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Langmuir 2008, 24, 2960-2962
Release and Transfer of Polystyrene Dewetting Pattern by Hydration Force Nobuhiko J. Suematsu,†,‡ Satoshi Nishimura,*,† and Tomohiko Yamaguchi† Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology, AIST Central 5-2, 1-1-1 Higashi, Tsukuba 305-8565, Japan ReceiVed December 7, 2007. In Final Form: January 22, 2008 We studied the release and transfer of polystyrene (PS) microdots on mica. The PS dots were released from the mica in water. The release was affected by the density of K+ ions on the mica surface, which was controlled by pretreatment with K2CO3 solutions. The release of PS dots became dominant at the concentrations above [K+]crit ) 1 × 10-4 M in the K2CO3 solution. In this concentration region, repulsive forces appeared as a result of the hydration of K+ ions on the mica (J. Colloid Interface Sci. 1981, 83, 531). This result suggests that the repulsive hydration force causes the release of the PS dots. Followed by the release process, we successfully demonstrated the transfer of the array of PS dots from the mica to another substrate.
Introduction Dewetting of a dilute polymeric solution is one of the promising techniques for preparation of two-dimensional patterns.1-3 These polymeric patterns play a role as scaffolds for arrangement of nanoparticles4 and dies.5 From the technical aspect, the substrates used for the preparation of the patterns have been often limited to mica or silicon wafers, because the dewetting process is highly sensitive to the flatness and chemical properties of the substrates. To extend this kind of patterning techniques beyond such a restriction, it is required to release the patterns from the substrates and transfer them onto other substrates that are even unfavorable for the dewetting process. In this paper, we showed that the release process of ordered arrays of polystyrene (PS) microdots depends on the ionic condition of substrate surfaces. After comparing muscovite mica with slide glass, it will be noted that the repulsive hydration force induced by surface K+ ions plays a crucial role in the release of the dots. We verify that the release of the PS dots can be controlled by the surface density of K+ ion on the mica surface. Finally, we exhibit a successful example of the transfer of dewetting patterns. Experimental Section Polystyrene (PS; Mw ) 570 000, Mw/Mn < 1.01) was purchased from Aldrich. Toluene, hydrochloric acid (HCl) solution, and potassium carbonate (K2CO3) were of reagent grade and used without further purification. Millipore Ultrapure water was used in all experiments. We used two kinds of planar substrates in this study. One of them was commercially available slide glass plate (Matsunami) that was cleaned by a concentrated HCl solution, followed by rinsing with water prior to use. The other was muscovite mica (Niraco) cut into a suitable size prior to use. Freshly cleaved mica (K-mica) were immersed in a dilute HCl solution (5 × 10-5 M, pH ) 4.6, 100 mL) for 2 h in order to replace * To whom correspondence should be addressed. E-mail: s.nishimura@ aist.go.jp. † Nanotechnology Research Institute, AIST. ‡ Graduate School of Pure and Applied Science, University of Tsukuba. (1) Karthaus, O.; Ijiro, K.; Shimomura, M. Chem. Lett. 1996, 25, 821-822. (2) Karthaus, O.; Grasjo, L.; Maruyama, N.; Shimomura, M. Chaos 1999, 9, 308-314. (3) Yabu, H.; Shimomura, M. AdV. Funct. Mater. 2005, 15, 575-581. (4) Suematsu, N. J.; Ogawa, Y.; Yamamoto, Y.; Yamaguchi, Y. J. Colloid Interface Sci. 2007, 310, 648-652. (5) Hashimoto, Y.; Karthaus, O. J. Colloid Interface Sci. 2007, 311, 289-295.
the K+ ions completely with H+ on the basal plane6,7 and is referred to as H-mica. After drying, the H-mica was immersed in K2CO3 solutions (5 × 10-6 to 5 × 10-4 M, 100 mL) for 2 h in order to change the ratio of K+/H+ on the basal plane. This mica is referred to as K/H-mica. For testing the surface densities of the K+ ion on the mica surfaces, contact angles of a water droplet on them were measured using a goniometer (Pocket Goniometer PG-2, FIBRO System). A dilute PS solution in toluene (0.5 mg/mL) was cast on the mica sheets or a slide glass plate at room temperature to obtain ordered arrays of PS microdots. The mica sheets and the glass plate with the array of PS dots were rinsed with water or ethanol (EtOH) for 1 min under an optical microscope (Olympus STM with objective lens ULWD MS-Plan50, Olympus). The digitized images were taken and analyzed on a PC using ImageJ 1.36. For the transfer of the array of the PS dots, a TEM grid covered with Formvar film (JEOL) was placed on the K-mica with the PS dot pattern, and a drop of water was introduce to penetrate into the gap intervening between them. After a few minutes, the grid, on which the PS dots were transferred, was taken up.
Results and Discussion Ordered arrays of PS microdots were produced on a glass plate, a freshly cleaved mica (K-mica) sheet, and a HCl-treated mica (H-mica) sheet (Figure 1; The PS dots appeared as white spots on these substrates). The PS dots on the glass plate remained where they were after counting with water (Figure 1a), while the dots on the K-mica were immediately released from the surface by the contact with water (Figure 1b). In addition, for the mica, such a release of the dots did not occur in ethanol even under ultrasonic treatment. The values of Hamaker constant are calculated to be 1.8 × 10-20, 0.8 × 10-20, and 1.4 × 10-20 J for the PS/H2O/mica, the PS/ H2O/MeltQuartz, and the PS/EtOH/mica systems, respectively.8 Assuming that the PS dots are released against adhesion caused by the van der Waals attractive force, the PS dots are most unlikely to be released in the case of the PS/H2O/mica system because of the highest value of the Hamaker constant. This prediction was inconsistent with our observation. The van der Waals force is not related to the release of the PS dots from the substrates. (6) Claesson, P. M.; Herder, P.; Stenius, P.; Eriksson, J. C.; Pashley, R. M. J. Colloid Interface Sci. 1986, 109, 31-39. (7) Osman, M. A.; Suter, U. W. J. Colloid Interface Sci. 2000, 224, 112-115. (8) Israelachvili, J. N. Intermolecular and Surface Force, 2nd ed.; Academic Press Limited: London, 1992; Chapter 11.
10.1021/la703823d CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008
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Figure 1. PS dots observed under optical microscope before and after rinsing with water. The PS dots appear as white spots. The substrates are (a) glass plate, (b) fresh mica (K-mica), and (c) HCltreated mica (H-mica). Average diameters of the dots are (a) 3.0, (b) 1.1, and (c) 1.1 µm.
It has been known that both muscovite mica and a glass plate expose silicon oxide surface. However, K+ ions present in the interlayers of the mica remained on the basal surface after cleavage, while the glass surface is covered with silanol groups. Pashley reported that the presence of alkali cations at mica/water interfaces yields a repulsive hydration force between two mica sheets.9 The repulsive force induced by hydration of alkali cations in the interlayers of clays is also one of the reason for the osmotic swelling of clays at the interlayers.10 Analogous to the swelling of clays, the repulsion at the PS/mica interface must be induced by the hydration of K+ ions on the mica. Thus, we infer that the release of the PS dots would strongly depend on the hydration of the K+ ions. We examined the release of the PS dots by controlling the surface density of the K+ ions on the mica surface. Unlike the case of the K-mica, the dots on the H-mica remained as they were even after the exposure to water (Figure 1c), which is same as for the glass plate. This result provides a further justification for our hypothesis that the hydration of the K+ ions would cause the release of the PS dots from the substrates. The K/H-mica surface prepared by ion exchange of H+ with + K ion was assessed using contact angle measurements. The contact angle of water (θ) on the H-mica was around 26 °, while a drop of water on the K-mica spread out (θ ∼ 0). The values of the contact angle on the K/H-mica decreased with increasing the concentration of K2CO3 solution where the H-mica was immersed (Figure 2). This result indicates that the mica surface became more hydrophilic as the surface density of K+ ion was increased by the treatment with higher concentration of K2CO3 solution. We observed the release of the PS dots from the K/H-mica surface under microscope. The number of the released PS dots was counted after the contact with water for 1 min. They were released from the mica stochastically. We defined a removal ratio as the measure of the release of the dots as follows: (9) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531-546. (10) Van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed.; Wiley-Interscience: New York, 1963; Chapter 10.
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Figure 2. Contact angle of water and the removal ratio of PS dots. They are plotted against the logarithmic concentration of K+ ion in the treating solution (K2CO3). The removal ratio is defined as the ratio of the number of the released dots divided by the number of total dots in a fixed area of 7500 µm2 (eq 1). The thin solid line is the fitting curve of the removal ratio calculated from the sigmoid function (eq 2). The critical concentration of release is assessed to be 1 × 10-4 M.
R)
Number of released PS dots Total number of PS dots
(1)
The removal ratio (R) increased with increasing the concentration of K+ ion (Figure 2). To estimate the critical concentration of K+ ion for the release of the PS dots, we applied the empirical sigmoid function to our data
R)
1 1 + exp[-∆(log[K + ] - log[K + ]crit)]
(2)
where ∆ is the slope and [K+]crit is the critical concentration corresponding to R ) 0.5. The eq 2 with ∆ ) 4 and [K+]crit ) 1 × 10-4 M was fitted well to the experimental values shown in Figure 2. In addition, the release time of the dot depended on the difference in the surface density of the K+ ion on the K/H-mica. For example, it took about 20 s to release 50% of the dots in the observation area at the mica surface treated with the K2CO3 solution of the critical concentration, whereas all of the dots went away from the K-mica soon after the addition of water. We confirmed that there is little difference in the release time of the dots with diameters from 1 to 10 µm in this study. Pashley suggested that the repulsive force acting between the two mica surfaces overcame the van der Waals attractive force when the concentration of K+ ion was higher than 4 × 10-5 to 3 × 10-4 M (the critical hydration concentration: CHC).9 The CHC agreed well with the critical concentration of [K+]crit ) 1 × 10-4 M in this study. This agreement implies that the repulsive hydration force plays a key role in the release of the PS dots from the mica surfaces. It was also reported that the distance between two H-mica surfaces in water is jumped in the primary minimum position that is predicted by the DLVO theory.9 This indicates that the van der Waals attraction becomes dominant in the absence of the repulsive hydration force for the H-mica. Similarly, we considered that there is little repulsive hydration force between the PS dots and the H-mica surface, which is evident from our observation that the PS dots were not released from the H-mica surface.
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of the dots showed an ordered pattern. After the transfer, this pattern was kept on the Formvar film. This release and transfer technique of two-dimensional polymer pattern is quite convenient and highly reproducible. We believe that these results could expand the application field of the self-organizing process including the dewetting.
Conclusion
Figure 3. Transfer of the ordered array of PS dots from (a) the K-mica to (b) Formvar film on a grid for transmission electron microscopy.
In summary, the release of the PS dots on the mica surface could be controlled by the ionic condition of the surface. This simple method enables us to transfer the dots array from the K-mica to another substrate. The ordered array of the PS dots on the K-mica was released and transferred to the Formvar film surface on a grid for TEM (Figure 3). Such a transfer is indispensable for us to observe nanostructures formed in the PS dots.4 On the K-mica, the arrays
The release of the PS dots on the mica surface could be induced by the repulsive hydration force controlled by the surface K+ ion. This was supported by the fact that the removal ratio of the dots rise at the critical hydration concentration of K+ ion.9 Finaly, under the appropriate condition for the release of the PS dots, we successfully demonstrated the transfer of the dewetting dot pattern. Supporting Information Available: The diameters of the PS dots were varied from 1 to 10 µm. As shown in Figure S1, the difference of the dot size was less effective to the release. This material is available free of charge via the Internet at http://pubs.acs.org. LA703823D
