Alignment Control of Self-Assembled Organosiloxane Films Derived

pubs.acs.org/Langmuir © 2009 American Chemical Society

Alignment Control of Self-Assembled Organosiloxane Films Derived from Alkyloligosiloxane Amphiphiles )

Jenny Du,† Madoka Fukushima,‡ Shigeru Sakamoto,‡ Mikako Sakurai,‡ Takashi Suzuki,‡ Atsushi Shimojima,§ Hirokatsu Miyata, Cathleen M. Crudden,† and Kazuyuki Kuroda*,‡,^ Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada, ‡Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan, §Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Frontier Research Center, Canon Inc., 3-30-2 Simomaruko, Ohta-ku, Tokyo 146-8501, Japan, and ^Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan )

†

Received June 3, 2009. Revised Manuscript Received August 11, 2009 Transparent and continuous organosiloxane films with macroscopically oriented mesostructures were prepared by dip-coating a substrate, on which a rubbing-treated polyimide film is formed, with hydrolyzed solutions of oligosiloxane precursors (CnH2nþ1Si(OSi(OMe)3)3). The structure of the films depends on the alkyl chain length of the precursors such that films with two-dimensional (2D) hexagonal and lamellar structures are obtained when n = 10 and 16, respectively. In the 2D hexagonal film, the cylindrical organic moieties are aligned perpendicular to the rubbing direction in the plane of the film over the whole film thickness. On the other hand, the lamellar film changes its orientation with increased distance from the substrate surface. While the orientation of the lamellae at the surface of the film is parallel to the film-air interface, they are perpendicularly aligned in the vicinity of the substrate with the layer normal parallel to the rubbing direction. The observed unique orientation of the mesostructures is attributed to the anisotropic hydrophobic interactions between the alkyl chains of the hydrolyzed oligosiloxane molecules and the polymer chains of the polyimide layer oriented by the rubbing treatment.

Introduction Since the discovery of periodically ordered, surfactant-templated mesostructured silica,1-3 there has been significant interest in exploiting the unique physical and chemical properties of these and related materials in areas such as catalysis, optics, and sensing, to name a few. Many of these applications4-6 require convenient access to well-defined, mesostructured thin films7 and films possessing tailored organic functionality.8-10 Such films can be easily produced by the evaporation-induced self-assembly process11-14 or by heterogeneous deposition under hydrothermal *Corresponding author: e-mail [email protected]; Tel þ81-3-5286-3199; Fax þ81-3-5286-3199.

(1) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Cortial, G.; Siutkowski, M.; Goettmann, F.; Moores, A.; Boissi
ere, C.; Grosso, D.; Le Floch, P.; Sanchez, C. Small 2006, 2, 1042. (5) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (6) Nguyen, T.-Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (7) Etienne, M.; Quach, A.; Grosso, D.; Nicole, L.; Sanchez, C.; Walcarius, A. Chem. Mater. 2007, 19, 844. (8) Nicole, L.; Boissi
ere, C.; Grosso, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 15, 3598. (9) Soler-Illia, G. J. A. A.; Innocenzi, P. Chem.;Eur. J. 2006, 12, 4478. (10) Hoffmann, F.; Cornelius, M.; Morell, J.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (11) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (12) Ogawa, M. Chem. Commun. 1996, 1149. (13) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (14) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Adv. Mater. 1999, 11, 579. (15) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703.

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conditions onto substrates.15 Although the mesostructures in the film are oriented with respect to the substrate surface, the in-plane orientation is, in general, not fully controlled. Additionally, a high loading of organofunctional silanes in the starting solution generally causes a dramatic decrease in the structural order and uniformity. To maximize the utility of these mesostructured materials in advanced applications, control of their in-plane structures over large length scales is essential. To address this issue, we have reported the preparation of continuous silica films with macroscopically oriented 2D and 3D hexagonal mesostructures on substrates bearing anisotropic polymer coatings such as Langmuir-Blodgett films and rubbing-treated polyimide.16-18 Although obtaining mesostructured films using surfactants as structure-directing agents is relatively facile, the process involves complex chemistry and is known to be highly sensitive to external conditions such as temperature and humidity during the film formation,19 in addition to the composition of the precursor solutions. Recently, we reported a new approach using siloxane-organic nanohybrid molecular building blocks for the preparation of films with highly ordered mesostructures. We employed specially designed oligomeric siloxane-based precursors (as exemplified by 1Cn in Scheme 1),20,21 which are comprised (16) Miyata, H. Microporous Mesoporous Mater. 2007, 101, 296, and references therein. (17) Miyata, H.; Kawashima, Y.; Itoh, M.; Watanabe, M. Chem. Mater. 2005, 17, 5323. (18) Suzuki, T.; Miyata, H.; Kuroda, K. J. Mater. Chem. 2008, 18, 1239. (19) Grosso, D.; Cagnol, F.; Soler-Illia, G. J. de A. A.; Crepaldi, E. L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.; Sanchez, C. Adv. Funct. Mater. 2004, 14, 309. (20) Shimojima, A.; Kuroda, K. Angew. Chem., Int. Ed. 2003, 42, 4057. Shimojima, A.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Kuroda, K. J. Am. Chem. Soc. 2005, 127, 14108. (21) Shimojima, A.; Kuroda, K. Chem. Rec. 2006, 6, 53.

Published on Web 09/02/2009

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Scheme 1. Preparation of Macroscopically Oriented, Mesostructured Films by (i) Hydrolysis of the Precursor 1Cn, (ii) Dip-Coating of the Hydrolyzed Solution onto a Rubbing-Treated Substrate, and (iii) Self-Assembly and Polycondensation of Amphiphilic Hydrolyzed Speciesa

a

The white arrow indicates the rubbing direction.

of hydrophobic alkyl chains and alkoxysilyl groups. Hydrolysis of such precursors produces amphiphlic molecules having both selfassembling and cross-linking abilities. This design eliminates the need to employ separate templating agents and polymerizable matrix precursors, in contrast to the conventional synthesis, which assures the reproducible formation of a uniform structure at the molecular scale. In the hybrid materials of our previous studies, the molecularscale positions of the organic moiety with respect to the inorganic one, as well as the organic/inorganic ratio, were precisely controlled by carefully designing the starting molecules. This methodology holds promise for the preparation of advanced and highly functional nanoscale hybrid materials by introducing appropriate functional groups in the starting molecules. For example, ester groups incorporated into the organic moiety undergo postsynthetic cleavage under acidic conditions to generate a hybrid mesoporous material with well-defined -COOH group distribution within the pores.22 To effectively exploit these improved material properties achieved through the thoughtful design of molecules and their self-assembly processes, control of the structure of the hybrid materials at macroscopic scales is of paramount importance.23,24 Herein, we report the preparation of continuous organosiloxane films with hierarchically controlled structures using molecular precursors 1Cn as building units. These films are formed on substrates bearing a rubbing-treated polyimide layer by a dipcoating method (Scheme 1). The in-plane orientation of the mesostructures in these films is controlled as a result of the initial interfacial interactions between the hydrolyzed 1Cn and the anisotropic polymeric film. When the alkyl chain is relatively short (n = 10), a 2D hexagonal structure is formed with strictly controlled in-plane alignment of the cylinders perpendicular to the rubbing direction (Scheme 1). When the length of the alkyl chain is increased to n = 16, a lamellar structure with alternating organic and siloxane layers is formed.20 In this case, the average direction of the layer normal is parallel to the rubbing direction near the substrate-film interface. However, the normal of the lamellae tends to be parallel to the surface normal in the proximity of the film-air interface (Scheme 1). The present results show that the macroscopic alignment direction of various mesostructures can be controlled on an anisotropic polymer surface. Combining appropriately designed building blocks with an anisotropic substrate should permit the creation of hybrid materials with specific anisotropic functionalities resulting from control over the film structure from the molecular to the macroscopic scale. (22) Goto, R.; Shimojima, A.; Kuge, H.; Kuroda, K. Chem. Commun. 2009, 6152. (23) Fukuoka, A.; Miyata, H.; Kuroda, K. Chem. Commun. 2003, 284. (24) Molenkamp, W. C.; Watanabe, M.; Miyata, H.; Tolbert, S. H. J. Am. Chem. Soc. 2004, 126, 4476.

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Experimental Section Substrate Preparation. The rubbing-treated polyimide coated substrates were prepared according to a previously published procedure.16 The polyimide (poly(hexamethylenepyromellitimide)) film is prepared by spin-coating the polyamic acid onto clean silicon or quartz substrates and then heated at 200 C for 1 h in air to allow for the formation of the polyimide layer (∼10 nm in thickness) via thermal imidization. The polyimidecoated substrates are then subjected to a rubbing treatment by fixing the substrates to a height-controllable stage and contacting the polyimide layer with a nylon-covered cylindrical roller of 24 mm radius rotating at a rate of 1000 rpm. Precursor Synthesis. The precursors tris(trimethoxysilyloxy)(decyl)silane and tris(trimethoxysilyloxy)(hexadecyl)silane were prepared according to previously published procedures20 under a nitrogen atmosphere using standard Schlenk techniques. In a typical procedure for the synthesis of 1C10, 12.5 g of decyltrichlorosilane in diethyl ether (120 mL) was slowly added to a mixture of anhydrous THF (180 mL), diethyl ether (180 mL), deionized water (2.7 mL), and aniline (13.7 mL) under vigorous stirring for 2 h at 0 C. Aniline hydrochloride precipitate was removed from the reaction mixture by vacuum filtration to give a transparent solution. Solvent exchange with hexane was conducted under vacuum and repeated three times, affording the crystallization of decylsilanetriol, which was collected by vacuum filtration and dried in vacuo. Decylsilanetriol was then dissolved in anhydrous THF (300 mL) and added to a solution of tetrachlorosilane (50 mL) and hexane (60 mL). The solvent volume was reduced under vacuum, followed by the addition of anhydrous MeOH, hexane, and anhydrous pyridine for the methanolysis of C10H21Si(OSiCl3)3. Excess solvent and reagents were removed under vacuum to yield the crude product. Pure 1C10 was isolated as a clear, colorless liquid by vacuum distillation. Similar conditions were used for the synthesis of the 1C16 precursor. Film Synthesis. The coating solution was prepared by hydrolysis of 1Cn in a starting sol with the reagents respectively present in the following molar ratios: 1Cn:THF:H2O:HCl = 1.0:50:18:0.002. The sol was allowed to stir at 25 C for 12 h before adding additional H2O in a ratio of H2O/1Cn=32. Rubbing-treated polyimide-coated substrates were coated with the solution by dip-coating, with the withdrawal direction set perpendicular to the rubbing direction, at a rate of 2 mm s-1 in air at a relative humidity of ∼50%. The films were then dried under ambient conditions for 2 days. Characterization. The surface texture of the films was observed by an optical microscope (Olympus BX50). The X-ray diffraction (XRD) in Bragg-Brentano geometry was performed with a MAC Science M03XHP22 diffractometer using Mnfiltered Fe KR radiation at 40 kV and 20 mA. In-plane XRD patterns were recorded with a RIGAKU ATX-G diffractometer with a 4-axis goniometer using Cu KR radiation at 50 kV and 300 mA. A soller slit with a vertical divergence of 0.48 was used to obtain a parallel beam. The incident angle of the X-ray in the inplane geometry was 0.2. Samples were prepared for crosssectional TEM using focused ion beam (FIB) (SEIKO EG&G DOI: 10.1021/la901983m

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Figure 1. XRD patterns (measured in Bragg-Brentano geometry) of (a) 1C10-derived film and (b) 1C16-derived film measured with the rubbing direction parallel to the projection of the incident beam.

Figure 3. (A) In-plane j-2θχ scanning profiles for the 1C10-

Figure 2. Optical micrographs of (a) 1C10-derived film and (b)

derived film with the rubbing direction oriented (a) parallel and (b) perpendicular to the projection of the incident X-ray beam at j = 0. (B) The corresponding j-scanning profile at 2θχ = 4.76. Inset (A): scanning axes of the in-plane XRD geometry.

1C16-derived film.

SMI2050), and cross-sectional TEM images were recorded with a JEOL JEM-2010 microscope operated at 200 kV.

Results and Discussion Figure 1a shows the conventional XRD profile for the 1C10derived film measured in Bragg-Brentano geometry with the rubbing direction of the substrate oriented parallel to the projection of the incident X-ray beam. The XRD pattern shows three peaks assigned to the (10), (20), and (30) lattice planes (d=2.81, 1.41, and 0.93 nm, respectively), which closely correspond to the XRD pattern previously observed for the 2D hexagonally ordered film prepared from 1C10.20 The optical micrograph image of this film is shown in Figure 2a. The observed texture, aligned perpendicularly to the rubbing direction, is similar to that previously reported for uniaxially aligned surfactant-silica composite films prepared on a rubbing-treated polyimide-coated substrate.16,17 These results strongly suggest alignment of the 2D hexagonal structure in the direction perpendicular to the rubbing direction. It should be noted, however, that the intensity of the out-ofplane XRD peaks and the quality of the alignment texture obtained for these films show some dependence on the dip-coating rate. From Figure S1, the XRD peak intensities improve as the dip-coating rate goes from 0.5 to 2.0 mm s-1. Correspondingly, the improvement in the alignment texture observed by optical microscope followed the same trend (Figure S2). It appears that a fine balance between the rate of solvent evaporation and structure formation exists, which controls the quality of the final mesophase obtained. The alignment of the cylindrical assemblies is estimated using grazing incidence in-plane XRD.16 The in-plane radial (j-2θχ) scanning profile clearly shows the anisotropic nature of the film. 13616 DOI: 10.1021/la901983m

No peak is observed in the profile when the rubbing direction is set parallel to the projection of the incident beam (Figure 3A, trace a), whereas two diffraction peaks assigned to the (11) and (21) lattice planes are observed at 2θχ = 2.50 and 4.76, respectively, when the rubbing direction is set perpendicular to the incident X-rays (Figure 3A, trace b). The observed anisotropy in the in-plane XRD patterns provides convincing evidence for the presence of strong anisotropy in the mesostructure over the whole volume of the film.16,17 The distribution of the in-plane alignment of the cylindrical assemblies can be quantitatively estimated by measuring the in-plane j-scanning profile (in-plane rocking curve) with the detector fixed at the (21) peak position. The profile in Figure 3B shows two strong and sharp peaks separated by an interval of 180, which supports the formation of a 2D hexagonal mesostructure uniaxially aligned perpendicular to the rubbing direction. The uniaxial alignment of the cylindrical assemblies is also demonstrated by cross-sectional TEM. Figure 4 shows the crosssectional view of the 1C10-derived film when the film is cut parallel to the rubbing direction. The film is about 400 nm in thickness and complete alignment of the structure is achieved over the whole thickness of the film from the substrate-film interface (lower right) to the film-air interface (upper left). The alignment of the long axis of the cylinders is also observed when the film is cut perpendicular to the rubbing direction (Figure S3 in the Supporting Information). We have previously shown that oligosiloxane precursors with relatively long alkyl chains (C14 to C18) yielded materials with lamellar structure where alkyl chains are interdigitated.20 This can be explained using simple geometric arguments. A longer alkyl chain (when paired with the same oligosiloxane headgroup used for 1C10) results in a decreased interfacial curvature of the Langmuir 2009, 25(23), 13614–13618

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Figure 4. Cross-sectional TEM image of the 1C10 film sliced parallel to the rubbing direction. The polyimide layer is about 10 nm thick. The inset shows the enlarged image (2) of the central region of the film.

micelles, favoring the formation of a lamellar structure. Here, we prepared films using the 1C16 oligomer on the same rubbingtreated polyimide-coated substrate to compare the resultant structure to that obtained with the 1C10 oligomer. The XRD profile measured under the Bragg-Brentano geometry (Figure 1b) is consistent with the formation of a conventional lamellar structure where the lamellae lie parallel with respect to the substrate surface. However, the optical micrograph image of this film shows some alignment texture (Figure 2b). Further investigation by in-plane XRD revealed the presence of lamellae that are not parallel to the substrate surface (Figure 5A, trace b). The in-plane radial (j-2θχ) scanning profile (Figure 5A) shows definite in-plane structural anisotropy of the film. The lattice spacing estimated from the Bragg-Brentano geometry (d01 = 3.35 nm, Figure 1b) is slightly different from that estimated from the in-plane geometry (d01=3.53 nm, Figure 5A, trace b). This inconsistency may be attributed to the shrinkage of the film in the direction normal to the substrate surface during the drying process. Such anisotropic shrinkage only affects the lattice spacing measured under the Bragg-Brentano geometry and has been observed for many mesostructured films.17 The in-plane j-scanning profile of this 1C16 film (Figure 5B) shows two sharp peaks separated by an interval of ∼180, indicating that the lamellae that contribute to the in-plane diffraction are aligned perpendicularly with respect to the rubbing direction in the plane of the film (see Scheme 1, right, for a possible structural model). Figure 6 shows the TEM image of the film (cross-sectioned along the rubbing direction) near the substrate-film interface. The perpendicular alignment of lamellae extending from the substrate surface (bottom right) is evident. However, near the film-air interface, complete alignment of the lamellae parallel to the interface is observed (Figure S4 in the Supporting Information). Thus, the in-plane alignment of the lamellae observed near the substrate surface is not maintained through the entire thickness of the film. A transition region is observed where the orientation of the lamellae changes between the two different orientations so as to minimize the formation of structural defects, Langmuir 2009, 25(23), 13614–13618

Figure 5. (A) In-plane j-2θχ scanning profiles for the 1C16derived film with the rubbing direction oriented (a) parallel and (b) perpendicular to the incident X-ray beam at j = 0. (B) The corresponding j-scanning profile.

Figure 6. Cross-sectional TEM image of the 1C16 film sliced parallel to the rubbing direction with the substrate-film interface located at the bottom right corner of the image.

which is evidenced by the curved stripes in Figure 6 between the two interfaces. The overall lamellar structure of the film is influenced by the two interfaces (substrate-film and film-air), each of which gives a different orientation of the lamellae. The lamellae should lie parallel to the film-air interface as shown in Figure 7b, top image. In contrast, the lamellae can be formed perpendicularly to the anisotropic rubbing-treated polyimide surface because the DOI: 10.1021/la901983m

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interfaces, alignmment control by the rubbing-treated polyimide layer through anisotropic hydrophobic interactions between the rubbed polyimide and 1C10 oligomer molecules is facilitated. As described above, the formation of a uniformly anisotropic film depends on the two boundary conditions at the air-film and the film-substrate interfaces. If the conditions of the two interfaces are consistent, the anisotropic structure can be achieved over the whole film. To achieve total uniformity, the mesostructure should be optimized through careful design of the shape of the building units.

Conclusions

Figure 7. Illustration of the arrangements of hydrolyzed 1Cn molecules in the (a) 1C10 and (b) 1C16 systems near the film-air interface (top) and substrate-film interface (bottom) viewed from the direction normal to the rubbing direction.

hydrolyzed 1C16 molecules adsorb parallel to the surface to maximize their hydrophobic interactions with the elongated alkyl moieties of the polyimide (Figure 7b, bottom image). Note that such perpendicular orientation of the lamellae near the substrate surface was not achieved when the film was coated on a polyimide film without a rubbing treatment. Although the XRD profile measured under the Bragg-Brentano geometry is quite similar to that for the film prepared on a rubbing-treated substrate, no obvious peak is observed in the in-plane radial (j-2θχ) scanning profile (Supporting Information, Figure S5), suggesting that the lamellae are oriented parallel to the substrate over the whole thickness. In contrast to the partial alignment of the lamellar film, the 2D-hexagonal film prepared from 1C10 has a highly aligned cylindrical mesostructure over the whole film because the parallel orientation of the 2D hexagonal structure satisfies both of the boundary conditions for the two interfaces. At the substrate surface, hydrolyzed 1C10 molecules adsorb parallel to the elongated polyimide chains by hydrophobic interactions to form aligned hemicylindrical surface micelles,25 which causes total uniaxial alignment (Figure 7a). It is plausible that as a result of the unnecessary change in the alignment between the two (25) Miyata, H.; Kuroda, K. Chem. Mater. 2000, 12, 49.

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Amphiphilic, oligomeric alkylsiloxane precursors (1Cn) easily form mesostructured films with uniaxially aligned 2D hexagonal mesostructure and partially aligned lamellar structure by dipcoating a prehydrolyzed sol of 1Cn onto a substrate with a rubbing-treated polyimide layer. In these films, the position of the organic moiety can be precisely defined on the substrate; therefore, the introduction of more complex functional groups in the organic moiety is of great interest. The ease and high reproducibility of this method are promising for the preparation of inorganic-organic hybrid films with improved functionality as a direct result of the control over the structural order on both microscopic and macroscopic scales. An improvement in the chemical stability of these hybrid materials in comparison to conventional silica-surfactant composites could also be anticipated due to the covalent nature of the interface between the siloxane and organic moieties, which could impact their usefulness in advanced applications. Acknowledgment. J.D. is thankful for financial support from the Global COE Program (MEXT, JAPAN), Queen’s University, and the Natural Sciences and Engineering Research Council of Canada (NSERC) in terms of a Canada Graduate Scholarship (CGS-D). K.K. acknowledges the support by a Grant-in-Aid for Scientific Research (A). The A3 Foresight Program “Synthesis and Structural Resolution of Novel Mesoporous Materials” supported by the Japan Society for Promotion of Science (JSPS) is also acknowledged. Supporting Information Available: Figures S1-S5. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2009, 25(23), 13614–13618