Porous poly(2-octyl cyanoacrylate): a facile one-step preparation of superhydrophobic coatings on different substrates

Journal of Materials Chemistry A View Article Online Published on 29 November 2012. Downloaded by University of St Andrews Library on 13/01/2014 11:04:11. COMMUNICATION Cite this: J. Mater. Chem. A, 2013, 1, 1026 Received 1st November 2012 Accepted 28th November 2012 View Journal | View Issue Porous poly(2-octyl cyanoacrylate): a facile one-step preparation of superhydrophobic coatings on different substrates† Xin Du,a Junsheng S. Li,ab Linxian X. Liac and Pavel A. Levkin*ab DOI: 10.1039/c2ta00934j www.rsc.org/MaterialsA Superhydrophobic surfaces hold great promise in a variety of applications where the extreme water repellency can lead to novel properties and functionalities. Most of the existing techniques, however, require multi-step and laborious procedures as well as are only applicable to certain substrates. We present a facile one-step (“paint-like”) method for creating superhydrophobic porous polymer coatings. The approach is based on the anionic polymerization of octyl cyanoacrylate in the presence of aqueous ethanol. This leads to the formation of a highly porous superhydrophobic polymer film. The morphology of the porous structure can be controlled by varying the ethanol/water ratio. The method is fast, convenient, does not require any special equipment, and can be performed in the presence of oxygen. We show that the technique can be used to coat variety of materials, is applicable to three-dimensional substrates and leads to the formation of stable and strongly adherent superhydrophobic coatings. Introduction Superhydrophobic surfaces, i.e. surfaces with both advancing and receding water contact angles (WCAs) above 150 , have attracted a lot of attention during the last decade mainly because of their unique water repellent and self-cleaning properties.1–8 Owing to their properties, superhydrophobic surfaces can nd numerous applications in a variety of industrial and research elds ranging from coatings for solar cells and biotechnological reactors to coatings for microuidic devices and microarrays. During the past decade, a number of methods for the fabrication of superhydrophobic surfaces have been reported.9–22 However, despite the progress in the eld, most of the Institute of Toxicology and Genetics, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany. E-mail: levkin@kit.edu a Department of Applied Physical Chemistry, University of Heidelberg, 69120 Heidelberg, Germany b c Department of Organic Chemistry, University of Heidelberg, 69120 Heidelberg, Germany † Electronic supplementary information (ESI) available: Fig. S1, Video S1 and S2. See DOI: 10.1039/c2ta00934j 1026 | J. Mater. Chem. A, 2013, 1, 1026–1029 methods still require multi-step procedures,19 harsh conditions, UVirradiation or oxygen-free conditions.21–23 Clearly, development of one-step paint-like methods, i.e. methods equally applicable to different substrates and non-planar surfaces under ambient air environment, is important to accelerate the implementation of superhydrophobic surfaces in various applications. Alkyl cyanoacrylates are well known for their use as “super-glue” and as a surgery adhesive.24 Poly(alkyl cyanoacrylates) are biocompatible and biodegradable, making them suitable for applications in biology and medicine.25 Cyanoacrylates are also known to be more reactive than the corresponding acrylates or methacrylates and they polymerize instantaneously via anionic polymerization in the presence of traces of water, usually forming a strong bond to the substrate. Here, we present a novel facile one-step method that can be used to make superhydrophobic porous polymer coatings on virtually any substrate. The approach is based on the anionic polymerization accompanied by phase separation of a layer of octyl cyanoacrylate upon treatment with aqueous ethanol. This leads to the formation of a highly porous superhydrophobic poly(octyl cyanoacrylate) lm that is strongly adhered to the substrate. The method is applicable to various substrates and both at or shaped surfaces. The morphology of the porous structure can be controlled by varying the ethanol/water ratio. In addition, as anionic polymerization of cyanoacrylates is not inhibited by oxygen, formation of superhydrophobic porous polymer coatings can be performed under ambient oxygen rich conditions. Experimental section Materials and methods Ethyl cyanoacrylate (containing 5–10% poly(methyl methacrylate)) and butyl cyanoacrylate (98%) were obtained from WPI Inc. (USA). 2-Octyl cyanoacrylate was obtained from GluInc (Canada). Ethanol (abs.), acetone, diethyl ether, methanol, n-hexane, water, tetrahydrofuran (THF), dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Aldrich (Germany) and used without further purication. This journal is ª The Royal Society of Chemistry 2013 View Article Online Published on 29 November 2012. Downloaded by University of St Andrews Library on 13/01/2014 11:04:11. Communication Fig. 1 Schematic representation of the method for making a superhydrophobic porous poly(2-octyl cyanoacrylate) coating on different substrates. SEM images were obtained with the LEO 1530 scanning electron microscope (Zeiss, Germany). Water contact angles (8 ml water droplets were used) were determined using ImageJ soware with a DropSnake plugin. The size of polymer globules was determined from SEM images using ImageJ. Preparation of superhydrophobic porous coatings The procedure for the preparation of superhydrophobic porous poly(alkyl cyanoacrylate) layers consists of spreading the monomer on the surface, followed by immersing the surface into aqueous ethanol (Fig. 1). This leads to the formation of a highly porous polymer layer on the surface of a substrate. The thickness of the monomer layer could be controlled by the application method. Manual spreading of the liquid monomer on a substrate using, for example, a glass slide leads to relatively thick polymer lms with thicknesses in the range of tens of micrometers. An application of a spin-coater can reduce the thickness to less than ten micrometers. However, the minimal thickness would then depend on the size of polymer globules and the polymer morphology. As the polymerization can be initiated by water present in air, both the humidity and the time between the application of monomers and the treatment with ethanol should be controlled. Different water/ethanol volume ratios were used for different cyanoacrylate monomers: 1/100 for ethyl cyanoacrylate, 8/100 for butyl cyanoacrylate, and various ratios for 2-octyl cyanoacrylate. The immersion time was varied between 5 and 40 s. The substrate was then removed from the solvent and dried in air, which gave a thin highly porous polymer lm attached to the substrate. In order to test the reproducibility of our method, the preparation of a poly(2-octyl cyanoacrylate) superhydrophobic surface was repeated 10 times, the average static WCA was 159 Æ 3 . Journal of Materials Chemistry A monomers to create highly porous polymer coatings. However, since superhydrophobicity is a result of the combination of surface roughness with hydrophobicity of the material, application of butyl cyanoacrylate or ethyl cyanoacrylates led to less hydrophobic surfaces with static, advancing and receding WCAs being 152 Æ 3 , 163 Æ 3 , 146 Æ 4 , and 126 Æ 1 , 130 Æ 2 , 0 , respectively (Table 1). The polymerization of a liquid layer of the monomer spread on the surface is triggered by water present in the ethanol solution. However, since the produced polymer is not soluble in the ethanol solution, the polymerization is accompanied by simultaneous phase separation resulting in the formation of a highly porous polymer network.20 Fig. 2 shows scanning electron microscopy (SEM) images of the produced poly(alkyl cyanoacrylate) lms. Poly(butyl cyanoacrylate) and poly(2-octyl cyanoacrylate) based surfaces exhibit similar morphology. However, the poly(ethyl cyanoacrylate) surface shows much smaller globule and pore sizes – in the order of 110 Æ 30 nm. This could be caused by the higher activity of the ethyl cyanoacrylate than that of the other two monomers, which resulted in more nucleation sites at the onset of polymerization. Another possible explanation is that the ethanol–water mixture is a better solvent for the initially formed poly(ethyl cyanoacrylate) chains, leading to a later onset of the phase separation and, thus, smaller pores and globules. As the morphology of the porous structure may depend on the amount of initiator26 as well as on the composition of the porogen,22,27,28 we analyzed how the ethanol/water ratio inuenced the morphology and hence superhydrophobicity of the produced porous poly(2-octyl cyanoacrylate). Six water–ethanol mixtures have been tested. As shown in Fig. 3a, the receding WCA of the samples increases from 77 to 153 as the amount of water increases from 2% to 16.7%. The static and advancing WCAs change only slightly. When the water concentration reaches about 16.7% (10 ml in 50 ml ethanol), the surface exhibits the most superhydrophobic behavior, with static, advancing and receding WCAs being 159 Æ 3 , 164 Æ 2 , and 153 Æ 2 , respectively. The reason for the difference in hydrophobicity becomes clear when the morphologies of the corresponding porous structures are compared (Fig. 3b and c). As shown in Fig. 3b, the size of the pores and polymer globules increases gradually upon the increase of water concentration, thereby resulting in larger multi-scale roughness of the surfaces and more pronounced superhydrophobicity. These results conrm the previously described correlation between superhydrophobicity and surface morphology.22,29 The long term stability of the superhydrophobic property is very important for practical applications and can be easily compromised even by slight hydrophilization of the surface, caused, for example, by naturally occurring UV irradiation. We tested the stability of the superhydrophobic poly(2-octyl cyanoacrylate) surface in indoor and outdoor (exposed to UV, dust, rain) environments (see ESI†). The Results and discussion The procedure for the preparation of a superhydrophobic polymer coating consists of spreading 2-octyl cyanoacrylate monomer on a surface, followed by immersing the surface into aqueous ethanol (Fig. 1). This one-step method instantaneously leads to the formation of a highly porous superhydrophobic polymer layer on the substrate. This method can be used with other cyanoacrylate This journal is ª The Royal Society of Chemistry 2013 Table 1 Static, advancing and receding water contact angles (WCAs) of the obtained porous poly(alkyl cyanoacrylate) surfaces Alkyl group Static WCA Advancing WCA Receding WCA Ethyl Butyl Octyl 126 Æ 1 152 Æ 3 159 Æ 3 130 Æ 2 163 Æ 3 164 Æ 2 0 146 Æ 4 153 Æ 2 J. Mater. Chem. A, 2013, 1, 1026–1029 | 1027 View Article Online Published on 29 November 2012. Downloaded by University of St Andrews Library on 13/01/2014 11:04:11. Journal of Materials Chemistry A Communication Fig. 2 (a) Photographs of the porous polymer films made by anionic polymerization of ethyl-, butyl- and 2-octyl cyanoacrylates. Water/ethanol volume ratio for each sample: ethyl cyanoacrylate (1 : 100), butyl cyanoacrylate (8 : 100), 2-octyl cyanoacrylate (20 : 100). (b) SEM images of the porous polymer films (cross-section and top view). Scale bars: 20 mm (top left), 30 mm (left column, middle and bottom), 5 mm (middle column), and 2 mm (right column). Fig. 3 (a) Correlation between water contact angles (WCAs) and the concentration of water in the water–ethanol mixture used to produce porous poly(2-octyl cyanoacrylate) surfaces. (b) Morphologies of the samples produced using different water–ethanol mixtures. Scale bar 5 mm. (c) Relation between average polymer globule size and the water content. WCAs decreased only by $2 in the case of the 4-week indoor test (Fig. S1a†). The tested surface still exhibited superhydrophobicity aer 5 months, with static, advancing and receding WCAs being 154 Æ 3 , 162 Æ 2 , and 152 Æ 2 , respectively. The 8-week outdoor experiment resulted in a decrease in static advancing, and receding WCAs by 8 , 5 and 9 , respectively, thereby showing in part faster deterioration of the superhydrophobic properties of the produced coating (Fig. S1b†). The ability to create strongly adherent superhydrophobic coatings on different and non-at substrates is important for many applications. However, most of the reported methods are still limited to only specic substrates.10,30,31 An advantage of the method described here is that poly(alkyl cyanoacrylates) usually form a strong bond with a substrate. This is known from applications of 1028 | J. Mater. Chem. A, 2013, 1, 1026–1029 Fig. 4 Superhydrophobic poly(2-octyl cyanoacrylate) coatings on different flat and nonflat substrates. Pictures show water droplets dyed with rhodamine. (a) TLC plate, (b) acrylic coated cloth tape, (c) steel, (d) paper, (e) polypropylene, (f) cotton gauze, (g) mesh-like plastic surface, (h) curved tube-like surface, and (i) cotton fibers. “super-glue”, which is based on polymerization of ethyl cyanoacrylate. Fig. 4 shows examples of different substrates coated with superhydrophobic porous poly(2-octyl cyanoacrylate) (see also Video S1 and S2†). The polymer lm adhered well to materials such as acrylic coated cloth tape, paper, cotton cloth, glass and wood. The Table 2 Water contact angles (WCAs) on porous poly(2-octyl cyanoacrylate) coatings prepared on different materials Static WCA Acrylic tape TLC plate (silica) Polypropylene Cotton gauze Steel Paper Wood Advancing WCA Receding WCA 161 157 162 162 159 155 157 162 163 164 165 164 163 165 157 149 152 145 151 141 143 This journal is ª The Royal Society of Chemistry 2013 View Article Online Published on 29 November 2012. Downloaded by University of St Andrews Library on 13/01/2014 11:04:11. Communication superhydrophobic coating could even be formed on skin. Static, advancing and receding WCAs on different substrates aer coating are summarized in Table 2. The superhydrophobic surfaces are stable in methanol, ethanol and n-hexane, however, they can be damaged by diethyl ether, acetone, dimethyl sulfoxide, tetrahydrofuran, chloroform and dimethyl formamide. Repetitive washing of the substrates with methanol, followed by drying with a nitrogen gun, did not lead to the detachment of lms or to formation of cracks, showing good adherence of the superhydrophobic lms to the substrate. The obtained surfaces also showed relatively good stability to scratching, which was attributed to the inherent bulk porosity of the polymer lm. Conclusions In conclusion we demonstrated a novel, fast and convenient “paintlike” method for coating surfaces with a superhydrophobic porous polymer lm. By using commercially available and biocompatible 2-octyl cyanoacrylate as a monomer, and a water–ethanol mixture as both the porogen and initiator, a variety of different substrates could be made superhydrophobic within seconds. Some limitations of the method include relatively weak mechanical stability of the coating due to the porous structure of the polymer and, as with most of the porous materials, the produced superhydrophobic coating is not completely transparent. Nevertheless, the method does not require any complicated equipment, inert atmosphere, or harsh conditions, and can be applied to surfaces with complex nonat geometries. We expect that this low-cost, fast and convenient one-step method will nd numerous applications in a variety of research and industrial areas. Acknowledgements The research is supported by the Helmholtz Association's Initiative and Networking Fund (grant VH-NG-621). The authors thank the Institute of Nanotechnology (Dr Thorsten Scherer) of the Karlsruhe Institute of Technology for the use of SEM. P.A.L. is grateful to Dr Svec and Prof. J.M.J. Fr´chet for providing a creative environment e during 2007–2009 when the ideas for this paper were born. References 1 X. M. Li, D. Reinhoudt and R. M. Crego-Calama, Chem. Soc. Rev., 2007, 36, 1350–1368. 2 P. Roach, N. J. Shirtcliffe and M. I. Newton, So Matter, 2008, 4, 224–240. 3 L. Feng, S. H. Li, Y. S. Li, H. J. Li, L. J. Zhang, J. Zhai, Y. L. Song, B. Q. Liu, L. Jiang and D. B. Zhu, Adv. Mater., 2002, 14, 1857–1860. 4 X. J. Feng and L. Jiang, Adv. Mater., 2006, 18, 3063–3078. 5 T. L. Sun, L. Feng, X. F. Gao and L. Jiang, Acc. Chem. Res., 2005, 38, 644–652. This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry A 6 K. Koch and W. Barthlott, Philos. Trans. R. Soc., A, 2009, 367, 1487–1509. 7 X. F. Gao and L. Jiang, Nature, 2004, 432, 36. 8 M. Callies and D. Qu´r´, So Matter, 2005, 1, 55–61. ee 9 Y. Li, L. Li and J. Q. Sun, Angew. Chem., Int. Ed., 2010, 49, 6129–6133. 10 H. Ogihara, J. Xie, J. Okagaki and T. Saji, Langmuir, 2012, 28, 4605–4608. 11 N. J. Shirtcliffe, G. McHale, M. I. Newton, G. Chabrol and C. C. Perry, Adv. Mater., 2004, 16, 1929–1932. 12 Z. G. Guo, J. Fang, J. C. Hao, Y. M. Liang and W. M. Liu, ChemPhysChem, 2006, 7, 1674–1677. 13 H. S. Hwang, S. B. Lee and I. Park, Mater. Lett., 2010, 64, 2159–2162. 14 H. Kinoshita, A. Ogasahara, Y. Fukuda and N. Ohmae, Carbon, 2010, 48, 4403–4408. 15 K. K. S. Lau, J. Bico, K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne, G. H. McKinley and K. K. Gleason, Nano Lett., 2003, 3, 1701–1705. 16 S. S. Latthe, H. Imai, V. A. Ganesan and V. Rao, Appl. Surf. Sci., 2009, 256, 217–222. 17 R. F¨rstner, W. Barthlott, C. Neinhuis and P. Walzel, u Langmuir, 2005, 21, 956–961. 18 Y. L. Zhang, J. N. Wang, Y. He, Y. Y. He, B. B. Xu, S. Wei and F. S. Xiao, Langmuir, 2011, 27, 12585–12590. 19 C. P. Pennisi, V. Zachar, L. Gurevich, A. Patriciu and J. J. Struijk, Conf. Proc. IEEE Eng. Med. Biol. Soc., 2010, 2010, 3804–3807. 20 L. Zhang, N. Zhao, X. F. Li, Y. H. Long, X. L. Zhang and J. Xu, So Mater., 2011, 7, 4050–4054. 21 S. Kato and A. J. Sato, J. Mater. Chem., 2012, 22, 8613–8621. 22 P. A. Levkin, F. Svec and J. M. J. Fr´chet, Adv. Funct. Mater., e 2009, 19, 1993–1998. 23 K. Matyjaszewski, S. Coca, S. G. Gaynor, W. Mingli and B. E. Woodworth, Macromolecules, 1998, 31, 5967–5969. 24 C. J. J. McLean, J. Accid. Emerg. Med., 1997, 14, 40–41. 25 D. M. M. D. Toriumi, K. O' Grady, D. M. D. Desai and A. M. D. Bagal, Plast. Reconstr. Surg., 1998, 102, 2209– 2219. 26 C. Serbutoviez, J. G. Kloosterboer, H. M. J. Boots and F. J. Touwslager, Macromolecules, 1996, 29, 7690–7698. 27 A. S´fr´nya, B. Beilera, K. L´szl´b and F. Svec, Polymer, 2005, a a a o 46, 2862–2871. 28 J. S. Li, E. Ueda, A. Nallapaneni, L. X. Li and P. A. Levkin, Langmuir, 2012, 28, 8286–8291. 29 M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto and T. Watanabe, Langmuir, 2000, 16, 5754–5760. 30 Z. J. Chen, Y. B. Guo and S. M. Fang, Surf. Interface Anal., 2010, 42, 1–6. 31 H. Y. Erbil, A. L. Demirel, Y. Avcı and O. Mert, Science, 2003, 299, 1377–1380. J. Mater. Chem. A, 2013, 1, 1026–1029 | 1029