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Langmuir 1997, 13, 2538-2540
Two-Dimensional Arrays from Polymer Spheres in Nanoscale Prepared by the Langmuir-Blodgett Method H. Du, Y. B. Bai,* Z. Hui, L. S. Li, Y. M. Chen, X. Y. Tang, and T. J. Li Department of Chemistry, Jilin University, Changchun, 130023, P.R.C. Received July 8, 1996. In Final Form: February 13, 1997X Densely packed monolayers of latex particles with a diameter of 70 nm were obtained by means of the Langmuir-Blodgett method with a polymer colloid directly used as a subphase. Different kinds of spreading molecules were tested to transfer the latex particle monolayer. Cospreading molecule monolayers were proved to be the best choice. The pH value of the subphase was a key factor in controlling the interaction between the particles and the spreading molecule monolayers. The π-A isotherms were in good agreement with transmittance electron microscopy images, indicating that in the case of acidic condition, the interattraction force was strong enough to make polymer particles form a monolayer and to pull it onto the substrate.
Introduction Efforts to organize materials on microscopic scales are playing a more and more important role in nanoscience.1,2 The densely packed monolayers of polymer colloid particles have aroused much interest in recent years because of their potential use as imaging substances,3 a highly ordered mask for metal dispersion prior to removal of polymer particles with an organic solvent,4 etc. Different techniques, including spin coating, electrophoresis and self-assembly via solvent evaporation, have been used to obtain highly ordered latex monolayers.5-7 Burns et al. have even accomplished creating polystyrene particle arrays by the interaction between standing wave optical fields and the dielectric objects.8 The Langmuir-Blodgett technique represents one of the most attractive tools for the formation of latex arrays. For example, micrometer size latex particles are dispersed in organic solvents such as benzene or ethanol and then dropped on the water surface. The microspheres floating on the water-air interface are compressed into densely packed monolayers.4,9,10 However, very little has been reported on the latex particles monolayer with a polymer colloid as the subphase and amphiphilic molecules as spreading monolayers. In this experiment, the polymer colloid was synthesized by emulsion polymerization and was diluted without any further treatment. The pH value of the subphase was controlled to get the monolayers of nanosize latex particles. The behavior of the films was studied by π-A isotherms and TEM. Experimental Section Polymer latex particles with a diameter of 70 nm were prepared by emulsion copolymerization of styrene, butyl acrylate, and acrylic acid.11 The emulsion was dialyzed against water for 24 X
Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (2) Anczykowski, B.; Chi, L. F.; Fuchs, H. Surf. Interface Anal. 1995, 23, 416. (3) Hayashi, S.; Kumamoto, Y.; Suzuki, T.; Hirai, T. J. Colloid Interface Sci. 1991, 144 (2), 538. (4) Lenzmann, F.; Li, K.; Kitai, A. H.; Stover, H. D. H. Chem. Mater. 1994, 6, 156. (5) Deckman, H. W.; Dunsmuir, J. H.; Garoff, S.; McHenry, J. A.; Peiffer, D. G. J. Vac. Sci. Technol. 1988, B6 (1), 333. (6) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (7) Denkov, N. D.; Velve, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (8) Burns, M. M.; Fournier, J.; Golovchenko, J. A. Science 1990, 249, 749. (9) Kumaki, J. Macromolecules 1988, 21, 749. (10) Fulda, K.; Tieke, B. Adv. Mater. 1994, 6 (4), 288.
S0743-7463(96)00671-3 CCC: $14.00
h and then diluted with ultrapure water (18 MΩ). This polymer colloid was used as the subphase with a concentration of about 0.3 mg/mL. The pH value of the colloid was adjusted by hydrochloric acid and sodium hydrate. After recrystallization, octadecyl amine, stearic acid, and methyl octadecylate were used as spreading or cospreading molecules. They were dissolved in distilled chloroform before used. In the LB deposition, a JC-1 LB film balance (produced by Jilin University, Changchun) was used. The trough (39 × 12.7 cm2) used was carefully washed with ethanol and chloroform before use on the experiment. A spreading molecule solution of 50 µL was spread onto the subphase using a syringe, adding small drops of the spreading molecule solution at different locations. After the solvent volatilized, the floating film at the air-water interface was compressed continuously at a speed of 12.7 cm2/min. The surface pressure was simultaneously monitored by a film balance, and a pressure-area (π-A) isotherm of the sample was obtained. The surface layer was transferred onto the substrate by vertical dipping at a surface pressure of 20 mN/m. The experiments were carried out at 19-21 °C. The latex particle monolayer was observed with a transmittance electron microscope (JEM-2000FX). The TEM grids covered by a Formvar film were attached on a glass plate. The deposition of particles was described as above.
Results and Discussion The polymer colloid used as subphase was prepared by the semicontinuous emulsion copolymerization of styrene, butyl acrylate, and acrylic acid.11 Among the three kinds of monomers, acrylic acid is the most hydrophilic one. According to emulsion polymerization theory, many carboxyls are distributed on the surface of each particle. Moreover, the anion emulsifier (sodium dodecyl sulfate) forms a negative charge layer due to -SO4- groups around each sphere. It is mainly because of the electric charge layer that the latex particles disperse in water steadily, although these particles that only dissolved in the aqueous phase cannot form a closely packed monolayer on the surface by themselves. When there were no spreading molecules, the π-A isotherm was a horizontal line with a surface pressure of zero and a latex monolayer cannot be obtained. So the spreading molecules are needed to attract the latex to the surface, and the interattraction makes latex film formation possible. From this point, the electrostatic interaction between the spreading molecule monolayer and the surface groups of latex particles plays an important role in the system. In the beginning, octadecyl amines were chosen as spreading molecule monolayers since the -NH2 group can attract the negative (11) Zhang, G.; Zhang, L.; Liu, F.; Du, H.; Li, T.; Tang, X. Acta Sci. Nature Univ. Jilin. 1996, 1, 99.
© 1997 American Chemical Society
Two-Dimensional Arrays from Polymer Spheres
Figure 1. π-A isotherms of C18H37NH2 for various pH values with pure water as the subphase: (a) pH ) 5.0; (b) pH ) 7.6; (c) pH ) 9.3.
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Figure 4. π-A isotherms of 18NA on latex subphase: (a) pH ) 1.5; (b) pH ) 3.0; (c) pH ) 7.0.
Figure 2. π-A isotherms of mixed spreading molecules on pure water subphase: (a) pH ) 1.5 (18NA); (b) pH ) 3.0 (18NA); (c) pH ) 5.4 (18NA); (d) pH ) 1.5 (18NA2).
Figure 3. π-A isotherms of mixed spreading molecules on latex subphase (pH ) 1.5): (a) 18NA; (b) 18NA2.
latexes so that a polymer particle monolayer can be formed at the air-water interface. π-A isotherms of C18H37NH2 on a pure water subphase are shown in Figure 1. In the alkaline subphase, there was no phase transfer point at the π-A isotherm with a collapse pressure at about 53 mN/m. The limit area per moleule, which was defined by extrapolating the maximum slope of the π-A curve to zero surface pressure, was 19 Å2. When the pH value was higher than 9.0, the π-A curve of C18H37NH2 was hardly changed, which means that the C18H37NH2 chains have been closely packed into a monolayer. However, with the decreasing of pH value, the π-A isotherms have a collapse pressure, 35 mN/m. It is supposed that in an acidic solution, -NH2 groups are protonated. On compression, the free molecules become nearer and nearer. The positive charges repulse each other and prevent the same groups from packing closely. So the film cannot be stable at a higher pressure and collapses quickly. In order to improve the stability of the spreading molecule monolayer dependent on pH, another kind of molecule is required as “plugs”. Therefore, steric acid (C17H35COOH) and methyl octadecylate (C17H35COOCH3)
Figure 5. TEM image of LB monolayers of latex particles (π ) 20 mN): (a) pH ) 1.5; (b) pH ) 3.0.
are respectively added into octadecyl amine solution as mixed spreading molecules. C17H35COOCH3 alone is not a good amphiphilic molecule for film formation, and also, it has no direct interaction with C18H37NH2. It is only used as a plug to make a vacant space and weaken the repulsive force between C18H37NH2 molecules. So only a small amount of C17H35COOCH3 is added, and the mole ratio of it to C18H37NH2 is 1:9. However, the results were not satisfactory. Good π-A isotherms were not obtained in the range of pH values from 1.0 to 5.4, and the films collapsed at a low pressure. We think that another kind of molecule that can interact with C18H37NH2 may be efficient to improve the film formation. Thus, C17H35COOH has been selected to fit this purpose. The mole ratio of C18H37NH2 to C17H35COOH was 2:1 (18NA), 4:1 (18NA2), and 8:1 (18NA3). The π-A isotherms with water as the subphase (curves a and d of Figure 2) indicate that 18NA and 18NA2 can form good films when pH ) 1.01.5. But as far as 18NA3 is concerned, the film collapses at a very low pressure. This is because the C18H37NH2 monolayer itself is not stable under acidic conditions and
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the amount of C17H35COOH in 18NA3 is too small to improve the stability. With the polymer colloid as the subphase, the π-A isotherm of 18NA2 had a lower collapse pressure and a smaller limit area per molecule than that of 18NA (Figure 3). So the 18NA cospreading molecule is regarded as the best choice. It is appropriate at various pH values and can keep balance automatically by the interaction of -NH2 and -COOH groups (curves a-c of Figure 2). In the case of the latex colloid as the subphase, it was found that the pH value was still a key factor and its effect on π-A isotherms was much stronger. This has been proved by the apparent differences of π-A isotherms at various pH values (Figure 4). The area per molecule of the spreading molecule was 70 Å2 at pH 1.5, which changes apparently compared with that in the pure water subphase. Although pH ) 3.0, the area per molecule was 45 Å2, which has few changes compared with Figure 2b. When the pH value increased to 7.0, the area per molecule decreased to 20 Å2, which is approximate to that with pure water as the subphase. The monolayers under the three conditions were transferred onto copper grids and were probed with TEM. As indicated by TEM images, most areas were covered by polymer spheres at pH ) 1.5 and the particles were densely packed (Figure 5a), whereas at pH ) 3.0, a few particles and many interstices were observed (Figure 5b). Under the third condition, hardly any spheres could be found. On the basis of these results, it is concluded that relative densely packed monolayers of latex particles can be formed and transferred under acidic condition (pH ) 1.5). With the increase of alkalinity, the attraction between the spreading molecule monolayer
Du et al.
and the particles becomes weaker and weaker so that no particle monolayer can be obtained. It is supposed that under alkaline conditions with many hydroxyls, OHgroups, rather than the negative charges on the surface of polymer spheres, are attracted by the -NH2 group, since the particle is too large and heavy. The -NH2OHgroups repulse the negative particles so that the monolayer cannot be formed. In contrast, -NH3+ groups are dominant under acidic conditions. Their interactions with polymer particles are enhanced, and many particles are attracted to the air-water interface. The strong interactions between the particles and spreading molecules result in the increase of the area per molecule to 70 Å2. As mentioned above, the formation of polymer particle monolayers is caused by the electrostatic interaction between the spreading molecule monolayer and the particle surfaces. Conclusions Polymer latex particles with a diameter of 70 nm can be densely packed into relative ordered monolayers using the LB method. The behavior of monolayers was studied by means of π-A isotherms and TEM. The choice of spreading molecules depends on the interaction between them and the latex particles. Mixed spreading molecules can be used to improve the film stability at various pH values. In the case of the acidic subphase, the attraction between latex particles and spreading molecules is strong enough to pull the latex particles on the substrate and two-dimensional arrays of latex particles are obtained. LA9606714
