Composite Thin Film by Hydrogen-Bonding Assembly of Polymer

A simple method (Filmetric F20) was introduced to determine the thickness and refractive index of the films. The film thickness was found to be a line...
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Langmuir 2006, 22, 338-343

Composite Thin Film by Hydrogen-Bonding Assembly of Polymer Brush and Poly(vinylpyrrolidone) Shuguang Yang, Yongjun Zhang, Li Wang, Song Hong, Jian Xu,* and Yongming Chen* State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

Chengming Li UniVersity of Mainz, Welderweg 11, 55099 Mainz, Germany ReceiVed June 13, 2005. In Final Form: October 16, 2005 Based on hydrogen-bonding layer-by-layer (LBL) assembly in aqueous solution, poly(vinylpyrrolidone) (PVPON) and a spherical polymer brush with a poly(methylsilsesquioxane) (PSQ) core and poly(acrylic acid) (PAA) hair chains were used to fabricate composite multilayer thin films. Hydrogen bonding as the driving force was confirmed by FT-IR spectrometry. A simple method (Filmetric F20) was introduced to determine the thickness and refractive index of the films. The film thickness was found to be a linear function of the number of bilayers. The average increase in thickness per bilayer is 28.3 nm. The film morphology was characterized with scanning electron microscopy and atomic force microscopy. The images obtained from the two instruments show a great resemblance. The films were further calcined to get an inorganic film by removing the organic components, or treated with tetrabutylammonium fluoride (TBAF) to remove the PSQ core and get an organic film. The optical properties and morphological changes induced by these treatments were also studied.

Introduction In the past several decades, materials science has been rapidly developed into an interdisciplinary field that encompasses organic, polymeric, and biological components in addition to the classic metals and inorganics.1 Composite materials, such as reinforced plastics, alloys, and even animal bones, combine two or more desirable properties to make them endowed with better performance in contrast to single-component materials. Higher functional systems, such as photochemical energy conversion, require accurate control of molecular orientation and organization in nanoscale. It is highly desirable to develop methods to design and construct multicomponent nanostructures. Multilayer composite thin film is a very simple but important model in nanoscale composites. It can help us understand how the molecules assemble, what structure they will form, and how the structure determines the properties. As a major method to fabricate multilayer thin films, the Langmuir-Blodgett (LB) technique had dominated for about 60 years until Decher and Hong introduced the layer-by-layer (LBL) technique in 1991.2 LB technique can finely control the molecule orientation in the nanostructured films; however, it requires special equipment and is only suitable for amphiphilic molecules. Meanwhile it has severe limitations with respect to substrate size and topology as well as film stability. Generally, LBL technique is a kind of fuzzy nanoassembly. Usually the LBL assembly films do not have a clear layered structure.1 However, the LBL technique does not need complicated equipment. There is no restriction on the size and morphology of the substrate on which the film is constructed. The LBL method has been proved to be a simple but versatile method to fabricate composite multilayer thin films.3 * Corresponding author. E-mail: [email protected] (J.X.); ymchen@ iccas.ac.cn (Y.C.). (1) Decher, G. Science 1997, 277, 1232. (2) Decher, G.; Hong, J. D. Makromol. Chem. Macromol. Symp. 1991, 46, 321.

The most widely investigated LBL technique is the alternate deposition of cationic and anionic polyelectrolytes, i.e., the socalled electrostatic self-assembly (ESA) method. Driven by electrostatic force, ESA has attracted the most attention due to the fact that electrostatic interaction is the simplest and has the least steric demand among all chemical bonds. Other LBL techniques which employ different driving forces, such as hydrogen bonding,4 coordination bonding,5 charge-transfer interaction,6 and covalent bonding7 have also been developed, but received less attention. Hydrogen bonding plays a crucial role in the basic processes of life, such as protein synthesis, recognition, and DNA duplication. It also plays an important role in composite materials. It has been used to make the different components in blended plastics more compatible. In 1997, Rubner4a and Zhang4b reported the preparation of hydrogen-bonding multilayer film. Recently, dendrimers,8 Au,9 and CdSe nanoparticles10 were reported to be successfully incorporated into hydrogen-bonded LBL multilayer films. In this paper, composite multilayer films were fabricated from poly(vinylpyrrolidone) (PVPON) and a spherical polymer brush (3) (a) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (b) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430. (4) (a) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (b) Wang, L.; Wang, Z.; Zhang, X.; Shen, J.; Chi, L.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (5) (a) Hao, E.; Wang, L.; Zhang, J.; Yang, B.; Zhang, X.; Shen, J. Chem. Lett. 1999, 5. (b) Mwaura, K.; Thomsen, D. L.; Phely-Bobin, T.; Taher, M.; Theodoropulos, S.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2000, 122, 2647. (6) (a) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (b) Zhang, Y.; Cao, W. Langmuir 2001, 17, 5021. (7) (a) Chen, J.; Luo, G.; Cao, W. Macromol. Rapid Commun. 2001, 22, 311. (b) Serizawa, T.; Nanameki, K.; Yamamoto, K.; Akashi, M. Macromolecules 2002, 35, 2184. (c) Zhang, Y.; Guan, Y.; Liu, J.; Xu, J.; Cao, W. Synth. Met. 2002, 128, 305. (8) Huo, F.; Xu, H.; Zhang, L.; Fu Y.; Wang, Z.; Zhang, X. Chem. Commun. 2003, 874. (9) Hao, E.; Lian, T. Chem. Mater. 2000, 12, 3392. (10) Hao, E.; Lian, T. Langmuir 2000, 16, 7879.

10.1021/la051581e CCC: $33.50 © 2006 American Chemical Society Published on Web 11/18/2005

Polymer Brush and PVPON Hydrogen-Bonding Assembly

with a poly(methylsilsesquioxane) (PSQ) core and poly(acrylic acid) (PAA) hair chains based on hydrogen-bonding LBL technique. Polymer bushes, an assembly of polymer chains, which are tethered at one end to a surface or an interface, began to attract attention in the 1950s when it was found that grafting polymer molecules to colloidal particles was a very effective way to prevent flocculation.11 In the present work, polymer brushes were utilized to incorporate PSQ particles, the second kind of component, into the composite film. The thickness of the composite film can easily approach a level of λ/4 of visible light at which the film could have potential applications in optical instruments.12 In addition, according to its composite feature, selective dissolution and calcination were imposed on the film to adjust its composition and optical properties Experimental Section Materials. Spherical polymer brush (PSQ-PAA) with a PSQ core and PAA hair chains (PSQ-PAA, Mw ) 4.03 × 107, Mw,hair ) 1.26 × 105, Rg ) 95.7 nm, Rh ) 83.6 nm, number of hairs ) ∼357, d core ) ∼25 nm (double check the Rg and Rh values; here Rg > Rh)) was prepared previously in Schmidt’s group at the University of Mainz, Germany.13 PVPON (K30) was received from Beijing Chemical Reagent Co. PAA (sodium salt, Mw ) 8000) and tetrabutylammonium fluoride (TBAF) were purchased from Aldrich. The polymer solutions and rinsing solutions were prepared with Millipore water (resistivity ) 18.2 MΩ cm, Milli Q-plus system). Preparation of Composite Film. The self-assembled films were fabricated either on quartz or silicon slides according to the different characterization methods. For films subjected to FT-IR or morphology analysis, silicon slides were used because they are transparent in the IR range and their surface is very flat. When monitoring the film growth with UV-visible spectrometer, quartz slides were chosen. Before use, both substrates were first immersed in a boiling H2SO4/H2O2 mixture (7:3 (v/v)) for 30 min, then rinsed with deionized water thoroughly, and finally dried with a stream of N2. The concentrations of PSQ-PAA solution and PVPON solution were both at 1.0 mg/mL. They were adjusted to pH ) 3 by adding HCl. The films were fabricated on an automated device (North Tianfu Ltd., Beijing, China). The substrates were immersed in PVPON and PSQ-PAA (or PAA when fabricating PVPON/PAA multilayer films) alternately, with three rinses in the water (pH ) 3) to remove the excess polymers. The deposition time and rinse time were 4 and 1 min, respectively. A PVPON/PSQ-PAA repeated unit is called a bilayer. The number of bilayers was set according to the practical demand. The symbol (PVPON/PSQ-PAA)n means that the film was made from PVPON and PSQ-PAA and has n bilayer(s). Calcination. The films were calcined in a tube stove at 300 or 400 °C in air. The temperature was raised from room temperature at a rate of 15 °C/min-1, then held at the set temperature for 2 h, and allowed to cool spontaneously. Selective Dissolution. The samples were put into a TBAF solution (0.1 M) and incubated for 30 min. Then they were rinsed thoroughly with deionized water and blown dry with N2. Characterization. UV-visible spectra were recorded on a Shimadzu UV-1601 PC spectrophotometer. IR spectra were measured on a Bruke Equinox 55 FT-IR/FAR 106 spectrophotometer. Scanning electron microscopy (SEM) images were obtained on a S-4300F electron microscope (Hitachi, Japan). Atomic force microscopy (AFM) measurements were carried out in air at room temperature on a Nanoscopy IIIA (Digital Instruments, Inc.) in the tapping mode. Commercial silicon probes (model TESP-100) with a typical resonant frequency of approximately 300 kHz was used to obtain the images. XPS characterization was performed on an ESCALab220I-XL spectrometer (VG Scientific) with an Al KR X-ray source (1486.6 eV). The thickness and optical parameter of the film were determined (11) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. (12) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (13) Li, C.; Schmidt, M.; Chen, Y. Abstr. Pap. Am. Chem. Soc. 2003, 225, U623 511-POLY (Part 2).

Langmuir, Vol. 22, No. 1, 2006 339 on a Filmetrics F20 (Filmetrics, Inc.) in the reflectance mode with a blank silicon slide as reference.

Result and Discussion It was well-known that PVPON and PAA form an interpolymer complex in aqueous solution through hydrogen bonding.14 Erasable multilayer thin films from PVPON and PAA based on hydrogen bonding were reported by Granick and Sukhishvili recently.15 PSQ-PAA used in this work is a spherical polymer brush, with a PSQ core that is covered with many PAA chains. It can be considered as a derivative of PAA, and its interaction with PVPON should be similar to that of PAA. The fabrication of PVPON/PSQ-PAA multilayer films is shown schematically in Scheme 1. The fabrication process was monitored using a UV-visible spectrometer. As shown in Figure 1, the absorbance of the film increases with the increasing number of the assembly cycles. The peak at 197 nm, which was attributed to the π-π transition of PVPON, increases linearly as a function of the assembly cycle (inset of Figure 1), indicating that the assembly process is successful. To confirm that hydrogen bonding is the driving force for the film assembly, FTIR spectra of the film were measured. For comparison, IR spectra of pure PVPON and PSQ-PAA were also measured. As shown in Figure 2, PSQ-PAA presents a strong absorption band at 1710 cm-1 accompanied with two shoulders. The shoulder at 1755 cm-1 is assigned to the stretching of the uncharged monomeric carboxylic group, while the main peak is associated with the stretching of the uncharged dimerized or associated form of carboxylic groups.15-17 The shoulder at 1660 cm-1 might root from the short-range interactions of carbonyl dipoles18 or hydrogen bonding between more than two carboxylic groups.16b The results indicate that most of the carboxylic groups in PSQ-PAA are bonded with intramolecular hydrogen bonds. PVPON presents a strong absorption band at the 1680 cm-1 which was assigned to the CdO stretching.19 Two bands were found in the IR spectra of the (PVPON/PSQ-PAA)20 film. They are centered at 1720 and 1648 cm-1 and can be assigned to the carbonyl stretching of PSQ-PAA and PVPON, respectively. Compared with pure PSQ-PAA, the carbonyl stretching of PSQPAA in the film shifts to a higher frequency, indicating intermolecular hydrogen bonds formed between PSQ-PAA and PVPON at the expense of partial detachment of intramolecular hydrogen bonds among the carboxylic groups of PSQ-PAA. The same phenomenon was reported when PVPON hydrogen-bonded with poly(methacrylic acid).15 The carbonyl stretching PVPON in the (PVPON/PSQ-PAA)20 film is 32 cm-1 lower than that of the pure PVPON, confirming again that strong intermolecular hydrogen bonds formed between the two components. The film thickness was obtained on a Filmetrics F20, which measures the reflective spectrum of a thin film. Because of their optical path difference, the light reflected from the air-film interface and the film-substrate interface will interfere, and a peak will form when λ ) (2nd)/i and a valley will form when λ ) 4nd/(2i - 1), where λ is the wavelength, n the refractive index, d the film thickness, and i an integer. By analyzing the period and amplitude of these oscillations, the F20 can determine (14) Yan, S.; Chen, Z.; Song, Z.; Pao, Q. Water Soluble Polymer (in Chinese); Chemical Industry Press: Beijing, 1988; Chapter 4. (15) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (16) (a) Lu, X.; Weiss, R. A. Macromolecules 1995, 28, 3022. (b) Dong, J.; Ozaki, Y.; Nakashima, K. Macromolecules 1997, 30, 1111. (17) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 346. (18) Painter, P. C.; Pehlert, G. J.; Hu, Y. H.; Coleman, M. M. Macromolecules 1999, 32, 2055. (19) Zhang, Y.; Guan, Y.; Yang, S.; Xu, J.; Han, C. C. AdV. Mater. 2003, 15, 832.

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Scheme 1. Fabrication of the LBL Assembly Composite Films Based on Hydrogen Bonding from PVPON and PSQ-PAA

thickness and optical properties of a thin film. As shown in Figure 3A, the measured and calculated data fit well (the correlation of the fit is 0.9936). The thickness of a (PVPON/ PSQ-PAA)16 film was measured to be 452.4 nm, and the refractive index at 632.8 nm is 1.63. The thicknesses of a series of films with different assembly cycle numbers were measured. In accordance with the UV-vis result, good linear relationship of the thickness and assembly cycle number was found (Figure 3B). The average increase per bilayer was found to be 28.3 nm, which is close to the diameter of the core of the polymer brush (∼25 nm), indicating that only a monomolecular layer was added per layer.20 The morphologies of the multilayer films that were fabricated on silicon slides were studied with SEM and AFM. The SEM image of a (PVPON/PSQ-PAA)10 multilayer film is shown in Figure 4, which reveals that many small particles homogeneously disperse in the film. The diameter of these particles is about 25

Figure 1. UV-visible spectra of the LBL hydrogen-bonding films with different numbers of assembled bilayers (from bottom to top: 0, 2, 4, 6, 8, and 10 bilayers). Inset: Film absorbance at 197 nm as a function of the numbers of the assembled bilayer.

nm, which corresponds to the size of the polymer brush PSQPAA. The AFM image of a (PVPON/ PSQ-PAA)10 film (Figure 5A) exhibits a similar morphology. It was long known that polyelectrolyte multilayer films and some hydrogen bonded multilayer films exhibit a morphology with small islets distributed in the films.21-23 The islets were considered to be polymer complex coacervates formed by the two assembly polymers.22 In general, the polymer complex coacervates would heterogeneously spread in the film, and their size is polydisperse. However, in the present case, the islets homogeneously spread around, and their size is almost monodisperse, indicating they are from single spherical polymer brushes, rather than polymer complex coacervates. To further prove this, a (PVPON/PAA)10 film was fabricated under the same condition. Its AFM image shows that the (PVPON/PAA)10 film is very flat; no islet was found in the film (Figure 5B). Therefore the small particles in the (PVPON/ PSQ-PAA)10 film can be attributed to the single spherical polymer brushes immobilized in the multilayer film. It is noteworthy that the morphology of the PVPON/PSQ-PAA films does not change

Figure 2. FTIR spectra of PVPON, PSQ-PAA, and a (PVPON/ PSQ-PAA)20.

Polymer Brush and PVPON Hydrogen-Bonding Assembly

Figure 3. (A) Measured and calculated reflectance spectra of a (PVPON/PSQ-PAA)16 film using the Filmetrics F20. (B) Film thickness as a function of the numbers of assembled bilayer. The film thickness was obtained by analysis of the reflectance spectra of A.

Figure 4. SEM microphotograph of a (PVPON/PSQ-PAA)10 multilayer film.

with the assembly cycle numbers. As shown in Figure 5C, the (PVPON/PSQ-PAA)16 film presents a morphology similar to that of the (PVPON/ PSQ-PAA)10 film (Figure 5A). (20) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (21) Zhang, Y.; Yang, S.; Liu, C.; Dai, X.; Cao, W.; Xu, J.; Li, Y. New J. Chem. 2002, 26, 617. (22) Fu, Y.; Bai, S.; Cui, S.; Qiu, D.; Wang, Z.; Zhang, X. Macromolecules 2002, 35, 9451. (23) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414.

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Figure 5. AFM images (1 µm × 1 µm) of (A) a (PVPON/PSQPAA)10 film, (B) a (PVPON/PAA)10 film, (C) a (PVPON/PSQPAA)16 film, (D) a (PVPON/PSQ-PAA)16 film after calcination at 300 °C for 2 h, and (E) a (PVPON/PSQ-PAA)16 film after calcination at 400 °C for 2 h.

The composite feature of the film determines that the different components in the film can be further processed according to their chemical and physical properties.24 The (PVPON/PSQPAA)16 films were put into a tube stove to burn the organic component out. AFM images of the films calcined at 300 and 400 °C are shown in Figure 5D,E, respectively. Compared with the original film (Figure 5C), after calciation at 300 °C the film exhibits smaller particle size but higher particle population density (Figure 5D). The smaller particle size can be explained by the heat-induced removal of some PAA hairs from the polymer brushes. As a result of the breaking down of the PVPON strata by heat, the particles stacked more densely. However, at 300 °C, the organic component was not completely burned out. The film still exhibits an absorption peak of C-H in its FTIR spectrum (Figure 6A). Meanwhile, the film appears brown in color and exhibits a broad band in its UV-vis spectrum (Figure 6B), indicating that the film was somewhat carbonized. The carbonization of the film was also proved by XPS spectrum that exhibits peak of the amorphous carbon. When the calcinations temperature was increased to 400 °C, the calcined film is transparent. Its absorption in UV-vis spectrum is rather low (Figure 6B). No C-H band was found in its IR spectrum (Figure 6A). All the results indicate that the organic component was completely (24) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111.

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Figure 7. Refractive indexes of the films as a function of wavelength: (a) (PVPON/PAA)16 film; (b) (PVPON/PSQ-PAA)16 film; (c) (PVPON/PSQ-PAA)16 film after 2 h calcination at 400 °C; (d) (PVPON/PSQ-PAA)16 film after 2 h calcination at 300 °C; (e) (PVPON/PSQ-PAA)16 film after 30 min immersion in the TBAF solution.

Figure 6. FTIR (A) and UV-visible (B) spectra of (PVPON/ PSQ-PAA)16 films before and after calcination at different temperature.

removed by heat. The XPS data show the film still contains some carbon, but the carbon content is much lower than that of the film calcined at 300 °C. The AFM image (Figure 5E) shows that the morphology of the film is obviously different from the original film (Figure 5C) and the film calcined at 300 °C (Figure 5D). The film is composed of densely packed particles, since the organic components were completely burned out. The high temperature also made some particles conglutinate and increased the particle size. The film thickness before and after calcinations was measured on the Filmeric F20. The thickness of the original (PVPON/ PSQ-PAA)16 film is 451.4 nm. After calcination at 300 °C, the film thickness was reduced to 135.6 nm. When the calcination temperature was increased to 400 °C, the film thickness was reduced further to 108.4 nm. The changes of the film thickness induced by calcination further interpret the increased particle population density. From Filmetric F20, other properties of the film, such as refractive index, extinction coefficient, and roughness, can also be obtained. In this research, the film materials can be regarded as an isotropic insulator; therefore the extinction coefficient can be regarded to be zero according to Maxwell theory. Figure 7 compares the refractive indexes of different films. The (PVPON/ PAA)16 film and the (PVPON/PSQ-PAA)16 film are both prepared on the basis of hydrogen bonding between the carboxyl group of PVPON and the hydroxyl group of PAA; however, their refractive indexes are obviously different. The refractive index of a film is dependent on its composition and structure. The incorporation of PSQ particles into the film with the polymer

Figure 8. UV-visible (A) and FTIR (B) spectra of the (PVPON/ PSQ-PAA)16 film before and after 30 min incubation in the TBAF solution.

brushes leads to their composition and structure diversity. After the (PVPON/PSQ-PAA)16 film was calcined, its composition changed and so did its refractive index. The refractive index of the film calcined at 300 °C is the highest. It is even over 2.3 at some wavelengths, because PSQ, silicon oxide, organic remainder, and amorphous carbon coexist in the film. Compared with the film calcined at 300 °C, the refractive index of the film

Polymer Brush and PVPON Hydrogen-Bonding Assembly

Figure 9. AFM images (1 µm × 1 µm) of a (PVPON/PSQ-PAA)16 film: (A) Before dipping in the TBAF solution; (B) after 15 min incubation; (C) after 30 min incubation (left, height image; right, phase image).

calcined at 400 °C declined, because the content of amorphous carbon was largely reduced. The Si-O bonds in the PSQ core can be broken by TBAF, which has been widely used to etch Si-O bond-containing polymers, such as poly(dimethylsiloxane).25 In a control experiment, the pure PSQ particles dissolve quickly in a TBAF solution. Here we chose TBAF to remove the PSQ core, the second component in the composite film. At the same time it does not destroy the organic component in the film, as proved by UV-visible spectra. No big difference was found in the UV-vis spectra of the film before and after immersion in TBAF solution (Figure 8A). Meanwhile, the IR spectra in Figure 8B exhibits that, after the dissolution of PSQ, the ratio between the intensity of the PAA carbonyl peak and that of the PVPON carbonyl peak increased, indicating that although the organic component in the film remains intact, the specific hydrogen(25) (a) Liaw, D. J. Polymer 1997, 38, 5217. (b) Childs, W. R.; Nuzzo, R. G. AdV. Mater. 2004, 16, 1323.

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bonding structure may have changed. After the removal of the PSQ cores, the resultant film is the same as the (PVPON/PAA)16 film in terms of components; however, the refractive indices of the two films are different (Figure 7a,e), indicating they have a different structure. This difference may stem from two aspects. One is that the PAA hair chains of the polymer brush are longer than the PAA chains in the (PVPON/PAA)16 film. The other is that the spherical brush structure in the (PVPON/PSQ-PAA)16 film may still remain to some extent after the removal of the PSQ core, which is different from that in the (PVPON/PAA)16 film. AFM was applied to observe the morphology change after the removal of the PSQ core. Figure 9 presents the height image and phase image together. In phase imaging, the phase log of the cantilever oscillation, relative to the drive signal, is simultaneously monitored with topography data. The phase lag is very sensitive to variation in many material properties, so phase imaging is an effective means to distinguish the different components in composite materials.26 Both the height image and the phase image can differentiate the two components in the original (PVPON/ PSQ-PAA)16 film; however, the phase image manifests the component’s difference more clearly (Figure 9A). After the film was immersed in TBAF for 15 min, most of the particles disappeared (Figure 9B). As shown in the phase image of Figure 9B, there is a twinkling dot in the center of the remaining particles. The twinkling dots could be attributed to the remains of the PSQ. Incubation in TBAF for 15 min is not long enough to dissolve all the PSQ cores, and the phase image recorded this intermediate process of the dissolution. When the incubation time was prolonged to 30 min, the PSQ cores were completely removed. Both the height and phase images do not exhibit any islets, and the film becomes rather flat, as shown in Figure 9C. After the PSQ cores were dissolved, the thickness of the film reduced from 452.4 to 383.9 nm, and the root-mean-square (RMS) roughness of the film also reduced from 49.3 to 19.4 nm. The dissolution of the PSQ cores gave the PAA hairs freedom to rearrange in the film and led to flatter film.

Conclusion Using the hydrogen-bonding LBL technique, composite multilayer films were successfully fabricated from the PVPON and a spherical polymer brush, PSQ-PAA. The film thickness can be precisely controlled by the number of the assembled cycles. Further calcination can decompose the organic components of the film, and an inorganic UV-visible transparent film can be obtained. By selectively dissolving the PSQ cores of the spherical polymer brushes, an organic film was obtained. The resultant film became thinner and flatter with the removal of the PSQ cores. Acknowledgment. The National Natural Science Foundation (NFSF) of China (Y.Z., Grant No. 20204017) and 2004 Outstanding Young Foundation of NFSF (J.X., Grant No. 5045312) are acknowledged for their financial support of this work. LA051581E (26) Kim, Y.; Lieber, M. C. Science 1992, 257, 357.