Langmuir 2007, 23, 9695-9698
9695
From Superhydrophilicity to Superhydrophobicity: The Wetting Behavior of a Methylsilicone/Phenolic Resin/Silica Composite Surface Weixin Hou†,‡ and Qihua Wang*,† State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China, and Graduate School, Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed April 8, 2007. In Final Form: June 26, 2007 A methylsilicone/phenolic resin/silica composite surface was prepared by a casting method. The wetting behavior of the surface was investigated. It was found that the as-prepared surface can be varied from superhydrophilicity to superhydrophobicity as the drying temperature increased. Methylsilicone/silica and phenolic resin/silica composite surfaces were also prepared as comparisons. Both of them cannot achieve superhydrophobicity. A mechanism was proposed to explain this phenomenon.
Introduction It is well-known that some plant leaves (such as lotus) and insect wings (such as cicada) show self-cleaning properties, and water droplets can slide immediately when they are dropped on these surfaces. The superhydrophobic surface (identified as water contact angle larger than 150°, sliding angle less than 5°) has aroused great interests in recent years. Many methods have been used to prepare superhydrophobic surfaces, such as plasma treatment,1,2 sol-gel method,3 chemical etching,4,5 chemical vapor deposition (CVD),6,7 layer-by-layer (LBL),8,9 template method,10-12 electrospun polymer fibers,13 and so on. Among these methods, the wetting switching surface was focused on specifically, for example, photoresponsive materials, such as ZnO,14 spiropyram;15 pH-responsive surface;16 temperature-responsive polymer;17 and * To whom correspondence should be addressed. E-mail: Wangqh@ lzb.ac.cn. † Lanzhou Institute of Chemical Physics. ‡ Chinese Academy of Sciences. (1) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432-3438. (2) Fresnais, J.; Benyahia, L.; Chapel, J. P.; Poncin-Epaillard, F. Eur. Phys. J. Appl. Phys. 2004, 26, 209-212. (3) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626-5631. (4) Qian, B. T.; Shen, Z. Q. Langmuir 2005, 21, 9007-9009. Guo, Z.; Zhou, F.; Hao, J.; Liu, W. J. Am. Chem. Soc. 2005, 127, 15670-15671. (5) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824-3827. (6) Lau, K. K. S.; Bico, J.; Teo, K. B. K., et al. Nano Lett. 2003, 3, 1701-1705. (7) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. Langmuir 2004, 20, 56595661. (8) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349-1353. (9) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713-4716. (10) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Adv. Mater 2005, 17, 1977-1981. (11) Feng, L.; Li, S. H.; Li, H. J.; Zhai, J.; Song, Y. L.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2002, 41, 1221-1223. (12) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929-1932. Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (13) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5545-554. (14) Feng, X. J.; Feng, L.; Jin, M. H. et al. J. Am. Chem. Soc. 2004, 126, 62-63. (15) Rosario, R.; Gust, D.; Garcia, A. A.; et al. J. Phys. Chem. B 2004, 108, 12640-12642. (16) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; et al. AdV. Mater. 2005, 17, 12891293. (17) Sun, T. L.; Wang, G. J.; Feng, L.; et al. Angew. Chem. Int. Ed. 2004, 43, 357-360.
electrical potential-responsive conducting polymer films.18 Recently, Xia et al.19 prepared a dual-responsive surface that switched between superhydrphilicity and superhydrophobicity and had responsivity to both temperature and pH. In addition, silica has been used for preparing superhydrophobic surfaces in many reports,20-21 but multiexperiment steps and expensive materials have restricted the application of superhydrophibic surfaces. The casting method is a simple and convenient method, and superhydrophobic surfaces obtained from solution casting of polymers have been reported widely.22 Here, we report a simple casting method to prepare superhydrophobic surfaces. The single experiment step and common materials are beneficial to the application, and the surface wettability can be adjusted from superhydrophilicity to superhydrophobicity just by controlling the drying temperature. Experimental Section Preparation. Phenolic resin (PF6291, Jinan Shengquan Corp) and methylsilicone with different weight percentages were dissolved in 20 mL of tetrahydrofuran (THF), and then 0.20 g of silica nanoparticles (20 nm) were dispersed in the solution. The mixture was stirred with a magnetic stirrer about 1 h, and then an ∼50 µL droplet was cast-coated on a glass slide at ambient temperature and was dried in an oven about 10 min. Five different drying temperatures were studied: 20, 60, 100, 140, and 180 °C. In addition, phenolic resin and methylsilicone films were prepared with the same method as references. Characterization. The microstructure of the as-prepared surface was characterized using a JSM-5600LV scanning electron microscope (JEOL, Japan). The contact angle was measured with a KYOWA contact-angle meter using a 5 µL water droplet as the indicator. An average contact angle value was obtained by measuring the same sample at five different positions. The chemical compositions of the as-prepared surface were investigated using X-ray photoelectron spectroscopy (XPS), which was conducted on a PHI-5702 electron spectrometer using an Al KR line excitation source with the reference of C1s at 285.00 eV. The take off angle of XPS is 90°. (18) Xu, L. B.; Chen, W.; et al. Angew. Chem. Int. Ed 2005, 44, 6009-6012. (19) Xia, F.; Feng, L.; Wang, S. T.; et al. AdV. Mater. 2006, 18, 432-436. (20) Ming, W.; Wu, D.; et al. Nano Lett. 2005, 5, 2298-2301. (21) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662-6665. (22) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 13771380. Liu, X. Y.; Zhang, C. C.; Hang, Y. C. Macromol. Rapid Commun. 2004, 25, 1606-1610. Zhao, Z.; Xu, J.; Xie, Q. D.; et al. Macromol. Rapid Commun 2005, 26, 1075-1080. Xie, Q. D.; Xu, J.; Feng, L.; et al. AdV. Mater. 2004, 16, 302-305.
10.1021/la7010259 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007
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Figure 1. SEM image of phenolic resin film, the insert is photograph of water droplet on the surface.
Figure 3. Variation of water contact angle with the drying temperature at different concentrations of phenolic resin. Error bar of contact angle is (1°. Table 1. C/O/Si Atom Ratio of the As-Prepared Surface when Dried at 60 and 180 °Ca 33%/0%/67% 30%/3%/67% 0%/33%/67%
Figure 2. Variation of water contact angle with the drying temperature at different weight percentages of phenolic resin/ methylsilicone/silica. Error bar of contact angle is (1°.
Results and Discussion A smooth transparent film was obtained when the phenolic resin solution was cast-coated on a glass slide as shown in Figure 1. The as-prepared surface has a water contact angle of about 60° (90°, hydrophobic material) was also prepared with the same method. An interesting wetting phenomenon was observed by combining methylsilicone, phenolic resin, and silica nanoparticles. The content of silica (0.20 g) and the weight of polymers (0.10 g) are constant, but different weight percentages of phenolic resin/methylsilicone/ silica (0%/33%/67%, 16.5%/16.5%/67%, 30%/3%/67%, and 33%/0%/67%) were investigated. Figure 2 gives the variation of water contact angle with drying temperature at different weight percentages: when the phenolic resin was not present in the coating, the resulting water contact angle varied from 45° to 137° as the drying temperature increased from 20 to 180 °C; the superhydrophobic surface was not prepared. However with the weight percentage changed to 16.5%/16.5%/67%, the hydrophobicity of the coating increased noticeably. Compared to 137°, the maximum water contact angle of the as-prepared surface increased to 142°. When the weight percentage of phenolic resin was increased further, a special wetting phenomenon was observed. The as-prepared surface showed a transition between superhydrophilicity and superhydrophobicity as the drying temperature increased when the weight percentage of the phenolic resin increased to 30%. However, if there is no methylsilicone present in the coating, the as-prepared surface is superhydrophilic even dried at 180 °C. From the above discussion, it is found that the concentration of phenolic resin has an obvious impact on the water contact angle of the coating. Figure 3 shows the variation of water contact
60 °C
180 °C
C/O/Si 22%/62.2%/15.8% C/O/Si 23.0%/59.5%/17.5% C/O/Si 14.7%/65.2%/20.1%
C/O/Si 22.3%/62.8%/16% C/O/Si 33.7%/50.4%/15.9% C/O/Si 17.1%/63.7%/19.2%
a The weight percentages of phenolic resin/methylsilicone/silica are 33%/0%/67%, 30%/3%/67%, and 0%/33%/67%, individually.
angle with the drying temperature at different concentrations of phenolic resin (0.09 g/20 mL, 0.30 g/20 mL, and 0.50 g/20 mL), while the concentration of methylsilicone (0.01 g/20 mL) and the content of silica (0.20 g) are constant. The surface wettability has been discussed above, when the concentration of phenolic resin is 0.09 g/20 mL, the resulting water contact angle varied from 0° to 151° as the drying temperature increased. The sliding angle was only about ∼3°. As the concentration increased to 0.30 g/20 mL, the water contact angle reached 138°, even if the sample was dried at 20 °C, the hydrophobicity of the surface enhanced apparently. Furthermore, the resulting static water contact angle of the coating is not easy to measure when it was dried at 180 °C. Water droplets can almost slide immediately on the surface. On the other hand, the adhesive force of the coating increased obviously with the addition of phenolic resin, which was usually used as an adhesive agent. When the concentration of phenolic resin was further increased to 0.50 g/20 mL, the surface wettability also changed. The resulting water contact angle is about 148° and almost was not varied with the drying temperature. However, the sliding angle increased abruptly: when the sample was tilted, even upside down, the droplet stayed pinned to the surface. That is to say, a higher concentration of phenolic resin is beneficial to the increase of coating adhesion but also provided higher adhesion to the water droplet. The microstructure of the as-prepared surface was observed by a scanning electron microscope. Figure 4 shows SEM images of the as-prepared surface with different weight percentages of phenolic resin/methylsilicone/silica. The coating is composed of silica nanoparticles. Since the dimensions of silica particles are nanometers, micro-nano binary structures can be observed as shown in Figure 4. Moreover, the aggregation of silica nanoparticles is easier with the addition of polymers, which is beneficial to the formation of hierarchical structures and lead to the low sliding angle. The microstructure of the composite surface with
From Superhydrophilicity to Superhydrophobicity
Langmuir, Vol. 23, No. 19, 2007 9697
Figure 4. SEM images of the as-prepared surface with different weight percentages of phenolic resin/methylsilicone/silica, the inserts are photographs of water droplet on the surface: (a) 0%/33%/67%, dried at 180 °C; (b) 33%/0%/67%, dried at 180 °C; (c) 30%/3%/67%, dried at 60 °C; (d) 30%/3%/67%, dried at 180 °C. (e) Composite surface dried at 180 °C, the concentration of phenolic resin is 0.30 g/20 mL.
high concentration of phenolic resin was also investigated as shown in Figure 4e. It was almost the same with other images as shown in Figure 4. Compared to the flat meyhylsilicone film, the roughness of the as-prepared surface increased obviously. According to the Cassie equation,23 the water droplet does not completely wet the rough substrate, and the presence of air pockets located over the microrough surface decreased the contact area between the water drop and the substrate, which is useful to increase the contact angles. The wettability of a surface was determined by two factors: surface compositions and the microstructure. Since the microstructures of the as-prepared surface are almost the same, we believed it is the different compositions on the coating that induced the different wetting behavior of the composite surface. The compositions of the as-prepared surface were investigated with X-ray photoelectron spectroscopy (XPS). Table 1 shows the C/O/ Si atom concentration ratio of the coating with different weight percentages of phenolic resin/methylsilicone/silica when the sample was dried at 60 and 180 °C. Three conclusions can be made. First, the variation of the C/O/Si atom concentration ratio with the drying temperature is not obvious if methylsilicone was not present in the coating. Second, the aggregation of the polymer on the top surface of the coating can be achieved if phenolic resin was not present but not apparent. Third, when the phenolic resin and methylsilicone coexisted in the coating, the atom concentration ratio of C/O/Si varied from 23.0%/59.5%/17.5% to 33.7%/50.4%/15.9% as the drying temperature increased from 60 to 180 °C, and the intensity of C1s peak increased obviously. Compared to 60 °C, the polymer aggregates at the surface when (23) Cassie, A.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-550.
Figure 5. XPS spectra of C1s peak at different drying temperatures: (a) 60 °C; (b) 180 °C. The weight percentage of phenolic resin/ methylsilicone/silica is 30%/3%/67%.
9698 Langmuir, Vol. 23, No. 19, 2007
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Figure 6. Possible mechanism of formation of the composite surface at different weight percentages of phenolic resin/methylsilicone/silica.
dried at 180 °C. The C1s peak was further investigated as shown in Figure 5 at high resolution. The intensity of C-Si (belonging to methylsilicone; 284.6 eV) increased obviously with the drying temperature; inversely, the intensity of C-C (belonging to phenolic resin; 285.3 ev) and C-O (belonging to phenolic resin; 286.8 ev) peaks showed almost no change, which indicated that it is the methylsilicone aggregated on the surface and not the phenolic resin. This can also explain the wetting switching between superhydrophilicity and superhydrophobicity. (High resolved spectra of O1s and C1s peaks of the as deposited layers can be seen in Figure S1, see the Supporting Information.) In addition, the variation of sliding angle with the concentration of the phenolic resin can also be explained. A high concentration of phenolic resin is beneficial to the aggregation of methylsilicone on the top surface, but if the concentration of phenolic resin is too high, the sliding angle increases abruptly as discussed above. This may be because the phenolic resin is a hydrophilic material, and contact angle hysteresis was influenced by the functional groups on the surface.24 According to above discussion, a novel mechanism was proposed to explain the wetting behavior of the as-prepared surface as shown in Figure 6. In Figure 6a, when methylsilicone was not present in the solvent, since silica and phenolic resin are both hydrophilic material, they mixed very well, and phenolic resin distributed homogeneously in the coating. The as-prepared surface is superhydrophilic. In Figure 6b, phenolic resin was not present, (24) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759-3766; Hennig, A.; Eichhorn, K.-J.; Staudinger, U.; Sahre, K.; Rogalli, M.; Stamm, M.; Neumann, A. W.; Grundke, K. Langmuir 2004, 20, 6685-6691.
and the circumstance is the same, but the as-prepared surface is hydrophilic not superhydrophilic when dried at 60 °C, since methylsilicone is a hydrophobic material. Figure 6, panels c and d, shows that the aggregation of methylsilicone on the top surface is easier with the addition of phenolic resin. The cooperative action may occurr between phenolic resin and methylsilicone especially at high drying temperature, which lead to the switching between superhydrophilicity and superhydrophobicity of the asprepared surface.
Conclusion A composite surface was prepared by a casting method. The wetting behavior of the as-prepared surface can be controlled by the drying temperature and the concentration of phenolic resin. A novel mechanism was proposed to explain the special phenomenon. In addition, the adhesive problem was also considered in this experiment for further application. This method can be used for a variety of polymers and nanoparticles, and will be useful to the application of superhydrophobic surface. Acknowledgment. The authors acknowledge the financial support of the Innovative Group Foundation from NFSC (Grant No.50421502) and the National Natural Science Foundation of China (Grant No.50475128). Supporting Information Available: High resolved spectra of O1s and C1s peaks of the as deposited layers. This material is available free of charge via the Internet at http://pubs.acs.org. LA7010259
