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The Importance of a Direct in Situ Evaluation of an Amphiphilic Diblock Copolymer Monolayer. The Similarity and Difference between Its Nanostructures on Water and on Solid Substrates Examined by X-ray Reflectometry and Atomic Force Microscopy† Keitaro Kago, Hideki Matsuoka, Ryuji Yoshitome, Emiko Mouri, and Hitoshi Yamaoka* Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, Japan Received September 8, 1998. In Final Form: April 6, 1999 X-ray reflectometry (XR) was carried out on amphiphilic diblock copolymer poly(R-methylstyrene)block-poly(decyl 4-vinylpyridine) monolayers on a water surface and on a glass plate. From XR data, the layer thickness and surface and interface roughness were determined. For comparison, atomic force microscopy (AFM) was carried out on the samples deposited on solid substrates (glass and mica). From XR data, it was quantitatively clarified that the monolayer became thicker and rougher by the deposition on solid substrates than on the water surface. AFM revealed aggregates formed on the solid substrates. The structural difference between the samples on a water surface and on solid substrates may be due to the preparation procedure of the sample on solid substrates. These findings indicate that the correct and precise information for the structure of the monolayer on water surface can be evaluated only by in situ experiments such as XR.
Introduction X-ray reflectometry (XR) is a technique to measure the dependence of X-ray reflection intensity on the incident/ reflection angles of X-ray.1,2 By analysis of specular XR data, the electron density profile normal to the surface can be determined. Hence, the film thickness and surface and interface roughness can be estimated by XR measurement with a very high resolution on the order of angstroms. Nondestructive and in situ measurements can be performed by XR. To observe the monolayer on a water surface, pretreatment is often necessary such as deposition on solid substrates or doping of chromophores. However, XR can be performed on an adsorbed layer on a liquid surface without any modification or treatment. Using this advantage, some authors have performed XR at the liquid-vapor interface.3-8 In contrast to XR, atomic force microscopy (AFM) provides the lateral, microscopic structure of the surface with resolution on the order of nanometers. It can investigate the topology of the deposited layer and even the dynamics of the isolated single polymer chain on a solid substrate.9 The use of a specific cantilever tip, for example, the end of which has been attached to a glass * To whom correspondence should be addressed. † Presented at Polyelectrolytes ‘98, Inuyama, Japan, May 31June 3, 1998. (1) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (2) Stamm, M. Adv. Polym. Sci. 1992, 100, 357. (3) Lu, B. C.; Rice, S. A. J. Chem. Phys. 1978, 68, 5558. (4) Vaknin, D.; Kjær, K.; Als-Nielsen, J.; Lo¨sche, M. Biophys. J. 1991, 59, 1325. (5) Vaknin, D.; Kjær, K.; Ringsdorf, H.; Blankenburg, R.; Piepenstock, M.; Diederich, A.; Lo¨sche, M. Langmuir 1993, 9, 1171. (6) Hazallah, B.; Bosio, L.; Cortes, R.; Errafii, N. J. Chem. Phys. 1996, 93, 1202. (7) Wu, X. Z.; Ocko, B. M.; Deutsch, M.; Sirota, E. B.; Sinha, S. K. Physica B 1996, 221, 261. (8) Chou, C. H.; Regan, M. J.; Pershan, P. S.; Zhou, X. L. Phys. Rev. E 1997, 55, 7212.
sphere, makes it possible to measure the interaction between the glass and solid plane directly.10 The structure of the adsorbed layer, such as the amphiphilic polymer monolayer and monolayer-polymer complex, on the water surface in a “wet” state is not always the same as that on a solid substrate in a “dried” state. This is because the layer may undergo a conformational change during the deposition procedure on a solid substrate due to the interaction between the monolayer and the substrate and/or by drying. Thus, to investigate the structure of the adsorbed layer on a water surface, in situ measurement is necessary. We have constructed an “Air-Water Interface X-ray Reflectometer” which has a compact size for laboratory use, not requiring the use of a synchrotron.11-17 This apparatus is equipped with an X-ray generator which a has high power, an accurate positioning sample stage, a Langmuir-Blodgett (LB) trough with surface pressure sensor, and so on. These systems made it possible to perform in situ XR on monolayers on a water surface. By using this apparatus, we could estimate the nanostructure and its change of lipid monolayer on a water surface as a function of surface pressure.11,12,15 The structural change (9) Kumaki, J.; Nishikawa, Y.; Hashimoto, T. J. Am. Chem. Soc. 1996, 119, 3321. (10) Kurihara, K.; Mizukami, M. Prog. Colloid Polym. Sci. 1997, 106, 2266. (11) Matsuoka, H.; Yamaoka, H. Proc. Risø Int. Symp. Mater. Sci. 1997, 18, 437. (12) Yamaoka, H.; Matsuoka, H.; Kago, K.; Endo, H.; Eckelt, J. Physica B 1998, 248, 280. (13) Kago, K.; Endo, H.; Matsuoka, H.; Yamaoka, H. Polym. Preprints Jpn. 1998, 47, 1040. (14) Kago, K.; Matsuoka, H.; Endo, H.; Eckelt, J.; Yamaoka, H. Supramol. Sci. 1998, 5, 349. (15) Yamaoka, H.; Matsuoka, H.; Kago, K.; Endo, H.; Eckelt, J.; Yoshitome, R. Chem. Phys. Lett. 1998, 295, 245. (16) Kago, K.; Fu¨rst, M.; Matsuoka, H.; Yamaoka, H.; Seki, T. Langmuir 1999, 15, 2237. (17) Kago, K.; Matsuoka, H.; Yoshitome, R.; Yamaoka, H.; Ijiro, K.; Shimomura, M. Submitted for publication.
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of the monolayer of poly(vinyl alcohol) containing photochromic side chains with azobenzene by the irradiation of ultraviolet and visible light on a water surface was detected by in situ XR.16 We also observed the DNA structure in complex with lipid on a water surface and on a solid substrate: the total thickness of the DNA layer of the dimethyldioctadecylammonium/DNA complex on the air-water interface was about 40 Å, although the thickness of the DNA layer on the solid substrate was determined to be about 11 Å.17 This reminds us of the importance of in situ experiments. In this study, we have performed in situ XR on an amphiphilic diblock copolymer monolayer on a water surface. By analysis of the XR data, the structure of the adsorbed layer was investigated. We have also made XR measurements on the samples deposited on solid substrates. The structural differences between wet and dried states were investigated. For the samples on solid substrates, AFM observations were also performed. AFM data, especially for surface roughness, were compared with the estimations made by XR measurements. Experimental Section Material. The polymer used was an amphiphilic diblock copolymer consisting of hydrophobic poly(R-methylstyrene) and hydrophilic poly(decyl 4-vinylpyridine) with iodine as a counterion (P(RMSt)-b-P(4VP-C10H21I)). Previously, we reported that (P(RMSt)-b-P(4VP-C10H21I) formed a monolayer on water surface.14 This sample was synthesized by living anionic polymerization following the procedures of Zhu et al.18 The degree of polymerization was 50 for both hydrophobic and hydrophilic parts. For in situ XR, a suitable amount of 3:1 (v/v) chloroform/2propanol solution was spread on the water surface in the LB trough of the X-ray reflectometer to prepare the monolayer. The samples deposited on the glass plate and mica substrate were prepared by the Langmuir-Blodgett technique. The solution was spread on the water surface, and the substrates were dipped and lifted vertically at a certain speed (say, 10 mm/min) under a constant surface pressure. Since both glass and mica surfaces are hydrophilic, a Z-type deposition should occur. XR. The XR was performed by an “Air-Water Interface X-ray Reflectometer” in our laboratory. This apparatus was constructed by modification of a RINT-TTR θ-θ rotating-anode X-ray system (Rigaku Corporation, Tokyo, Japan). The X-ray generator is a rotating anode type with a Cu target, and its maximum power is 60 kV to 300 mA. The wavelength of incident X-ray is 1.5406 Å (Cu KR1). On the sample stage, a Langmuir film balance and LB trough (USI System, Fukuoka, Japan) are mounted. The Langmuir film balance has a trough barrier to change the surface pressure and the Wilhelmy type surface pressure sensor. The details of the apparatus have been already described elsewhere.11,14,15 X-ray reflectivity is a function of refractive indices of substances in the system. The complex refractive index n of a substance for X-rays is written as
n ) 1 - δ - iβ
(1)
where
Figure 1. XR curves for spread monolayers of P(RMSt)50-bP(4VP-C10H21I)50 on a water surface at different surface pressures. The curves are shifted by 1 decade to avoid superposition. In the inset, a Rq4 vs q (q ) 4π/λ sin θ) plot for the data at a surface pressure of 35 mN/m is shown. Simulation curves are also shown assuming the thickness of the hydrophobic part to be 9.1 (-5), 19.1 (+5), and 24.1 Å (+10 Å). are distinguished from each other only by the difference of their refractive indices. To obtain the film thickness, surface and interface roughness, and density, the XR profile was subjected to curve fitting. Calculations and data analysis for XR were made using a scientific software program invented by Rigaku Corporation and scientific program “MUREX118 (Multiple Reflection of X-rays)”.19 The procedure of data fitting and simulation is based on the theory of Parratt20 and Sinha et al.21 AFM. The samples deposited on a glass plate and mica substrate were examined by AFM using the SPI3800 probe station and SPA300 unit system of a Scanning Probe Microscope System SPI3800 series (Seiko Instruments, Tokyo, Japan). The cantilevers were made of Si (Olympus, Tokyo, Japan), and their spring constant was 3 and 20 N/m for the measurement of the sample on glass plate and mica substrate, respectively. The measurements were performed in dynamic force mode (noncontact mode).
Results and Discussion δ)
2
F λ r (f + ∆f′) 2π e M
(2)
λ2 F r (∆f′′) 2π e M
(3)
β)
re is the classical electron radius, λ the wavelength of the X-ray, (f + ∆f′ + i∆f′′) the complex atomic scattering factor, F the density, and M the atomic mass. The refractive index is a unique parameter to represent the character of the substance; the layers (18) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583.
Figure 1 shows XR curves for spread monolayers of P(RMSt)50-b-P(4VP-C10H21I)50 on the water surface at different surface pressures.14 The data were plotted after subtraction of background intensity. Reflectivity was detected down to 10-6-10-7. Theoretically, the reflectivity has to be almost unity below the critical angle since total (19) Program for calculation/analysis of X-ray reflectivity, fluorescence intensity from multilayered thin films in grazing incidence/exit X-ray experiments written by K. Sakurai, National Research Institute for Metals, Tsukuba, Japan. (20) Parratt, L. G. Phys. Rev. 1954, 95, 359. (21) Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. Phys. Rev. B 1988, 38, 2297.
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Table 1. Fitting Parameters for the XR Curves for Spread Monolayers of P(rMSt)50-b-P(4VP-C10H21I)50 on a Water Surface at Different Surface Pressures δ β thickness roughness (×10-6) (×10-6) (Å) (Å) 35 mN/m P(RMSt)50 P(4VP-C10H21I)50 water 36 mN/m P(RMSt)50 P(4VP-C10H21I)50 water 37 mN/m P(RMSt)50 P(4VP-C10H21I)50 water
2.44 5.22 3.54 2.44 5.22 3.54 2.44 5.22 3.54
0.003 0.192 0.012 0.003 0.192 0.012 0.003 0.192 0.012
14.1 20.8 17.0 21.0 20.8 22.0
10.6 0.5 4.1 10.7 1.7 3.9 10.6 1.7 3.9
reflection should occur, but the experimental data show lower reflectivity than unity. This may be due to the fact that the water surface is not perfectly flat; because the size of the LB trough is small, an X-ray beam will hit the curved surface which is produced by the surface tension. At low surface pressures, XR curves were similar to that for a pure water surface. However, at higher surface pressures shown in Figure 1, a broad Kiessig fringe, which is the result from the interference between X-rays reflected at the surface and interface, was observed above the critical angle at about 1.5°. (The “belly” of the curve corresponds to the fringe. Because of the thinness of the layer, clear Kiessig fringes which have a clear maximum and minimum cannot be observed.22) Moreover, the second fringe was observed at about 2.5°. With increasing surface pressure, the Kiessig fringe became clearer and its position shifted toward lower angles. This indicates that compression made the monolayer denser and thicker. By exposure to X-rays, the polymer might undergo radiolysis (especially in the case of R-methyl alkyl (or allyl) vinyl compound) and/or the introduction of a polar (water soluble) group.23 If so, the layer would seem to become thinner. To estimate the effect of the radiation damage, XR measurement was made after the previous one at the same surface pressure. As a result, no difference was observed between the data obtained. So the radiation damage of the polymer can be ignored. The data were subjected to curve fitting. Least-squares fits to the experimental data were tried by using software with various initial parameters to find the best fit. The solid lines in Figure 1 are the best fit to the experimental data by model calculation. The fitting results for three different surface pressure conditions are summarized in Table 1. The thickness of the hydrophobic part increased from about 14 to 21 Å with increasing the surface pressure from 35 to 37 mN/m while that of the hydrophilic part is almost constant at 21 Å. The thickness of the hydrophobic part is expected to be between 16 (radius of gyration of poly(R-methylstyrene)) and 126 Å (full length in all-trans conformation). Since the thickness of the hydrophobic part obtained by XR was 14-21 Å, the molecular chains should be packed in a rather compact form in the monolayer on the water surface. The inset of Figure 1 shows the preciseness of fitting procedure. The simulation curves with different thicknesses of the hydrophobic part are shown and compared with the best fit curves. It is clear that only 5 Å difference in thickness causes a significant disagreement at about q ) 0.1-1, in this case. It has been clearly shown that only a several angstrom change in thickness was detectable by our previous study.11,12,15,16 The density of the hydrophilic part estimated by the curve (22) Vaknin, D.; Als-Nielsen, J.; Piepenstock, M.; Lo¨sche, M. Biophys. J. 1991, 60, 1545. (23) Baltes, H.; Schwendler, M.; Helm, C. A.; Heger R.; Goedel W. A. Macromolecules 1997, 30, 6633.
Figure 2. XR curves of P(RMSt)50-b-P(4VP-C10H21I)50 deposited on a glass plate at different surface pressures. The curves are shifted by 1 decade to avoid superposition.
fitting procedure was almost the same as that for decyl 4-vinylpyridine iodide in the bulk state (1.4 g/cm3). Conversely, the density of the hydrophobic part was estimated to be about 0.7 g/cm3, which is smaller than that in the bulk state. The surface roughness, i.e., airhydrophobic part interface roughness, was estimated to be about 10 Å. The result seems extraordinary that the P(4VPC10H21I)50 part did not change its molecular conformation although the P(RMSt)50 part increased in thickness by 50% upon compression from 35 to 37mN/m. This may be due to the simplicity of the fitting model. It is thought that the polymer layer does not form a simple two-layered structure but forms a more complex one, including the lateral inhomogeneity, non-Gaussian interface profile, etc. This point is now under investigation by using different polymer samples quaternized by alkyl chains of different length. Figure 2 shows XR curves for P(RMSt)50-b-P(4VPC10H21I)50 deposited on a glass plate from the water surface at different surface pressures. Like the data shown in Figure 1, reflectivity was lower than unity below the critical angle. This may be also due to the fact that the glass surface is not perfectly flat, i.e., due to a waviness of the surface in large dimension, not due to the “surface roughness” in small dimension. The Kiessig fringe was observed in XR curves. With increasing surface pressure, the Kiessig fringe became clearer and its position shifted toward lower angles. This indicates that a thicker monolayer was formed by compression. The critical angles of water and glass were different from each other due to the difference of refractive indices of water and glass; the former is 0.14° and the latter 0.22°. Thus the thickness and roughness of the sample on a water surface and that on a glass plate cannot be compared simply by superposing the XR curves for each sample. By curve fitting, the thickness of the hydrophobic part was found to increase from about 13 to 36 Å and that of
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Figure 3. (a) AFM images of P(RMSt)50-b-P(4VP-C10H21I)50 deposited on a glass plate at different surface pressures and (b) typical examples of their height profiles. Table 2. Fitting Parameters for the XR Curves for P(rMSt)50-b-P(4VP-C10H21I)50 Deposited on Glass Plates at Different Surface Pressures
Table 3. Structural Parameters for P(rMSt)50-b-P(4VP-C10H21I)50 Deposited on Glass Plates at Different Surface Pressures Estimated by AFM
δ β thickness roughness (×10-6) (×10-6) (Å) (Å) 10 mN/m P(RMSt)50 P(4VP-C10H21I)50 Glass 25 mN/m P(RMSt)50 P(4VP-C10H21I)50 Glass 34 mN/m P(RMSt)50 P(4VP-C10H21I)50 Glass
1.06 1.65 7.44 1.80 2.93 7.44 2.01 3.59 7.44
0.001 0.060 0.096 0.002 0.110 0.096 0.003 0.132 0.096
13.0 10.8 25.5 14.6 35.9 15.2
15.0 0.2 11.7 18.0 4.2 11.0 20.3 4.4 10.1
the hydrophilic part from about 11 to 15 Å with increasing surface pressure from 10 to 34 mN/m, as is summarized in Table 2. Indeed the range of surface pressure in which the sample had been prepared was different from that for water surface measurements, but the monolayer was thicker on a glass plate; the total thickness of the monolayer on the water surface (35 mN/m) and on a glass plate (34 mN/m) was about 35 and 51 Å, respectively. But the thickness of hydrophilic part was smaller on the glass plate than on the water surface. From this result, the hydrophilic chains might extend more laterally on the glass surface than on the water surface. The surface roughness (the air-hydrophilic part interface roughness)
20 mN/m 25 mN/m 34 mN/m
surface roughness (Å)
distance between aggregates (nm)
4.8 7.4 11.2
40 31 26
was estimated to be about 20 Å for the sample deposited at a surface pressure of 34 mN/m, which is larger than that on a water surface. The density in the hydrophobic part was estimated to be about 0.6 g/cm3 (34 mN/m). The value is slightly smaller than that obtained for the hydrophobic part on the water surface (0.7 g/cm3). This is qualitatively reasonable because the total volume occupied by the molecules became larger, i.e., the monolayer expanded, and accordingly the density became smaller. Thus the XR measurements indicated that the structure in a dried state was different from that in a wet state; the monolayer was thicker and its surface was rougher in the dried state. Next, we have applied the AFM technique to the samples on solid substrates to observe the surface structure. Figure 3a shows AFM images for P(RMSt)50-b-P(4VPC10H21I)50 deposited on a glass plate at different surface
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Figure 4. (a) AFM images of P(RMSt)50-b-P(4VP-C10H21I)50 deposited on a mica substrate at different surface pressures and (b) typical examples of their height profiles.
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Table 4. Structural Parameters for P(rMSt)50-b-P(4VP-C10H21I)50 Deposited on Mica Substrates at Different Surface Pressures Estimated by AFM
3 mN/m 10 mN/m 28 mN/m 30 mN/m
surface roughness (Å)
distance between aggregates (nm)
6.6 8.9 14.0 10.0
36 31 27 24
pressures. Some molecules formed aggregates. The size of these aggregates was roughly 20-30 Å in diameter. The domain of the aggregate came closer with increasing surface pressure, and its height increased. However, no significant change in domain size was observed. This indicates that the number of molecules in each aggregate was not largely affected by a change in surface pressure. Figure 3b shows height profiles for the AFM images in Figure 3a. They were the cross sections which pass through the nearest-neighboring aggregates. Judging from these images, the distance between the nearest-neighboring aggregates is determined to change from 36 nm (10 mN/ m) to 24 nm (34 mN/m) in the surface pressure range measured. The structural parameters obtained by AMF measurement are summarized in Table 3. The surface roughness was also determined from AFM data, as summarized in Table 3. It is the root mean square of the deviation of the height from the standard plane which has been determined by averaging the height of the monolayer. The surface roughness became larger with increasing surface pressure. The values were smaller than those obtained by XR measurements for the sample deposited on a glass plate at surface pressures of 25 and 34 mN/m, although we used just the same samples for XR and AFM measurements. In the case of the AFM measurement, it may be possible that the top of cantilever could not reach the bottom of the narrow space between aggregates. If so, the roughness might be determined to be smaller by AFM measurement. Moreover, the thickness and roughness are calculated by averaging these in the coherence area (of the order of µm2) in the case of XR,24,25 though in a smaller scan area (500 nm × 500 nm) in the case of the AFM measurement. These technical differences may cause the deviation of roughness obtained by XR and AFM measurements. To check the effect of monolayer-substrate interaction on monolayer structure, we used one more substrate material, i.e., mica. XR could not be applied to the samples on mica substrate since it was impossible to prepare the sample of which the surface was flat in the projected area of X-ray (2 mm (width of X-ray beam) × 13-1 mm (projected length of X-ray beam which depends on incident angle)) due to nonflatness of mica surface; indeed the mica surface is molecularly smooth but not macroscopically flat, unlike the case of silicon wafer. AFM could be performed for mica systems. Parts a and b of Figure 4 show AFM images for P(RMSt)50-b-P(4VP-C10H21I)50 deposited on mica substrate at different surface pressures and height profiles, respectively.14 As in the case for those deposited on a glass plate, aggregates were found. The numerical data are summarized in Table 4. Indeed the roughness and the distance between the nearest-neighboring aggregates do not exactly match those for the sample on a glass plate; the trend of changing is qualitatively the same. (24) Braslau, A.; Pershan, P. S.; Swislow, G.; Ocko, B M.; Als-Nielsen, J. Phys. Rev. A 1988, 38, 2457. (25) Helm, C. A.; Tippmann-Krayer, P.; Mo¨hwald, H.; Als-Nielsen, J.; Kjær, K. Biophys. J. 1991, 60, 1457.
Table 5. Comparison of Surface and Interface Roughness of P(rMSt)50-b-P(4VP-C10H21I)50 Monolayer on Water Surfaces and Solid Substrates Estimated by XR and AFM roughness (Å) XR on water surface (35 mN/m) XR on glass plate (34 mN/m) AFM on glass plate (34 mN/m) AFM on mica substrate (30 mN/ m)
P(RMSt)50 P(4VP-C10H21I)50 water P(RMSt)50 P(4VP-C10H21I)50 glass P(RMSt)50 P(RMSt)50
10.6 0.5 4.1 20.3 4.4 10.1 11.2 10.0
Since no large difference could be observed for the aggregate structure between glass plate and mica substrate, this aggregate formation is universal for hydrophilic surfaces. The surface roughness became larger with increasing surface pressure, but it was decreased with increasing surface pressure from 28 to 30 mN/m. At these higher surface pressures, the arrangement of aggregates looked ordered and the packing became denser. As was the case for the glass plate, the space between aggregates became narrower and the top of the cantilever might not reach the bottom. Moreover, since the lower part of the aggregates overlapped, the height from the bottom of the monolayer became apparently lower and the depth of the “valley” between aggregates became shallower. Consequently, the surface roughness may become apparently smaller. For convenience, Table 5 summarizes the numerical data for the surface roughness of the sample on the water surface and solid substrates obtained by XR and AFM measurements at the surface pressure of 30-35 mN/m. The above assumptions are confirmed. The smaller roughness for the identical sample on a glass plate estimated by AFM than by XR is likely because the top of the cantilever may not reach the bottom of the narrow space between aggregates. Thus the roughness seems to be estimated more reliably by XR. The samples on solid substrates showed a larger surface roughness than those on a water surface by XR. The possible reason for the roughness increase is the aggregate formation on solid substrates. Hence, the formation of aggregates might occur during the preparation of the sample on a solid substrate but not on a water surface. The AFM results observed for the sample on a glass plate and mica substrate were similar. If they were different, the interaction between solid substrate and polymer should have affected the structure of the polymer: The formation of aggregates might not be induced by the interaction between solid substrate and polymer and the properties of the substrate. The structural difference between the samples on water surface and solid substrate may be due to the preparation procedure of the sample on a solid substrate, that is to say, deposition from the water surface onto the solid substrate and drying. Conclusion We quantitatively clarified that the sample on a water surface was different in structure from that on a glass plate by XR. The aggregates which were observed in the sample on a solid substrate might have formed during the deposition and drying procedure. The structure obtained by AFM was almost the same for both glass and mica
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states, we can obtain useful information on the structure of monolayers.
systems. Although the roughness obtained by AFM was different from that by XR, this might be due to the technical differences of the measurements. The present study clearly reminds us of the importance of in situ experiments for air-water interface systems. However, there are few studies comparing the structures on a water surface and solid substrate by XR and the results obtained by XR and AFM. Similar attempts such as this study might be necessary for other systems. It is interesting to note that it has been possible to make in situ AFM measurements of the Langmuir film at the air/ liquid interface.26 By combining XR and AFM, as well as XR and neutron reflectometry,4,27 in both wet and dried
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(26) Eng, L. M.; Seuret, Ch.; Loose, H.; Gu¨nter P. J. Vac. Sci. Technol., B 1996, 14, 1386.
(27) Su, T. J.; Styrkas, D. A.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 6892.
Acknowledgment. This work was financially supported by the New Energy Development Organization (NEDO) project of the Ministry of International Trade and Industry of Japan and also by a Grant-in-Aid for Scientific Research on Priority Areas (Molecular Superstructure-Design and Creation) by the Ministry of Education, Science, Sports and Culture of Japan (08231237, 09217230) to whom our sincere gratitude is due.
