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Covalently Attached Multilayer Assemblies by Sequential Adsorption of Polycationic Diazo-Resins and Polyanionic Poly(acrylic acid) Junqi Sun, Tao Wu, Feng Liu, Zhiqiang Wang, Xi Zhang,* and Jiacong Shen Key Lab of Supramolecular Structure and Spectroscopy, Department of Chemistry, Jilin University, Changchun, 130023, P. R. China Received November 11, 1999. In Final Form: January 31, 2000 Diazo-resins (DAR) as polycation and poly(acrylic acid) (PAA) as polyanion were alternately assembled into a multilayer structure by using the layer-by-layer self-assembly technique. PAA as a weak polyelectrolyte was very sensitive to the pH value of the solution, and that pH could be used to tune the thickness of the PAA layer. Upon UV irradiation, a fraction of the carboxylate groups reacted with diazonium groups at the adjacent interfaces of the multialyer, so partially covalently attached multilayer assemblies could be formed. Their stability improved greatly compared with the fraction that was not UV-irradiated. The photoreaction occurring between the layers was confirmed by means of UV-visible and Fourier transform infrared spectroscopy. The thickness of the UV-irradiated films was characterized with X-ray diffraction, results from which showed that the thickness of the films can be adjusted in nanometer scale by simply changing the pH value of the solutions.
Introduction Layer-by-layer (LbL) self-assembly technique allows multicomposites to be formed easily by sequential adsorption.1-3 The advantages of this technique are diversified4-8 and can be illustrated as follows: (i) The preparative procedure is simple and no elaborate instrument is required. (ii) The building blocks can be easily available, and almost all the charged materials can be assembled directly or indirectly. (iii) The thickness of individual layers can be easily controlled by many factors such as immersion time, concentration/ionic strength of the solution, pH value of the solution, molecular weight of the polyelectrolyte, etc. (iv) There are no restrictions with respect to the substrate type, size, and topology. Any charged surface is useable. (v) It is easy to build multicomponent layer structures. There is no doubt that this technique is a rapid and experimentally very simple way to produce complex layered structures with precise control of layer composition and thickness. This may be the reason this technique received much attention from the fields of chemistry, material science, biology, physics, and electronics.9-14 Although this technique has numerous (1) Decher, G. Science 1997, 277, 1232. (2) Decher, G. In Comprehensive Supramolecular Chemistry; Sauvage, J.-P., Ed.; New York, 1996; Vol 9. (3) Zhang, X.; Shen, J. C. Adv. Mater. 1999, 11, 1139. (4) Schmitt, J.; Decher. G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (5) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (6) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (7) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515. (8) Sun, Y. P.; Hao, E. C.; Zhang, X.; Yang, B.; Shen, J. C.; Chi, L. F.; Fuchs, H. Langmuir 1997, 13, 5168. (9) Sun, J. Q.; Sun, Y. P.; Zou, S.; Zhang, X.; Sun, C. Q.; Wang, Y.; Shen, J. C. Macromol. Chem. Phys. 1999, 200, 840. (10) Caruso, F.; Donath, E.; Moehwald, H. J. Phys. Chem. B 1998, 102, 2011. (11) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304. (12) Hodak, J.; Etcheniqe, R.; Calvo, E. J. Langmuir 1997, 13, 2708. (13) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13. (14) Moon, J. H.; Choi, J. U.; Kim, J. H.; Chung, H.; Hahn, H. J.; Kim, S. B.; Park, J. W. J. Mater. Chem. 1996, 6, 365.
advantages, there are some disadvantages, too. It is wellknown that the LbL process is an adsorption and desorption balance of charge-containing materials at the solid/liquid interfaces. The stability of the resulting films can be affected by many factors such as the type of solvents used, the ionic strength, concentration of the solution, the pH value of the solvent, temperature, etc. For example, the films can be etched, especially in the salt-added solvents.15 Consequently, how to improve the stability of the assembled films has always been a challenge to chemists. In our previous publications, we reported a novel way to fabricate covalently attached multilayer films.16,17 This technique could lead to the improvement of the stability of the films because of the covalent interaction formed between the neighboring layers. The materials needed include a polycation containing diazonium groups (DAR) and a polyanion containing sulfonic acid groups (PSS). The procedure is simple and involves only two steps: first, ionic self-assembly, which forms the basic multilayer needed; second, the post UV irradiation disposition, which converts the ionic interaction between the neighboring layers to a covalent one. This method is an effective way to fabricate covalently attached multilayer films by alternating deposition of materials containing the diazonium groups with materials containing sulfonate, including polyanions and small molecules. In this article, we want to enlarge the varieties of materials that can be used in this type of technique. We investigated if this technique is applicable to polyanions containing the carboxylic acid group, because carboxylic acid groups are usually negatively charged species. If it were the same as we expected, its applications would be extended widely. We selected poly(acrylic acid) (PAA) as a model material. Fourier transform infrared (FTIR) experiments showed that the amount of PAA deposited (15) Mao, G. Z.; Tsao, Y.; Tirrell. M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1995, 11, 942. (16) Sun, J. Q.; Wu. T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (17) Sun, J. Q.; Wang, Z. Q.; Sun, Y. P.; Zhang, X.; Shen, J. C. Chem. Commun. 1999, 693.
10.1021/la991482z CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
Multilayer of Polycationic DAR and Polyanionic PAA
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Scheme 1. Structures of Materials Used in the Deposition Process. (a) DAR; (b) PAA; (c) PDDA
could be influenced by the pH value of the solution. FTIR spectra showed that the driving force for DAR/PAA assemblies was not constrained to electrostatic interaction, hydrogen bonding interaction also existed between the -NH of DAR and the -COOH of PAA. After UV irradiation, the diazonium groups and the carboxylate groups reacted to form carboxylate ester. Therefore, the stability of the films improved greatly compared with that of nonirradiated ones, as confirmed by the solvent-etching experiments.
Figure 1. UV-Vis absorption spectra of multilayer films of DAR/PAA fabricated from pH 3.0 solutions.The number of bilayers is 2, 4, 6, and 8 from the lower to the upper profiles.
Experimental Section Materials. The following chemicals were used as supplied. Poly(diallyldimethylammonium chloride) (PDDA) was from Aldrich. Polycationic DAR was kindly provided by Prof. Weixiao Cao (College of Chemistry and Molecular Engineering, Peking University, Beijing).18 The molecular weight (Mn) of the DAR was ca. 2640. The synthesis of PAA was reported elsewhere.19 [η] ) 109 (in 1,4-dioxane at 30 °C), i.e., Mn is larger than 8.2 × 105. The structures of polycations and polyanion used were shown in Scheme 1. Deionized water was used throughout. Instrument and Measurement. UV-visible (Vis) spectra were obtained using a Shimadzu 3100 UV-Vis-NIR spectrophotometer. FTIR spectra were collected at a 4 cm-1 resolution on a Bruker IFS66V FTIR instrument equipped with a DTGS detector. A minimum of 100 scans was coadded. X-ray diffraction was performed on a Rigaku X-ray diffractometer (D/max γA, using Cu KR radiation with a wavelength of 1.542 Å). A 30-W medium mercury lamp was used to irradiate the films at a distance of 13 cm. Process to Produce Partially Covalently Attached Multilayer Films. A quartz wafer was immersed in a fresh piranha solution (v/v ) 1:3, 30%H2O2/98%H2SO4) and heated until no bubbles released. After being rinsed with ample water and dried, the resulted substrates were immersed in aqueous cationic solution of 0.9 vol % PDDA for 20 min to obtain a positively charged surface. In general, the fabrication of covalently attached multilayer films involves two steps: LbL assembly and the post UV irradiation. Take a positively charged substrate as an example. The substrate was dipped alternately in aqueous solutions of PAA and DAR for 20 min, with intermediate water rinsing and N2 drying. Multilayer films can be formed by repeating these two steps in a cyclic fashion. Next, the abovefabricated films were exposed under UV light for a given time to ensure that the photoreaction proceeded completely. In this way, the partially covalently attached multilayer films were obtained. The deposition process of the films was conducted in the dark to avoid the decomposition of the DAR.
Results and Discussion Preparation of Covalently Attached Multilayer Assemblies. Because the PAA used here is a type of weak polyelectrolyte with pKa of 4.5,20 three types of conditions (18) Cao, W. X.; Ye, S. J.; Cao, G. S.; Zhao, C. Macromol. Rapid Commun. 1997, 18, 983. (19) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H.; Macromol. Rapid Commun. 1997, 18, 509. (20) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309.
Figure 2. Dependence of the absorbance at 380 nm on the number of bilayers with pH values of 2.0 (squares), 3.0 (filled squares), and 5.0 (triangles).
were chosen to produce the assemblies. The concentration of DAR (1.0 mg/mL) and PAA (1.0 mg/mL) was fixed, whereas their pH values were set at 2.0, 3.0, and 5.0, respectively. In all cases, the pH values of the DAR and PAA were the same, and the deionized water used for rinsing was adjusted to the same pH value to avoid the shift in the conditions in the solution. Meanwhile, the pH value of the solution was monitored with a pH meter every few cycles of the assembly. UV-Vis spectroscopy was used to observe the LbL self-assembled multilayer fabrication process. Figure 1 shows the UV-Vis absorption spectra of 2, 4, 6, and 8 bilayers of DAR/PAA assembled on a quartz slide at a pH value of 3.0. The absorbance at 380 nm was attributed to the π-π* transition of the diazonium group.16-18,21 The linear increase in absorbance at 380 nm with the number of layers indicates a progressive deposition with almost an equal amount of deposition of the DAR in each cycle. The pH value of the solution can produce a dramatic effect on the assembly behavior of both PAA and DAR. As shown in Figure 2, when the pH value of the PAA solution is 2.0, the amount of DAR adsorbed in each cycle was the largest. When the pH value is 3.0 and 5.0, the amount of DAR adsorbed decreased, whereas the change of the pH value from 3.0 to 5.0 produced only a slight effect on the DAR adsorption. Because no chromophore is present in PAA, the amount of PAA deposited in each cycle cannot be monitored precisely by UV-Vis methods. The influence of pH values on the deposition behaviors of DAR and PAA will be
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Figure 3. UV-Vis absorption spectra of eight bilayers of DAR/ PAA (fabricated from the pH 5.0 solution) upon irradiation with UV light for (a) 0 s, (b) 20 s, (c) 30 s, (d) 1 min, (e) 1.5 min, (f) 3.5 min, and (g) 10 min.
discussed later combined with the FTIR results, which gave the amount of PAA clearly. Figure 2 also shows clearly that in each case, a linear increase of the absorbance at 380 nm can be obtained, which implies that the film deposition process is uniform in all cases. It is well-known that the diazonium group is very active.21 With heating or under UV irradiation, it can decompose rapidly. As for the compound of DAR used here, it has been shown that under UV irradiation, diazonium groups decompose, while cationic phenyl groups can be formed in the polyion chains, which can react easily with nucleophile-containing compounds. Here the carboxylate group in PAA is nucleophilic, so it is conjectured that this type of group can react easily with diazonium groups under UV irradiation.18,21,22 Based on this consideration, the above-assembled PAA/DAR films were irradiated with UV light to observe the changes occurring in the films. Taking an 8-bilayer PAA/DAR film assembled at pH 5.0 for example, the film was irradiated with a 30-W medium mercury lamp at a distance of 13 cm. As indicated in Figure 3, under UV irradiation, the diazonium groups decomposed gradually with the decrease of the absorbance at 380 nm and concomitant increase of the absorbance at 292 nm. An isosbestic point at 335 nm appeared. Throughout the experiments, we found that the decomposition of this 8-bilayer DAR/PAA film had proceeded completely within 10 min. Similar results were obtained on films with pH values of 2.0 and 3.0. Driving Forces for the Assemblies. Although UVVis spectroscopy showed the decomposition of the diazonium groups under UV irradiation in the films, this result is not enough to judge what type of reaction occurred between the diazonium and carboxylate groups. FTIR spectroscopy was applied to show the changes taking place in the films before and after UV irradiation. Figure 4 shows the FTIR spectra of multilayer films of PAA/DAR deposited on cationic PDDA-modified CaF2 plates. The bottom three are spectra of films formed at pH values of 2.0, 3.0, and 5.0 before UV irradiation (from the lower to the upper), whereas the upper three are corresponding spectra after UV irradiation. From the spectra before UV irradiation, we could find that the spectra with different pH values (21) Patai, S. The Chemistry of Diazonium and Diazo Groups; John Wiley & Sons: New York, 1978; p 351. (22) Lee, W. E.; Calvert, J. G.; Malmberg, E. W. J. Am. Chem. Soc. 1958, 80, 1588.
Figure 4. IR spectroscopy of DAR/PAA multialyer films on CaF2 substrates before and after UV irradiation. Before UV irradiation, (a) pH ) 2.0, (b) pH ) 3.0, (c) pH ) 5.0; after UV irradiation, (d) pH ) 2.0, (e) pH ) 3.0, (f) pH ) 5.0.
were similar in peak positions except for their intensities. Films formed at pH 2 are given as an example. Before UV irradiation (Figure 4a), two absorption peaks at 2222 and 2168 cm-1 can be found which originate from the symmetric and asymmetric stretching vibrational modes of -N≡N+.23 Another peak at 1111 cm-1 corresponds to the N-O stretching vibrational mode of complexes of diazonium and carboxylate groups (-N≡N+fOCO-).24 The peak at 1579 cm-1 was asymmetric and would overlap of several peaks. Second derivative spectra were used to show the peaks involved in this region. Figure 5 Aa shows the second derivative plot of pH 2.0 film before UV irradiation from 1650 to 1500 cm-1. The asymmetric peak at 1579 cm-1 comprises two peaks. One peak at 1610 cm-1, which is the asymmetric stretching vibrational mode of COO-, 19 and the other strong peak at 1579 cm-1, which was related to the phenyl groups connected with diazonium group in DAR because of the strong conjugation of phenyl and diazonium groups. The peak at 1724 cm-1 is assigned to the vibrational mode of carboxylic acid (-COOH),19,20 which also comprises several peaks as evidenced by the second derivative spectrum. Figure 5Ba shows the second derivative plot of pH 2.0 film before UV irradiation in the range from 1800 to 1675 cm-1. Three peaks are found at 1734, 1767, and 1681 cm-1, which corresponded to CdO when different types of hydrogen bonds formed.25 After UV irradiation (shown in Figure 4d), the peaks at 2222, 2168, and 1111 cm-1 disappeared completely, which meant the decomposition of the diazonium groups in the films. These results were consistent with the UV-Vis spectroscopy. The intensity of the peak at 1610 cm-1 decreased markedly, as shown in Figure 5 Ab. The peak at 1579 (23) Nuttall, R. H.; Roberts, E. R.; Sharp, D. W. A. Spectrochimica 1961, 17, 947. (24) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: New York, 1990; p 239. (25) Dong, J.; Ozaki, Y.; Nakashima, K. Macromolecules 1997, 30, 1111.
Multilayer of Polycationic DAR and Polyanionic PAA
Langmuir, Vol. 16, No. 10, 2000 4623 Scheme 2. Schematic Representation for Structure Changes upon UV Irradiation
Figure 5. Second derivative plots of pH 2.0 DAR/PAA film in the region (A) from 1650 to 1500 cm-1 and (B) from 1800 to 1675 cm-1, with (a) before UV irradiation and (b) after UV irradiation.
cm-1 also disappeared, whereas another peak appeared at 1599 cm-1. This should be the decrease of the extent of conjugation of the phenyl groups. What could lead to these changes? The only reasonable change under UV irradiation is that the diazonium groups decompose and cationic phenyl groups form. Then a SN1 reaction takes place by the carboxylate (COO-) groups, and the carboxylate esters form (-Ph-OCO-). This change causes both the disappearance of COO- groups and the decrease of the conjugation of phenyl groups due to the formation of an ester structure, which leads to the shift of the absorption of phenyl groups to 1599 cm-1. The shift toward higher frequency of phenyl groups after UV irradiation was consistent with our previously reported systems.16,17 In the second derivative plot of pH 2.0 film after UV irradiation, as shown in Figure 5 Bb, the CdO peaks change a little because of the change of the hydrogen bond states (as discussed below). The shift of the CdO stretching from 1739 to 1746 cm-1 is also an evidence of the formation of carboxylate ester. As described before,16 the reaction takes place in two steps as usual SN1 reaction: first, the decomposition of the diazonium groups upon UV irradiation and the formation of the cationic phenyl group; second, the nucleophilic reaction by the carboxylate groups and the formation of the carboxylate ester. The photoreaction after UV irradiation was confirmed further by solvent etching experiment, which will be discussed in the next paragraph. The reaction that occurs in films between the DAR and PAA is shown in Scheme 2.
Because the DAR used is positively charged, it seems that the PAA used here should act as negative and the driving force for the films assembly should be electrostatic interaction. But with a careful analysis of the FTIR spectra, it is easy to find that there is also hydrogen bonding between -COOH and the -NH group in DAR. As a comparison, the pure PAA has a broad absorption band around 3000 cm-1 and an absorption peak at 1707 cm-1, which indicates that the carboxyl group in pure PAA is not in a free but in an associated state.19 After alternate deposition with DAR, the absorption at 1707 cm-1 shifted to about 1724 cm-1. Meanwhile, OH bands also shifted to 1938 cm-1 and the vicinity of 2510 cm-1. The N-H stretching band in a free state in DAR peaks at 3231 cm-1. While in the assembled film, N-H shows two peaks at 3231 and 3099 cm-1. The shift of the N-H peaks toward lower wavenumber is the hydrogen-bonded N-H stretch.26 We can infer that hydrogen bonds form between -COOH and -NH groups, where the -COOH group acted as a proton donor and the -NH group acted as a proton acceptor.19,26 Hence, the driving force for DAR and PAA assemblies is the combination of electrostatic interaction and hydrogen bonding. UV irradiation destroyed some hydrogen bonds as indicated by the decrease in the intensity at 1938 cm-1 in FTIR spectra. This should be attributed to the change of configuration upon UV irradiation. It is essential that with the increase of pH value, the intensity of hydrogen bonding decreased, i.e., there is some relationship between the hydrogen bonding and the amount of the free carboxylic acid. Another phenomenon observed is the influence of pH value on the FTIR spectra of PAA/DAR films. With the increase of the pH, the relative intensity at 1724 cm-1 of carboxylic acid group decreased. This result is not unexpected: at higher pH, the fraction of carboxylate groups increases, too. In general, when the pH reached 5.0, most of the carboxyl groups should exist in the form of carboxylate as follows from their pKa value.20 However, in our case, the content of the carboxylic acid is still large, as shown in Figure 4c. This may be due to the influence of the proton from the -NH groups. We tried to increase pH values of both the solutions of PAA and DAR so that most of the carboxyl groups would be in the carboxylate form. Unfortunately, the diazonium group was unstable when the pH value of the solution approached 6.0. So this attempt was not successful. Meanwhile, we can observe from the FTIR spectroscopy that with the increase of pH, the amount of PAA assembled increases; this fact will be discussed in the next section. Fine-tuning of the Films Thickness of DAR/PAA Assemblies. One of the advantages of the weak polyelectrolyte in LbL assembly is that the single layer thickness can be easily controlled by simply changing the (26) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717.
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Figure 6. X-ray diffraction patterns of 10-bilayer DAR (1.0 mg/mL)/PAA (1.0 mg/mL) films deposited on quartz slides. (a) pH ) 2.0, (b) pH ) 3.0, and (c) pH ) 5.0.
pH value of the solution. To prove this speculation, the three types of PAA/DAR films with pH values of 2.0, 3.0, and 5.0 were investigated by means of X-ray diffraction measurements. As shown in Figure 6, a series of wellresolved Kiessig fringes were found in all cases, which proved the homogeneity of the corresponding films. The existence of only Kiessig fringes, but no Bragg peaks, indicates an interpenetrated structure between the layers. This was governed by the property of typical polyelectrolyte multilayer based on electrostatic/hydrogen bonding interaction, but not by the post UV irradiation induced reaction which turned the structure into a partially covalent network.19,20,27,28 For a quartz plate covered with 10 bilayers of PAA/DAR deposited from solutions at different pH values, the total thickness of the films were 57.4 (pH ) 2), 34.7 (pH ) 3.0), and 33.3 nm (pH ) 5.0), respectively. That is, the thickness of one bilayer of PAA/ DAR film can be well tuned from 3.3 to 5.7 nm by only adjusting the pH of the solution. Hence, we have an easy way to control the deposition of PAA and DAR and the film structure. Combining the UV-Vis spectroscopy, FTIR spectroscopy and small-angle X-ray diffraction results, we can conclude that both the amounts of DAR and PAA assembled in each dipping changed with pH value. When the pH value of the solution decreased, the amount of PAA adsorbed in each cycle clearly increased. PAA is a weak polyelectrolyte with a pKa of ∼4.5. When the pH value of PAA solution increases, the amount of carboxylate groups formed increases, so does the negative charge density in the PAA chain. The increased charge density makes the PAA chain adopt an extended configuration because of the repulsion between the charged groups. In this way, the polymer adsorbed must form a planar structure and its amount be less. When the pH value of PAA solution is low (for example, pH ) 2.0), repulsion between the groups is much weaker and the PAA chain adopts a random coil configuration, so the adsorbed chain is loopy and forms a slightly thicker layer. In one set of experiments, at pH value of 2.0, the largest amount of DAR was deposited, whereas at pH 3.0 and 5.0, the (27) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (28) Lu¨tt, M.; Fitzsimmons, M. B.; Li, D. Q. J. Phys. Chem. B 1998, 102, 400.
Sun et al.
amounts were almost equal. We assumed that the configuration of the DAR chain in solution is less sensitive to the change of the pH than that of PAA. This is probably a complex phenomenon caused by the combination of the hydrogen bonding between the carboxylic acid (-COOH) and the imine group, the morphology of the PAA layer underneath and some other unknown factors that are currently under investigation. Stability Analysis. The LbL assembled films of PAA/ DAR were etched by immersing them into a ternary mixture of H2O-dimetylformamide-ZnCl2 (3:5:2, w/w/ w) to assess their stability. The ternary system was chosen because of its high solubility toward polyelectrolyte complexes. The amount of films etched out can be estimated easily from the change of the UV-Vis spectroscopy at 380 nm (i.e., the amount of the DAR etched out). Different film fabricated at different pH solution displays different stability. After 5 min immersed in this ternary solvent, the amount of DAR scaled off is 5% (pH ) 2.0), 7% (pH ) 3.0), 11% (pH ) 5.0), respectively. It seems that the much thicker the film is, the higher is its stability. Overall, this type of LbL assembled film is not adequately stable in the solvent above. The film can be etched out because the type of solvent applied is able to dissociate the ionic/hydrogen bonds linking the multilayers to the solid surface. Here, ZnCl2 can act as dissociating agent. As a comparison, we etched the UV-irradiated films by immersing them into the same ternary solvent while sonicating them for 0.5 h. No detectable damage of all three types of films was found, according to the absorbance at 292 nm, which indicates a much greater stability. These results show that the irradiation is an effective approach to produce much more stable films with different thicknesses. Why is the stability of the films enchanced by UV irradiation? According to the results of the FTIR and UVVis spectroscopy, the covalent carboxylate ester (-PhOCO-) already formed between the carboxylate groups (of PAA) and phenyl groups (of DAR) should be responsible for it. Although carboxylate groups do not dominate in the pH 2.0 films, it seems that comprehensive covalent bonding is not necessary to stabilize the films. Even a small fraction of carboxyl groups that connect covalently with the sublayer can fix the entire chain on the underneath layer and significantly improve its stability. Such a partially covalent attached structure makes the resulting films more stable in all solvents, such as MeOH, dimethyl sulfoxide, dimethyl formamide, CHCl3, and the above-mentioned ternary solvent. Conclusion Multilayer films formed by polycationic DAR and PAA were produced. The driving force for the film assembly is the combination of electrostatic interaction and hydrogen bonding. Photoreaction between the acrylate and diazonium-connected phenyl groups stabilizes the films under UV irradiation This is a simple and efficient way to produce highly stable multilayer structures with precise control of film compositions and thickness. The film thickness can be well tuned by only adjusting the pH of the solution. Acknowledgment. The work is supported by National Natural Science Foundation of China. LA991482Z
