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Langmuir 2006, 22, 7956-7959
Direct Catalytic Route to Superhydrophobic Polyethylene Films Wei Han,† Di Wu,‡ Weihua Ming,‡ Hans (J. W.) Niemantsverdriet,† and Peter C. Thu¨ne*,† Schuit Institute of Catalysis and Laboratory of Materials and Interface Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands ReceiVed May 19, 2006. In Final Form: July 7, 2006 Polyethylene films grow on a flat silica surface modified by the bis(imino)pyridyl iron(II) catalyst during ethylene polymerization in toluene solvent. The resulting films show superhydrophobic properties. Advancing water contact angle as high as 169° and sliding angles as low as 2° are obtained on these films. SEM images reveal special surface structures of these films containing micrometer-sized islands, submicrometer particles on the islands, and stress nanofibers between the islands, which render superhydrophobicity to the polyethylene surfaces. After the submicrometer particles and stress nanofibers are removed by annealing, the superhydrophobic properties of the polymer films disappear.
Introduction It is well known that some plant leaves show extreme waterrepellent and self-cleaning effects due to their unique topographic surface structures. Such surfaces with a water contact angle (CA) of greater than 150° and a sliding angle (R) of less than 5° are classified as superhydrophobic surfaces and are of great interest in both fundamental and applied research. Currently, superhydrophobic surfaces are produced via two main approaches. One is to introduce roughness onto a hydrophobic surface employing techniques such as plasma treatment,1,2 chemical etching,3,4 chemical vapor deposition (CVD),5,6 micropatterning with templates,7-10 electrospun polymer nanofibers,6,11 and so forth. Surfaces with dual-size (micro- and nanostructure) or hierarchical structures have been fabricated and proven to be very effective in generating superhydrophobicity and particularly small water sliding angles.9,12-16 The other main approach is to decrease the surface energy of a rough surface by chemically bonding lowsurface-energy species to the surface.1,17-19 A somewhat different approach involves the synthesis of superhydrophobic surfaces * To whom correspondence should be addressed. E-mail: p.c.thuene@ tue.nl. Phone: +31(0)402474997. Fax: +31(0)402473481. † Schuit Institute of Catalysis. ‡ Laboratory of Materials and Interface Chemistry. (1) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (2) Fresnais, J.; Benyahia, L.; Chapel, J. P.; Poncin-Epaillard, F. Eur. Phys. J.: Appl. Phys. 2004, 26, 209. (3) Qian, B. T.; Shen, Z. Q. Langmuir 2005, 21, 9007. (4) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824. (5) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365. (6) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742. (7) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. AdV. Mater. 2005, 17, 1977. (8) 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. (9) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. AdV. Mater. 2004, 16, 1929. (10) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (11) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549. (12) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298. (13) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (14) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14, 1857. (15) Han, J. T.; Xu, X. R.; Cho, K. W. Langmuir 2005, 21, 6662. (16) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966. (17) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754.
employing catalysis. An example was presented by Jiang and co-workers who prepared suprahydrophobic surfaces by growing aligned carbon nanotube (ACNT) films on quartz substrates using an iron catalyst.20 In this procedure, the superhydrophobic structure self-assembled on the surface without the need for any post-treatment. Polyolefins form an attractive class of materials for superhydrophobic coatings because they are cheap, chemically stable, and nontoxic. Superhydrophobic surfaces have been prepared from polyethylene, and polypropylene surfaces have been prepared from polymer solutions21-23 or by surface treatment with CF4 plasma.2 However, to our knowledge no direct catalytic route to superhydrophobic polyolefin coatings has yet been reported, even though polyolefins are largely made by catalytic polymerizations over heterogeneous catalysts. Herein, we describe a simple one-step method for creating superhydrophobic polyethylene (PE) films from a flat-silica-supported catalyst after polymerizations in solvent. The superhydrophobic property of the PE films is induced by their special micro/nanostructures. Our approach demonstrates a new method for creating superhydrophobic PE films directly from ethylene gas and a very small amount of catalyst, which does not require a template, extensive use of expensive chemicals (fluorinated compounds, carbon nanotubes), and/or additional complex treatments (plasma modification, chemical grafting). Experimental Section The preparation of the PE films on flat wafer surfaces is similar to that reported previously,24 as illustrated in Scheme 1 by the following processes: (1) A chlorosilane-functionalized bis(imino)pyridyl ligand was allowed to anchor to the wafer by the reaction with the hydroxyl groups on the wafer surface. (2) Iron chloride was coordinated to the ligand after the ligand-modified wafer was treated with an FeCl2‚4H2O solution in THF. (3) Polymerizations were performed at room temperature in toluene in the presence of (18) Xie, Q. D.; Xu, J.; Feng, L.; Jiang, L.; Tang, W. H.; Luo, X. D.; Han, C. C. AdV. Mater. 2004, 16, 302. (19) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (20) Li, H. J.; Wang, X. B.; Song, Y. L.; Liu, Y. Q.; Li, Q. S.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 1743. (21) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (22) Lu, X. Y.; Zhang, C. C.; Han, Y. C. Macromol. Rapid Commun. 2004, 25, 1606. (23) Lu, X. Y.; Zhang, J. L.; Zhang, C. C.; Han, Y. C. Macromol. Rapid Commun. 2005, 26, 637. (24) Han, W.; Mu¨ller, C.; Vogt, D.; Niemantsverdriet, J. W.; Thu¨ne, P. C. Macromol. Rapid Commun. 2006, 27, 279.
10.1021/la061414u CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
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Langmuir, Vol. 22, No. 19, 2006 7957 Scheme 1. Preparation of Superhydrophobic PE Films by the Catalytic Routea
a
The polymerization catalyst used here was the supported bis(imino)pyridyl iron(II) catalyst.
Figure 1. Images of a 10 µL water droplet on different PE surfaces: (a) on PE film 1 produced from solution-phase polymerization using the bis(imino)pyridyl iron(II) catalyst and (b) on smooth, melt-crystallized PE film 3 produced from gas-phase polymerization using a Cr/SiO2 Phillips catalyst. triisobutylaluminum (TIBA) at 2 or 10 bar ethylene pressure. Polymerization was stopped by removing the wafer from the solution after the pressure was reduced. The PE-coated wafer was washed with toluene. Film 1 was obtained with a thickness of 3.5 µm after 1 h of polymerization under 2 bar ethylene pressure. Film 2 showed a thickness of 90 µm after a 3 h of polymerization under 10 bar ethylene pressure. Scanning electron microscopy (SEM) was performed using a Philips XL-30 ESEM FEG environmental scanning electron microscope (Philips, The Netherlands, now Fei Co.) in high-vacuum mode at an acceleration voltage of 2 or 3 kV. Contact angles and sliding angles were measured with deionized water on a Dataphysics OCA 30 instrument at room temperature (∼21 °C). All of the contact angles and sliding angles were determined by averaging values measured at three different points on each sample surface.
Results and Discussion The surface wettability is determined by measuring water contact angles (CAs) on the as-grown PE films. Films 1 and 2 demonstrate superhydrophobic behavior. For film 1, the water advancing and receding CAs are (164.2 ( 0.7)° and (161.1 ( 0.3)°, respectively, and the sliding angle R is 3°. PE film 2 also shows an extremely water-repellent property, with an advancing CA of (169.2 ( 0.6)°, a receding CA of (164.2 ( 0.4)° and an R of 2°. Figure 1a shows the shape of a 10 µL water droplet on investigated PE surface 1. As a comparison, PE film 3 with a smooth surface, produced from polymerization at 145 °C in the gas phase,25 was also subjected to wettability measurement. This melt-crystallized PE film 3 exhibits normal hydrophobic behavior (as shown in Figure 1b) with advancing/receding CAs of (107.6 ( 0.5) and (83.9 ( 1.7)°, respectively. Superhydrophobic PE films 1 and 2 can maintain their superhydrophobicity long term when stored in air; the water CAs remain essentially constant after 3 months. However, the PE films appear to lose their superhydrophobicity temporarily if they are submerged in water for 24 h: when the wet samples are immediately subjected to wetting measurements, the advancing water CAs drop to around 142°. However, after the soaked (25) van Kimmenade, E. M. E.; Kuiper, A. E. T.; Tamminga, K.; Thu¨ne, P. C.; Niemantsverdriet, J. W. J. Catal. 2004, 223, 134.
samples are allowed to dry, they regain their superhydrophobicity (CAs above 160°). SEM images in Figure 2 clearly show three morphological features for as-grown films 1 and 2 (i.e., micrometer-sized polymer islands, submicrometer polymer particles on top of the islands, and polymer stress fibers between the islands). The as-grown PE films are ruptured into islands (Figure 2a and b). The average island area of film 1 is around 50 µm2, and between these islands appear valleys with an average width of 5 µm. For film 2, the island area is 25 times larger than the one from film 1, and the gaps between islands are around 25 µm. These islands are connected with each other by stress fibers with a typical diameter of 20 nm as shown in Figure 2b. From the side view (Figure 2c), it is apparent that the initial homogeneous polymer film has broken up with crevices down to the silica surface. This morphology is a result of the shrinkage of the polymer film after the evaporation of the solvent, which is trapped inside the polymer during polymerization.24 Moreover, on top of the PE islands, a secondary structure exists, as shown from the high-magnification side view for film 2 (Figure 2d). The polymer islands appear to be covered by small polymer particles of around 200 nm to 1 µm in diameter. We propose that these particles are indeed the ends of polyethylene crystal strands, which grow perpendicular to the flat catalyst surface as reported earlier.26 The combined effect of the described morphological features is to increase the surface roughness of the polyethylene films. Theoretical models have been proposed to describe the surperhydrophobicity of rough surfaces. Wenzel’s model indicates that the roughness factor r of a surface will geometrically enhance both the water CA and hysteresis on a hydrophobic surface; here r is defined as the ratio between the actual surface area and the projected surface area.27,28 The increase in the water CA hysteresis is suggested to result in a larger sliding angle. However, when the roughness factor r exceeds a certain level, the hysteresis starts to decrease because of a transition from the Wenzel regime (26) Loos, J.; Lemstra, P.; van Kimmenade, E. M. E.; Niemantsverdriet, J. W.; Ho¨hne, G. W. H.; Thu¨ne, P. C. Polym. Int. 2004, 53, 824. (27) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (28) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466.
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Figure 2. SEM images of PE films produced from the catalytic route on the flat silica surface: (a) top view for polymer film 1, (b) top view for film 2, (c) side view for film 2, and (d) high-magnification side view for film 2. Ethylene polymerization was performed at room temperature at 2 bar for 1 h for film 1 and at room temperature at 10 bar for 3 h for film 2.
to the Cassie regime,29 in which the liquid droplet sits on a composite surface made of solid and air. The fact that the air remains trapped beneath the droplet greatly enhances the surface hydrophobicity. The CA (θf) on a rough surface in Cassie’s model can be expressed as
cos θf ) fs cos θ - fv
(1)
Here, θ is the CA on the smooth surface; fs and fv are the fractional areas of the liquid-solid interface and the liquid-vapor interface, respectively (i.e., fs + fv )1). Equation 1 demonstrates that a large liquid-vapor interface will result in the superhydrophobic property of the surface. One can estimate the fv of surface 1 in Figure 2a to be about 0.32 from the SEM image if the valleys on the ruptured PE are merely considered to be the places where the liquid-vapor interfaces form. By applying eq 1 and fv ) 0.32, the advancing CA of surface 1 would be calculated to be 141.0°, whereas the observed advancing CA is much higher (close to 170°). This discrepancy indicates that the roughness introduced by the polymer islands alone is not large enough to explain the extreme water repellence of our PE films. Therefore, there are other factors that enhance the hydrophobicity of the PE films. First, dual-size surface structures have been proven to be very efficient in generating superhydrophobic surfaces in nature and man-made materials.9,12-15,30,31 Hence the secondary structure generated by the submicrometer PE particles sitting on the island surface should also be taken into account. The presence of these particles would offer extra air pockets between the liquid droplet and the island surface. The dual-size surface structure introduced by these particles and polymer islands could efficiently ensure a large liquid-vapor fraction on the interface. Second, during the break (29) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
Figure 3. Side view of SEM images for annealed PE film 1.
up of the PE films, valleys emerge, and stress fibers form between the polymer islands. These hydrophobic fibers may prevent water droplets from penetrating into the valleys, which could help reduce the CA hysteresis. To confirm the importance of the secondary structure and the stress fibers to the superhydrophobicity of the PE films, film 1 was heated to 160 °C for 3 h and then allowed to cool to room temperature. After the annealing process, the PE islands’ morphology remains, and the surfaces of the PE films become smooth (Figure 3). In addition, the stress fibers that connected the polymer islands have disappeared. This annealed film is subjected to the wetting measurement and shows advancing/ receding CAs of (135.6 ( 0.4) and (76.7 ( 0.3)°. This advancing CA agrees better with the calculated CA of 141° using eq 1 as discussed earlier. The results from the wetting measurements clearly show that the surface of the annealed sample, without secondary roughness and stress fibers, lacks the superhydrophobic
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Langmuir, Vol. 22, No. 19, 2006 7959
property and its wetting behavior falls into the Wenzel regime (a higher advancing CA and a lower receding CA). We can tune the morphology of the PE films by applying different polymerization conditions, such as temperature, pressure, and solvent, or by employing careful annealing experiments afterward. Currently, we attempt to create PE surfaces with only dual-size roughness but without stress nanofibers or smooth micrometer-sized islands with only stress nanofibers in order to gain further insight into the contributions of each factor.
examples of chemically anchored polymerization catalysts, which allow polymer films to grow on the surfaces of supports, have been reported.32-34 A variety of materials, such as glass, silicon wafers, aluminum oxide, metals with hydroxyl groups on the surface, and functionalized polymers, can serve as supports. These available techniques can be adopted to create superhydrophobic polymer surfaces via the method described in this letter. In addition, a great potential use of this approach is to create a superhydrophobic surface on an irregular surface, where certain techniques (i.e., common coating, microlithography) cannot apply.
Conclusions In summary, we developed a facile one-step method for fabricating superhydrophobic PE films from the very common material ethylene via a direct catalytic route. The PE films are produced from polymerization in toluene solution on wafer surfaces with preanchored catalysts. These films show superhydrophobicity with water advancing contact angles of greater than 164° and sliding angles of less than 3°. We believe that the combination of the dual-size surface structure and the PE stress nanofibers is responsible for the superhydrophobicity of the asgrown PE films. Increasing numbers of well-defined and efficient
Acknowledgment. This work was performed under the auspices of NIOK, The Netherlands Institute for Catalysis Research. We acknowledge financial support from The Netherlands Technology Foundation (STW), no. 790.35.706. LA061414U (30) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (31) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (32) Hlatky, G. G. Chem. ReV. 2000, 100, 1347-1376. (33) Choplin, A.; Quignard, F. Coord. Chem. ReV. 1998, 180, 1679-1702. (34) Severn, J. R.; Chadwick, J. C.; Duchateau, R.; Friederichs, N. Chem. ReV. 2005, 105, 4073.
