10624
Langmuir 2003, 19, 10624-10627
Notes Ultra-Water-Repellent Poly(ethylene terephthalate) Substrates Katsuya Teshima,*,†,‡ Hiroyuki Sugimura,† Yasushi Inoue,§ Osamu Takai,| and Atsushi Takano‡ Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan, Advanced Technology Laboratory, Research & Development Center, Dai Nippon Printing Co., Ltd., Kashiwa 277-0871, Japan, Research Center for Nuclear Materials Recycle, Nagoya University, Nagoya 464-8603, Japan, and Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan Received February 17, 2003. In Final Form: August 26, 2003
Introduction Wettability of a solid surface is governed by two factors, that is, surface energy and morphology.1,2 One of the promising approaches is incorporation of fluorine atoms onto the surface, since fluorine provides a low surface energy due to its small atomic radius and large electronegativity.3,4 An ideally flat surface covered with regularly aligned and closely packed CF3 groups shows a water contact angle of about 120°.4 Such a surface has the lowest surface energy among all the solid surfaces. To obtain a water-repellent surface showing a water contact angle greater than this value, a proper surface texture is crucial. Indeed, surfaces with a water contact angle greater than 150° have been successfully fabricated so far by providing both a proper surface roughness and a low surface energy.5-14 * To whom correspondence should be addressed. † Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University. ‡ Dai Nippon Printing Co., Ltd.. § Research Center for Nuclear Materials Recycle, Nagoya University. | Center for Integrated Research in Science and Engineering, Nagoya University. (1) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (2) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (3) Schaub, T. F.; Kellogg, G. J.; Mayes, A. M.; Kulasekere, R.; Ankner, J. F.; Kaiser, H. Macromolecules 1996, 29, 3982. (4) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. Langmuir 1999, 15, 4321. (5) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (6) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 1040. (7) Takai, O.; Hozumi, A.; Sugimoto, N. J. Non-Cryst. Solids 1997, 218, 280. (8) Hozumi, A.; Takai, O. Thin Solid Films 1997, 303, 222. (9) Bonnan, M. P.; Burnside, B. M.; Little, A.; Reuben, R. L.; Wilson, J. B. Chem. Vap. Deposition 1997, 3, 201. (10) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (11) Wu, Y.; Sugimura, H.; Inoue, Y.; Takai, O. Chem. Vap. Deposition 2002, 8, 47. (12) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (13) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395.
For fabricating water-repellent optical devices including windows, eyeglasses, and displays,6-12,15 surface treatment techniques for optically transparent substrates are of primary importance. In particular, these devices are persistently demanded to be fabricated from polymeric materials, since, by the use of such materials, lightweight devices can be made and their production costs are potentially reduced. However, in general, polymeric materials are less heat-resistant compared with other transparent inorganic materials. Thus, polymeric optical devices must be treated at a low temperature. Here, we report a novel method by which ultra-waterrepellency can be provided to poly(ethylene terephthalate) (PET) substrates without degrading their optical transparency. We have provided two crucial properties to PET substrates. First, to form a proper surface texture on a PET substrate, a domain-selective oxygen plasma treatment was employed. Through this treatment, noncrystalline domains in the PET substrate were preferentially etched. Subsequently, to reduce its surface energy, a hydrophobic layer was formed on the nanotextured substrate by means of chemical vapor deposition (CVD). The water-repellency of the modified PET substrates is discussed in terms of their surface nanotextures and chemical compositions. Experimental Section Ultra-water-repellent PET substrates were fabricated by a two-step process. PET substrates were treated with oxygen plasma in order to form a rough surface and introduce hydrophilic functional groups. Subsequently, the nanotextured PET substrates were coated with a hydrophobic layer by means of CVD in order to reduce their surface energies. A capacitively coupled RF discharge system was used for the oxygen plasma treatment. The apparatus used for these experiments has been described elsewhere.16 Biaxially oriented PET sheets (UNITIKA Ltd.) of 12 µm in thickness were used as substrates. In the first step, nanotextured and hydrophilic PET surfaces were prepared by oxygen plasma treatment. After evacuation below 5.0 × 10-5 Pa, oxygen was introduced into the plasma system. The pressure of oxygen was regulated to be 5 Pa by controlling the oxygen supplying rate with a mass flow controller. An rf power of 100 W was applied to generate oxygen plasma for 10 min. In the second step, hydrophobic layers were prepared by CVD using four types of organosilane precursors, that is, heptadecafluoro-1,1,2,2-tetrahydrodecyl-1-trimethoxysilane [namely, fluoroalkylsilane, FAS, Shin-etsu Chemicals, CF3(CF2)8(CH2)2Si(OCH3)3], n-octadecyltrimethoxysilane [ODS, Tokyo Kasei Organic Chemicals, CH3(CH2)17Si(OCH3)3], methyltrimethoxysilane [MTMOS, Shin-etsu Chemicals, CH3Si(OCH3)3], and trimethylmethoxysilane [TMMOS, Shin-etsu Chemicals, CH3OSi(CH3)3]. Figure 1 shows the chemical structures of these precursors. The treated PET substrates were placed together with a glass cup filled with 0.2 cm3 organosilane liquid into a 65 cm3 Teflon container. The container was sealed with a cap and (14) Coulson, S. R.; Woodward, I.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. J. Phys. Chem. B 2000, 14, 8836. (15) Hong, B. S.; Han, J. H.; Kim, S. T.; Cho, Y. J.; Park, M. S.; Dolukhanyan, T.; Sung, C. Thin Solid Films 1999, 351, 274. (16) Teshima, K.; Inoue, Y.; Sugimura, H.; Takai, O. Thin Solid Films 2002, 420, 324.
10.1021/la034265d CCC: $25.00 © 2003 American Chemical Society Published on Web 11/08/2003
Notes
Langmuir, Vol. 19, No. 25, 2003 10625
Figure 1. Chemical structures of organosilane precursors.
Figure 2. XPS C 1s spectra obtained from (a) the untreated PET surface and (b) the PET surface treated with oxygen plasma. placed for 3-5 h in an oven maintained at 100 °C or for 1-3 days in an oven maintained at 30 °C.
Results and Discussion Figure 2 shows XPS C 1s spectra obtained from (a) the untreated PET surface and (b) the oxygen plasma-treated PET surface. The PET repeat unit is composed of eight H atoms, ten C atoms, and four O atoms. The C 1s spectrum of the untreated PET surface (Figure 2a) is deconvoluted into five peaks corresponding to carbon atoms of the benzene rings unbonded to the ester group (peak C1 at 284.7 eV), carbon atoms of the benzene rings bonded to the ester group (peak C2 at 285.2 eV), carbon atoms singly bonded to oxygen (peak C3 at 286.5 eV), ester carbon atoms (peak C4 at 289.1 eV), and a weak peak (just below 292.0 eV) resulting from a π f π* shake-up process.17-27 The relative component concentrations determined from these peak areas are similar to the stoichiometric values for PET (2:1:1:1 for C1/C2/C3/C4). On the other hand, the C 1s spectrum as shown in Figure 2b reveals that the PET surface was chemically altered due to the oxygen plasma treatment. Two new components (C5 at 287.9 eV and C6 at 289.6 eV) are clearly observable in spectrum b. The C5 and C6 components originate from isolated carbonyl groups (CdO) and OsCOsO groups.21,23,26 The C1-4 peaks of the (17) Lippitz, A.; Friedrich, J. F.; Unger, W. E. S.; Schertel, A.; Wo¨ll, Ch. Polymer 1996, 37, 3151. (18) Chataib, M.; Roberfroid, E. M.; Novis, Y.; Pireaux, J. J.; Caudano, R.; Lutgen, P.; Feyder, G. J. Vac. Sci. Technol., A 1989, 7, 3233. (19) Gerenser, L. J. J. Vac. Sci. Technol., A 1990, 8, 3682. (20) Wong, P. C.; Li, Y. S.; Mitchell, K. A. R. Appl. Surf. Sci. 1995, 84, 245. (21) Cueff, R.; Band, G.; Benmalek, M.; Besse, J. P.; Butruille, J. R.; Jacquet, M. Appl. Surf. Sci. 1997, 115, 292. (22) Paynter, R. W. Surf. Interface Anal. 1998, 26, 674. (23) Koprinarov, I.; Lippitz, A.; Friedrich, J. F.; Unger, W. E. S.; Wo¨ll, Ch. Polymer 1998, 39, 3001. (24) Sandrin, L.; Sacher, E. Appl. Surf. Sci. 1998, 135, 339. (25) Ektessabi, A. M.; Yamaguchi, K. Thin Solid Films 2000, 377, 793. (26) Vasquez-Borucki, S.; Achete, C. A.; Jacob, W. Surf. Coat. Technol. 2001, 138, 256. (27) Me´dard, N.; Soutif, J. C.; Poncin-Epaillard, F. Langmuir 2002, 18, 2246.
untreated PET are broadened, indicating that each peak includes more than one unique species. These species are ascribed to be formed through chemical reactions of the polymer chains with activated oxygen species. The broadening of C1 and C2 peaks is associated with destruction of aromatic rings in PET.23,24,28 These peaks are thought to include signals from polar groups such as -CsCdO, -CsCOO, or -CsCsO as well.21,23 The peak resulting from the π f π* transition, attributable to the phenyl groups, decreased due to the oxygen plasma treatment. However, since the shake-up peak does not disappear completely, the aromatic components remained on the surface region to the same extent.23,28 These XPS results may support the idea that oxygen plasma induced the phenyl ring-opening reaction leading to the formation of carbonyl groups. Consequently, hydrophilic functional groups were introduced on the PET surface. The water contact angle of the PET substrate changed from approximately 80° before treatment to less than 10° afterward. Since such hydrophilic functional groups have chemical affinities to organosilane molecules,29 organosilane molecules chemisorb onto the plasma-treated PET substrate when the substrate is exposed to organosilane vapor. Thus, organosilane layers were formed on the plasma-treated PET substrate by means of CVD (Figure 3).30 Figure 4 shows XPS C 1s spectra obtained from the nanotextured PET surfaces covered with (a) an FAS layer, (b) an ODS layer, (c) an MTMOS layer, and (d) a TMMOS layer. On the FAS-coated sample, three new components (C7 at 290.5 eV, C8 at 292.2 eV, and C9 at 294.5 eV) are clearly observed, as shown in spectrum a. The C7, C8, and C9 components originate from the -CF2-CH2-, -CF2-CF2-, and CF3CF2- groups, respectively.18,30 The CF3 and CF2 components could be clearly resolved from the other components because of fluorine’s great electronegativity. The spectrum of the ODS layer (Figure 4b) consists of a single peak centered at 285.0 eV (C10) besides the components caused by the PET surface treated with oxygen plasma, indicating that a hydrocabon layer corresponding to its precursor was formed on the nanotextured surface.31 On the other hand, the spectra of the MTMOS layer (Figure 4c) and the TMMOS layer (Figure 4d) consist of the components originated from the nanotextured PET surface and the two new components, that is, Si-Cx at 283.8 eV (Si-C networks: C11) and Si-CHx at 284.8 eV (C12).32 In these cases, the signal intensities from the PET surface were stronger than those in the cases of FAS and ODS, so many hydrophilic functional groups were readily observed on the PET surface coated with the MTMOS layer or the TMMOS layer. These results are ascribable to the very thin thicknesses of these organosilane layers. In particular, TMMOS molecules are thought to hardly form a closely packed monolayer on the nanotextured surfaces due to steric hindrance between trimethylsilyl groups, -Si(CH3)3. Figure 5 shows atomic force microscope (AFM) images of (a) the untreated PET surface, (b) the PET surface treated with oxygen plasma, and (c) the PET surface coated with FAS after the oxygen plasma treatment. The surface of the untreated PET substrate appears to be compara(28) Ton-That, C.; Teare, D. O. H.; Campbell, P. A.; Bradley, R. H. Surf. Sci. 1999, 433, 278. (29) Plueddemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum Press: New York and London, 1991. (30) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600. (31) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550. (32) Finot, E.; Roualdes, S.; Kirchner, M.; Rouessac, V.; Berjoan, R.; Durand, J.; Goudonnet, J.-P.; Cot, L. Appl. Surf. Sci. 2002, 187, 326.
10626
Langmuir, Vol. 19, No. 25, 2003
Notes
Figure 3. Formation models of an FAS hydrophobic layer on the nanotextured and hydrophilic polymer surface by low-temperature CVD.
Figure 4. XPS C 1s spectra obtained from the nanotextured PET surfaces covered with (a) an FAS layer, (b) an ODS layer, (c) an MTMOS layer, and (d) a TMMOS layer.
tively smooth with a root-mean-square roughness (Rrms) of 1.0 nm, as shown in Figure 5a. On the other hand, many protrusions greater than 10 nm in height are observed on the PET surface treated with oxygen plasma. Its Rrms was estimated to be 9.1 nm. Since PET has two domains, that is, crystalline and noncrystalline domains, and the noncrystalline domains are more readily etched than the crystalline domains, these protrusions were probably formed due to the domain-selective plasma etching. As can be seen in Figure 5b, the protrusions were orderly formed on the PET surface. The origin of this arrangement is considered to be the orientation and sectional crystallinity of the biaxially stretched PET substrate used in this study.17 Although many protrusions still remain on the PET surface even after the hydrophobic coating (Figure 5c), these protrusions are much larger in height and diameter than those shown in Figure 5b. Rrms of the PET surface with the hydrophobic layer, which was
Figure 5. AFM images of the PET surfaces (a) untreated, (b) treated with oxygen plasma, and (c) coated with FAS after the oxygen plasma treatment.
estimated to be 15.3 nm, is larger than that of the plasmatreated PET surface without the hydrophobic layer. A
Notes
Langmuir, Vol. 19, No. 25, 2003 10627
with transmittance over 90% in the visible range. Indeed, the printed characters beneath the substrate are clearly readable. In addition, the nanotextured PET surface with the ODS layer or MTMOS layer showed ultra-waterrepellency with contact angles greater than 150° as well, although the nanotextured PET surface with the TMMOS layer was less water-repellent compared with the others, showing a water contact angle of 140°. This smaller water contact angle is caused by the incompleteness of the TMMOS layer and its less hydrophobic property, as shown in Figure 4d. Conclusions
Figure 6. Photographs of water droplets on the transparent and ultra-water-repellent PET substrate.
distance between adjoining prominences is about 50-200 nm. FAS molecules were considered to chemisorb and polymerize onto the nanotextured and hydrophilic PET surface produced by the oxygen plasma etching, resulting in the formation of a hydrophobic layer as well as other organosilane molecules, that is, ODS and MTMOS. Figure 6 shows photographs of a water drop on the PET substrate treated with the oxygen plasma followed by the hydrophobic FAS layer coating. As clearly demonstrated in these photographs, the modified PET surface is certainly ultra-water-repellent, showing a water contact angle greater than 150°. The modified substrate is transparent
We have succeeded in providing ultra-water-repellency to PET substrates without degrading their optical transparency. Appropriate nanotextures were fabricated on the PET substrates through oxygen plasma etching. Simultaneously, hydrophilic functional groups were introduced on the substrates. These functional groups promoted adhesion of a hydrophobic layer consisting of organosilane molecules. Since the nanotextures remained after the organosilane layer coatings to some extent, the substrates showed ultra-water-repellency with water contact angles greater than 150°. Furthermore, the nanotextures were fine enough without scattering visible light so that the PET substrates preserved optical transparency. Since our process can be conducted at a low temperature below the glass transition temperatures of various optical plastics, the technique will be favorable for the fabrication of ultra-water-repellent optical devices made of polymeric materials. LA034265D
