.-J.; Yang, S.-M.; Seung-Man Langmuir 2007, 23, 7981. (41) Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K.; Cohen, R.; Rubner, M.; Rutledge, G.; Gregory Adv. Mater. 2007, 19, 255. (42) Zhu, Y.; Feng, L.; Xia, F.; Zhai, J.; Wan, M.; Jiang, L. Macromol. Rapid Commun. 2007, 28, 1135–1141. (43) Zhu, M.; Zuo, W.; Yu, H.; Yang, W.; Chen, Y. J. Mater. Sci. 2006, 41, 3793. (44) Han, D.; Steckl, A. Langmuir 2009, 25, 9454. (45) Valtola, L.; Koponen, A.; Karesoja, M.; Hietala, S.; Laukkanen, A.; Tenhu, H.; Denifl, P. Polymer 2009, 50, 3103.

Published on Web 12/09/2010

DOI: 10.1021/la102808w

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Article Scheme 1. Structure of the Perfluorinated P(FDA-co-AA)-b-PAN Diblock Copolymer

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selected to ensure the stability of the structured electrospun coating (thus the roughness) during annealing, thanks to the infusibility of PAN.

Materials and Methods

developed for preparing such surfaces. Due to its ease of implementation, its low cost, its rapidity, and its applicability to a wide range of polymer matrices, electrospinning has emerged as a promising technique to produce surfaces with appropriate roughness, morphology, and/or porosity. For instance, Acatay et al. were the first to report on the use of electrospinning for the preparation of superhydrophobic surfaces by deposition of fibers starting from a poly(acrylonitrile-coR,R-dimethyl meta-isopropenyl benzyl isocyanate) (poly(AN-coTMI)/fluorolink-D (perfluorinated chain with CH2OH end functional group) mixture followed by the annealing of these materials in order to enable the migration of the perfluorinated groups to the solid-air interface.36 Singh et al. prepared superhydrophobic nanofibers by electrospinning of an organic-soluble poly[bis(2,2,2trifluoroethoxy)phosphazene].37 Rutledge et al. described the preparation of superhydrophobic surfaces by combining electrospinning of poly(ε-caprolactone) (PCL) and initiated chemical vapor deposition of perfluoroalkyl ethyl methacrylate.21 The same group also demonstrated that such surfaces could be prepared by deposition of polydimethylsiloxane based diblock copolymers, that is, poly(styrene-b-dimethylsiloxane) block copolymer, or by deposition of poly(styrene-b-dimethylsiloxane)/polystyrene mixtures.38 In 2006, polyaniline/polystyrene conductive composite films with a lotus-leaf-like structure were prepared via a simple electrospinning method by Zhu et al.39 Strategy involving the preparation of multiscale surface morphology was also reported by Lim et al.40 The electrospun nanofibers were produced out of aqueous solutions which contained water-soluble polymers and different colloids: polystyrene microspheres for larger particles and monodisperse silica nanoparticles for smaller particles. Selective removal of organic materials by calcination and subsequent treatment with fluorinated silane coupling agent by vapor-phase reactions enabled the creation of superhydrophobic surfaces with extremely low sliding angle. The Rutledge group reported on the preparation of double-roughened electrospun fibers by directly introducing nanometer-scale porous structures onto electrospun fiber surfaces or decorating the electrospun fibers with fine nanoparticles using layer-by-layer (LBL) deposition techniques.41 Even though electrospinning is an appropriate technique for preparing rough surfaces involving superhydrophobic character, adhesion of the polymer film onto the substrate is generally extremely weak, so that for coating applications with long-term durability it remains useless. In this paper, we report on the preparation of electrospun mats of a specifically tailored diblock perfluoropolymer, that is, poly(heptadecafluorodecylacrylate-co-acrylic acid)-b-poly(acrylonitrile) (PFDA-co-AA)-b-PAN) (Scheme 1), on aluminum surfaces in order to promote the surface with durable superhydrophobicity and with the goal to test the possible improved corrosion resistance. The specific block copolymers, prepared by reversible additionfragmentation chain transfer (RAFT) in supercritical carbon dioxide (scCO2), are made of a first perfluorinated block containing acrylic acid groups that allow the anchoring of the copolymer on the aluminum surface that is necessary for the durability of the functionality. The second block, poly(acrylonitrile) (PAN), has been 336 DOI: 10.1021/la102808w

Materials. Acrylic acid (AA, Aldrich), 1H,1H,2H,2H-heptadecafluorodecylacrylate (FDA, Aldrich), acrylonitrile (AN, Aldrich), asobisisobutyronitrile (AIBN, Aldrich), dimethylformamide (DMF, Aldrich), R,R,R-trifluorotoluene (TFT, Aldrich), and CO2 (N48, air liquide) were used as received. S-1-DodecylS-(R,R0 -dimethyl-R00 -acetic acid)trithiocarbonate was prepared according to the procedure described by Shea et al. Poly(acrylonitrile) (Mn app = 7000 g/mol; Mw/Mn = 1.7) was prepared by conventional free radical polymerization at 65 °C for 2 h in DMF using AIBN as initiator. Substrate Cleaning. The substrates are made of aluminum plates (AL2024 T3 plates, an alloy of aluminum) currently used and provided by the aerospace company SONACA (Belgium). They were used after a cleaning process that consists of a degreasing in isopropanol and acetone (1/1, Aldrich), followed by a basic attack in NaOH (100 mg/mL, Aldrich) for 45 min, and an acidic attack in HNO3 (100 mg/mL, Aldrich) for a few seconds. Characterization. The water contact angles were measured using a Kr€ uss DSA100 drop shape analyzer at 25 °C with 5 μL sessile drops. Advancing contact angles (θa) were measured just before the onset of motion of the meniscus of a differentially incremented 5 μL water droplet moves. Next, the volume of this drop was differentially reduced until the meniscus moves and receding contact angles (θr) were determined. The contact angle hysteresis (H) was estimated by the difference between θa and θr. Scanning electron microscopy (SEM) analyses were performed using a 515 Philips scanning electron microscope. Polymer Synthesis. Synthesis of Poly(heptadecafluorodecyl acrylate-co-acrylic acid) Copolymers. In a typical experi-

ment, 1.2576 g of S-1-dodecyl-S-(R,R0 -dimethyl-R00 -acetic acid)trithiocarbonate (3.4  10-3 mol), 0.0405 g of AIBN (2.39  10-4 mol), and 0.9686 g of acrylic acid (1.345  10-2 mol) were introduced in a 35 mL high pressure cell and deoxygenated by a CO2 flux for 5 min. Then, 10 mL of deoxygenated FDA (3.14  10-2 mol) was added under a CO2 flux with the help of a syringe before equilibrating the high pressure cell at 80 °C and 300 bar for 18 h. After polymerization, the pressure was slowly released and the polymer was collected as a yellow powder that was purified by two cycles of solubilization in TFT/precipitation in methanol. The polymer was finally dried at 60 °C under vacuum for 24 h, and the monomer conversion was determined gravimetrically (94%). The purified copolymer was then analyzed by 1H NMR (250 MHz, 298 K, 50/50 v/v CFC113/CDCl3 mixture) δ (ppm): 5.12 (1H, CH-S-C(S)-S- chain end), 4.55 (2H, CH2-O-C(O)of FDA), 3.53 (2H, C 11H23 -CH2 -S-C(S)-S- chain end), 2.65 and 2.85 (1H, CH-C(O)-O- of FDA and AA, respectively), 2.2 < δ < 1.7 (2H, -CH2-CH-C(O)-O- of P(FDAco-AA)), 1.5 (20H, CH3-C10H20-CH2-S-C(S)-S- of the chain end), 1.3 (6H, CH3-C-CH3 of the chain end), 1.05 (3H, CH3-C10H20-CH2-S-C(S)-S- of the chain end). The Mn was calculated by comparison of the relative intensities of the peak characteristic of the methylene protons of the chain end (CH2-S-C(S)-S, δ = 3.56 ppm) and the peaks corresponding the CH group (δ = 2.64 ppm) of AA and the methylene protons (CH2-O-C(O), δ = 4.52 ppm) as well as the CH groups (CH-C(O)-O, δ = 2.45 ppm) of FDA, whereas the composition was deduced from the relative intensities of the CH groups (δ = 2.64 ppm) of AA and the methylene protons as well as the CH groups of FDA (CH2-O-C(O), δ = 4.52 ppm and CH-C(O)-O, δ = 2.45 ppm, respectively): Mn,RMN = 6000 g/mol; [FDA]/[AA] in the copolymer = 85/15.

Synthesis of Poly(heptadecafluorodecyl acrylate-co-acrylic acid)-b-polyacrylonitrile Diblock Copolymers. In a typical experiment, 4.155 g of P(FDA-co-AA) (Mn = 6600 g/mol, Langmuir 2011, 27(1), 335–342

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Figure 1. SEM characterization of polymers film obtained by electrospinning of P(FDA-co-AA)/PAN mixtures of different compositions

before (top) and after annealing at 130 °C for 48 h (bottom). Electrospinning conditions: tip-to-plate distance = 15 cm, voltage = 20 kV, flow rate = 0.8 mL/h, time = 30 min.

FDA/AA exp = 85/15) was introduced in a 35 mL high pressure cell in the presence of 0.0140 g of AIBN (8.28  10-5 mol). The high pressure cell was then flushed with CO2 for 5 min before adding 10 mL of acrylonitrile (0.157 mol, expected Mn for the PAN block = 13 000 g/mol). The high pressure cell was then equilibrated at 300 bar and 80 °C for 18 h. After polymerization, the pressure was slowly released and the diblock copolymer was collected as a powder. Residual acrylonitrile was removed under vacuum before characterization by 1H NMR spectroscopy and size exclusion chromatography (SEC) in DMF (Mn,app, SEC = 43 000 g/mol, Mw/Mn = 1.19). The purified copolymer was then analyzed by 1H NMR (250 MHz, 298 K, 50/50 v/v CFC113/DMF mixture) δ (ppm): 5.12 (1H, CH-S-C(S)-S- chain end), 4.55 (2H, CH2-O-C(O)- of FDA), 3.53 (2H, C11H23-CH2-S-C(S)-Schain end), 3.42 (1H, CH-CN of PAN), 2.65 and 2.85 (1H, CH-C(O)-O- of FDA and AA, respectively), 2.2 < δ < 1.7 (2H, -CH2-CH-C(O)-O- of P(FDA-co-AA) and PAN block), 1.5 (20H, CH3-C10H20-CH2-S-C(S)-S- of the chain end), 1.3 (6H, CH3-C-CH3 of the chain end), 1.05 (3H, CH3-C10H20CH2-S-C(S)-S- of the chain end). Mn was calculated by 1H NMR spectroscopy (250 MHz, 298 K, 50/50 v/v CFC113/DMF mixture) by comparison between the relative intensities of the methyl end group of the polymer (CH3, δ = 0.95 ppm) and the CH group of acrylonitrile (CH-CN, δ = 3.42 ppm): Mn,NMR = 13 000 g/mol.

Preparation of Polymer Nanoparticles by Electrospinning. Micro/nanosized particles of perfluorinated based copolymers were prepared by electrospinning. In a typical run, solutions of perfluorinated polymer (5 or 10 wt % compared to the solvent) in DMF were electrospun at 0.8 mL/h using a tip to collector distance ranging from 5 to 15 cm. A plastic syringe (5 mL) with a nozzle of 0.1 mm diameter was filled with 2 mL of DMF solution. An electrode connected to a high voltage supply that generates positive DC voltages (up to 20 kV) was connected to the syringe. A grounded aluminum plate was used as a collector. Corrosion Tests. Aluminum plates of 10  7.5 cm2 covered by annealed coating obtained from solutions of 25/75 P(FDA-co-AA)/ PAN homopolymers mixtures, 25/75 P(FDA-co-AA)-b-PAN/PAN diblock/homopolymer mixtures, and pure P(FDA-co-AA)-b-PAN diblock copolymers were further exposed for 100 or 200 h to acetic acid salt spray (5% NaCl, 1-3% HAc, pH 3.1-3.3, 35 ( 1 °C)

Results In order to impart superhydrophobicity to a surface, the coating of a highly hydrophobic polymer (such as a partially perfluorinated polymer) is not sufficient. Both hydrophobicity and roughness of the surface at the nano- and microscale level have to be combined.1 Furthermore, when long-term applications are searched for, Langmuir 2011, 27(1), 335–342

adhesion of the coating to the surface is of prime importance. A perfluorinated polyacrylate containing some acrylic acid groups (poly(heptadecafluorodecylacrylate-co-acrylic acid), P(FDA-coAA)) was thus considered in our study in order to combine in the same polymer both hydrophobicity (thanks to the perfluorinated structure) and adhesion to aluminum (thanks to the carboxylic acid groups, known for adhesion onto aluminum54). Electrospinning of this functional partially perfluorinated polymer was then chosen as a possible way for imparting roughness to the coating on the surface by creating a 3D electrospun mat. To promote adhesion, an annealing step above Tg of the copolymer is expected to favor contact of the carboxylic groups with the surface. This annealing step is however detrimental for the coating roughness due to the melting of the mats. Therefore, we decided to incorporate an infusible polymer (PAN) in these mats as a blend or as a block copolymer that is expected to preserve the morphology of the coating obtained by electrospinning during annealing. Indeed, PAN is an infusible polymer that is thus expected to keep morphology during annealing in contrast to the fluorinated polymers. Thus, the effect of annealing on the coating morphology has been studied for different systems that include PAN as an infusible component. Different systems were investigated, and their efficiencies to impart superhydrophobicity to the aluminum surface were studied, while the coating adhesion is of prime importance. Finally, the ability of the coatings to impart corrosion resistance to aluminum was considered. The three studied systems are (i) a blend of the functional perfluorinated copolymer (P(FDA-co-AA)) with the infusible polymer (PAN), (ii) a block copolymer (P(FDA-co-AA)-bPAN) containing the functional perfluorinated copolymer (P(FDAco-AA) as the first block and PAN as the second block, and (iii) a blend of this (P(FDA-co-AA)-b-PAN) copolymer with PAN. Polymer Synthesis. The synthesis of the P(FDA-co-AA) random copolymer (FDA/AA molar ratio = 85/15 and molecular weight = 6000 g/mol) was carried out by reversible additionfragmentation chain transfer (RAFT) in supercritical carbon dioxide (scCO2) as a green polymerization medium using S-1dodecyl-S-(R,R0 -dimethyl-R00 -acetic acid)trithiocarbonate as RAFT agent in the presence of a small amount of AIBN at 80 °C and 300 bar for 24 h as reported elsewhere.26 This copolymer was then used in similar conditions as a macroinitiator for the polymerization of acrylonitrile, resulting in the synthesis of the P(FDA-co-AA)-b-PAN diblock copolymer (6000-13000 g/mol). As described in the experimental section, 1H NMR spectroscopy allowed determination of the copolymer composition, and SEC confirmed the formation of the diblock copolymer. DOI: 10.1021/la102808w

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Figure 2. SEM characterization of electrospun mats obtained from P(FDA-co-AA)-b-PAN/PAN mixtures of different compositions: solution in DMF (100 mg/mL). Electrospinning conditions: tip-to-plate distance = 15 cm, voltage = 20 kV, flow rate = 0.8 mL/h, electrospinning time = 30 min. Table 1. Static Contact Angle Values Obtained for Film Deposited by Electrospinning of P(FDA-co-AA)-b-PAN/PAN Mixtures of Different Compositions before and after Annealing at 130°C for 48 h: Voltage = 20 kV, Flow Rate = 0.8 mL/h, Time = 30 mina P(FDA-co-AA)-b-PAN/PAN composition 100/0

75/25

50/50

25/75

tip-to-plate distance (cm)

θ (deg) before annealing (H)

θ (deg) after annealing (H)

θ (deg) before annealing (H)

θ (deg) after annealing (H)

θ (deg) before annealing (H)

θ (deg) after annealing (H)

θ (deg) before annealing (H)

θ (deg) after annealing (H)

5

155.7 (6.6)

154.4 (11.2)b

153.7 (2.3)

152.1 (5.3)

rolling drop (4.1)

155.7 (1.2)

152.3 (4.9)

rolling drop (2.5)

10

153.9 (5.6)

152.2 (7.2)

152.9 (2.4)

152.5 (4.8)

155.5 (9.1)

152.4 (2)

153.2 (2)

151 (1.8)

15

152.5 151.9 151.2 151.3 155.6 154.8 156.2 147.5 (12.1)b (6.2) (3.3) (3.9) (11.1)b (7.1) (5.2) (11.7)b b (H) = hysteresis = (advancing contact angle - receding contact angle), values in parentheses. Pinning of the water droplet was observed during the measurement of the receding contact angle. a

Electrospinning of P(FDA-co-AA) and PAN Mixtures. Due to the limited solubility of the perfluorinated copolymer in DMF, only solutions of low concentration (100 mg/mL in DMF) were used in electrospinning. Indeed, it is well-known that perfluorinated polymers exhibit very limited solubility in common nonfluorinated solvents whereas PAN is insoluble in the fluorinated ones. Electrospinning of P(FDA-co-AA) random copolymer (Mn = 6000 g/mol, FDA/AA molar ratio = 85/15) and PAN (Mn = 13 000 g/mol) mixtures in DMF of different compositions was performed onto cleaned aluminum plates at a flow rate of 0.8 mL/h, a spinning voltage of 20 kV, and a tip-to-plate distance of 15 cm. By using these electrospinning conditions, an appropriate micro- and nanostructuration of the coating occurs. The SEM characterization of the deposited films, before annealing, reveals that the morphology of the electrospun mats changed from particles of undefined morphology for P(FDA-co-AA) rich solution (P(FDA-co-AA)/PAN = 75/25 w/w) to spherical particles for PAN rich solutions (P(FDA-co-AA)/PAN = 25/75 w/w) (Figure 1). This morphology imparts superhydrophobicity to the surface as proved by the surface contact angles which are all above 150°. Such coatings are however poorly adhesive to Al and can be easily removed by finger. In a second step, the polymer films were thus annealed at 130 °C, a temperature higher than the Tm of the P(FDA-co-AA) (Tm ∼ 80 °C, determined by DSC) in order to favor contact of the first block with the aluminum surface, therefore promoting adhesion through the carboxylic 338 DOI: 10.1021/la102808w

Figure 3. SEM characterization of electrospun P(FDA-co-AA)b-PAN mats at different concentrations: tip-to-ground distance = 15 cm, voltage = 20 kV, flow rate = 0.8 mL/h, electrospinning time = 3 h.

acid groups. After annealing, the structuration of the electrospun mats obtained from a P(FDA-co-AA)-rich solution (P(FDA-coAA)/PAN = 75/25) or 50/50 w/w P(FDA-co-AA)/PAN mixture) has disappeared almost completely during annealing due to the melting of the P(FDA-co-AA) copolymer. As a consequence, a decrease of the roughness was observed and the superhydrophobic behavior of the surfaces before annealing shifted to hydrophobic after annealing with static water contact angles of 125.6° and 138.2° when 75/25 or 50/50 w/w P(FDA-co-AA)/PAN mixtures are, respectively, deposited and annealed. Only surfaces obtained from a PAN rich P(FDA-co-AA)/PAN mixture (P(FDA-co-AA)/PAN = 25/75 w/w) still demonstrate superhydrophobic behavior (θ = 151.7°) after annealing as a result of Langmuir 2011, 27(1), 335–342

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Figure 4. Effect of the annealing as a function of time on the morphology of the electrospun P(FDA-co-AA)-b-PAN mats obtained from 10 wt % solution in DMF: tip-to-ground distance = 15 cm, voltage = 20 kV, flow rate = 0.8 mL/h, electrospinning time = 3 h. Table 2. Static Contact Angle Measurement of Aluminium Plates Coated by Electrospun P(FDA-co-AA)-b-PAN Mats: Electrospinning Time = 3 h, Annealing Time = 24 ha electrospinning conditions concentration

flow rate (ml/h)

voltage (kV)

tip-to-ground distance (cm)

θ (deg) before annealing (H)

θ (deg) after annealing (H)

50 mg/mL

0.8 0.8 0.8

20 20 20

5 10 15

152.6 (1.0) 151.4 (2.1) 150.2 (1.6)

153.2 (0.1) 151.2 (1.4) 151.4 (1.6)

0.8 20 5 151.6 (1.2) 0.8 20 10 151.3 (1.4) 0.8 20 15 152.0 (1.7) a (H) = hysteresis = (advancing contact angle - receding contact angle), values in parentheses.

150.2 (1.6) 150.7 (1.2) 151.3 (1.5)

100 mg/mL

the pronounced roughness of the coating. Nevertheless, in such conditions, the melting of the electrospun mats is insufficient to improve the adhesion of the film onto the aluminum support, and after annealing the coating can be very easily rubbed out with a brush. Electrospinning of P(FDA-co-AA)-b-PAN and PAN Mixtures. In order to prepare rough surfaces with preserved morphology and better adhesion after annealing by electrospinning of polymer mixtures, the P(FDA-co-AA) random copolymer was replaced by a P(FDA-co-AA)-b-PAN (Mn = 6000-13000 g/mol, FDA/AA molar ratio =85/15) diblock copolymer. Indeed, in view of practical industrial developments, it is of interest to decrease the cost of a process, and in this approach the cost mainly comes from the fluorinated component. Therefore, beside electrospinning of the pure P(FDA-co-AA)-b-PAN, mixtures with PAN homopolymers have also been studied. Electrospinning onto an aluminum plate of solutions (100 mg/mL in DMF) composed of P(FDA-co-AA)-b-PAN/PAN (Mn of PAN homopolymer = 13 000 g/mol) mixtures of different compositions was investigated. Since the fluorinated block of the P(FDA-co-AA)-b-PAN has lower surface tension, it is expected to be exposed to the outer surface of the structured coating. Such a surface segregation of the fluorocarbon species in electrospun fibers has already been observed,46-51 and a similar strategy was proposed by Rutledge et al. for the electrospinning of PDMS/PS-b-PDMS mixtures.38 After electrospinning deposition, the SEM characterization reveals that the morphology of the electrospun mats changes from quasi spherical microparticles coexisting with elongated particles (for pure solution of P(FDA-co-AA)-b-PAN)) to spherical particles with the increase of the PAN content (Figure 2). These observations probably result from the decrease of the solution viscosity with the increase in PAN content. In a second step, the (46) Deitzel, J.; Kosik, W.; McKnight, S.; Beck Tan, N; Desimone, J. M.; Crette, S. Polymer 2002, 43, 1025. (47) Gaines, G. L. J. Chem. Phys. 1969, 73, 3143. (48) Schmitt, R.; Gardella, J.; Magill, J.; Salvati, L. Macromolecules 1985, 18, 2675. (49) Pan, D.; Prest, W J. Appl. Phys. 1985, 58, 2861. (50) Bhatia, Q.; Pan, D; Koberstein, J Macromolecules 1988, 21, 2166. (51) Thomas, R.; Anton, R; Graham, D; Darmon, W; Sauer, M; Stika, B; Swartzfager, K Macromolecules 1997, 30, 2883.

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Table 3. Thickness of the Electrospun Film Obtained from Solution of P(FDA-co-AA)-b-PAN (100 mg/mL in DMF) after Annealing for 24 h at 130°C electrospinning conditions flow rate (ml/h)

voltage (kV)

tip-to-ground distance (cm)

0.8 0.8 0.8 0.8

20 20 20 20

15 15 15 15

electrospinning thickness time (h) (μm) 0.5 1 3 4

3.1 ( 2.5 4.1 ( 2.6 15.3 ( 2.6 23.2 ( 8.2

polymer films were annealed at 130 °C for 48 h, and the resulting polymer films were characterized by SEM. In case of these block copolymer-based mixtures, SEM analysis clearly shows the persistence of the structuration of the coating after annealing due to the possible anchoring of the fluorinated copolymer in the PAN matrix thanks to the second PAN block. Superhydrophobicity of the aluminum substrate before and after annealing was evidenced by static water contact angle measurements (Table 1). Moreover, advancing contact angles (θa) were also measured just before the onset of motion of the meniscus of a differentially incremented 5 μL water droplet. Next, the volume of this drop was differentially reduced until the meniscus moved and receding contact angles (θr) were determined. Contact angle hysteresis (H) was estimated by the difference between θa and θr. The results show that all surfaces are characterized by low hysteresis values. All surfaces, before and after annealing, present a pronounced superhydrophobic behavior characterized by a rolling drop even for PAN rich solutions, in line with the migration of the perfluorinated copolymer at the surface of the blend. It should be mentioned that all the values presented in Table 1 correspond to the less superhydrophobic areas of the surface. However, from these annealed electrospun mixtures on Al, it was evidenced that only films prepared from pure P(FDA-co-AA)-b-PAN can sufficiently melt in order to improve the adhesion of the polymer film onto the aluminum plate. Indeed, even for mixtures of low PAN content, the annealed electrospun mats can be very easily rubbed out with a brush whereas rough films obtained from solution of pure diblock DOI: 10.1021/la102808w

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Figure 5. Dynamic contact angle measurements for thick film of 23.2 ( 8.2 μm (left) and thinner film of 3.1 ( 2.5 μm (right).

Figure 6. Cross cut adhesion test of a 3 μm annealed coating onto Al substrate.

copolymer showed stronger adhesion. These observations will be discussed in more detail in the next paragraph. Electrospinning of Pure Diblock Copolymers. From the previous result, electrospinning of solutions of pure P(FDA-coAA)-b-PAN seems to offer the best compromise in terms of structuration after annealing and adhesion. Therefore, electrospinning of a pure solution of the diblock copolymer was optimized and adhesion of the superhydrophobic film was demonstrated by crosscut adhesion test and by accelerated corrosion test using acetic acid salt spray test. So, solutions of various concentrations of pure P(FDA-co-AA)-b-PAN diblock copolymer (Mn = 6000-13 000 g/mol, FDA/AA molar ratio = 85/15) were electrospun onto cleaned aluminum plates. For each solution, the processing parameters, that is, the tip-to-ground distance, the flow rate, and the spinning voltage, were changed in order to obtain electrospun mats of designed roughness so that the superhydrophobicity is achieved. From Table 2, it appears that whatever the tip-to-ground distance and the concentration of the solution (50 mg/mL or 100 mg/mL), working at maximum flow rate of 0.8 mL/h and a minimum voltage of 20 kV is suitable to obtain suitable electrospun mats. The SEM characterization of the polymer film revealed that elongated particles or quasi spherical particles coexisting with elongated particles are formed when 50 and 100 mg/mL in DMF solutions are electrospun, respectively (Figure 3). In a second step, the polymer films were annealed at 130 °C for different times and the resulting polymer films were characterized by SEM. The results shown in Figure 4 clearly evidence the effect of the annealing time of the copolymer. After 48 h, the structuration is less sharp but a tridimensional morphology of the coating remains, which keeps the surface superhydrophobic. 340 DOI: 10.1021/la102808w

Figure 7. (A) Neat aluminum plate (10  5 cm2) and (B) the same neat Al plate after exposure for 200 h to acetic acid salt spray test (5% NaCl, 1-3% HAc, pH 3.1-3.3, 35 ( 1 °C).

Superhydrophobicity of the aluminum substrate before and after annealing was evidenced by static water contact angle measurements. All surfaces, before and after annealing, have a pronounced superhydrophobic behavior characterized by rolling droplet (all the values presented in Table 2 correspond to the less superhydrophobic areas of the surface, Table 2). The effect of the thickness of the polymer film on the superhydrophobicity was also investigated by electrospinning of a solution of P(FDA-co-AA)-b-PAN (100 mg/mL in DMF) for different times ranging from 30 min to 4 h. First of all, SEM analysis has evidenced that the morphology of the electrospun mats was found to be independent of the electrospinning time. After deposition by electrospinning, the thickness of the polymer film was measured by ellipsometry. As expected, an increase of the electrospinning time involves an increase of the thickness of the polymer films (Table 3). More interestingly, thicker films show a more pronounced superhydrophobic character as evidenced by the dynamic contact angle measurements performed on aluminum plates covered by a thin film of 4.1 ( 2.6 μm or a thicker film of 23.2 ( 8.2 μm (Figure 5). These results can be explained by a more pronounced roughness of surfaces covered by thicker films. From the results previously shown, it appears that films prepared by annealing of electrospun mats obtained from pure P(FDA-co-AA)-b-PAN diblock copolymers solutions in DMF represent the best compromise in terms of adhesion (due to the melting of the P(FDA-co-AA) block) and stability of the structuration (due to the infusibility of the PAN sequence). Adhesion of Langmuir 2011, 27(1), 335–342

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Figure 8. Corrosion test of aluminum plate covered by pure P(FDA-co-AA)-b-PAN diblock copolymer exposed to acetic acid salt spray test (5% NaCl, 1-3% HAc, pH 3.1-3.3, 35 ( 1 °C): (A) Neat aluminum plate. (B) Surface coated by a coating of 3.1 ( 2.5 μm thickness, after exposure for 200 h, black spots = corrosion. (C) Surface coated by a coating of 23.2 ( 8.2 μm thickness after exposure for 200 h.

coatings onto aluminum plates was first estimated by the cross-cut adhesion test (ASTM D3359) that consists of making perpendicular cross-cracks in the polymer film followed by the removal of the film using tape and finally counting the number of squares for which the polymer was removed.52 Whatever their thickness (after annealing for 24 h), all coatings showed good adhesion onto the aluminum surfaces with no squares being removed after removal of the tape (Figure 6). Optical observation and EDAX analysis of the surface during SEM observation after peeling confirmed the remaining of the fluorinated coating on the Al surface and that only few cohesive ruptures occur within the electrospun coating. As it is known that fluorinated coating presents antisticking or limited sticking properties, a second test, that is, the acetic acid salt spray test, an accelerated anticorrosion test, was realized in order to confirm the good adhesion of the coating onto the support. Aluminum plates covered by annealed coating obtained from solutions of 25/75 P(FDA-co-AA)/PAN mixtures, 25/75 P(FDA-co-AA)-b-PAN/PAN diblock/homopolymer mixtures, and pure P(FDA-co-AA)-b-PAN diblock copolymers were further exposed for 100 or 200 h to acetic acid salt spray (5% NaCl, 1-3% HAc, pH 3.1-3.3, 35 ( 1 °C). After exposition, the corrosion resistance of the aluminum plate was visually observed. Aluminum plates coated by films of very poor adhesion obtained from electrospun solutions of homopolymer and diblock copolymer/homopolymer mixtures were rapidly corroded after a few hours due to the removal of the coating. On the other hand, electrospun mats of pure P(FDA-co-AA)-b-PAN diblock copolymer showed improved resistance against corrosion after annealing (24 h). Indeed, for thin coatings (3.1( 2.5 μm), corrosion of the aluminum surface was only observed after 200 h, whereas aluminum plates covered by a thicker film (23.2 ( 8.2 μm) show very strong resistance against corrosion with no visible traces of corrosion after this period of time (Figures 7 and 8). These observations are in agreement with some results reported by Fir et al. who showed that the corrosion inhibition was depending on the coating thickness53 and further confirmed by contact angle measurements on the Al substrate exposed to the accelerated (52) Montefusco, F.; Bongiovanni, R.; Priola, A.; Ameduri, B. Macromolecules 2004, 37, 9804. (53) Fir, M.; Orel, B.; Vuk, A. S.; Vilcnik, A.; Jese, R.; Francetic, V. Langmuir 2007, 23, 5505.

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acetic acid salt spray test. Indeed, for Al substrates covered by an homogeneous film of fluorinated polymer, the contact angle remains unchanged after accelerated acidic treatment showing the remaining of the fluorinated coating. Indeed, perfluorinated materials are usually known for chemical stability, and due to their hydrophobic character the hydrolysis of the ester group is most probably deeply slowed down. Very interestingly, the careful observation of the corrosion on the thin electrospun coating shows that corrosion only occurs at the location of defects (no coating at some locations due to the partial dissolution during electrospinning by solvent droplets sputtering) and that corrosion is deeply delayed as compared to conventional anticorrosion coatings that would be applied with a thickness of only about 3 μm. This clearly shows the beneficial effect of superhydrophobicity on corrosion protection of Al. As the fluorinated products are known to show poor adhesion onto a substrate, these additional results confirm that, after annealing, the carboxylic acid groups of the P(FDA-co-AA) block were able to interact with the hydroxyl group of the aluminum surface with the formation of interfacial carboxylate complexes allowing the improvement of the film adhesion. Such an assumption relies on the work of Alexander et al. who characterized the formation of such complexes in the case of the deposition of polyacrylic films onto aluminum surfaces by X-ray photoelectron and Fourier transform infrared spectroscopy.54

Conclusion Successful preparation by electrospinning of aluminum superhydrophobic surfaces with good film adhesion was reported. The strategy relies on the deposition of a perfluorinated P(FDA-coAA)-b-PAN diblock copolymer that was specifically designed for being composed of a first fusible perfluorinated sequence containing carboxylic acid groups and a second infusible PAN sequence. Therefore, annealing of the electrospun mats at 130 °C allows the melting of the fluorinated sequence, improving the adhesion of the polymer film onto the aluminum substrate due to the presence of the COOH groups, whereas the structuration of the surface is maintained, thanks to the infusible PAN sequence. All surfaces show, before and after annealing, a superhydrophobic behavior that is more pronounced for thicker films. The adhesion (cross-cut (54) Alexander, M; Payan, S.; Duc, T. M. Surf. Interface Anal. 1998, 26(13), 961.

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adhesion test) as well as the corrosion inhibition (acetic acid salt spray test) of the annealed superhydrophobic films were demonstrated to be improved by the superhydrophobicity effect. Thick films (∼20 μm) exhibit the best adhesion and excellent corrosion resistance.

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Acknowledgment. This work has been supported by Region Wallonne (CORRONET Project), the Science Policy Office of the Belgian Federal Government (PAI 6/27), and FRS-FNRS. C.D. is “Senior Research Associate” by the National Funds for Scientific Research (FRS-FNRS), Belgium.

Langmuir 2011, 27(1), 335–342