pubs.acs.org/Langmuir © 2010 American Chemical Society
Netlike Knitting of Polyelectrolyte Multilayers on Honeycomb-Patterned Substrate Wei Sun, Liyan Shen, Jiaming Wang, Ke Fu, and Jian Ji* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Received May 26, 2010. Revised Manuscript Received July 4, 2010 The pH-amplified exponential growth layer-by-layer (LBL) self-assembly process was directly performed on honeycomb-patterned substrate for achievement of “guided patterning” of polyelectrolyte multilayers. Polyethylenimine (PEI) and poly(acrylic acid) (PAA) were used as polyanions, and their pH were carefully tuned to achieve pHenhanced exponential growth. Guided by underlying hexagonally patterned islandlike poly(dimethylsiloxane) (PDMS) arrays, the diffusive polyelectrolytes rapidly interweaved into linear, multilayered structures distributed along the grooves between the patterned protuberate and formed a regular network of multilayered film with uniform mesh size. Netlike “knitting” of polyelectrolyte multilayers on honeycomb-patterned substrate has been realized by following this procedure. Superhydrophobic surfaces could be readily obtained after several bilayers of LBL assembly (with thermal cross-linking and surface fluorination by chemical vapor deposition), indicating that successful fabrication of functional micro- and nanoscale hierarchical structures can be achieved. Both high- and low-adhesion superhydrophobic surfaces (“petal effect” and “lotus effect”) can be obtained with different bilayers of assembly, proving that different levels of nano- to microstructural hierarchy can be realized using this method. Furthermore, we were able to get topographically asymmetric, free-standing, polyelectrolyte multilayer films in the case that we performed more than eight bilayers of assembly. This research reported template-directed LBL patterning assembly for the first time. It provides a beneficial exploration for the surface patterning technique for the LBL assembly process.
Introduction Layer-by-layer (LBL) assembled polymeric multilayer films based on electrostatic interactions as the driving force of selfassembly are of significant interest for a broad range of applications, such as antireflection coatings,1 biosensors,2 nonlinear optics,3 biomedical coatings,4 organic light-emitting devices,5 and so on. Developed by Decher and co-workers early in 1991,6 the LBL assembly technique has emerged as an approach of great versatility for the construction of functional layered composite films on substrates with different geometries. Though LBL technique has been thoroughly studied as a methodology in various aspects, how to achieve surface patterning of LBL assembled multilayer films with ease and reliability is still one of the subjects which are lack of attention. Patterning of the assembled polymeric ultrathin films is one of the critical steps that determine the final applications of these films, especially in miniaturized devices.7 To achieve either chemical or topographical patterning on LBL assembled films, conventional lithographic methods, including microcontact *Corresponding author. E-mail:
[email protected]. (1) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nature Mater. 2002, 1, 59. (2) Calvo, E. J.; Danilowicz, C.; Wolosiuk, A. J. Am. Chem. Soc. 2002, 124, 2452. (3) Van Cott, K. E.; Guzy, M.; Neyman, P.; Brands, C.; Heflin, J. R.; Gibson, H. W.; Davis, R. M. Angew. Chem., Int. Ed. 2002, 41, 3236. (4) (a) Hu, X. F.; Ji, J. Langmuir 2010, 26, 2624. (b) Wang, X. F.; Ji, J. Langmuir 2009, 25, 11664. (c) Lin, Q. K.; Ding, X.; Qiu, F. Y.; Song, X. X.; Fu, G. S.; Ji, J. Biomaterials 2009, 31, 4017. (5) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067. (6) (a) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. 1991, 95, 1430. (7) (a) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (b) Shi, F.; Dong, B.; Qiu, D. L.; Sun, J. Q.; Wu, T.; Zhang, X. Adv. Mater. 2002, 14, 805. (c) Hua, F.; Shi, J.; Lvov, Y.; Cui, T. Nano Lett. 2002, 2, 1219. (8) Kidambi, S.; Sheng, L.; Yarmush, M. L.; Toner, M.; Lee, I.; Chan, C. Macromol. Biosci. 2007, 7, 344.
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printing,8 photolithography,9 capillary transfer lithography,10 microfluidic templating,11 and nanoimprinting,12 have been extensively employed. All of the above fabrication procedures fall into the category of the so-called “top-down strategy” which is known as complex and time-consuming. The self-assembly process, which is a bottom-up strategy commonly adopted in biological systems, has been actively pursued in the area of patterning for efficient structuring of functional surfaces.13 A variety of microand nanostructures can be obtained by spontaneous development of such self-assembled patterns.14 Recently, guided assembly of assembling objects on patterned templates has attracted remarkable attentions as a promising method for achieving patterning through self-assembly in a controllable fashion. For example, ordered ensembles of nanoparticles, which can be seemed as patterned arrays, can be readily prepared through multiple-step hybrid approaches based on the strategy of template-guided self-assembly.15-17 In this study, we attempted to realize “guided patterning” of polyelectrolyte multilayers by directly performing pH-amplified exponential growth LBL assembly on honeycomb-patterned substrate. (9) Shi, F.; Wang, Z. Q.; Zhao, N.; Zhang, X. Langmuir 2005, 21, 1599. (10) Ko, H. H.; Jiang, C. Y.; Tsukruk, V. V. Chem. Mater. 2005, 17, 5489. (11) Reyes, D. R.; Perruccio, E. M.; Becerra, S. P.; Locascio, L. E.; Gaitan, M. Langmuir 2004, 20, 8805. (12) Lu, Y. X.; Chen, X. L.; Hu, W.; Lu, N.; Sun, J. Q.; Shen, J. C. Langmuir 2007, 23, 3254. (13) Shimomura, M.; Sawadaishi, T. Curr. Opin. Colloid Interface Sci. 2001, 6, 11. (14) Whitesides, G. M.; Grzybowski., B. Science 2002, 295, 2418. (15) Junkin, M.; Watson, J.; Geest, J. P. V.; Wong, P. K. Adv. Mater. 2009, 21, 1247. (16) Hoogenboom, J. P.; Retif, C.; de Bres, E.; van de Boer, M.; van LangenSuurling, A. K.; Romijn, J.; van Blaaderen, A. Nano Lett. 2004, 4, 205. (17) Ruan, W. D.; Wang, C. X.; Ji, N.; Lu, Z. C.; Zhou, T. L.; Zhao, B.; Lombard, J. R. Langmuir 2008, 24, 8417.
Published on Web 08/04/2010
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Picart et al. have first reported a special kind of LBL assembly process, in which the deposited mass and the thickness of polyelectrolyte multilayers increase exponentially rather than linearly with the number of deposition cycles.18 Attributed to the ability of the polyelectrolyte to diffuse into and out of the multilayers during each deposition step, one can conveniently construct hierarchical topographic features both in the micro- and nanometer size.19 Recently, some of us reported new ways to amplify the exponential growth of multilayers by either adding silver nitrate or adjusting the pH of the polyelectrolyte pair.20,21 pH-amplified exponential growth LBL assembly takes advantage of the synergistic effect of the pH-dependent tunable charge density and diffusivity of the weak polyelectrolytes, resulting in the facile fabrication of thicker films in a very limited number of depositing cycles. It becomes a reliable and versatile tool to be applied in the rapid fabrication of hierarchical topographic features based on LBL assembly. The present paper describes a novel strategy based on pHamplified exponential growth LBL assembly on honeycombpatterned substrate, aiming to realize spontaneous “directed patterning” by a one-step, self-assembly process. Polyethylenimine (PEI) and poly(acrylic acid) (PAA) were used as polyanions, and their pH were carefully tuned to achieve pH-enhanced exponential growth. It is turned out that the polyelectrolytes showed site-selective exponential deposition with the assembly circles, leading to a netlike patterned morphology of the obtained multilayered film after only three-bilayer assembling. Guided by underlying hexagonally patterned islandlike poly(dimethylsiloxane) (PDMS) arrays, the diffusive polyelectrolytes rapidly interweaved into linear, multilayered structures distributed along the grooves between the patterned protuberate and formed a regular network of multilayered film with uniform mesh size. Superhydrophobic surfaces could be readily obtained after several bilayers of assemby, indicating that successful fabrication of functional micro- and nanoscale hierarchical structures can be achieved using this guided-assembly patterning method. Both high- and low-adhesion superhydrophobic surfaces (“petal effect” and “lotus effect”) can be obtained depending on the number of the assembly cycles. Furthermore, we were able to prepare topographically asymmetric, free-standing, polyelectrolyte multilayer film in the case that we performed more than eight bilayers of assembly. It is the first report of template-directed LBL patterning assembly.
Experimental Section Materials. Polystyrene (PS) (Mw =1.4 105) was kindly provided by Prof. Qiang Zheng. Monodispersed silica particles with mean diameters of 200 nm were prepared by hydrolysis of tetraethoxysilane in an alcohol medium in the presence of water and ammonia following the well-known St€ ober method.22 The poly(dimethylsiloxane) (PDMS) prepolymer and curing agent were purchased from Dow Corning (Sylgard 184). Poly(acrylic (18) (a) Picart, C.; Lavalle, Ph.; Hubert, P.; Cuisinier, F.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414. (b) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G.; Schaaf, P.; Voegel, J.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (19) (a) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J. Langmuir 2003, 19, 440. (b) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J.; Mesini, P.; Schaaf, P. Macromolecules 2004, 37, 1159. (c) Richert, L.; Lavalle, P.; Payan, E.; Shu, X.; Prestwich, G.; Stoltz, J.; Schaaf, P.; Voegel, J.; Picart, C. Langmuir 2004, 20, 448. (d) Lavalle, P.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635. (e) Choi, J.; Rubner, M. Macromolecules 2005, 38, 116. (f) Boudou, T.; Crouzier, T.; Ren, K.; Blin, G.; Picart, C. Adv. Mater. 2010, 22, 441. (20) Ji, J.; Fu, J. H.; Shen, J. C. Adv. Mater. 2006, 18, 1441. (21) Fu, J. H.; Ji, J.; Shen, L. Y.; Kuller, A.; Rosenhahn, A.; Shen, J. C.; Grunze, M. Langmuir 2009, 25, 672–675. (22) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
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acid) (PAA, Mw = 100 000) and polyethylenimine (PEI, Mw = 2.5 104, water-free) were obtained from Sigma-Aldrich. (Tridecafluoroctyl)triethoxysilane was purchased from Degussa Co. (Germany). Preparation of the Patterned PDMS Substrate. The breath figures (BF) method was employed to prepare patterned substrate for further molding replication of PDMS. In a typical BF-based preparation process, the casting solution was first obtained by mixing PS chloroform solution with silica particle ethanol suspension. The casting solution was then transferred onto a clean glass substrate dropwise. At the same time, a humidified flow of air was directed onto the liquid film on the glass substrate. After solidification, the obtained film was dried at room temperature. Fabrication of PDMS negative replicas from polymer film templates was achieved by means of soft lithography. Specifically, PDMS prepolymer was mixed with a curing agent at a 10:1 weight ratio and was carefully poured onto the BF film. After curing at 130 °C for 2 h and removal of the PS mold by dissolution in chloroform, PDMS negative replica were obtained. Flat PDMS substrate was prepared by directly replicating from the glass slide.
Performance of pH-Amplified Exponential Growth Layerby-Layer Assembly on the Honeycomb-Patterned Substrate. The multilayer films were constructed by alternately depositing polycation (1 mg mL-1 PEI) and polyanion (3 mg mL-1 PAA) aqueous solutions onto patterned or flat PDMS substrate. The pH of the PEI and PAA solutions were adjusted to 9.0 and 2.85 for multilayer deposition, respectively.25 The multilayer films were built by first immersing the substrate in the PAA solution for 15 min, followed by rinsing with pure water (with a pH of ∼5.5) three times; the substrates were then immersed in PEI solution for 15 min, followed again by washing three times with pure water. By repetition of this process in a cyclic fashion, a multilayer film of PAA/PEI was obtained. The multilayered films were thermally cross-linked at 180 °C for 2 h to preserve the obtained surface morphological features before further characterization. Hydrophobic Treatment of the Obtained Surfaces. Chemical vapor deposition of (tridecafluoroctyl)triethoxysilane was performed to obtain a low-surface-energy surface. The multilayer films on the PDMS substrate were placed in a sealed chamber together with (tridecafluoroctyl)triethoxysilane, after which the sealed chamber was placed in an oven at 130 °C for about 2.5 h. Then, the samples were withdrawn from the chamber and placed in an oven at 180 °C for 1.5 h to remove the unreacted silane molecules. Characterizations. The surfaces of the structured films were characterized with a field-emission scanning electron microscope (FESEM; FEI, SiRion100), operating at a 25 kV accelerating voltage. Samples were made conductive by deposition of a gold layer in a vacuum chamber. Contact angles of the samples toward distilled water were measured by sessile drop technique using contact-angle measurement equipment (KRUSS DSA 10-MK2). A water droplet of 4 μL was dispensed onto the substrate investigated; five different positions were measured to get the average contact angle.
Results and Discussion The BF method is a well-known bottom-up surface patterning technique.23-26 The method is based on evaporative cooling and subsequent water droplet templating to form porous film with ordered, honeycomb-like pore arrays. We employed here the particle-assisted BF method in which nanoparticles were used as second component to function as water droplet stabilizer in order to make high-quality BF arrays.27 Figure 1a shows the surface (23) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (24) Stenzel, M. H. Aust. J. Chem. 2002, 55, 239. (25) Bunz, U. H. F. Adv. Mater. 2006, 18, 973. (26) Fan, D. W.; Jia, X. F.; Tang, P. Q.; Hao, J. C.; Liu, T. B. Angew. Chem., Int. Ed. 2007, 46, 3342. (27) Sun, W.; Ji, J.; Shen, J. Langmuir 2008, 24, 11338.
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Figure 1. (a) SEM image of microporous polymeric film fabricated by using the silica nanoparticle-assisted BF method. Such patterned substrate was used as molding template of PDMS replication. (b) SEM image of PDMS substrate with islandlike arrays replicated from structure in (a) via soft lithography.
Figure 2. SEM images of PAA2.85/PEI9.0 films constructed on flat PDMS substrate in bilayers of 2-5 (a-d).
Figure 3. SEM images of PAA2.85/PEI9.0 films constructed on honeycomb-patterned PDMS substrate in bilayers of 2-5 (a-d). Prior to SEM investigation, the assembled multilayered films were thermally cross-linked to preserve desirable surface morphological features. (e) Relation curve between contact angle and the number of deposition cycles. (f) Shape of the water droplet on the surface with structures shown in (c) when it is turned upside down.
morphology of silica nanoparticle-assisted polystyrene BF film. Hexagonally patterned arrays of open pores can be observed over several square centimeters of the surface area of the obtained film. Using the patterned polymeric film as molding template, we fabricated replicated PDMS substrate via soft-lithography procedure described in experimental details. Figure 1b evidences the faithful reproduction of the patterned, PDMS negative substrate which is composed of neat arrays of islandlike protuberances. The typical size of protuberances of the patterned substrate is 3-4 μm. The polyelectrolyte multilayers were constructed by alternate deposition of PEI at high pH and PAA at low pH. As a control, we first studied morphological changes in the case that we applied the pH-amplified exponential growth LBL assembly on flat substrate. Figure 2 contains SEM images showing the morphological features after 2-5 bilayers of assembly (correspond to Figure 2a-d) on the flat PDMS substrate. The pH pair we adopted for the polyelectrolytes was PAA2.85/PEI9.0, which makes it a typical pH-amplified exponential growth LBL assembly process.21 Morphology of the exponential growth multilayer went through significant change among just several bilayers of assembly, reflecting rapid increase of the deposited polyelectrolyte mass and film thickness with the number of deposited bilayers. Polyelectrolyte deposition only exhibits morphological topography of nanosized knots and wrinkles after two bilayers of assembly (Figure 2a). While after one more bilayer of assembly, submicroscale isletlike morphology was formed (Figure 2b). The islets are composed of nanowrinkles and display coil-like morphology. The size of the islets reaches several micrometers after four bilayers (Figure 2c), while the polyelectrolyte assembly still maintained discontinuous. The merge of the separated blocks took place after five bilayers of assembly (Figure 2d), showing carpets of vermiculate patterns which is in good accordance with our previous reports.20,21 The growth of the polyelectrolyte multilayers shows no directed tendency of patterning.
Interesting patterning features were found when we applied the same LBL assembly process on the honeycomb-patterned PDMS substrate with arrays of protuberances. Two bilayers of assembling decorated the protuberances with nanobumps just like the case of early stage of assembly on the flat substrate (Figure 3a). Closer examination would reveal that continuous depositions of the polyelectrolyte had formed around the protuberances. They wrapped the arrays from the bottom of the substrate, even formed thin bands bridging the neighboring protuberances. After three bilayers of assembly, the void space of the grooves between the protuberance arrays were completely occupied by the deposited assembly of the polyelectrolytes (Figure 3b). Every hemisphereshaped protuberance was surrounded by multilayered polyelectrolyte assemblies, forming a microsized ringlike pattern. The depositions along the grooves were linked continuously with each other throughout the entire substrate, developing into netlike, patterned morphology. While the assemblies covering the upper face of the hemisphere arrays were much less prominent, showing nanoscale wrinklelike morphology. The linear network of polyelectrolyte multilayer becomes even thickened and twisted into wormlike assembly after four bilayers of assembly (Figure 3c), and the wrinklelike assemblies on the topside grow denser and thicker as well. Guided by underlying hexagonally patterned islandlike PDMS arrays, the diffusive polyelectrolytes rapidly interweaved into linear structures distributed along the grooves between the patterned protuberate and formed a regular network of multilayered films with uniform mesh size. Owing to the siteselective exponential LBL deposition, we have constructed hierarchical, patterned topographic features of polyelectrolyte multilayers in both the micro- and nanometer size. After five bilayers of assembly, the depositions in the grooves begin to spread over the whole substrate, forming typical vermiculate sheets covering the protuberance arrays (Figure 3d). Overall, unlike the case of assembly on flat substrate, template-directed LBL assembly process
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resulted in spontaneous patterning of the pH-amplified exponential growth multilayers. We think that the uneven alternative deposition of the polyelectrolytes on this honeycomb-patterned substrate with islandlike, fluctuating arrays may directly lead to the template-directed assembling topography. The LBL assembly process involves steps of water cleaning after every assembly cycle. The water-cleaning step would remove a considerable amount of the polyelectrolyte deposition on the top of the PDMS protuberances considering the poor adsorptive capacity of PDMS due to its low surface energy. On the other hand, the deposition of the polyelectrolytes in the sunken part of the arrays, being the grooves, was harder to be influenced in the water-cleaning step of the LBL assembly. So the distribution of the mass of polyelectrolytes becomes highly uneven between the grooves and the protuberances. The smaller area (the grooves) got more polyelectrolyte deposition than the larger area (the protuberances), becoming “reservoirs” for the polyelectrolytes. So the assembly in the grooves will reach the regime of exponential growth much faster than that on the protuberances. Much thicker assembly in the grooves can be achieved under just a few cycles of assembly. The difference of the assembly rate between different topographical areas eventually generates the substratedirected patterned assembling characteristics. We also studied the wettability properties of the obtained LBLassembled, patterned surfaces with different number of deposition cycles. Figure 3e shows the corresponding contact angles for the structures shown in Figure 3a-d. The topography of the multilayer structure was fixed by thermal cross-linking and treated by the chemical vapor deposition of (tridecafluoroctyl)triethoxysilane before we characterized the contact angles. It should be noted that the treated samples show no topographic difference comparing with the untreated ones. It appears that as the number of bilayers increases, the contact angle grows accordingly. With two and three bilayers of assembly, the contact angles were not significantly increased upon the assembly compared with the patterned PDMS film without LBL assembly (with contact angle of about 130°). While with four bilayers of assembly, the contact angle dramatically increased into 150.4°, which reveals the superhydrophobicity of the obtained structure. It seems that the difference of surface roughness between three- and four-bilayered substrate leads to very different wettability behavior. The cooperation of microsized, wormlike network and nanosized wrinkles can result in optimized topographical parameter for fabricating a superhydrophobic surface. Although the topographical feature between three- and four-bilayered substrate resembles a lot, the relatively small difference in roughness could result in a big variation in contact angles. Nano- to microstructural hierarchy is more mature for four-bilayered substrate than the three-bilayered one. After one more bilayer of assembling, a roughness on both microand nanolength scales was induced, and an increase of contact angles from 134.5° to 150.4° was observed (Figure 3e). It is important to point out that the superhydrophobic surface we obtained was with high adhesion, and the water droplet could be pinned on the surfaces at any tilted angle. High-adhesion superhydrophobic surfaces can be biologically inspired by the surface of gecko feet and rose petals.28-30 Jiang et al. have provided detailed discussion on the biomimetic approaches for fabricating high-adhesion superhydrophobic surfaces,31 among which is to (28) Greiner, C.; Arzt, E.; del Campo, A. Adv. Mater. 2010, 22, 2125. (29) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Adv. Mater. 2005, 17, 1977. (30) Feng, L.; Zhang, Y. A.; Xi, J. M.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Langmuir 2008, 24, 4114. (31) Liu, M. J.; Zheng, Y. M.; Zhai, J.; Jiang, L. Acc. Chem. Res. 2010, 43, 368.
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Figure 4. SEM images of PAA2.85/PEI9.0 films constructed on honeycomb-patterned PDMS substrate in bilayers of 13 with reverse side (a, b) showing honeycomb patterned porous structure and upper side (c) showing vermiform patterned rough topography. (d) is the cross-sectional image of the free-standing film.
construct hierarchical structures to form a partial Wenzel state. The Wenzel state is a pinning solid/liquid contact mode showing high liquid adhesion, just like the case of rose petals.32 Studies revealed that the microstructure of the rose petal was composed of a periodic array of micropapillae with nanoscaled cuticular folds on the top.30 The hierarchical, patterned topographic features of multilayered films on the top of PDMS microarray as shown in Figure 3c highly mimic the structures of the rose petal, except that the grooves between the protuberances are stacked with wormlike, twisted polyelectrolyte multilayers. We assume that the patterned network of multilayered film, which well encloses the protuberance arrays, create well-like, microscaled structure, so the water droplet can realize full penetration. The nanoscaled, wrinklelike morphology on the top of PDMS protuberances holds air gaps between the nanostructures just like the case of rose petals, thus forming a partial Wenzel state. So here by simple LBL assembly on the patterned substrate we create a functional surface with “sticky superhydrophobicity” showing so-called “petal effect”. Such biomimetic high-adhesion superhydrophobic surfaces may find applications in liquid transportation, biochemical separation, etc.33-35 Interestingly, after the PDMS array was completely covered by the multilayers with eight bilayers of assembly, the adhesion between solid and liquid was dramatically reduced. For such typical topography of the pH-amplified exponential growth LBL assembly (Figure 4c) similar to our previous reports, it presents high superhydrophobicity and small sliding angle.20,21 It can be explained that at highest level of hierarchy a transition from the Wenzel to Cassie regime occurred.36-40 Rather different hierarchical structures result in different solid-liquid interface contact area and three-phase contact line, which further influence the water-droplet mobility.41 Hence, one can obtain (32) Lafuma, A.; Quere, D. Nature Mater. 2003, 2, 457. (33) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089. (34) Winkleman, A.; Gotesman, G.; Yoffe, A.; Naaman, R. Nano Lett. 2008, 8, 1241. (35) Zhao, N.; Xie, Q. D.; Kuang, X.; Wang, S. Q.; Li, Y. F.; Lu, X. Y.; Tan, S. X.; Shen, J.; Zhang, X. L.; Zhang, Y.; Xu, J.; Han, C. C. Adv. Funct. Mater. 2007, 17, 2739. (36) Li, C.; Guo, R. W.; Jiang, X.; Hu, S. X.; Li, L.; Cao, X. Y.; Yang, H.; Song, Y. L.; Ma, Y. M.; Jiang, L. Adv. Mater. 2009, 21, 4254. (37) Lai, Y. K.; Gao, X. F.; Zhuang, H. F.; Huang, J. Y.; Lin, C. J.; Jiang, L. Adv. Mater. 2009, 21, 3799. (38) Huang, X. J.; Kim, D. H.; Im, M.; Lee, J. H.; Yoon, J. B.; Choi, Y. K. Small 2009, 5, 90. (39) Samanta, B.; Ofir, Y.; Patra, D.; Rotello, V. M. Soft Matter 2009, 5, 1247. (40) Yang, J.; Zhang, Z. Z.; Men, X. H.; Xu, X. H.; Zhu, X. T. J. Colloid Interface Sci. 2010, 346, 241. (41) Gao, X. F.; Yao, X.; Jiang, L. Langmuir 2007, 23, 4886.
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either sticky or slippery superhydrophobic surfaces based on this template-directed LBL assembly process, depending on how many bilayers of assembly are performed. When we performed the pH-amplified exponential growth LBL assembly more than eight cycles, the polyelectrolyte would be thick enough to be easily peeled off the PDMS substrate by simple thermal treatment. Mechanically stable free-standing polyelectrolyte film can be readily prepared (Figure 4d). Such film fabricated on the top of a patterned substrate has asymmetric topography between two sides of the film. The reverse side (the one in contact of PDMS substrate) of the film faithfully replicated the exact structure of the original BF template by the molding and transfer steps (Figure 4a,b). Honeycomb-patterned, highly porous structure on the polyelectrolyte film has been successfully prepared. The other side of the films exhibit vermiform patterned rough topography which has been proved to be an optimal hierarchical structure with superhydrophobic property (Figure 4c).20,21
Conclusions In conclusion, we directly performed pH-amplified exponential growth LBL assembly on the honeycomb-patterned substrate. The polyelectrolytes showed site-selective exponential deposition, leading to a netlike patterned morphology of the obtained multilayered films after only three bilayers of assembling. Guided by underlying hexagonally patterned islandlike PDMS arrays, the diffusive polyelectrolytes rapidly interweaved into linear structures distributed along the grooves between the patterned protuberate and formed a regular network of multilayered films with uniform mesh size. Netlike “knitting” of the polyelectrolyte multi(42) Ma, Y.; Sun, J. Chem. Mater. 2009, 21, 898. (43) Wang, H. L.; Gao, J.; Sansinena, J. M.; McCarthy, P. Chem. Mater. 2002, 14, 2546.
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layers on honeycomb-patterned substrate has been realized by following this procedure. Superhydrophobic surfaces could be readily obtained after four bilayers of LBL assembling, indicating the successful fabrication of functional micro- and nanoscale hierarchical structures can be achieved. Both high- and low-adhesion superhydrophobic surfaces (“petal effect” and “lotus effect”) can be obtained with different bilayers of assembly, indicating that different levels of nano- to microstructural hierarchy can be realized using this method. The site-selective exponential deposition with different growth rate of the multilayer was responsible for the templated assembly of the patterned LBL assembly. Furthermore, we were able to get topographically asymmetric, free-standing, polyelectrolyte multilayer films in the case that we performed more than eight bilayers of pH-amplified exponential growth LBL assembly. One side of the obtained free-standing polyelectrolyte multilayer films possess honeycomb-patterned microporous structure replicated from PDMS substrate, while the other side show typical vermiculate patterns with rough wrinkles. Such free-standing polyelectrolyte multilayer film with asymmetric patterned structure may find potential application in some areas requiring special properties like smart materials.42,43 This research reported template-directed LBL patterning assembly for the first time. It provides a beneficial exploration for the surface patterning technique for the LBL assembly process. Acknowledgment. Financial support from the Natural Science Foundation of China (NSFC-20774082, 50830106), Programs Foundation of Ministry of Education of China (No. 20070335024), the Fundamental Research Funds for the Central Universities(2009QNA4039), Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM200911), and Natural Science Foundation of China of Zhejiang Province (Y4080250) is gratefully acknowledged.
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