Hydrogen-Bonding-Directed Layer-by-Layer Assembly of Dendrimer and Poly(4-vinylpyridine) and Micropore Formation by Post-Base Treatment

Transcription

1 Hydrogen-Bonding-Directed Layer-by-Layer Assembly of Dendrimer and Poly(4-vinylpyridine) and Micropore Formation by Post-Base Treatment Hongyu Zhang, 1 Yu Fu, 1 Dong Wang, 1 Liyan Wang, 1 Zhiqiang Wang *2 and Xi Zhang *1 1 Key Lab for Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, , P. R. China 2 Department of Chemistry, Tsinghua University, Beijing, , P. R. China [*] Prof. Dr. Xi Zhang Key Lab for Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, , P. R. China Fax: or xi@jlu.edu.cn 1

2 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 poly-ether dendrimer (DEN-CH). The PVP/DEN-CH multilayer film was fabricated by layer-by-layer (LbL) assembly of PVP and DEN-CH from methanol solution. UV-vis spectroscopy revealed a uniform deposition process. The interaction between PVP and DEN-CH was identified as hydrogen bonding through Fourier Transform Infrared (FT-IR) spectroscopy. Meanwhile, the composition change of a PVP/DEN-CH multilayer film in a basic solution was detected by X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy, and the morphology variation was observed by atomic force microscopy (AFM). A two-step variation was observed: the dissolution of DEN-CH from the multilayer into the basic solution, and the gradual reconformation of PVP polymer chains remaining on the substrate, which produced a micropourous film. Interestingly, compared with our previous PVP/poly(acrylic acid) (PAA) system, under the same conditions, the release of DEN-CH from PVP/DEN-CH 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. 2

3 Introduction Self-assembly can offer rational design and construction of highly ordered mesoand 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 towards the characterization, modification, and processing of ultrathin films and multilayered structures constructed by self-assembly due to 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, ultra-low-cost techniques. The materials can be small organic molecules 6 or inorganic compounds, 7 12 macromolecules, 13,14 biomacromolecules such as proteins, 15,16 DNA 17,18 or even colloids Although the ultrathin multilayers fabricated by LbL method commonly cannot achieve a satisfactory well-defined layer structure, because of the interfacial interpenetrating 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 3

4 multilayer films, 4-21 other weak interactions, such as hydrogen bonding, have also been employed as driving forces for the LbL assembly. For example, Rubner et al. 22 and Zhang et al. 23 reported simultaneously the formation of ultrathin films via H-bonding attraction by LbL assembly technique. ne 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 photoreactive 26 polyelectrolyte multilayers were successfully constructed. Granick and co-workers prepared erasable hydrogen-bonded multilayers containing weak polyacids, which could be assembled at low ph and subsequently dissolved at higher ph as a consequence of increasing the ionization degree of the weak polyacids. 27,28 Lian et al. prepared polymer and nanoparticle composite multilayer based on hydrogen bonding. 29 More noticeably, on the basis of the hydrogen-bonded erasable system, Rubner et al. 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 hydrogen-bonded PVP and PAA and electrostatically formed poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) layers via 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 anti-reflection coating. 3 For example, Rubner 4

5 et al. 32 and Caruso et al. 33 demonstrated that PAH/poly(acrylic acid) (PAH/PAA) films could form microporous structures upon exposure to solutions with different ph values or ionic strengths. In addition, Bruening and co-works reported that poly(amidoamine) (PAMAM) dendrimer/pah multilayers were also capable of forming such microporous films by simply exposing multilayers to acidic aqueous solutions. 34 bviously, with the proper choice of assembly conditions or treatment conditions covering a wide range of ph and/or ionic strength, 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 poly(4-vinylpyridine)/poly(acrylic acid) (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. The aim of the present article is attempting not only to confirm the formation mechanism of the microporous film as mentioned above, but also to find a new way to control the fabrication of the microporous film. Hence, we have employed the carboxyl-terminated poly-ether dendrimer (DEN-CH) as a hydrogen donor and constructed a multilayer film by alternating deposition of poly(4-vinylpyridine) (PVP) and DEN-CH via hydrogen bonding in a cyclic fashion. We are wondering if a microporous film can be formed, when the multilayer film of PVP/DEN-CH is 5

6 immersed in a basic solution. We anticipate that a comparison study between PVP/DEN-CH and PVP/PAA will be helpful and constructive for the forthcoming discussion about the formation of microporous film. Experimental Section Materials. Poly(ethyleneimine) (PEI, M w = 50,000), and (4-aminobutyl)-dimethylmethoxysilane were obtained from Aldrich and used without further treatment. Carboxyl-terminated poly-ether dendrimer (DEN-CH), 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 Poly(4-vinylpyridine) (M w = 180,000) was synthesized as previously described. 38 Film Preparation. The LbL film was assembled on a quartz slide or a calcium fluoride (CaF 2 ) plate. The quartz slide was used for UV-vis, XPS, and AFM measurements, and the CaF 2 plate for FT-IR. The quartz slide and CaF 2 plate need to be modified before LbL deposition. The quartz surface was modified with (4-aminobutyl)-dimethylmethoxysilane, resulting in a -tailored surface, and the CaF 2 surface was modified with a precursor layer of poly(ethyleneimine) (PEI). The -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-CH methanol solution for 10 min, to add a DEN-CH 6

7 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-CH) n, where n is the number of deposition cycles. To investigate the influence of basic aqueous solution on the hydrogen-bonding-directed multilayer film, the resulting LbL film was immersed in NaH 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-CH multilayers were collected on a Bruker IFS 66V instrument equipped with a DTGS detector at 4 cm -1 resolution. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB Mark II (VG company, UK) photoelectron spectrometer using a monochromatic Mg Kα X-ray source. Atomic force microscopy (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-CH multilayer buildup. Figure 2 displays 7

8 the UV-vis absorption spectra of (PVP/DEN-CH) n multilayers (with n = 1-12) assembled on a -tailored quartz surface. As shown in Figure 2, the DEN-CH absorption is clearly identified by the characteristic peaks at 234 and 281 nm due to the π - π* transition of the benzene of DEN-CH, substantiating the incorporation of DEN-CH molecules into the multilayers. Unfortunately, due to the strong absorption of DEN-CH in UV region, the overlapping spectra of the DEN-CH and PVP does not allow assignment of a unique absorption band of the multilayer film solely to the PVP. The insert of Figure 2 shows the absorbance of quartz-supported (PVP/DEN-CH) 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-CH is deposited for each adsorption procedure and that the PVP/DEN-CH LbL films grow uniformly with each deposition cycle. However, the observed growth at 281 nm is non-linear in Figure 2, which could be accounted for the formation of DEN-CH aggregates within the multilayer. Similar phenomena was observed in the electrostatic LbL self-assembly of dye molecules. 39,40 In addition, it is found that there is almost no desorption of DEN-CH during the multilayer buildup. In order 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 DEN-CH onto a quartz substrate previously coated with a (PVP/DEN-CH) 2 PVP precursor film. It is shown that, under the conditions used, the deposition of a single DEN-CH layer 8

9 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-CH and PVP solutions on the adsorption behavior. It is found that the amount of DEN-CH adsorbed per bilayer grows with increasing the concentration of DEN-CH 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-CH is also increased accordingly when using a fixed concentration of DEN-CH solution (0.08 mg/ml). We found that the amount of DEN-CH adsorbed is strongly dependent on the concentration of the dipping solution, with higher concentrations resulting in a greater amount of adsorbed DEN-CH at equilibrium. The driving force for the construction of the PVP/DEN-CH 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 H group. 41,42 Figure 5 a and b show the FT-IR spectra of the cast films of PVP and DEN-CH on CaF 2 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-CH, 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 Ar- bond. Figure 6 shows the FT-IR spectrum of an 9

10 8-bilayer PVP/DEN-CH film on a CaF 2 plate. In this figure, we can find clearly that a -H stretching vibration appears at 2470 and 1934 cm -1, indicating a strong hydrogen-bonding between the carboxylic acid of DEN-CH and 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-CH multilayer film, the absorption peaks could be assigned to the ring vibration of PVP or DEN-CH and the vibration of aryl- band of DEN-CH, and no position change of which was observed in comparison with pure PVP and pure DEN-CH. These results further provide the evidence that the multilayer film is assembled via the hydrogen bonding. To investigate the influence of a basic aqueous solution on the PVP/DEN-CH multilayer, X-ray photoelectron spectroscopy (XPS) was used to detect the composition variation of the LbL film in a NaH solution. Prior to immersion in a basic solution, there are two C 1s photopeaks at approximately and ev as shown in Figure 7a, the former weak peak is assigned to the carbon of carboxylic acid in DEN-CH. 44 Comparing the XPS spectra of (PVP/DEN-CH) 5 PVP LbL films before and after immersion in a ph = 12.5 NaH aqueous solution for 2 min, we can find that the distinct photopeak at ev corresponding to the carbon of carboxylic acid in DEN-CH disappears in the spectrum of the film after the base treatment as shown in Figure 7b. This change suggests that DEN-CH is removed from the multilayer film by the basic solution. Moreover, XPS also displays that the C 1s photopeak at ev could be weaken after one-minute immersion in the basic solution, which indicates that DEN-CH partially releases from multilayer film for 10

11 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 between the films before and after immersion was observed, which implies that the PVP still remains on the substrate. From the above discussions, we demonstrate that when the PVP/DEN-CH LbL film is immersed in a basic aqueous solution, one of the film components, DEN-CH, dissolves away and the other component, PVP, remains on the substrate. In order to study the release kinetics of dendrimers from the PVP/DEN-CH multilayer film, we measured the change of film absorbance at 234 nm as a function of ph of the basic solution and immersion time. Figure 8 shows the intensity change of the DEN-CH absorption with the immersion time in basic solutions with different ph values. It indicates that in the basic solutions PVP/DEN-CH multilayer films are not stable and prone to deconstruction, and the deconstruction process of the multilayer film depends sensitively on ph of basic solutions. From Figure 8, it can be seen, with increasing ph of the basic solutions from 11.0 to 13.0, the release rate of DEN-CH increases greatly. For the (PVP/DEN-CH) 5 PVP multilayer films immersed in the basic solution of ph =11.0 for 180 min, no DEN-CH release from the multilayer was observed. While at ph =13.0, at the very beginning of immersion, e.g. 1 min, approximately 80 % DEN-CH was released, and an equilibrium plateau was reached after 25-minute base treatment. The above analysis indicates that, when the LbL film is dipped into a basic aqueous solution, 11

12 DEN-CH 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-CH 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-CH are ionized by the basic solution, which leads to the destruction of hydrogen bonding between PVP and DEN-CH. After the hydrogen bonds are destroyed, DEN-CH leaves the film because of its solubility in the basic solution, while PVP remains due to its poor solubility in the basic solution. ne question is why DEN-CH can release from PVP/DEN-CH multilayer even slower than PAA from 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-CH and PAA are soluble in the basic solution, the solubility of DEN-CH should be less than that of PAA because of the existence of benzene rings in the DEN-CH, which must lower its release rate from the multilayer. The second possible reason is the molecular shape. Because of the branching structure, DEN-CH would be anchored in the PVP matrix, and is harder to escape out of the multilayer. However, in the case of PAA, the polymer chain is linear, and should be easier to be drawn from the film. Therefore, the result that PAA release faster than DEN-CH is reasonable. The morphology variation of the PVP/DEN-CH multilayer film in the basic aqueous solution was explored using AFM. The AFM image of the 12

13 (PVP/DEN-CH) 6 PVP 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-CH on a quartz plate exhibits a high coverage with granular structures with the size from 150 to 300 nm. The AFM images of (PVP/DEN-CH) 14 multilayer film after immersion in ph = 12.5 NaH aqueous solutions at 25 C for different periods of time are shown in Figure 10. After 10-minute immersion in basic solutions, the surface of 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 10-minute immersion. When immersing the (PVP/DEN-CH) 14 multilayer film in the basic solution for 30 min, the pores with 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 averagely 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 PVP/PAA multilayer 35 after base treatment under the same treatment conditions (ph = 12.5, 25 C). In contrast to the separate pores in PVP/PAA system, for the same immersion time (180 min), the distribution and shape of pores obtained in the present case is more uniform (Figure 10D). Moreover, the surface pore coverage is significantly higher than that obtained in PVP/PAA system. 35 The above analysis indicates that a time-controlled microporousity of multilayer film can be obtained by immersion of the PVP/DEN-CH film in a basic solution. It is known that ph and ionic strength during or after the film construction can 13

14 not only anneal surface roughness but also lead to more dramatic structural rearrangements, such as porosity in the multilayer structure. In this case, after DEN-CH is removed rapidly by the basic solution, at the beginning the remaining PVP should retain that extended state. However, with prolonged immersion time the extended PVP chains gradually rearrange due to 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 above-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-CH. Conclusions In this article, firstly we presented the fabrication and detailed characterization of the PVP/DEN-CH layer-by-layer film based on hydrogen bonding. Afterwards, the variety behavior of such multilayer in a basic solution was investigated, which indicated that a microporous film was formed by the rapid release of DEN-CH and slowed re-organization of remaining PVP on the substrate. Moreover, we compared the varieties of the PVP/DEN-CH and PVP/PAA mutilayers in basis solutions. An interesting finding is that the release rate of DEN-CH from PVP/DEN-CH multilayer is lower than that of PAA from PVP/PAA multialyer in a basic solution, and the resulting microporous morphologies are remarkably different as well. We presume that the phenomena could be accounted for the difference in the solubility 14

15 and molecular shape of DEN-CH and PAA. We can conclude from the above discussions that incorporating different building blocks as hydrogen-bonding donor into multilayer assembly is an effective way to adjust the release process and microporosity by immersion of layer-by-layer films into basic solutions. ur studies on microporous films resulting from hydrogen-bonding-directed multilayer, combined with other insights into hydrogen-bonded ultrathin films or porous thin films, may pave the way for further theoretical researches and potential applications in the future. Acknowledgment. This work is supported by the Major State Basic Research Development Program (G ), the National Natural Science Foundation of China ( ), and a key project of the Educational Ministry. The authors thank Mr. Fengwei Huo for helpful discussions during the experiments. References (1) Lehn, J. M. Supramolecular Chemistry Concepts and Perpectives; VCH: Weinheim, (2) Ulman, A. An Introduction to Ultrathin rganic Films: From Langmuir-Blodgett to Self-assembly; Academic Press: Boston, (3) Decher, G.; Schlenoff, J. B. Multilayer thin films Sequential Assembly of Nanocomposite Materials; VCH: Weinheim, (4) Decher, G.; Hong, J. D., Schmitt, J. Thin Solid Films 1992, 210/211, 831. (5) Decher, G. Science 1997, 277, (6) Zhang, X.; Gao, M. L.; Kong, X. X.; Sun, Y. P.; Shen, J. C. Chem. Commun. 1994, 15

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19 Figure captions Figure 1 Schematics of the layer-by-layer assembly of PVP and DEN-CH on a quartz substrate based on hydrogen bonding: (I) adsorption of PVP and (II) adsorption of DEN-CH. Figure 2 UV-vis spectra of (PVP/DEN-CH) n multilayer films with n = 0~12 on -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. Insert: absorbance at 234 nm vs the number of deposition cycles. Figure 3 UV absorbance at 234 nm recorded as a function of immersion time for deposition of a single monolayer of DEN-CH on a quartz substrate coated with (PVP/DEN-CH) 3 PVP precursor film. Figure 4 Influence of 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 5 FT-IR spectra of cast films of (a) pure PVP and (b) pure DEN-CH on CaF 2 plates. Figure 6 FT-IR spectrum of a (PVP/DEN-CH) n (n = 8) multilayer film on a PEI-modified CaF 2 plate. The insert shows a magnification of the FT-IR spectrum in the range from 1300 to 1900 cm

20 Figure 7 C 1s XPS spectra of (PVP/DEN-CH) 5.5 multilayer films before (a) and after (b) immersion in ph = 12.5 NaH aqueous solution at 25 C for 2 min. Figure 8 The decrease of absorbance at 234 nm of (PVP/DEN-CH) 5 PVP multilayer films vs. the immersion time in the NaH aqueous solutions with different ph values. Figure 9 AFM height image ( µm 2 ) of a (PVP/DEN-CH) 6 PVP multilayer film. Figure 10 AFM height images ( µm 2 ) of (PVP/DEN-CH) 14 multilayer films on a quartz substrate after immersion in a ph = 12.5 NaH aqueous solution at 25 C for 10 (A), 30 (B), 60 (C), and 180 min (D). 20

21 PVP Ⅰ N N NH N 2 N N N N N N N N DEN-CH Ⅱ N N NH N 2 N N H H H H H H N H H H N H H H N H H H H H H N H H H H N H H H H N H H H H H H Ⅰ Ⅱ H H H H H H H H H H H H H H H H DEN-CH CH 2 CH N n PVP Hydrogen bonding donor Hydrogen bonding acceptor Figure 1 21

22 Absorbance Absorbance at 234 nm Number of bilayers Wavelength/nm Figure 2 22

23 Absorbance Immersion time/min Figure 3 23

24 at 234 nm a Absorbance b c Number of bilayers Figure 4 24

25 Absorbance (a. u.) a b Wavenumber/cm -1 Figure 5 25

26 0.07 Absorbance Wavenumbers/cm * 1934 * Wavenumber/cm -1 Figure 6 26

27 Intensity (a. u.) a b DEN-CH * Binding Energy (ev) Figure 7 27

28 Figure 8 28

29 Figure 9 29

30 A B C D Figure 10 30