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Evidence for Spin Coating Electrostatic Self-Assembly of Polyelectrolytes Seung-Sub Lee,† Ki-Bong Lee,‡ and Jong-Dal Hong*,† Department of Chemistry, University of Incheon, 177 Dohwa-dong Nam-gu, Incheon 402-749, Korea, and Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea Received February 16, 2003. In Final Form: June 9, 2003 The influence of molecular mass on the formation of polyelectrolyte multilayers on an oppositely charged surface of a rotating substrate was explored for a combination of strong cationic and anionic polyelectrolytes, such as poly(1-(N-benzylpyridinio-2-yl)ethylene bromide) (PVP-2B) and ι-carrageenan. UV/visible spectroscopy and ellipsometry measurements confirmed that the amount of material deposited on a substrate is inversely proportional to the logarithm of the molecular weight of PVP-2B at a low concentration of 1 mM in the spin-coated as well as the solution-dipped multilayer assemblies. A quantitative evaluation of the data shows that an increase of the molecular weight Mw of PVP-2B from 3.8K to 6.0M leads to 50 and 23% decreases in the average amount and thickness per bilayer deposited using the spin coating electrostatic self-assembly technique, respectively. Studies were also carried out to determine the effect of polymer concentration and spin speed on the adsorption rate of high molecular weight (HMW) and low molecular weight (LMW) polyelectrolytes. Increase of the spin speed leads to almost the same decrease in the average deposition rate of LMW and HMW polymers on a substrate. In sharp contrast, increasing the concentration of the polymer solution causes a higher increase in the average amount of the layer pair for HMW polyelectrolytes than for LMW. It was also found that there exists a critical concentration in which an equivalent amount of polymer is adsorbed on a solid substrate for two different HMW and LMW polymers; more LMW polyelectrolyte is adsorbed until a critical concentration, above which the trends are reversed. The observation would indicate the fact that the spin coating is less and less determined by the selfassembly process with increasing the concentration, causing enhanced nonspecific interactions among polymer chains. Particularly, some evidence is found for the self-assembly process that plays an essential role in the formation of polyelectrolyte multilayers on a rotating substrate.
Introduction In the preceding papers of this series,1,2 we described the molecular-level layer-by-layer processing of polyelectrolytes via using the spin coating electrostatic selfassembly (SCESA) technique, especially for three selected typical multilayer model systems: poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH),1-3 ι-carrageenan and a main-chain polyionene poly(((4,4′-bis(6dimethylammonio)hexyl)-oxy)azobenzene bromide),1 and PAH and CdS nanoparticles.2 These studies suggested to us that most solution-dipped multilayer systems would be regenerated via using the SCESA technique. This was also shown to allow fast fabrication of a well-ordered structure of multilayer thin films and thus is believed to play a significant role in the rapidly expanding field of thin film fabrication employing the electrostatic selfassembly (ESA)4 that was devoted to realization of a variety of multilayer heterostructures on a solid substrate over the last several years, as well reported in review papers.5 * To whom correspondence should be addressed. Tel: 82-32770-8234. Fax: 82-32-770-8238. E-mail:
[email protected]. † University of Incheon. ‡ Pohang University of Science and Technology. (1) Lee, S.-S.; Hong, J.-D.; Kim, C. H.; Kim, K.; Koo, J. P.; Lee, K.-B. Macromolecules 2001, 34, 5358. (2) Cho, J.; Char, K.; Hong, J.-D.; Lee, K.-B. Adv. Mater. 2001, 13, 1076. (3) Hong, J.-D.; Cho, J.-H.; Char, K.-H. Korean Patent Application No. KR2001-41773. U.S. Patent No. 2003026898. (4) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. Decher, G.; Hong, J.-D. European Patent 0 472 990 A2, 1992. Decher, G.; Hong, J.-D. U.S. Patent 5,208,111, 1993. Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831.
“Self-assembly” is a term that by now figures prominently in the literature of nanostructured materials and nanofabrication. The term therefore carries a variety of implicit or explicit meanings. The term introduced here, “spin coating electrostatic self-assembly”, refers to the molecular organization that is established in a complex system of interlocking components, resulting in welldefined film formation of the component materials. Only, the formation of polyelectrolyte film by SCESA, unlike the solution dip process, is accelerated under the influence of parameters such as concentration and spin speed. Several other sophisticated physical techniques should be utilized in order to obtain and to describe the complete mechanism of adsorption and monolayer formation of a polyelectrolyte on a rotating substrate. However, the film formation of an anionic polyelectrolyte on a cationic surface would be at first glance considered to be similar to that of neutral polymers reviewed by Bornside et al.6 and Lawrence.7 In spin coating, the bulk of the solution flows off the disk due to radial outflow to reach a thickness of the film on the order of micrometers, while a polymer/ solvent on a substrate is accelerated to a rotational speed of several thousand revolutions per minute (rpm). Then, the centrifugally driven flow has drastically diminished and evaporation of the volatile solvent becomes the chief means of film thinning.8 It would not be too surprising to (5) Decher, G. Science 1997, 277, 1232. Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000; pp 505-564. (6) Bornside, D. E.; Macosko, C. W.; Scriven, L. E. J. Imaging Technol. 1987, 13, 122. (7) Lawrence, C. J. Phys. Fluids 1988, 31, 2786. (8) Bornside, D. E.; Macosko, C. W.; Scriven, L. E. J. Appl. Phys. 1989, 66, 5185.
10.1021/la034263t CCC: $25.00 © 2003 American Chemical Society Published on Web 07/15/2003
Spin Coating Electrostatic Self-Assembly
Figure 1. Chemical structures and illustrations for the polyelectrolytes used in this study.
expect that the polyelectrolyte chains are electrostatically anchored to the surface and realigned to pack densely in the time scale of film formation on a rotating substrate, because a polyion takes just a few seconds for bonding to the surface by some segments in solution dip.9 In addition, the rapid increase of polymer concentration that is caused by the evaporation of the solvent seems to enhance the interaction among polymer chains to drive dense formation of the monolayer. Afterward, the electrostatic repulsion among identically charged ions leads to desorption of polyelectrolytes weakly bound on the assembled film surface during washing, thereby resulting in a reversal of the surface charge. The reversal could be evidenced in an experiment in which further deposition of an anionic polyelectrolyte was not observed on an identically charged surface when the steps were successively repeated five times.1 Here, it should be emphasized that the washing steps play an essential role in controlling thickness of the layers in a similar level of the solution-dipped film. This also eliminates the thickness dependency on the viscosity of the polymer solution in spin coating, as theoretical approaches predicted.8 Thus, this allows us to predict and to control precisely the layer thickness as well as the surface roughness. However, note that the SCESA multilayer films should be sharply distinguished from a simple deposition of polyelectrolytes on a spin coater.10 The aim of this paper is to obtain essential information about the self-assembling mechanism in the formation of polyelectrolyte films on a rotating substrate, first with determining molecular weight effects of a strong polyelectrolyte, poly[1-(N-benzylpyridinio-2-yl)ethylene bromide] (PVP-2B), on the average absorbance and thickness per bilayer of multilayer systems. In addition, studies are performed to quantify the influence of polymer concentration and spin speed on the adsorption rate of high molecular weight (HMW) and low molecular weight (LMW) polyelectrolytes based on the bilayer thickness and the amount of films deposited on a substrate. Experimental Section PAH and ι-carrageenan (CAG), obtained from Aldrich, were used as received. PVP-2B was synthesized and kindly provided by S. Fo¨rster at the Max Planck Institute in Mainz for polymer research. The synthesis and the detailed characterization of PVP2B are described elsewhere in the previous literature.11 Note that the polymer was completely quaternized as tested by elementary analysis and counterion titration.11a Molecular structures of the polymers used in this study are shown in Figure 1. (9) Lvov, Y.; Decher, G. Crystallogr. Rep. 1994, 39, 628. Schwarz, S.; Eichhorn, K. J.; Wischerhoff, E.; Laschewsky, A. Colloids Surf., A 1999, 159, 491. Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. Kurth, D. G.; Osterhout, R. Langmuir 1999, 15, 4842. Raposo, M.; Ponter, R. S.; Mattoso, L. H. C.; Oliveira, O. N., Jr. Macromolecules 1997, 30, 6069. Tsukruk, V. V.; Bliznyuk, V. N.; Visser, D.; Campbell, A. L.; Bunning, R. J.; Adams, W. W. Macromolecules 1997, 30, 6615. (10) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H.-L. Langmuir 2002, 18, 168. (11) (a) Fo¨rster, S.; Schmidt, M.; Antoniette, M. Polymer 1990, 31, 781. (b) Kuhn, A.; Fo¨rster, S.; Losch, R.; Rommelfanger, M.; Rosenauer, C.; Schmidt, M. Makromol. Chem., Rapid Commun. 1993, 14, 433.
Langmuir, Vol. 19, No. 18, 2003 7593 The concentrations of PAH and CAG in H2O were 1 unit mM (pH 4.0) and 2 unit mM (pH 6.3), respectively. Solutions of different molecular weight PVP-2B were prepared by dissolving the polymers in water at a concentration of 1 unit mM (pH 4.5). The preparation of multilayer assemblies based on the SCESA method proceeds using well-established procedures described in the previous literature.1,2 In this case, the polymer/solvent solution (ca. 0.5 mL) was poured onto a cleaned substrate precoated with 5 bilayers of PAH and CAG, and then it was spun at a speed of 5000 rpm for 20 s. Subsequently, 1 mL of Milli-Q deionized water (18 MΩ) was put on the substrate and then spun again at 5000 rpm for 20 s in order to remove the weakly bound polyelectrolyte. The washing steps were repeated three times. For solution dip multilayers, the procedure used routinely in previous preparations of films12 was adopted with 20 min adsorption and 1 min rinsing steps. The amount of polymer material deposited at each step was estimated from its absorption spectrum taken with a Perkin-Elmer UV/visible spectrophotometer (Lambda 40). The measurement of film thickness was performed by using an optical ellipsometer (Rudolph/Auto EL, 632.8 nm line) and an X-ray diffractometer (18 kW Rigaku Ru-300 anode, λ ) 1.54Å).
Results and Discussion To explore film formation involving spin-accelerated adsorption processes, it is necessary to closely examine the multilayer buildup and the impact of growth conditions. At first, we examined deposition conditions for PVP2B films at a monomolecular level on an oppositely charged surface of a substrate rotating with a spin speed of 5000 rpm, adjusting the concentration of the polymer solution and the number of washing steps. Also, the solution dip ESA technique was utilized for a comparative study. In a quick review of Figure 2 and Table 1, successive deposition of a PVP-2B/CAG bilayer is monitored for both preparation techniques by UV/visible spectroscopy, and thereby, a linear increase in absorbance of a film at 269 nm indicates the regular proceeding of the deposition process. Apparently, increasing the molecular weight from 3.8K to 154K shows a much stronger decrease in the average amount of solution-dipped PVP-2B/CAG bilayer (Figure 2b) as compared to the spin-coated one (Figure 2a). This reveals that increasing the molecular weight of the polymer would appreciably influence the diffusionlimited adsorption of polyelectrolytes onto the oppositely charged surface in the solution dipping. In the previous study,1,2 it was found the thickness of a layer could be controlled in a dimension similar to that of the solution-dipped one, even though slightly more material was always deposited under an established preparation condition by the SCESA. Implicit in the interpretation of the results is the assumption that the interchain interactions developed between polyelectrolytes in solution and on a film surface are driving the reproducible layer-by-layer polymer deposition. To support this assumption, we examined the role that molecular weight plays in determining the average thickness per bilayer. Polymers with the lowest available molecular weight distributions were used in these particular studies in order to minimize the effects of molecular weight distribution. Molecular weights covering the range 3780-6 041 400 were examined in this study. All of the PVP-2B polymers have a narrow molecular weight distribution, Mw/Mn ≈ 1.10, with the exception of the Mw_6.0M sample (Mw/Mn ) 1.80). Also note that the monodispersity of the polymers provides an ideal condition for studying the influence of molecular weight on the thickness of the layer pair adsorbed on a surface. (12) Hong, J.-D.; Jung, B. D.; Kim, C. H.; Kim, K. Macromolecules 2000, 33, 7905.
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Figure 2. Plot of the optical absorbance (at 269 nm) vs number of PVP-2B/CAG bilayers deposited on a substrate by using SCESA (a) and solution dip techniques (b). The concentrations of PVP-2B and CAG were 1 and 2 mM, respectively. The spin speed was fixed at 5000 rpm for the spin coating self-assembly. The deposition process was observed for different molecular weights of PVP-2B: 3.8K (b), 154K ([), 2.1M (2), and 6.0M (9). Table 1. Optical Absorbance and Thickness per PVP-2B/ CAG Layer Pair as a Function of Molecular Weight samples (Mw) PVP-2B
Mw/Mn
Mw_3.8K Mw_154K Mw_2.1M Mw_6.0M
1.18 1.10 1.07 1.80
Mwa PVP
Mwb PVP-2B
spin
thickness of PVP-2B/ CAG bilayer (nm)
0.0065 0.0058 0.0055 0.0052
3.02 2.82 2.68 2.53
abs/bilayer dip
1440 3780 0.0018 58600 153900 0.0011 798300 2076900 0.0010 2300000 6041400 0.0009
a The mass-average mass of poly(2-vinylpyridine) (PVP) was determined by gel permeation chromatography in solvent DMF, with calibration by absolute molar masses from light scattering (ref 12b). b The mass-average mass Mw of PVP-2B is yielded by a common conversion equation: Mw_PVP-2B ) Mw_PVP × (M_VP2B/M_VP), where M_VP and M_VP-2B are the molar masses of 2-vinylpyridine and 2-vinylpyridinium benzylbromide, respectively.
Figure 3 shows how the average absorbance and thickness per layer pair vary with increasing molecular weight for PVP-2B/CAG multilayer systems (see also Table 1). First, it was quite amazingly found that the amount of material deposited on a substrate is inversely proportional to the logarithm of the molecular weight in the spincoated as well as the solution-dipped multilayer assemblies (Figure 3a). These experimental results are well consistent with the general notion13,14 that the polyelectrolyte chains tend to adsorb onto an oppositely charged surface in a strongly extended conformation and then spread out to occupy more area on the surface as the molecular weight increases. The higher the molecular weight, the larger the area a polyion chain occupies. That leads to a further thinning of the film. A quantitative evaluation shows that an increase of the molecular weight for Mw_3.8K to Mw_6.0M for PVP-2B leads to 19 and 53% decreases (from Figure 3a) in the average amount of bilayers deposited via using the solution dip and SCESA techniques, respectively. A linear fit of optical absorbances per layer pair yields an about 1.7 times higher average decrease of the optical density for the spin-coated film than that of the solution-dipped film. This suggests that a stronger dependence of molecular weight on the amount of the layer pair exists for the spin-coated multilayer systems. That is, according to our knowledge, a first successful experimental demonstration of the relationship between molecular weight and thickness of polyelectrolyte, even (13) Gramain, P. H.; Myard, P. H. J. Colloid Interface Sci. 1981, 84, 114. Fleer, G. J.; Lyklema, J. In Adsorption from solution at the solid/ liquid interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: London, 1983; p 153. Cohen Stuart, M. A. J. Phys. France 1988, 49, 1001. (14) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717.
Figure 3. (a) Plot of the average absorbances per PVP-2B/ CAG bilayer deposited by spin coating self-assembly (b) and the solution dip technique (O). (b) Ellipsometric thickness of a PVP-2B/CAG bilayer vs molecular weight. The bilayer thickness was estimated based on the thicknesses of 10 PVP-2B/CAG bilayers spin-assembled at 5000 rpm with 1 mM PVP-2B and 2 mM CAG solutions.
though it might result from the special natures of the polymer used in this study such as molecular weight range, hydrophobicity of the polymer chain, steric effect of bulky side groups, and so forth. In accordance with the UV/visible absorbance, the ellipsometric thickness of PVP-2B monolayers is also inversely proportional to the logarithm of the molecular weight, as shown in Figure 3b. The fact that the spincoated and static dip-cycled multilayers exhibit a tendency to adopt the identical relationship between the molecular weight and the adsorbed amount per layer pair would be reliable evidence for the SCESA process involved in film formation. The quantitative evaluation of these data showed that an increase of molecular weight from 3.8K to 6.0M leads to a decrease of about 23% in the average thickness of the PVP-2B monolayer formed on a rotating substrate. Now, a few other comments are worth making about the early findings related to our studies. First,
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Figure 4. Plots of average absorbance per PVP-2B/CAG bilayer vs concentration (a) and spin speed (b). Note that the average absorbance per layer pair was obtained from a linear fitting of the absorbance data in the buildup of 10 PVP-2B/CAG bilayers for a low (3.8K) and high molecular weight Mw of PVP-2B (1.7M and 2.1M), respectively.
Lo¨sche et al. reported that about 4.5 and 10% increases in layer thickness are observed with increasing the weightaverage molecular weight from 185K to 1.03M for the PSS/PAH bilayer systems that were adsorbed on a substrate from 0.5 and 2 M NaCl solution, respectively.15 For the PSS/polyaniline combination employed in the Stockton and Rubner study, it was found that the average thickness per bilayer is almost independent of the molecular weight of PSS covering the range 5K-1M.14 Both of these findings attest to the fact that reduction of segmental repulsion (shielding effect) tends to produce thicker layers with a greater fraction of the polymer chain present as loops and tails, and that the adsorption would not be describable by the general adsorption model of polyelectrolytes established from our study. Generally, the segmental repulsion effects can be reduced by increasing the ionic strength of the polymer solution and/or reducing the charge density along the chain by a structural or pH change.13 To further understand the molecular weight dependency of the spin-accelerated adsorption, there exists a necessity to determine the distinctive effect of concentration or spin speed on the deposition rate of the arbitrarily chosen LMW and HMW polymers on a rotating substrate. Figure 4 shows such results for a low (3.8K) and high molecular weight Mw of PVP-2B (1.7M and 2.1M) based on data obtained from the linear fitting of the absorbance data in buildup of 10 PVP-2B/CAG bilayers with increasing either concentration (Figure 1, Supporting Information) or spin speed (Figure 2, Supporting Information). The influence of concentration on the adsorption of PVP-2B has been examined in the range of 1-5 mM with 2 mM CAG solution and the spin speed fixed at 5000 rpm. The linear fitting of the data in Figure 4a showed that the average absorbance per layer pair grows linearly with increasing the concentration of both LMW and HMW samples, thereby giving rise to the larger deposition rate for the higher molecular weight polymer. Further, it was unexpectedly observed that the two spin-accelerated adsorption isotherms are intercrossing at a certain point (ca. 3.2 mM) referred to as “critical concentration” at which an equivalent amount of materials is adsorbed on a solid substrate for the two different HMW and LMW polymers. It would be very important to explore the physical meaning of the critical concentration in formation of SCESA polyelectrolyte layers. The observation would be interpreted as follows: Below the critical concentration, increasing the molecular weight of a highly charged polymer appreciably (15) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893.
prevents the adsorption of polyelectrolyte on a substrate due to the fully extended chain conformations. However, above the critical concentration, the interaction and entanglement among polymer chains are presumably enhanced, resulting in a reverse adsorption trend in favor of the higher molecular weight sample. In addition, the absorbance per layer pair does not decrease to zero for the infinite dilution of the polymer solution. Note that the adsorption of charged polymers in the infinite dilute concentration on a rotating substrate is now under investigation and will be described elsewhere in a forthcoming publication. In contrast to the concentration effect on polymer deposition, it can be interestingly observed that increasing spin speed causes an almost similar deposition rate for LMW and HMW polyelectrolytes and that the adsorbed amount decreases in exact proportion to the spin speed, as shown in Figure 4b. X-ray reflectivity measurements are employed to obtain comparative data about the thickness change of PVP-2B/ CAG bilayers in dependence on concentration and spin speed with a selected molecular weight sample (2.1M), as shown in Figure 5. The observance of typical interference fringes might be reliable evidence for the relatively smooth surface of the spin-assembled multilayer films. The results of the thickness calculation from minimums of the fringes according to Bragg’s equation were plotted versus the number of adsorbed layers (inset in Figure 5a). The thickness of the bilayers also shows a linear dependence on the concentration in accordance with the absorbance. Note that the thicknesses per layer pair spin-assembled at 6000 rpm with 1 mM PVP-2B and 2 mM CAG solutions are estimated to be 1.54, 1.89, 2.20, and 2.55 nm for 1-4 mM concentrations, respectively. In contrast, the thicknesses per layer pair decrease linearly with increasing the spin speed from 4000 to 5000 and 6000 rpm (inset of Figure 5b) in an identical manner, as shown in other bilayer systems.1,2 Conclusions In conclusion, we have attempted to assess the influence of molecular weight on film formation for a strong cationic polyelectrolyte PBP-2B on an oppositely charged surface of a rotating substrate. First, it has been found that the amount and thickness per layer pair are inversely proportional to the logarithm of the molecular weight of PVP2B. Indeed, this is in good agreement with the theoretical expectation for polyelectrolyte adsorption in solution. The results strongly suggested to us that the film formation could be attributed to self-assembly of polyelectrolytes under the adopted preparation conditions.
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Figure 5. X-ray diffractograms of 10 PVP-2B/CAG layer pairs spin-assembled on a silicon substrate at 6000 rpm with 2 mM CAG solution and PVP-2B (Mw ) 2.1M) concentration ranging from 1 to 4 mM (a) and also of films deposited from 1 mM PVP-2B and 2 mM CAG solutions with spin speeds ranging from 3000 to 6000 rpm (b). In the inset of the figures is shown a plot of the average bilayer thickness vs concentration or spin speed.
Increasing the spin speed leads to almost the same level of decreasing rate in the average amount of low and high molecular weight polyelectrolytes deposited on a substrate. In sharp contrast, increasing the concentration of the polymer solution causes a higher growth rate in the average amount of the layer pair for HMW polyelectrolytes than for LMW. In other words, the higher the molecular weight, the stronger the influence of molecular weight on the interaction among polymer chains and the adsorption rate of polyelectrolyte. However, more LMW polyelectrolyte is adsorbed in the dilute concentration region than HMW, but the trends are reversed above a certain critical concentration, in which an equivalent amount of the polymer is adsorbed on a solid substrate for two different HMW and LMW samples. In view of these observations, we interpret the experimental findings with a qualitatively consistent picture of the driving force behind polyelectrolyte monolayer formation on a rotating substrate. Polyion adsorption consists of pre- and post- adsorption stages: polymers are adsorbed onto a static substrate for several seconds from a roughly constant concentration of polymer solution, and then additionally from a continuously increasing one that was caused by the evaporation of a volatile solvent during
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spinning. Polyion continues to be adsorbed onto the substrate until its rate of diffusion stops at zero due to the drastically increased viscosity of the film. In the pre- stage, the polymer adsorption on a static substrate would be considered to be transport-limited and roughly proportional to polymer concentration, as reported by O ¨ dberg et al.16 and others.17,18 In the post- stage, spinning tends to induce an additional deposition and subsequently accelerates realignment of polymer chains to dense packing. Here, this additional adsorption seems to be affected by parameters such as concentration, molecular weight, and spin speed: First, the deposition of different molecular weight polymer depends linearly on the concentration, and also the lower the molecular weight, the smaller the deposition rate (see Figure 4). This is simply due to dense formation of a polymer monolayer in the pre- stage which gives rise to less influence of concentration on the additional adsorption. For high molecular weight polymers, the mechanically induced entanglement between polyelectrolyte chains seems to play a significant role in the additional adsorption stage, thereby giving rise to a larger growth rate of the polymer deposition. In addition, the higher the spin speed, the less the amount of the material deposited due to the fast freezing of the diffusion caused by the solvent evaporation. The influence of spin speed on the deposition rate is almost independent of the molecular mass of the polymer. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (Grant Number R05-2003-000-10848-0) and in part by the Research Foundation of the University of Incheon. The authors thank Professor H. Mo¨hwald at the Max Planck Institute of Colloids and Interfaces in Germany for informative comments and helpful discussions. Also, we are very grateful to Professor Kwan Kim of Seoul National University allowing us to use his ellipsometer facilities. Supporting Information Available: Figures showing absorbance at λmax as a function of the number of PVP-2B/CAG bilayers for different molecular weights of PVP-2B. This material is available free of charge via the Internet at http://pubs.acs.org. LA034263T (16) O ¨ dberg, L.; Sandberg, S.; Welin-Klintstro¨m, S.; Arwin, H. Langmuir 1995, 11, 2621. (17) Lvov, Y.; Decher, G. Crystallogr. Rep. 1994, 39, 628. Okubo, T.; Suda, M. J. Colloid Interface Sci. 1999, 213, 565. Okubo, T.; Suda, M. Colloid Polym. Sci. 1999, 277, 813. Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. Schlenoff, J. B.; Li, M. Ber. BunsenGes. Phys. Chem. 1996, 100, 943. Houska, M.; Brynda, E. J. Colloid Interface Sci. 1997, 188, 243. Shinbo, K.; Suzuki, K.; Kato, K.; Kaneko, F.; Kobayashi, S. Thin Solid Films 1998, 327-329, 209. Advincula, R. C.; Baba, A.; Kaneko, F. Polym. Mater. Sci. Eng. 1999, 81, 95. Advincula, R. C.; Park, M. K.; Baba, A.; Kaneko, F. Polym. Mater. Sci. Eng. 1999, 81, 77. (18) Hoogeveen, N. G.; Stuart Cohen, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133, Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. In Polymers at Interfaces; Chapman and Hall: London, 1993; p 279.
