Hydrogen-Bonding-Directed Layer-by-Layer Assembly of Dendrimer

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Hydrogen-Bonding-Directed Layer-by-Layer Assembly of Dendrimer and Poly(4-vinylpyridine) and Micropore Formation by Post-Base Treatment Hongyu Zhang,† Yu Fu,† Dong Wang,† Liyan Wang,† Zhiqiang Wang,*,‡ and Xi Zhang*,† Key Lab for Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130023, People’s Republic of China, and Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China Received June 12, 2003. In Final Form: July 15, 2003 We reported a way to fabricate microporous films by post-base treatment of hydrogen-bonding-directed multilayer films of poly(4-vinylpyridine) (PVP) and carboxyl-terminated polyether dendrimer (DENCOOH). The PVP/DEN-COOH multilayer film was fabricated by layer-by-layer assembly of PVP and DEN-COOH from a methanol solution. UV-visible spectroscopy revealed a uniform deposition process. The interaction between PVP and DEN-COOH was identified as hydrogen bonding through Fourier transform infrared (FT-IR) spectroscopy. Meanwhile, the composition change of a PVP/DEN-COOH multilayer film in a basic solution was detected by X-ray photoelectron spectroscopy and UV-visible spectroscopy, and the morphology variation was observed by atomic force microscopy. A two-step variation was observed: the dissolution of DEN-COOH from the multilayer into the basic solution and the gradual reconformation of PVP polymer chains remaining on the substrate, which produced a microporous film. Interestingly, compared with our previous PVP/poly(acrylic acid) (PAA) system, under the same conditions, the release of DEN-COOH from a PVP/DEN-COOH multilayer is slower than that of PAA, and the microporous morphology is also different, which indicates that the molecular structure of a building block has a remarkable influence on the variation of a hydrogen-bonding-directed film in a basic solution.

Introduction Self-assembly can offer rational design and construction of highly ordered meso- and nanoscale structures with defined physical properties and chemical functions. Various studies have been devoted to the realization of functionalized organic materials by artificial supramolecular self-assembly.1 In the past decade, there has been a tremendous surge toward the characterization, modification, and processing of ultrathin films and multilayered structures constructed by self-assembly as a result of their potential applications including catalysis, microelectronics, nonlinear optics, sensors, and display technologies.2,3 The other reason for the intense interest in this field is that multilayers can bridge the gap between monolayers and spun-on or dip-coated films. A simple technique for ultrathin multilayer film assembly is the alternate layer-by-layer (LbL) electrostatic deposition of oppositely charged polyelectrolytes.4,5 The fabrication of multicomposite films by the LbL procedure means literally the nanoscopic assembly of different materials in a single device using environmentally friendly, ultralow-cost techniques. The materials can be small organic molecules6 or inorganic compounds,7-12 macromolecules,13,14 biomacromolecules such as pro* Authors to whom correspondence should be addressed. Fax: 0086-431-8923907 or 8980729. E-mail: [email protected]. † Jilin University. ‡ Tsinghua University.

(1) Lehn, J. M. Supramolecular Chemistry - Concepts and Perpectives; VCH: Weinheim, 1995. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-assembly; Academic Press: Boston, 1991. (3) Decher, G.; Schlenoff, J. B. Multilayer thin films - Sequential Assembly of Nanocomposite Materials; VCH: Weinheim, 2003. (4) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (5) Decher, G. Science 1997, 277, 1232.

teins,15,16 DNA,17,18 or even colloids.19-21 Although the ultrathin multilayers fabricated by the LbL method commonly cannot achieve a satisfactory well-defined layer structure, because of the interfacial interpenetration between neighboring layers, the versatile method still challenges the traditional LB technique and opens new avenues to advanced materials with practical applications. Although electrostatic interaction has been most widely used to construct multilayer films,4-21 other weak interactions, such as hydrogen bonding, have also been employed as driving forces for the LbL assembly. For (6) Zhang, X.; Gao, M. L.; Kong, X. X.; Sun, Y. P.; Shen, J. C. Chem. Commun. 1994, 1055. (7) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (8) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H. N.; Waraksa, C. C.; Miwa, T.; Fujishima, A.; Mallouk, T. E. Chem. Mater. 1999, 11, 1526. (9) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (10) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101. (11) Liu, S. Q.; Kurth, D. G.; Bredenko¨tter, B.; Volkmer, D. J. Am. Chem. Soc. 2002, 124, 12279. (12) Lin, C.; Kagan, C. R. J. Am. Chem. Soc. 2003, 125, 336. (13) Laschewsky, A.; Mayer, B.; Wischerhoff, E.; Arys, X.; Joans, A. Thin Solid Films 1996, 284/285, 334. (14) He, J. A.; Valluzzi, R.; Yang, K.; Dolukhan, T.; Sun, C. M.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (15) Kong, W.; Zhang, X.; Gao, M. L.; Zhou, H.; Li, W.; Shen, J. C. Macromol. Rapid Commun. 1994, 15, 405. (16) Lvov, Y.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (17) Serizawa, T.; Yamaguchi, M.; Akashi, M. Angew. Chem., Int. Ed. 2003, 42, 1115. (18) Taton, T. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 2000, 122, 6305. (19) Gao, M. Y.; Gao, M. L.; Zhang, X.; Yang, Y.; Yang, B.; Shen, J. C. Chem. Commun. 1994, 2777. (20) Schmitt, J.; Decher, G. Adv. Mater. 1997, 9, 61. (21) Chen, Z. H.; Yang, Y. A.; Qiu, J. B.; Yao, J. N. Langmuir 2000, 16, 722.

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example, Stockton and Rubner22 and Zhang et al.23 reported simultaneously the formation of ultrathin films via H-bonding attraction by the LbL assembly technique. One of the advantages of the hydrogen-bonding-directed films is that the fabrication of the LbL film is allowed in an organic solvent. Later, hydrogen-bonding-directed electroactive,24 photochromic,25 and photoreactive26 polyelectrolyte multilayers were successfully constructed. Sukhishvili and Granick prepared erasable hydrogenbonded multilayers containing weak polyacids, which could be assembled at a low pH and subsequently dissolved at a higher pH as a consequence of increasing the ionization degree of the weak polyacids.27,28 Hao and Lian prepared a polymer and nanoparticle composite multilayer based on hydrogen bonding.29 More noticeably, on the basis of the hydrogen-bonded erasable system, Yang and Rubner combined the light-initiated chemical reaction with the dip-pen technique to fabricate a patterned surface.30 Very recently, Caruso et al. reported on the preparation of multilayer films comprising alternate stacks of hydrogenbonded poly(4-vinylpyridine) (PVP) and poly(acrylic acid) (PAA) and electrostatically formed poly(sodium 4styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) layers via the LbL assembly technique and their high pH sensitivity toward deconstruction.31 Microporous ultrathin films have received increasing attention recently due to their numerous applications, including low-dielectric-constant and low-refractive-index thin film coating, separation filters, biocompatible membranes for controlled release and encapsulation systems, and antireflection coating.3 For example, Rubner et al.32 and Caruso et al.33 demonstrated that PAH/PAA films could form microporous structures upon exposure to solutions with different pH values or ionic strengths. In addition, Kim and Bruening reported that poly(amidoamine) dendrimer/PAH multilayers were also capable of forming such microporous films by simply exposing multilayers to acidic aqueous solutions.34 Obviously, with the proper choice of assembly conditions or treatment conditions covering a wide range of pHs and ionic strengths, it should be possible to induce microporosity in electrostatic assembly systems. In our previous study, Zhang et al. investigated the structure variation of a hydrogen-bonding-directed PVP/PAA LbL film in a basic aqueous solution.35 In this case, a two-step variation was observed: the first step is the dissolution of PAA from the film into the basic solution; the second is the gradual reconformation of PVP polymer chains remaining on the substrate, which produces a microporous film. The novel and unique mechanism of microporous film construction is anticipated to have potential applications in materials science. (22) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (23) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (24) Wang, L. Y.; Fu, Y.; Wang, Z. Q.; Wang, Y.; Sun, C. Q.; Fan, Y. G.; Zhang, X. Macromol. Chem. Phys. 1999, 200, 1523. (25) Fu, Y.; Chen, H.; Qiu, D. L.; Wang, Z. Q.; Zhang, X. Langmuir 2002, 18, 4989. (26) Cao, T. B.; Cao, W. X. Chem. Lett. 2001, 800. (27) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (28) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (29) Hao, E. C.; Lian, T. Q. Chem. Mater. 2000, 12, 3392. (30) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (31) Cho, J.; Caruso, F. Macromolecules 2003, 36, 2845. (32) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (33) Fery, A.; Scho¨ler, B.; Cassagneau, T.; Caruso, F. Langmuir 2001, 17, 3779. (34) Kim, B. Y.; Bruening, M. L. Langmuir 2003, 19, 94. (35) Fu, Y.; Bai, S. L.; Cui, S. X.; Qiu, D. L.; Wang, Z. Q.; Zhang, X. Macromolecules 2002, 35, 9451.

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The aim of the present article is to attempt not only to confirm the formation mechanism of the microporous film as was mentioned previously but also to find a new way to control the fabrication of the microporous film. Hence, we have employed the carboxyl-terminated polyether dendrimer (DEN-COOH) as a hydrogen donor and constructed a multilayer film by alternating the deposition of PVP and DEN-COOH via hydrogen bonding in a cyclic fashion. We are wondering if a microporous film can be formed when the multilayer film of PVP/DEN-COOH is immersed in a basic solution. We anticipate that a comparison study between PVP/DEN-COOH and PVP/ PAA will be helpful and constructive for the forthcoming discussion about the formation of the microporous film. Experimental Section Materials. Poly(ethyleneimine) (PEI, Mw ) 50 000) and (4aminobutyl)-dimethylmethoxysilane were obtained from Aldrich and used without further treatment. Carboxyl-terminated DENCOOH, which has been used as a building block to fabricate a hydrogen-bonding-directed multilayer by self-deposition,36 was synthesized according to the literature.37 PVP (Mw ) 180 000) was synthesized as was previously described.38 Film Preparation. The LbL film was assembled on a quartz slide or a calcium fluoride (CaF2) plate. The quartz slide was used for UV-visible, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) measurements, and the CaF2 plate was used for Fourier transform infrared (FT-IR) spectroscopy measurements. The quartz slide and CaF2 plate need to be modified before LbL deposition. The quartz surface was modified with (4-aminobutyl)-dimethylmethoxysilane, resulting in a NH2tailored surface, and the CaF2 surface was modified with a precursor layer of PEI. The NH2-terminated substrate was first immersed in a PVP methanol solution (1 mg/mL) for 10 min. In this way, the substrate was covered with a PVP layer, and, thus, a surface tailored with hydrogen bonding acceptors (pyridine groups) was formed. After rinsing with pure methanol and drying under a nitrogen stream, the resulting substrate was transferred into a DEN-COOH methanol solution for 10 min to add a DENCOOH layer. By repetition of the above two steps in a cyclic fashion, the LbL multilayer film was fabricated. Figure 1 shows the schematic assembling process on a quartz slide. The resulting multilayer films can be expressed as (PVP/DEN-COOH)n, where n is the number of deposition cycles. To investigate the influence of a basic aqueous solution on the hydrogen-bonding-directed multilayer film, the resulting LbL film was immersed in a NaOH aqueous solution. After rinsing with water and drying by nitrogen, the samples were stored under ambient conditions prior to measurement. Methods. UV-vis spectra were obtained on a Shimadzu 3100 UV-vis/near-IR recording spectrometer. FT-IR spectra of PVP/ DEN-COOH multilayers were collected on a Bruker IFS 66V instrument equipped with a DTGS detector at 4-cm-1 resolution. XPS spectra were obtained on an ESCALAB Mark II (VG company, U.K.) photoelectron spectrometer using a monochromatic Mg KR X-ray source. AFM images were taken with a Dimension 3100 (Digital Instruments, Santa Barbara, CA) under ambient conditions. AFM was operated in the tapping mode with an optical readout using Si cantilevers.

Results and Discussion UV-vis spectroscopy has proved to be a useful and facile technique to evaluate the growth process of multilayers and was, thus, used in the present work to monitor the LbL assembly process of PVP/DEN-COOH multilayer buildup. Figure 2 displays the UV-vis absorption spectra (36) Huo, F. W.; Xu, H. P.; Zhang, L.; Fu, Y.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2003, 874. (37) Hawker, C. J.; Wooley, K. L.; Fre´chet, J. M. J. Chem. Soc., Perkin Trans. 1 1993, 1287. (38) Wang, L. Y.; Fu, Y.; Wang, Z. Q.; Fan, Y. G.; Zhang, X. Langmuir 1999, 15, 1360.

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Figure 1. Schematics of the LbL assembly of PVP and DEN-COOH on a quartz substrate based on hydrogen bonding: (I) adsorption of PVP and (II) adsorption of DEN-COOH.

Figure 2. UV-vis spectra of (PVP/DEN-COOH)n multilayer films with n ) 0-12 on NH2-modified quartz substrates. The lowest curve corresponds to the baseline (n ) 0). The other curves, from bottom to top, correspond to n ) 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12, respectively. Inset: absorbance at 234 nm versus the number of deposition cycles.

of (PVP/DEN-COOH)n multilayers (with n ) 1-12) assembled on a NH2-tailored quartz surface. As is shown in Figure 2, the DEN-COOH absorption is clearly identified by the characteristic peaks at 234 and 281 nm due to the π-π* transition of the benzene of DEN-COOH, substantiating the incorporation of DEN-COOH molecules into the multilayers. Unfortunately, due to the strong absorption of DEN-COOH in the UV region, the overlapping spectra of the DEN-COOH and PVP does not allow the assignment of a unique absorption band of the multilayer film solely to the PVP. The inset of Figure 2 shows that the absorbance of quartz-supported (PVP/ DEN-COOH)n multilayer films at characteristic wavelength (234 nm) increases proportionally with the number of deposition cycles, n. This nearly linear growth of the absorption peaks indicates that an approximately equal amount of DEN-COOH is deposited for each adsorption procedure and that the PVP/DEN-COOH LbL films grow uniformly with each deposition cycle. However, the observed growth at 281 nm is nonlinear in Figure 2, which could account for the formation of DEN-COOH aggregates within the multilayer. A similar phenomenon was observed in the electrostatic LbL self-assembly of dye molecules.39,40 In addition, it is found that there is almost

Figure 3. UV absorbance at 234 nm recorded as a function of the immersion time for the deposition of a single monolayer of DEN-COOH on a quartz substrate coated with a (PVP/DENCOOH)3PVP precursor film.

no desorption of DEN-COOH during the multilayer buildup. To understand the deposition process in more detail, we have studied the physical adsorption kinetics. Figure 3 shows how the optical absorbance varies with time during the process of adsorbing a single layer of DENCOOH onto a quartz substrate previously coated with a (PVP/DEN-COOH)2PVP precursor film. It is shown that, under the conditions used, the deposition of a single DENCOOH layer onto a PVP surface is more than 90% complete within the first 5 min of immersion and it reaches a plateau of saturate adsorption after 10 min. We also examined the dependence of the concentration of DEN-COOH and PVP solutions on the adsorption behavior. It is found that the amount of DEN-COOH adsorbed per bilayer grows with increasing the concentration of DEN-COOH from 0.08 (Figure 4b) to 0.16 mg/ mL (Figure 4a). When increasing the concentration of PVP from 0.5 (Figure 4c) to 1.0 mg/mL (Figure 4b), the adsorbed amount of the DEN-COOH is also increased accordingly when using a fixed concentration of the DEN-COOH (39) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (40) Rousseau, E.; Auweraer, M. V.; Schryver, F. C. Langmuir 2000, 16, 8865.

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Figure 4. Influence of the concentration on the UV absorption (at 234 nm) against the number of deposition cycles. (a, [PVP] ) 1 mg/mL, [DEN] ) 0.16 mg/mL; b, [PVP] ) 1 mg/mL, [DEN] ) 0.08 mg/mL; c, [PVP] ) 0.5 mg/mL, [DEN] ) 0.08 mg/mL).

Figure 6. FT-IR spectrum of a (PVP/DEN-COOH)n (n ) 8) multilayer film on a PEI-modified CaF2 plate. The inset shows a magnification of the FT-IR spectrum in the range from 1300 to 1900 cm-1.

Figure 5. FT-IR spectra of cast films of (a) pure PVP and (b) pure DEN-COOH on CaF2 plates.

Figure 7. C(1s) XPS spectra of (PVP/DEN-COOH)5.5 multilayer films before (a) and after (b) immersion in a pH ) 12.5 NaOH aqueous solution at 25 °C for 2 min.

solution (0.08 mg/mL). We found that the amount of DENCOOH adsorbed is strongly dependent on the concentration of the dipping solution, with higher concentrations resulting in a greater amount of adsorbed DEN-COOH at equilibrium. The driving force for the construction of the PVP/DENCOOH multilayer film was identified by FT-IR spectroscopy. Hydrogen-bonding formation between pyridine and carboxylic acid leads to characteristic splitting patterns in the IR absorption of the carboxylic acid OH group.41,42 Figure 5a,b shows the FT-IR spectra of the cast films of PVP and DEN-COOH on CaF2 plates, respectively. For the cast film of PVP, the peaks appearing at 1596, 1556, and 1450 cm-1 can be ascribed to the ring vibration of pyridine groups of PVP. For the DEN-COOH, the bands at 1693 and 1720 cm-1 can be separately assigned to the carbonyl vibrations of carboxylic acid groups in associated and free states.43 The strong absorbance band appearing at 1161 cm-1 can be attributed to the vibration of the Ar-O bond. Figure 6 shows the FT-IR spectrum of an eight-bilayer PVP/DEN-COOH film on a CaF2 plate. In this figure, we can find clearly that a O-H stretching vibration appears at 2470 and 1934 cm-1, indicating a strong hydrogen-bonding between the carboxylic acid of DEN-COOH and the pyridine groups of PVP.41,42 Furthermore, in the region from 1660 to 1110 cm-1 in the FT-IR spectrum of the PVP/DEN-COOH multilayer film, (41) Katim, T.; Kihara, H.; Uryu, T.; Fujishims, A.; Fre´chet, J. M. J. Macromolecules 1992, 25, 6838. (42) Kumar, U.; Kato, T.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1992, 114, 6630. (43) Dong, J.; Ozaki, Y. Macromolecules 1997, 30, 286.

the absorption peaks could be assigned to the ring vibration of PVP or DEN-COOH and the vibration of the aryl-O band of DEN-COOH, no position change of which was observed in comparison with pure PVP and pure DENCOOH. These results further provide the evidence that the multilayer film is assembled via hydrogen bonding. To investigate the influence of a basic aqueous solution on the PVP/DEN-COOH multilayer, XPS was used to detect the composition variation of the LbL film in a NaOH solution. Prior to immersion in a basic solution, there are two C(1s) photopeaks at approximately 288.75 and 284.75 eV, as is shown in Figure 7a; the former weak peak is assigned to the carbon of carboxylic acid in DEN-COOH.44 Comparing the XPS spectra of (PVP/DEN-COOH)5PVP LbL films before and after immersion in a pH ) 12.5 NaOH aqueous solution for 2 min, we can find that the distinct photopeak at 288.75 eV corresponding to the carbon of carboxylic acid in DEN-COOH disappears in the spectrum of the film after the base treatment, as is shown in Figure 7b. This change suggests that DEN-COOH is removed from the multilayer film by the basic solution. Moreover, XPS also displays that the C(1s) photopeak at 288.75 eV could be weakened after a 1-min immersion in the basic solution, which indicates that DEN-COOH partially releases from the multilayer film for a short immersion time. However, under the same condition used (pH ) 12.5, 25 °C), PAA can release thoroughly from multilayer films of PVP/PAA during immersion for 1 min in basic solutions.35 As for N(1s), no obvious difference (44) Beamson, G.; Briggs, D. The XPS of Polymers Database. Surface Spectra; Wiley: Manchester, 1992.

Dendrimer and Poly(4-vinylpyridine)

Figure 8. Decrease of absorbance at 234 nm of (PVP/DENCOOH)5PVP multilayer films versus the immersion time in the NaOH aqueous solutions with different pH values.

between the films before and those after immersion was observed, which implies that the PVP still remains on the substrate. From the previous discussions, we demonstrate that when the PVP/DEN-COOH LbL film is immersed in a basic aqueous solution, one of the film components, DEN-COOH, dissolves away and the other component, PVP, remains on the substrate. To study the release kinetics of the dendrimers from the PVP/DEN-COOH multilayer film, we measured the change of the film absorbance at 234 nm as a function of the pH of the basic solution and immersion time. Figure 8 shows the intensity change of the DEN-COOH absorption with the immersion time in basic solutions with different pH values. It indicates that in the basic solutions PVP/DEN-COOH multilayer films are not stable and prone to deconstruction, and the deconstruction process of the multilayer film depends sensitively on the pH of the basic solutions. From Figure 8, it can be seen that, with increasing the pH of the basic solutions from 11.0 to 13.0, the release rate of DEN-COOH increases greatly. For the (PVP/DEN-COOH)5PVP multilayer films immersed in the basic solution of pH ) 11.0 for 180 min, no DEN-COOH release from the multilayer was observed. While at pH ) 13.0, at the very beginning of immersion, for example, 1 min, approximately 80% of the DENCOOH was released, and an equilibrium plateau was reached after 25-min base treatment. The above analysis indicates that, when the LbL film is dipped into a basic aqueous solution, DEN-COOH can be removed from the film, and its releasing rate can be controlled by changing the pH of the base solutions. The above results indicate clearly that DEN-COOH is removed and PVP remains on the substrates. After the immersion of the multilayer film into the basic aqueous solution, the carboxylic acid groups of DEN-COOH are ionized by the basic solution, which leads to the destruction of hydrogen bonding between PVP and DEN-COOH. After the hydrogen bonds are destroyed, DEN-COOH leaves the film because of its solubility in the basic solution, while PVP remains due to its poor solubility in the basic solution. One question is why DEN-COOH can release from the PVP/DEN-COOH multilayer even slower than PAA from the PVP/PAA multilayer. Two possible factors, the solubility and molecular shape, could be responsible for the difference in the release rate. The first may be their solubility difference in a basic solution. Although both DEN-COOH and PAA are soluble in the basic solution, the solubility of DEN-COOH should be less than that of PAA because of the existence of benzene rings in the DENCOOH, which must lower its release rate from the multilayer. The second possible reason is the molecular

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Figure 9. AFM height image (4.0 × 4.0 µm2) of a (PVP/DENCOOH)6PVP multilayer film.

shape. Because of the branching structure, DEN-COOH would be anchored in the PVP matrix, making it harder to escape out of the multilayer. However, in the case of PAA, the polymer chain is linear and should be easier to draw from the film. Therefore, the result that PAA releases faster than DEN-COOH is reasonable. The morphology variation of the PVP/DEN-COOH multilayer film in the basic aqueous solution was explored using AFM. The AFM image of the (PVP/DENCOOH)6PVP multilayer film prior to immersion in a basic solution is shown in Figure 9. As can be seen from this figure, the LbL self-assembly film containing PVP and DEN-COOH on a quartz plate exhibits a high coverage with granular structures with sizes from 150 to 300 nm. The AFM images of the (PVP/DEN-COOH)14 multilayer film after immersion in pH ) 12.5 NaOH aqueous solutions at 25 °C for different periods of time are shown in Figure 10. After a 10-min immersion in basic solutions, the surface of the multilayer film is rougher than that before immersion in basic solution, and no porous structure is observed (Figure 10A). While in the PVP/PAA system,35 nanosized pores emerge already after a 10-min immersion. When immersing the (PVP/DEN-COOH)14 multilayer film in the basic solution for 30 min, the pores about 200 nm in diameter and 16 nm in depth appear. During the immersion time from 30 to 180 min, the diameter and depth of the pores increase on average from 200 to 380 nm and from 16 to 36 nm, respectively. It is noted that the morphology of the film is different from the microporous film resulting from the PVP/PAA multilayer35 after base treatment under the same treatment conditions (pH ) 12.5, 25 °C). In contrast to the separate pores in the PVP/ PAA system, for the same immersion time (180 min), the distribution and shape of the pores obtained in the present case is more uniform (Figure 10D). Moreover, the surface pore coverage is significantly higher than that obtained in the PVP/PAA system.35 The above analysis indicates that a time-controlled microporosity of the multilayer film can be obtained by immersion of the PVP/DEN-COOH film in a basic solution. It is known that the pH and ionic strength during or after the film construction can not only anneal the surface roughness45-47 but also lead to more dramatic structural rearrangements, such as porosity32-35 in the multilayer structure. In this case, after DEN-COOH is removed rapidly by the basic solution, at the beginning the remaining PVP should retain that extended state. How(45) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Buunsen-Ges. Phys. Chem. 1996, 100, 948. (46) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655. (47) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725.

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Figure 10. AFM height images (4.0 × 4.0 µm2) of (PVP/DEN-COOH)14 multilayer films on a quartz substrate after immersion in a pH ) 12.5 NaOH aqueous solution at 25 °C for 10 (A), 30 (B), 60 (C), and 180 (D) min.

ever, with a prolonged immersion time the extended PVP chains gradually rearrange because of their high surface tension in the basic solution. As a result, in the lateral direction the film coverage decreases and in the vertical direction the thickness increases, which results in the previously mentioned morphology variation. Therefore, we propose that the morphology variation is a result of the reconformation of PVP induced by the basic solution, after the escape of DEN-COOH. Conclusions In this article, first we presented the fabrication and detailed characterization of the PVP/DEN-COOH LbL film based on hydrogen bonding. Afterward, the variety of behavior of such a multilayer in a basic solution was investigated, which indicated that a microporous film was formed by the rapid release of DEN-COOH and the slow reorganization of the remaining PVP on the substrate. Moreover, we compared the varieties of the PVP/DENCOOH and PVP/PAA mutilayers in basic solutions. An interesting finding is that the release rate of DEN-COOH from the PVP/DEN-COOH multilayer is lower than that of PAA from the PVP/PAA multilayer in a basic solution,

and the resulting microporous morphologies are remarkably different as well. We presume that the phenomena could account for the difference in the solubility and molecular shape of DEN-COOH and PAA. We can conclude from the previous discussions that incorporating different building blocks as hydrogen-bonding donors into a multilayer assembly is an effective way to adjust the release process and microporosity by immersion of LbL films into basic solutions. Our studies on microporous films resulting from hydrogen-bonding-directed multilayers, combined with other insights into hydrogen-bonded ultrathin films or porous thin films, may pave the way for further theoretical research and potential applications in the future. Acknowledgment. This work is supported by the Major State Basic Research Development Program (G2000078102), the National Natural Science Foundation of China (20204003), and a key project of the Educational Ministry. The authors thank Mr. Fengwei Huo for helpful discussions during the experiments. LA035036U