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Superhydrophobic Composite Films Produced on Various Substrates Panagiotis N. Manoudis,† Ioannis Karapanagiotis,*,‡ Andreas Tsakalof,§ Ioannis Zuburtikudis,| and Costas Panayiotou† Department of Chemical Engineering, Aristotle UniVersity of Thessaloniki, 54124 Thessaloniki, Greece, Ormylia Foundation, Art Diagnosis Center, Ormylia, Chalkidiki 63071, Greece, Medical Department, UniVersity of Thessaly, Larissa 41222, Greece, Department of Industrial Design Engineering, TEI of Western Macedonia, Kozani 50100, Greece ReceiVed June 10, 2008. ReVised Manuscript ReceiVed July 2, 2008 Hydrophilic silica (SiO2) nanoparticles were dispersed in solutions of poly(methyl methacrylate) (PMMA) and in solutions of a commercial poly(alkyl siloxane) (Rhodorsil 224), and the suspensions were sprayed on glass surfaces. The effect of the particle concentration on the hydrophobic character of PMMA-SiO2 and Rhodorsil-SiO2 films was investigated and showed the following: (i) Static contact angles (θs), measured on surfaces that were prepared from dilute dispersions (particle concentration 90°) polymers such as poly(methyl methacrylate) (PMMA) and poly(alkyl siloxane) (Rhodorsil 224), respectively. Hsieh et al. investigated the applicability of the method using only a hydrophobic polymer (Zonyl 8740) and one type (TiO2) of nanoparticle. It is noted that Rhodorsil is a commercial product that has been investigated and used as a protective coating for monument and building conservation.32,33 Finally, we show that the size and nature of the nanoparticles do not have any substantial effect on the maximum θs or minimum hysteresis (θA - θR) measured on the surfaces of the polymer-particle composite films. (21) Yu¨ce, M. Y.; Demirel, A. L. Eur. Phys. J. B 2008, DOI: 10.1140/epjb/ e2008-00042-0. (22) Gao, L.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052–9053. (23) Gao, L.; McCarthy, T. J. Langmuir 2007, 23, 9125–9127. (24) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (25) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652. (26) Otten, A.; Herminghaus, S. Langmuir 2004, 20, 2405–2408. (27) Wagner, P.; Fu¨rstner, R.; Barthlott, W.; Neinhuis, C. J. Exp. Bot. 2003, 54, 1295–1303. (28) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857–1860. (29) Gao, X.; Jiang, L. Nature 2004, 432, 36. (30) Wagner, T.; Neinhuis, C.; Barthlott, W. Ac. Zool. (Stockholm) 1996, 77, 213–225. ¨ ner, D.; Youngblood, J.; (31) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O McCarthy, T. J. Langmuir 1999, 15, 3395–3399. (32) Appolonia L.; Fassina V.; Matteoli U.; Mecchi A. M.; Nugari M. P.; Pinna D.; Peruzzi R.; Salvadori O.; Santamaria U.; Scala A.; Tiano P.; Proceedings of International Colloquium on Methods of EValuating Products for the ConserVation of Porous Building Material in Monument Rome, 1995. (33) Tsakalof, A.; Manoudis, P.; Karapanagiotis, I.; Chryssoulakis, I.; Panayiotou, C. J. Cult. Herit. 2007, 8, 69–72.
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Consequently, our approach shows that the suggested strategy (spraying a mixture of polymer solution with nanoparticles) appears to be, to a large extent, independent of the nature of the polymer-nanoparticle binary mixture sprayed on a surface. Furthermore, we show that the substrate has almost no effect on the enhanced hydrophobic character of the applied coatings. Polymer-silica composites are applied on a large variety of substrates such as glass, silicon, concrete, metal (aluminum), silk, wood, and marble. In all cases, the static contact angles (θs) are found to be greater than 150 and 160° for the PMMA-SiO2 and Rhodorsil-SiO2 composites, respectively. For any treated substrate, the contact angle hysteresis (θA - θR) is e7°. According to the above, the suggested strategy appears to have the following important advantages: (i) it can be used to treat a large variety of different surfaces; (ii) it is a low-cost and very simple method because it is a one-step process and includes the use of some very common materials; and (iii) it can be used for the treatment of large surfaces (including buildings and monuments) because the deposition of the composite film is achieved by a simple, fast spraying technique.
Experimental Section Poly(methyl methacrylate) (PMMA, Aldrich) with a molecular weight of 120 000 g/mol was dissolved in toluene to prepare stock solutions of 5 wt %. Rhodorsil 224 (Rhodia Silicones), which is a 7 wt % solution of poly(alkyl siloxane) in white spirit, was used as received. Silica (SiO2) particles (fumed powder, Aldrich) with a 7 nm mean diameter were mixed with the polymer solutions. Mixtures were subsequently stirred vigorously for 20 min. The polymernanoparticle dispersions were immediately sprayed onto clean glass slides (Wheel Brand) for 2 s through a nozzle with a diameter of 733 µm using an airbrush system (Paasche Airbrush). Preliminary experiments showed that long spray times (>5 s) resulted in films that were poorly adhered by the glass substrates. However, short spray times ( 90°), such as siloxane Rhodorsil, increase with roughness. We note that the (Young) contact angles of smooth PMMA and Rhodorsil films spin coated onto glass substrates were measured to be 72 ( 1° and 102 ( 0.5°, respectively. The Cassie-Baxter scenario (in which a water droplet sits on a mixture of air and solid) suggests that the water contact angle of any material (either hydrophilic or hydrophobic) increases with roughness.35 It has been argued that both states are superhydrophobic but the Cassie-Baxter state is the only one that corresponds to nonsticking drops (small θA - θR): a Wenzel drop interacts with many defects so that the associated contact angle hysteresis is very large.36 The discussion of the results of Figure 4 is divided into two sections according to the particle concentration (x axis of Figure 4): (i) films prepared from dispersions of high particle concentration (>0.5% w/v) and (ii) films that correspond to low particle concentration (0.5% w/v) correspond to the Cassie-Baxter scenario. This is evidenced by the enhanced θs values and the dramatic decrease in hysteresis that reflects a transition to a “composite” surface characterized by such a large roughness that a liquid cannot completely wet the crevices.7,37 According to Figures 1d-f and 3a,b, a rough two-length-scale hierarchical structure is formed that resembles the lotus leaf. (35) Athanassiou, A.; Lygeraki, M. I.; Pisignano, D.; Lakiotaki, K.; Varda, M.; Mele, E.; Fotakis, C.; Cingolani, R.; Anastasiadis, S. H. Langmuir 2006, 22, 2329–2333. (36) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495–2532. (37) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872–876.
This is proven if we compare these Figures with corresponding SEM images collected for the lotus leaf.25 (ii) Figure 4 shows that θs of Rhodorsil surfaces prepared from dilute dispersions (particle concentration 90°) models or even by a mixed state,21 which was mentioned in the previous paragraph. The increase in θs with roughness observed for the PMMA-based films cannot be rationalized by the Wenzel model, which suggests that for materials with θY < 90°, θs decreases with roughness. For the PMMA-based films, the increase in θs can be explained either by the Cassie-Baxter model or by a mixed state where both the Cassie-Baxter and the Wenzel scenarios coexist.21 Both PMMA-particle and Rhodorsil-particle films were prepared using exactly the same process, which resulted in the development of similar micro- and nanostructures and similar contact angle (both static and hysteresis) relative variations as a function of particle concentration (i.e., roughness). We cannot envision any obvious reason that indicates that the wettability of the two composite films must be described by different models. Consequently, we may assume that the Wenzel scenario should be excluded to explain the increase of θs in region ii (particle concentration 0.5% wt. Experiments were performed with Rhodorsil-Al2O3 and Rhodorsil-SnO2 dispersions to assess the particle concentration, above which θs (and θΑ - θR) exhibits saturation behavior. It was found that elevated particle concentrations are necessary to obtain high/constant θs and low/constant θΑ - θR. The contact angles in Table 2 correspond to particle concentrations of 10% w/v for both Al2O3 and SnO2 particles; further increases (or slight decreases) in particle concentration did not have any effect on the reported values. Table 2 shows that the surfaces prepared with the three different nanoparticles exhibit clearly superhydrophobic properties. Interestingly, the measured θs and θΑ - θR values are not affected by the nature or size of the nanoparticles. We note, however, that the particle size or nature (e.g., specific surface area) may have an effect on the minimum particle concentration that is necessary to achieve superhydrophobicity. This will be investigated in a future study. Here we just report that superhyrophobic surfaces can be obtained using different hydrophilic nanoparticles embedded in a polymer matrix. This is further supported by previously published studies that reported that superhydrophobic polymer-nanoparticle composite films can be produced using TiO2 nanoparticles.18 According to Table 2, once superhydrophobicity is achieved the size or nature of the nanoparticle does not have any important effect on the wettability of the surfaces of the composite films.
Conclusions A simple, low-cost strategy for the production of superhydrophobic surfaces that can be applied on large surfaces (e.g., buildings) was presented. A polymer-particle dispersion is sprayed on a substrate, and this process can result in the formation of a rough two-length-scale hierarchical structure that exhibits water-repellent properties. The hydrophobic character of the composite films was investigated extensively for PMMA and Rhodorsil (commercial siloxane) mixed with SiO2 nanoparticles, applied on glass substrate surfaces. It was shown, however, that superhydrophobicity can be achieved by using (i) other nanoparticles, such as Al2O3 and SnO2, and (ii) other substrates, such as silicon, concrete, metal (aluminum), silk, wood, and marble. The effects of particle concentration on the hydrophobic character of PMMA-SiO2 and siloxane-SiO2 surfaces were investigated and resulted in the following conclusions: (i) Static contact angles (θs), measured on surfaces that were prepared from dilute dispersions (particle concentration 0.5% w/v) can be rationalized by the Cassie-Baxter model. We speculate that films produced from dispersions of low particle concentration
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may correspond to a mixed state where both the Wenzel and Cassie-Baxter regimes are partially realized.21 Acknowledgment. We thank the Greek Ministry of Development (GSRT) and European Social Fund (EU) for their financial support through the research program PENED 2003. The financial support of the Greek State Scholarship Foundation (P.N.M.) is also gratefully acknowledged. LA801817E
