Adhesion of Two Physically Contacting Planar Substrates Coated

Autumn , K., Liang , Y. A., Hsieh , S. A., Zesch , W., Chen , W. P., Kenny , T. W., Fearing , R. and Full , R. J. Nature 2000, 405, 681. [Crossref], [...
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Adhesion of Two Physically Contacting Planar Substrates Coated with Layer-by-Layer Assembled Films Daisuke Matsukuma,† Takao Aoyagi,‡ and Takeshi Serizawa*,†,§ †

Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, ‡Department of Nanostructure and Advanced Materials, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan, and § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Received March 17, 2009. Revised Manuscript Received April 29, 2009 Adhesives composed of synthetic and low-cost molecules that are based on simple chemical principles are attractive because of their versatility. In this article, we report adhesion between two planar substrates coated with layer-by-layer (LbL) assembled films of cationic poly(diallyldimethylammonium chloride) (PDDA) and anionic poly(sodium styrenesulfonate) (PSS) and perform lap shear measurements of the adhered substrates. Films prepared on the substrates functioned as adhesives when one substrate coated with the PDDA-surface film contacted the other surface coated with the PSS-surface film under adequate pressure in the presence of water droplets, suggesting that two films adhered on the basis of polyion complex formation. Observations suggested that the adhesives failed at the substratefilm interface rather than at the bulk films. The adhesion was compared between film-coated substrates and noncoated ones. Confocal laser scanning microscopic observation of adhesives composed of fluorescently labeled poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) revealed that the labeled PAH assembled on one substrate was well dispersed, even in a nonlabeled film assembled on another substrate. It was therefore confirmed that after adhesion in the presence of the water component, the polyelectrolytes became intermixed between the glassy films, resulting in changes in the adhesive structure at the substrate-film interface.

Introduction Rapid progress in nanotechnologies has produced miniaturized materials that are composed of tiny and complicated frameworks. Material frameworks should be handled without sacrificing their microscopic structures and physical properties. However, conventional polymer adhesives unfortunately require organic solvents and thermal treatments, which might collapse the frameworks. In addition, it is typically difficult to coat framework surfaces with adhesives of nanometer thickness to maintain the desired material architectures. Therefore, novel adhesives that operate successfully at nanometer thicknesses are strongly desired for current nanotechnologies. As potential alternatives to conventional polymer adhesives, biorelated adhesives, inspired by mussels (L-3,4-dihydroxyphenylalanine (dopa)-rich proteins)1-4 and geckos (pattern surfaces),5-8 as well as their combination9 have been studied extensively *Corresponding author. Tel/Fax: +81-3-5452-5225. E-mail: t-serizawa@ bionano.rcast.u-tokyo.ac.jp. (1) Waite, J. H.; Qin, X. Biochemistry 2001, 40, 2887. (2) Lee, H.; Scherer, N. F.; Messersmith, P. B. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 12999. (3) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318, 426. (4) Westwood, G.; Horton, T. N.; Wilker, J. J. Macromolecules 2007, 40, 3960. (5) Autumn, K.; Liang, Y. A.; Hsieh, S. A.; Zesch, W.; Chen, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, 681. (6) Mahdavi, A.; Ferreira, L.; Sundback, C.; Nichol, J. W.; Chan, E. P.; Carter, D. J. D.; Bettinger, C. J.; Patanavanich, S.; Chignozha, L.; Ben-Joseph, E.; Galakatos, A.; Pryor, H.; Pomerantseva, I.; Masiakos, P. T.; Fawuin, W.; Zumbuehl, A.; Hong, S.; Borenson, J.; Vacanti, J.; Langer, R.; Karp, J. M. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2307. (7) Sethi, S.; Ge, L.; Ci, L.; Ajayan, P. M.; Dhinojwala, A. Nano Lett. 2008, 8, 822. (8) Qu, L.; Dai, L.; Stone, M.; Xia, Z.; Wang, Z. L. Science 2008, 322, 238. (9) Lee, H.; Lee, B. P.; Messersmith, P. B. Nature 2007, 448, 338.

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because of their versatility and nontoxicity. Although those adhesives are sophisticated and promising, biomolecular amino acids and proteins are sometimes costly, and their chemical stability becomes a subject of discussion in the case of long-term utilization and under extreme conditions. In addition, special equipment is required to fabricate fine pattern surfaces to mimic a gecko’s feet. Therefore, ultrathin adhesives composed of chemically stable and low-cost molecules based on simple chemical principles are of great interest. Layer-by-layer (LbL) assembly10-17 is a versatile and established film-fabrication method that is simply achieved by the alternate immersion of variously shaped solid substrates into two or more solutions of interactive polymers without requiring any special polymers or extensive equipment. Polyion complexes formed between the oppositely charged polyelectrolytes are representatively utilized for the LbL assembly. The film’s chemical composition can be highly tuned depending on the component polymers used for the required assembly steps. The film’s thickness can also be controlled by the number of assembly steps, the salt concentration in the aqueous polymer solutions, the pH, and so on. Not only electrostatic interactions but also other intermolecular interactions, including chemical linkages, (10) Decher, G. Science 1997, 277, 1232. (11) (a) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 2319. (b) Ariga, K.; Hill, J. P.; Lee, W. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9, 014109. (12) Lvov, Y. Protein Archit. 2000, 125. (13) Decher, G.; Schlenoff, J. B. Multilayer Thin Films; Wiley-VCH: Weinheim, 2003. (14) Hammond, P. T. Adv. Mater. 2004, 16, 1271. (15) Angelatos, A. S.; Katagiri, K.; Caruso, F. Soft Matter 2006, 2, 18. (16) Serizawa, T.; Akashi, M. Polym. J. 2006, 38, 311. (17) Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. Adv. Mater. 2006, 18, 3203.

Published on Web 05/20/2009

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Figure 1. Illustration of the experimental procedure for the adhesion and lap shear tests.

have been utilized in LbL assembly.18-21 Regularly structured films, which are assembled on substrates or in aqueous phases, have been utilized for various technological and biomedical applications. LbL assembled films have great potential as nanometer-thick and surface-charged adhesives between substrates. Quantitative adhesion measurements using a surface force apparatus revealed that physical contact between two films of opposite surface charges generated polyion complexes at their interface,22 whereas those with the same surface charges caused entanglement of the polymer chains.23 Regarding macroscopic planar substrate systems, a preceding patent24 demonstrated adhesion of a planar substrate coated with LbL assembled films to a noncoated bare substrate and revealed that the films functioned as ultrathin adhesives responsive to the external aqueous environment. Although the aforementioned patent24 is significant because it was the first to demonstrate the potential if films behave as adhesives, it is worthwhile to investigate the fundamental aspects of the adhesion of solid substrates further by using LbL assembled films. As an alternative system, we focused on adhesion between two substrates: one coated with polycation-surface films and the other coated with polyanion-surface films. We anticipated strongly adhered substrates based on polyion complex formation, possibly at the contacted interface region. To form polyion complexes between the two film surfaces effectively, the mobility of the polymer chains, mechanical contacts between (18) (a) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (b) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (c) Wang, L.; Cui, S.; Wang, Z.; Zhang, X. Langmuir 2000, 16, 10490. (d) Hao, E.; Lian, T. Chem. Mater. 2000, 12, 3392. (19) (a) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (b) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768. (c) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Macromolecules 1999, 32, 8220. (20) (a) Serizawa, T.; Hamada, K.-I.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. J. Am. Chem. Soc. 2000, 122, 1891. (b) Serizawa, T.; Hamada, K.-I.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. Langmuir 2000, 16, 7112. (c) Serizawa, T.; Yamashita, H.; Fujiwara, T.; Kimura, Y.; Akashi, M. Macromolecules 2001, 34, 1996. (d) Hamada, K.-I.; Serizawa, T.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. Langmuir 2001, 17, 5513. (e) Serizawa, T.; Hamada, K.-I.; Kitayama, T.; Akashi, M. Angew. Chem., Int. Ed. 2003, 42, 1118. (f) Serizawa, T.; Arikawa, Y.; Hamada, K.-I.; Yamashita, H.; Fujiwara, T.; Kimura, Y.; Akashi, M. Macromolecules 2003, 36, 1762. (g) Serizawa, T.; Hamada, K.-I.; Akashi, M. Nature 2004, 429, 52. (21) (a) Decher, G.; Schmitt, J.; Heiliger, L.; Siegmund, H.-U. European Patent EP647477, 1995.(b) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (c) Sun, J.; Wu, T.; Liu, F.; Wang, Z.; Zhang, X.; Shen, J. Langmuir 2000, 16, 4620. (d) Dai, J.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931. (e) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Lee, P. A.; Armstrong, N. R.; Dutta, P.; Marks, T. J. Chem. Mater. 2001, 13, 15. (f) Serizawa, T.; Nanameki, K.; Yamamoto, K.; Akashi, M. Macromolecules 2002, 35, 2184. (g) Serizawa, T.; Nakashima, Y.; Akashi, M. Macromolecules 2003, 36, 2072. (h) Serizawa, T.; Matsukuma, D.; Nanameki, K.; Uemura, M.; Kurusu, F.; Akashi, M. Macromolecules 2004, 37, 6531.  Lavalle, Ph.; Voegel, J.-C.; Schaaf, P.; Kekicheff, P. Langmuir (22) Kulcsar, A.; 2004, 20, 282.  (23) Johansson, E.; Blomberg, E.; Lingstro¨m, R.; Wagberg, L. Langmuir 2009, 25, 2887. (24) Ono, S.; Decher, G. Laminates of Multilayered Films Bonded through Hydrogen Bond for Self-Supporting Thin Films. WO/2006/054668, 2006.

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the surfaces, and the release of inorganic salts derived from counterions have to be promoted. For this purpose, a water atmosphere and adequate contacting pressure is required for the adhering process. In this article, we adhered two planar substrates coated with LbL assembled films in the presence or absence of water under physical contact with adequate pressure and quantitatively analyzed the adhesion strength of the substrates by lap shear measurements. Cationic poly(diallyldimethylammonium chloride) (PDDA) and anionic poly(sodium styrenesulfonate) (PSS) were used as a representative strong base-acid combination for LbL assembly. To visualize the adhesive layers directly by confocal laser scanning microscopy (CLSM), combinations of fluorescently labeled poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), in which the assembly pattern and adhesion properties were similar to those of PDDA and PSS, were also utilized. Silicon wafer and polyimide (PI) substrates were utilized as inorganic and organic models, respectively. The two substrates successfully adhered only in the presence of the films and water. Mechanistic considerations revealed that the contacted films were restructured by an intermixing of the component polyelectrolytes. Our experiments are shown schematically in Figure 1.

Experimental Section Materials. PDDA (Mw = 100 000 to 200 000), PSS (Mw = 70 000), PAH (Mw = 56 000), and PAA (Mw = 1800) were purchased from Aldrich and used without further purification. Fluorescein isothiocyanate (FITC) was purchased from Fluka and used without further purification. FITC-labeled PAH (PAHFITC) was synthesized according to the methods described in a previous study.25 In brief, FITC was dissolved in dimethyl sulfoxide (final concentration: 2 mg mL-1), and we protected it from light by wrapping the tube in aluminum foil. The FITC solution (1 mL) was then added to each 25 mL aliquot of a 2 mg mL-1 PAH solution (pH ∼10) such that the FITC to -NH2 monomer ratio was 1:100. The mixture was stirred and allowed to react at 4 °C overnight. The solution was dialyzed and then lyophilized. The silicon wafers (0.60 to 0.65 mm thickness, Sumco) and PI films (0.075 mm thickness, Nilaco) were cut to a size of 40  5 mm and were used as substrates. Before use, the silicon wafers were cleaned with 99.5% ethanol (spectral grade) purchased from Nacalai, and the PI films were used as received. Film Fabrication. The substrate was immersed in an aqueous PDDA or PAH solution (1 mg mL-1 containing 1 M NaCl) for 5 min at ambient temperature, rinsed with pure water, and then dried under N2 gas. The substrate was immersed again in a PSS or PAA solution (1 mg mL-1 containing 1 M NaCl), and the same procedure was repeated. This operation was repeated for a specific number of steps. The solution pH values of PAH (or PAH-FITC) and PAA were adjusted to 7.5 and 3.5 using 0.1 M HCl and NaOH, respectively, according to the methods described in a (25) Caruso, F.; Yang, W.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932.

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Article previous study.26 The assembly was started with the cationic polyelectrolytes. Film Characterization. For a quantitative analysis of the amount assembled, the LbL assembled films were prepared on 9 MHz quartz crystal microbalance (QCM) electrodes (USI). We then estimated the amount of polymer deposition (Δm) by measuring the frequency shifts (ΔF) in air as follows: -Δm (ng) = 0.87ΔF (Hz).27 Before the film preparation, the QCM electrodes were cleaned with a piranha solution [H2SO4/H2O2 (30 wt % in water) = 3/1 v/v] for 3 min, followed by being rinsed with pure water and dried with N2. We determined the film thicknesses by assuming the film density to be 1.15 and 1.17 g cm-3 for the PDDA/PSS and PAH/PAA combinations, respectively, which were estimated by both the amount of assembled polymer measured by the QCM analysis as well as the thickness measured directly by scratching-mode atomic force miscopy (AFM, SPM9600, Shimadzu). The surface ζ potential was measured by an ELS-Z2-Z1 apparatus (Otsuka Electronics) in 0.01 M NaCl solution containing dispersed polystyrene latex as the standard at ambient temperature. Adhesion Test. The lap shear test was performed using a Tensilon RTG-1225 apparatus (A&D) in air at ambient temperature. After the adhered sample was held at both ends with two mechanical chucks, it was loaded to failure at 50 μm min-1, and the failure strain was measured. All samples displayed cohesive failure. Confocal Laser Scanning Microscopy Observation. The films were prepared on borosilicate glass slides (Matsunami) that had been precleaned with a piranha solution. After the adhesion of 29-step PAH-FITC/PAA films to nonlabeled 30-step PAH/PAA films under optimized conditions (contact pressure 0.35 MPa), the adhesive layers were visualized by CLMS. The observations were carried out on a Leica TCS-NT fluorescence microscope with a PL APO (40, 0.85 NA) in air. The FITC fluorescence was detected after excitation at 488 nm and an emission band-pass filter at 500-600 nm.

Results and Discussion Film Fabrication and Lap Shear Experiments. An analysis using QCM substrates, which can monitor the amount of assembled polymer by the frequency decreases of the substrates,27 confirmed the successive LbL assembly of PDDA and PSS in the presence of 1 M NaCl (Figure 2). The thickness increased exponentially until step 12 and almost linearly after this step. Goh et al. have originally reported the exponential increase in assembly amounts and the thickness-dependent evolution of film morphologies for PDDA/PSS combinations assembled in the presence of salts.28 It is currently accepted as the unique mechanism for the exponential regime that during LbL assembly, polyelectrolytes diffuse into films from the solution, whereas the oppositely charged polyelectrolytes, which have already assembled, diffuse out of the films, repeating the “in-and-out” diffusion processes.29 The growth regime was strongly dependent on polyelectrolyte species as well as assembly conditions due to (26) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (27) Sauerbrey, G. Z. Phys. 1959, 155, 206. (28) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655. (29) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, Ph. Proc. Natl Acad. Sci. U.S.A. 2002, 99, 12531. (30) Lavalle, Ph.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C.; Picart, C. Macromolecules 2002, 35, 4458. (31) Hbsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J.-C.; Schaaf, P. Langmuir 2004, 20, 1980. (32) Salomki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232. (33) Kujawa, P.; Moraille, Sanchez, J.; Badia, A.; Winnik, F. M. J. Am. Chem. Soc. 2005, 127, 9224. (34) Porcel, C.; Lavalle, Ph.; Ball, V.; Decher, G.; Senger, B.; Voegel, J.-C.; Schaaf, P. Langmuir 2006, 22, 4376.

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Figure 2. QCM analysis of LbL assembled PDDA and PSS in the presence of 1 M NaCl. The open and closed symbols indicate PDDA and PSS, respectively.

changes in diffusion coefficient of polyelectrolytes.28-37 In addition, the composition of polyelectrolyte mixtures similarly changed the growth regime from totally linear to partially exponential.31 Furthermore, the increase in temperature changed the growth regime.32 Accordingly, the initial exponential assembly for the PDDA/PSS combinations in this study suggests the presence of the aforementioned “in-and-out” processes, and the subsequent linear assembly shows that the number of polyelectrolytes that diffuse out of the films tends to be constant. The LbL assembled films were similarly fabricated onto the surfaces of the desired substrates. One substrate was coated with films with a cationic outermost charge, and the other was coated with an anionic outermost charge. Therefore, adhesion via polyion complex formation between the two films was anticipated by the physical contacts, which was enhanced by the molecular level interactions of the oppositely charged polyelectrolytes.38 Substrates with a constant area (5  5 mm) were then adhered under an air atmosphere under a pressure of 0.35 MPa for 60 min at ambient temperature in the presence of water droplets (5 μL), which fully spread out at the adhered area after physical contact. Water, which was evaporated within 60 min in air, is essential for improving the mobility of the polyelectrolytes as well as for the formation of polyion complexes based on releasing water-soluble NaCl. No adhesion was observed without any film coating or in the absence of water. Finally, the adhesion strength (GL) was quantified by a lap shear test. The load on the adhered films increased linearly with increasing displacement (Figure 3). The slope for the adhered silicon wafers was smaller than that for a single silicon wafer (Figure 3a), suggesting that the LbL assembled adhesives, but not the silicon wafers, were transformed with load. Therefore, the mechanical properties of the adhesives could be discussed for silicon wafers. (See below.) However, the slope for the adhered PI substrates was almost the same as that for a single PI substrate (Figure 3b), suggesting that the PI substrates were elongated with load. Subsequently, the adhered films failed at the region of the adhesives when they reached marginal loads, thereby determining the adhesion strength of the two substrates. It should be noted that this test estimates the failure strength triggered by mechanically weak points in the adhesives. (35) Porcel, C.; Lavalle, Ph.; Decher, G.; Senger, B.; Voegel, J.-C.; Schaaf, P. Langmuir 2007, 23, 1898. (36) Mertz, D.; Hemmerle, J.; Mutterer, J.; Ollivier, S.; Voegel, J.-C.; Schaaf, P.; Lavalle, Ph. Nano Lett. 2007, 7, 657. (37) Jourdainne, L.; Lecuyer, S.; Arntz, Y.; Picart, C.; Schaaf, P.; Senger, B.; Voegel, J.-C.; Lavalle, Ph.; Charitat, T. Langmuir 2008, 24, 7842. (38) Berndt, P.; Kurihara, K.; Kunitake, T. Langmuir 1992, 8, 2486.

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The lap shear experiments demonstrated that two planar substrates coated with LbL assembled films at nanometer thickness successfully adhered in the presence of water via polyion complex formation. The GL values for the silicon wafers or PI substrates coated with combinations of 7/8-step and 13/14-step assembled films (the numbers represent the number of LbL assembly steps for each substrate), in which the total thicknesses of the adhesives were estimated to be 60 and 318 nm, respectively, are shown in Figure 4. With silicon wafers, the substrates adhered on the order of megapascals, indicating that the aforementioned adhesion within the 5  5 mm area withstood approximately 9 and 16 kgf, respectively. In fact, some experiments failed because of breakage of the silicon wafers during the lap shear tests. The present adhesion strengths were smaller than those of conventional epoxy resins (90-120 MPa)39 but were comparable to those of biomolecular adhesives inspired by mussels (∼4 MPa)4,40 and geckos (∼1 MPa),6-9 although it might be difficult to compare them directly because of different experimental conditions. Consequently, these observations suggest that the LbL assembled films successfully functioned as thin film adhesives. In the case of stiff silicon wafers, the Young’s modulus can be estimated from their load-displacement curves. The resultant Young’s modulus values for adhesives composed of 7/8- and 13/14-step pairs were found to be 650 ( 23 and 673 ( 7 MPa, respectively. These values were similar to the previously reported values for LbL assembled films composed of linear polyelectrolytes41 and were smaller than composite films containing inorganic compounds.42 Accordingly, these observations suggest that the adhered interface of the LbL assembled films was assimilated and miscible into the bulk films. Parameters Affecting Adhesion Strength. Interestingly, the GL values were different between substrates, and the silicon wafers adhered much more strongly as compared with the PI substrates (Figure 4). If the adhesives failed at the adhesion interfaces or at the bulk LbL assembled films during the lap shear tests (GII or GIII in Figure 5, respectively), then the values should be independent of the substrate species. Accordingly, it was concluded that the present adhesives failed at the interface between the substrates and the films (GI in Figure 5) depending on the interactions between the films and the substrate surfaces. The analysis of the various parameters affecting adhesion strength supported this interpretation. To reveal more detail, we systematically adjusted the total thickness of the adhesives for the PI substrates by tuning the assembly steps (Figure 6a and Table S1 in the Supporting Information). The GL values increased with increasing thickness, indicating that the adhesion strength can be controlled by the thickness. This observation is surprising because the surface charge (ζ potential) of the films for each outermost surface was constant after seven or eight steps of assembly (Figure 7). Therefore, the GL values were independent of the outermost charge of the films. Considering the aforementioned Young’s modulus values and the substrate dependence, it was concluded that the LbL assembled films, even in their glassy states43 on the (39) Nair, C. P. R. Prog. Polym. Sci. 2004, 29, 401. (40) Yu, M.; Deming, T. J. Macromolecules 1998, 31, 4739. (41) Gao, C.; Donath, E.; Moya, S.; Dudnik, V.; Mo¨hwald, H. Eur. Phys. J. E 2001, 5, 21. (42) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Science 2007, 318, 80. (43) (a) Mueller, R.; Ko¨hler, K.; Weinkamer, R.; Sukhorukov, G.; Fery, A. Macromolecules 2005, 38, 9766. (b) Ko¨hler, K.; Mo¨hwald, H.; Sukhorukov, G. B. J. Phys. Chem. B 2006, 110, 24002. (c) Fortier-McGill, B.; Reven, L. Macromolecules 2009, 42, 247.

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Figure 3. Typical load-displacement curve obtained by the lap shear test for the adhesion of (a) silicon wafers and (b) PI substrates coated with 7/8-step assembled PDDA/PSS films. The solid lines show the adhesion test, and the dashed lines show the elongation of each single substrate.

Figure 4. GL values for silicon wafers (open) or PI (gray) substrates coated with 7/8- and 13/14-step assembled PDDA/PSS films.

Figure 5. Schematic illustrations of the possible failure regions (GI: near the interface between substrate and film, GII: bulk film, GIII: adhered interface).

substrates, were restructured during the LbL assembly, by physical contact, or both, thus resulting in thickness-dependent interfacial structures. DOI: 10.1021/la900924w

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Figure 8. GL values for the adhesion of film-coated PI substrates with noncoated ones.

Figure 6. Parameters affecting the adhesion strength of PDDA/ PSS adhesives. (a) GL values for PI substrates coated with films of different thickness. (b) GL values for PI substrates coated with different combinations of the films. (9/22-, 13/20-, and 16/17-steps represent 52/469, 146/389, and 238/278 nm thickness, respectively.) (c) Time course of the GL values for PI substrates coated with 13/14-step assembled films. (d) GL values for silicon wafers coated with films of cationic (7/7 steps), oppositely charged (7/8 steps), and anionic (8/8 steps) surfaces.

Figure 7. ζ potentials of the PDDA/PSS films used for adhesion.

When the total thickness of the adhesives was made approximately constant, the GL values were almost the same (Figure 6b). If the adhesion strength is simply determined by each film thickness before adhesion, then the GL values should decrease upon decreasing thickness of the one component film. In other words, the GL value for the 9/22-step assembled films should be the smallest if restructuring does not occur by physical contact. Therefore, we concluded that restructuring certainly occurred during the adhesion process. When the adhesion time was changed to 60 min (Figure 6c), the GL values for the PI substrates increased with increasing time. In fact, it took approximately 20 min to reach saturated values, which is similar to the time required for successive LbL assembly steps.10-17 This observation could also be interpreted by considering restructuring during the adhesion processes. It is 9828 DOI: 10.1021/la900924w

noted that a three-fold increase in adhesion pressure from 0.35 to 1.05 MPa did not affect the adhesion strength for the PI substrates. Unexpectedly, silicon wafers coated with films with the same outermost surface still adhered (Figure 6d), although the values became slightly smaller. This observation indicates that the outermost surface charge is not very important for adhesion. In other words, restructuring triggered by physical contact between two films of the same charge still seemed to occur because water droplets would improve the mobility of the polyelectrolytes, as mentioned above, because the polyelectrolytes present in the under layer would protrude to the outermost layer, or both.44 Comparison with Previous One-Side System. The adhesion of film-coated PI substrates with noncoated ones in the presence of water droplets was also analyzed (Figure 8 and Table S2 in the Supporting Information) following a recent patent.24 As compared with our “both-side” system, this can be considered to be a “one-side” system. Interestingly, the GL values became saturated against the film thickness, possibly because restructuring did not affect these values, which was different from our system. Saturation is reasonable considering that the GL values are determined by the efficiency of the contacts between the LbL assembled film and the bare substrate. In other words, the efficiency of the contacts between the film surface and the substrate might increase upon increasing the film thickness and become constant above the threshold thickness. It should be emphasized that our system might be better for controlling and increasing the adhesion strength. As a consequence, all of these experiments strongly supported the presence of restructuring during the adhesion process in our system. Confocal Laser Scanning Microscopy Observations. The present observations confirm that restructuring occurred when two glassy LbL assembled films were in physical contact with one another in the presence of water droplets. In fact, when a colloidal particle coated with an LbL assembled film physically contacted another LbL assembled film on a planar substrate, the intermixing of polyelectrolytes was previously proposed.45 This observation also suggests that polyelectrolytes have the potential to intermix between LbL assembled films on planar substrates when contacted with one another. To visualize the contact-driven intermixing directly, CLMS observations were used on fluorescently labeled adhesives accord(44) Chluba, J.; Voegel, J.-C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800. (45) Ko¨hler, G.; Moya, S. E.; Leporatti, S.; Bitterlich, C.; Donath, E. Eur. Biophys. J. 2007, 36, 337.

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Figure 9. (a) QCM analysis of LbL assembled PAH and PAA. The open and closed symbols indicate PAH and PAA, respectively. (b) GL values for silicon wafer substrates coated with PAH/PAA films. 29

ing to methods described in a previous report. Because it was difficult to label PDDA or PSS, the combination of PAH and PAA was employed instead. QCM analysis revealed that the thickness of the PAH/PAA assembly increased exponentially until 12 steps and almost linearly after that step (Figure 9a), and the adhesion strength for PI substrates increased with increasing adhesive thickness (Figure 9b). These observations are similar to those for the PDDA/PSS assembly; therefore, we considered that the PAH/PAA combination could be an alternative to PDDA/PSS. It should be noted that PAH and PAA also interdiffuse during the assembly process. In other words, the interdiffusion thickness seemed to be constant at the linearly growing region (Figure 9a). A 29-step LbL assembly between PAH-FITC (the starting polymer) and PAA on the glass substrate was prepared. The resulting fluorescent film with a 2.1 μm thickness was adhered to nonfluorescent 30-step films of 1.7 μm thickness to visualize the intermixing of the PAH-FITC in the nonfluorescent films (Figure 10a). As a control, the 29-step fluorescent films were also contacted on noncoated, bare glass substrates (Figure 10b). After contact, the former fluorescent layer became significantly thicker than the latter. The mean thickness of the latter fluorescent layer was apparently estimated to be 6.7 μm from the CLMS image, which was much larger than the real film thickness (2.1 μm) estimated from the QCM analysis. This observation indicates that the CLSM images overestimated the film thickness. In this case, we found the “bleeding” effect to be 4.6 μm by subtracting the real thickness from the CLSM thickness. Meanwhile, the mean thickness of the former fluorescent layer was apparently estimated to be 9.6 μm from the CLMS image. Assuming that the bleeding effect is the same between the aforementioned two systems (although there seemed to be greater bleeding in Figure 10a than in b), the real thickness of Figure 10a should be ∼5 μm, which is approximately close to the total thickness of the adhered films (2.1 + 1.7 μm). This estimation supports the proposed intermixing mechanism. It is surprising that polyelectrolytes can intermix by physical contact between two LbL assembled films in the presence of water. In fact, the intermixing thickness (over 2 μm) was much larger than the apparent thickness at each step during the in-andout LbL assembly (∼200 nm in Figure 9a) and was a similar level to the interdiffusion of highly permeable polyelectrolytes during LbL assembly processes.29,34,35 Two LbL assembled films swelled with water molecules in physical contact might reach an appropriate equilibrium following restructuring, thus resulting in the difference in adhesion strength. Accordingly, it was found that Langmuir 2009, 25(17), 9824–9830

Figure 10. CLMS images of (a) “both-side” system (29-step assembled PAH-FITC/PAA films adhered onto nonlabeled 30-step assembled PAH/PAA films) and (b) “one-side” system (29-step assembled PAH-FITC/PAA films adhered onto bare glass). Scale bars indicate 10 μm.

physical contact between two LbL films in the presence of water induced rapid restructuring of the entire film. Stability. The adhesion strength of the adhered PDDA/PSS substrates did not change, even after storage for 1 year in air, thus indicating high stability. However, the adhesion was unstable in water. For instance, when PI substrates adhered with 13/14-step assembled PDDA/PSS films were immersed in water for 1 to 2 h at ambient temperature, the original GL values were decreased by 43 and 15%, respectively. This observation might be explained by considering the possibility that the water molecules penetrated the interface between the substrate and the adhesive to weaken the adhesion strength. Surface modification of the polymer substrates before the LbL assembly would be effective in overcoming this stability problem in water. As a consequence, these LbL assembled films satisfactorily functioned as nanometer-thick adhesives for two substrates in the dried state.

Conclusions The adhesion strength between two LbL assembled films was quantitatively analyzed by lap shear tests. We successfully adhered planar substrates such as silicon wafers and PI films by coating them with films via polyion complex formation in the presence of water droplets. The Young’s modulus values of the adhesive layers, which were estimated from their load-displacement curves, were similar to previously reported values for bulk films,41 suggesting that the adhered interfaces are miscible and form homogeneous adhesives. The adhesion strength was dependent on and controllable by both the substrate species and the total thickness of the adhesives. This suggests that the adhesives failed at the interfacial region between the substrate and the film. A comparison of the adhesion of film-coated substrates versus noncoated substrates24 revealed the advantages of our system. It was first demonstrated that LbL assembled glassy films prepared on planar substrates were considerably intermixed by physical contact with one another. The intermixing thickness was much larger than the interdiffusion thickness for the in-and-out LbL assembly process of the present PDDA/PSS combination. Such thick intermixing, which is an essential mechanism for realizing strong adhesion, must represent a unique case when two LbL assembled films physically contact each other in the presence of water. LbL assembled adhesives showed excellent stability in the dried state. Our analysis suggested the aforementioned DOI: 10.1021/la900924w

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Matsukuma et al.

intermixing processes; however, further studies such as X-ray or neutron reflectivity experiments are necessary to confirm the water-swollen thickness clearly as well as the restructuring thickness in the near future. LbL assembly is versatile and can readily prepare adhesive layers on any solid substrate. We expect that these LbL adhesives will have potential applications in various nanotechnology areas.

Dr. A. Watanabe and Prof. H. Aburatani (University of Tokyo) for help with CLMS measurements. This study was partially supported by PRESTO (JST), Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists, and Grants-in-Aid for Scientific Research (B) no. 20350052 (The Ministry of Education, Culture, Sports, Science, and Technology of Japan).

Acknowledgment. We thank Dr. H. Matsuno (University of Tokyo) for helpful discussion, Mr. K. Tanaka and Mr. K. Sasa (Otsuka Electronics) for help with ζ potential measurements, and

Supporting Information Available: GL values summarized in Tables. This material is available free of charge via the Internet at http://pubs.acs.org.

9830 DOI: 10.1021/la900924w

Langmuir 2009, 25(17), 9824–9830