Structural Control of Multilayered Inorganic−Organic Hybrids Derived

Technology, Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo 169-0051, Japan. Received July 5, 2001. In Final Form: November 16, 2001. Multilayere...
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Structural Control of Multilayered Inorganic-Organic Hybrids Derived from Mixtures of Alkyltriethoxysilane and Tetraethoxysilane Atsushi Shimojima† and Kazuyuki Kuroda*,†,‡ Department of Applied Chemistry, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169-8555, Japan, and Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo 169-0051, Japan Received July 5, 2001. In Final Form: November 16, 2001 Multilayered inorganic-organic hybrids were prepared by cohydrolysis and polycondensation of alkyltriethoxysilanes (CnTES, n ) 8-18) and tetraethoxysilane (TEOS). Thin films were deposited from homogeneous solutions containing the oligomeric species by dip-coating. Self-assembly of co-condensed oligomers into lamellar structures depended essentially on the alkyl chain length of CnTES and the temperature of the solutions. By controlling the solution temperature during the dip-coating process, transparent and highly ordered films were formed from the mixtures of TEOS and CnTES with various alkyl chain lengths. The resulting hybrid materials were structurally different from those derived from CnTES alone and exhibited various interlayer structures depending on the alkyl chain length, which is due to the differences in the arrangements and conformations of the chains.

Introduction Inorganic-organic nanocomposite materials with ordered nanostructures have recently received increasing attention.1 Organoalkoxysilanes have been widely used as structural units to construct a variety of silica-based hybrid materials.2-4 Although simple sol-gel reactions usually result in the formation of amorphous materials,5 self-organization of organosilane molecules offers an opportunity to create ordered hybrid materials. Such an attempt is of great interest from the possibility to afford novel hybrid materials that cannot be obtained by utilizing inorganic host materials6,7 or organic assemblies7-11 as structure directors. Much attention has been focused on the formation of two-dimensional alkylsiloxane polymers, called self-assembled monolayers (SAMs).12 The formation is strongly related to the amphiphilic nature of hydrolyzed organosilane molecules containing both hydrophilic silanol * To whom correspondence should be addressed. Phone & Fax: +81-3-5286-3199. E-mail: [email protected]. † Department of Applied Chemistry, Waseda University. ‡ Kagami Memorial Laboratory for Materials Science and Technology, Waseda University. (1) Judeinstein, P.; Sanchez, C. J. Mater. Chem. 1996, 6, 511-525. (2) Schubert, U.; Hu¨sing, N.; Lorenz, A. Chem. Mater. 1995, 7, 20102027. (3) Sanchez, C.; Ribot, F.; Lebeau, B. J. Mater. Chem. 1999, 9, 3544. (4) Loy, D. A.; Baugher, B. M.; Baugher, C. R.; Schneider, D. A.; Rahimian, K. Chem. Mater. 2000, 12, 3624-3632. (5) Brinker, C. J.; Sherer, G. W. Sol-Gel Science; Academic Press: San Diego, CA, 1990. (6) Isoda, K.; Kuroda, K.; Ogawa, M. Chem. Mater. 2000, 12, 17021707. (7) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403-1419. (8) Sakata, K.; Kunitake, T. J. Chem. Soc., Chem. Commun. 1990, 504-505. (9) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256-260. (10) (a) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611-9614. (b) Guan, S.; Inagaki, S.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 5660-5661. (11) Lu, Y.; Fan, H.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258-5261. (12) Ulman, A. Chem. Rev. 1996, 96, 1533-1554.

groups and hydrophobic alkyl chains. Recent research has shown that the process is not limited to the reactions at interfaces. Controlled reactions in the solution states lead to a spontaneous organization into multibilayer aggregates.13-17 These are a new class of materials consisting of organic two-dimensional arrays and inorganic networks linked by covalent Si-C bonds. However, the products are normally obtained as powders. Therefore, morphological control of these materials is of practical importance. We have recently demonstrated the formation of layered hybrid films by the sol-gel reaction of alkyltrialkoxysilane with the chain lengths of n ) 6-12 in the presence of tetraalkoxysilane.18 This approach relies on the networkforming ability of tetraalkoxysilane to form transparent and continuous films. Several processing parameters affect the structures of the resulting hybrid films. Our preliminary report has shown that the amount of water in the system plays an essential role not only in the hydrolysis reaction but also in the self-assembly to form well-ordered materials.18 More recently, we reported that the degree of polycondensation of both alkoxysilanes in solutions governs the nanostructure and the macroscopic homogeneity of the resulting films.19 Despite the optimization of these conditions, well-ordered films were not obtained from the mixtures of tetraalkoxysilane and alkyltrialkoxysilanes with the alkyl chains of n g 14.18 These behaviors may arise from the difference in the amphiphilic properties of hydrolyzed alkyltrialkoxysilane molecules, depending on the alkyl chain length. (13) Fukushima, Y.; Tani, M. J. Chem. Soc., Chem. Commun. 1995, 241-242. (14) Ukrainczk, L.; Bellman, R. A.; Anderson, A. B. J. Phys. Chem. B 1997, 101, 531-539. (15) Whilton, N. T.; Burkett, S. L.; Mann, S. J. Mater. Chem. 1998, 8, 1927-1932. (16) Shimojima, A.; Sugahara, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2847-2853. (17) (a) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135-3143. (b) Wang, R.; Baran, G.; Wunder, S. L. Langmuir 2000, 16, 6298-6305. (18) Shimojima, A.; Sugahara, Y.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 4528-4529. (19) Shimojima, A.; Umeda, N.; Kuroda, K. Chem. Mater. 2001, 13, 3610-3616.

10.1021/la011016l CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002

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In this paper, we report the formation of inorganicorganic hybrid films with precisely controlled nanostructures and macroscopic morphologies by the self-assembly in alkyltrialkoxysilane-tetraalkoxysilane systems. Under well-regulated conditions, in particular at controlled reaction temperatures, we succeeded in preparing transparent and highly ordered multilayered thin films using the mixtures of tetraethoxysilane and alkyltriethoxysilanes with various alkyl chain lengths (n ) 8-18). This approach will extend the possibility to prepare highly organized architectures by a simple sol-gel route using various organoalkoxysilanes. Experimental Section Materials. n-Alkyltriethoxysilanes with various chain lengths (CnH2n+1Si(OC2H5)3, CnTES, n ) 8, 10, 12, 14, 16, and 18) were prepared as follows: decyl-, dodecyl-, tetradecyl-, and hexadecyltriethoxysilane were synthesized by the ethanolysis of decyltrichlorosilane (Chisso Co.), dodecyltrichlorosilane (Tokyo Kasei Co.), tetradecyltrichlorosilane (Tokyo Kasei Co.), and hexadecyltrichlorosilane (Chisso Co.), respectively, and their purities were checked by 1H, 13C, and 29Si NMR. Octyltriethoxysilane (Tokyo Kasei Co.) and octadecyltriethoxysilane (Chisso Co.) were vacuum-distilled prior to use. Tetraethoxysilane (Si(OC2H5)4, TEOS) was purchased from Tokyo Kasei Co. and used without further purification. Synthesis. Cohydrolysis and polycondensation were performed by mixing CnTES and TEOS in ethanol, followed by the addition of an aqueous HCl solution. The molar ratio of the reaction mixtures was CnTES/TEOS/EtOH/H2O/HCl ) 1:4:50: 19:0.03, where the amount of water corresponds to the stoichiometric condition for the total hydrolysis of ethoxy groups. The reaction temperature was controlled at 20 or 40 °C. The reactions at 20 °C led to the precipitation when the alkyl chain length (n) in CnTES was n g 16, though homogeneous solutions were formed when the length was shorter than 14. In contrast, homogeneous solutions were obtained by the reactions at 40 °C for 90 min for all the CnTES-TEOS systems (n ) 8-18), although white gels were formed when the solution temperature decreased after the reaction (below ∼15, ∼22, and ∼33 °C for n ) 14, 16, and 18, respectively). The solid products derived from the C18TES-TEOS system were centrifuged and air-dried to form white powders. Thin films were deposited on glass substrates by dip-coating at a withdrawal rate of 60 mm min-1, using the homogeneous solutions prepared by the reaction at 40 °C for 90 min. During the dip-coating, the precursor solutions were kept at various temperatures ranging from 10 to 65 °C. In the cases of n ) 14, 16, and 18, the deposition at higher temperature regions was required (above 20, 30, and 40 °C, respectively) to avoid the formation of aggregates in the solutions. As-deposited films were air-dried at room temperature for 1 day so that polycondensation proceeded further, accompanied by the evaporation of volatile components. To ensure the reproducibility of the film formation, we carefully performed every dipping procedure at the surrounding temperature of 22 ( 2 °C and under a relative humidity of 30 ( 5%. Characterization. Liquid-state 1H and 29Si NMR spectra of the reaction mixtures were recorded on a JEOL Lambda-500 spectrometer with resonance frequencies of 499.10 and 99.05 MHz, respectively. Sample solutions were prepared in 5 mm glass tubes, where tetramethylsilane (TMS) was added for the internal reference and C2H5OD was used for obtaining lock signals. X-ray diffraction (XRD) patterns of the products were taken on a Mac Science M03XHF22 diffractometer using Mnfiltered Fe Kr radiation. Transmission electron microscopy (TEM) images were taken with a JEOL JEM-100CX microscope at an accelerating voltage of 100 kV. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4500S field emission microscope at an accelerating voltage of 15 kV. The samples for SEM were prepared by sputter-coating of Pt-Pd onto the cross section of cracked films. Solid-state 29Si magic angle spinning (MAS) NMR spectra were recorded on a JEOL JNM-CMX-400 spectrometer at a resonance frequency of 79.42 MHz with a 45° pulse and a recycle delay of 100 s. Solid-state 13C CP/MAS NMR

Figure 1. XRD patterns of the products formed in the C18TES-TEOS system by the reaction (a) at 20 °C for 30 min and (b) at 40 °C for 90 min followed by cooling to 20 °C. spectra were recorded on a JEOL GSX-400 spectrometer at a frequency of 100.40 MHz with a recycle delay of 5 s. Chemical shifts for both 13C and 29Si were expressed with respect to tetramethylsilane. Fourier transform infrared (FT-IR) spectra were obtained using a Perkin-Elmer Spectrum One spectrometer with a nominal resolution of 0.5 cm-1. The films were scraped off from the substrates and powdered for the solid-state NMR and IR measurements.

Results and Discussion Incorporation of TEOS-Derived Units in the SelfAssembly Process of Hydrolyzed CnTES. In the present study, both alkoxysilanes were fully hydrolyzed at early stages of the reaction, independent of the chain length in CnTES. Even when the reaction was performed at 20 °C, the 1H NMR spectra of the reaction mixtures revealed that the signals due to ethoxy groups (SiOCH2CH3, at around 3.8 ppm) almost disappeared within the first period of 15 min. The solutions were homogeneous at 20 °C for a long period when CnTES (n e 14) was used, whereas precipitation occurred in the case of CnTES with longer chains (n ) 16 and 18). This behavior is very similar to that observed in our previous study on the hydrolysis and polycondensation of CnTES alone in EtOH-H2O-HCl mixtures,16 where spontaneous organization of hydrolyzed silane molecules occurred for n g 16. Figure 1a shows the XRD pattern of the precipitates formed during the first 30 min of the reaction at 20 °C in the C18TES-TEOS system. The product exhibits a typical layered structure with a basal spacing of 5.30 nm, in close agreement with that of a layered material obtained without the addition of TEOS.16 As shown in Figure 2a, the 29Si MAS NMR spectrum of the precipitates displays the signals at the T1 (-50 ppm), T2 (-60 ppm), and T3 (-70 ppm) regions, and no signals are detected at the Q region (-80 to -120 ppm). These results clearly suggest that the product is predominantly composed of C18TES-derived units. The selfassembling ability of hydrolyzed CnTES molecules should become higher as the alkyl chain becomes longer, due to the strong hydrophobic interactions between alkyl chains.16 Even in the presence of TEOS, hydrolysis of CnTES containing longer alkyl chains (n ) 16 and 18) results in the rapid segregation of CnTES alone before co-condensation with TEOS. On the other hand, cohydrolysis and polycondensation in a homogeneous solution were observed for all the

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Figure 2. 29Si MAS NMR spectra of the products formed in the C18TES-TEOS system by the reaction (a) at 20 °C for 30 min and (b) at 40 °C for 90 min followed by cooling to 20 °C.

systems (n ) 8-18) at 40 °C. The 29Si NMR spectra of the solution in the C18TES-TEOS system revealed that the signals assigned to the monomeric species of both alkoxysilanes disappeared after 90 min of the reaction. Although the signals due to oligomeric species were not clearly resolved, the co-condensation between CnTES and TEOS should occur because sufficient evidence has already been reported in similar systems.20 The solutions of the CnTES-TEOS systems reacted at 40 °C for 90 min afforded gel products in the cases of n g 14 when the solution temperature decreased after the reaction. As described above, this behavior depends essentially on the chain length and the solution temperature, which can be related to the gel-to-liquid crystalline transition observed for general amphiphiles. Figure 1b shows the XRD pattern of the solid product formed in the C18TES-TEOS system on cooling the reaction mixture to 20 °C after the reaction at 40 °C. A peak with the d spacing of 3.41 nm and the peak due to the second-order diffraction are observed. The layered structure of the product was confirmed by the increase in the d value upon adsorption of decyl alcohol. The 29Si MAS NMR spectrum of the product (Figure 2b) shows the signals at -92, -102, and -111 ppm, corresponding to the Q2, Q3, and Q4 units, respectively, in addition to the signals at the T region (-50 to -70 ppm). The total intensity ratio (Q/T) of those signals is approximately 1.4. The Q signals can be attributed to the TEOS-derived units co-condensed with the T units during the reaction, because solid products never formed when only TEOS was reacted under the present conditions. Thus, a layered hybrid, structurally different from that prepared from C18TES alone, was formed by the coassembly in the C18TES-TEOS system. These results suggest that it is necessary to suppress the segregation of hydrolyzed CnTES molecules and to form co-condensed species for the formation of layered products containing Q units in the siloxane layers. This was attained by the reactions at 40 °C, probably due to the increased solubility of hydrolyzed species. The precise role of the co-condensation in the self-assembly was discussed elsewhere.19 In addition to the structural difference in the resulting hybrids, the incorporation of the Q units also contributed to the thermal stability. The (20) (a) Delattre, L.; Babonneau, F. Mater. Res. Soc. Symp. Proc. 1994, 346, 365-370. (b) Rodrı´guez, S. A.; Colo´n, L. A. Chem. Mater. 1999, 11, 754-762.

Figure 3. XRD patterns of the hybrid films prepared from the C10TES-TEOS system at various solution temperatures: (a) 10 °C, (b) 15 °C, (c) 20 °C, (d) 25 °C, (e) 30 °C, and (f) 35 °C.

layered hybrid derived from C18TES alone melted into an amorphous state upon heating above ∼110 °C.16 In contrast, the layered product derived from the C18TESTEOS system retained the structure even at 170 °C, suggesting that the product was composed of more stable siloxane networks. Formation of Multilayered Films. Morphological control into thin films was performed by dip-coating the reaction mixtures obtained by the reaction at 40 °C for 90 min. The starting composition of the silicate species (molar ratio of TEOS/CnTES) is expected to be maintained in the resulting films. As reported previously, rapid evaporation of the solvent during the coating procedure plays a crucial role in the self-assembly of alkylalkoxysilane-derived constituent.18 This process was utilized for the system containing CnTES with various alkyl chain lengths (n ) 8-18) to form ordered hybrid films. However, the structural ordering and the macroscopic homogeneity of the films were strongly affected by the solution temperature during the deposition. The XRD patterns of the hybrid films derived from the C10TES-TEOS system deposited at various solution temperatures are shown in Figure 3. Transparent films exhibiting a sharp diffraction peak (d ) 3.48 nm) were formed at around room temperature (20-25 °C). The layered structure of the film was confirmed by TEM (not shown). These results are consistent with our preliminary report on the layered hybrid films derived from decyltrimethoxysilane-tetramethoxysilane systems.19 Although the films prepared at lower temperatures (10 and 15 °C) also exhibited sharp diffraction peaks, they were not homogeneous and the d values were rather variable. In contrast, the deposition from the solutions at higher temperatures up to 30 °C results in the formation of homogeneous films with a disordered nanostructure. Similar trends were observed for the systems with n ) 8 and 12 (see Figures S1 and S2 in Supporting Information). In the case of n g 14, neither transparent nor wellordered films were deposited from the solution at around room temperature. The XRD patterns of the hybrid films derived from the C14TES-TEOS system are shown in

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Figure 4. XRD patterns of the hybrid films prepared from the C14TES-TEOS system at various solution temperatures: (a) 20 °C, (b) 25 °C, (c) 30 °C, (d) 35 °C, (e) 40 °C, and (f) 45 °C.

Figure 6. Optical micrographs of the hybrid films prepared from the C18TES-TEOS system at the solution temperatures of (a) 40 °C and (b) 55 °C; scale bar ) 20 µm.

Figure 5. XRD patterns of the hybrid films prepared from the C18TES-TEOS system at various solution temperatures: (a) 40 °C, (b) 45 °C, (c) 50 °C, (d) 55 °C, (e) 60 °C, and (f) 65 °C.

Figure 4. When the solution temperatures were below 25 °C, the deposited films were not homogeneous and displayed very broad peaks in the XRD patterns. However, the intensity of the peaks progressively increased as the solution temperature increased, and transparent and highly ordered films were obtained at 30-35 °C. The effect of the solution temperature was more clearly observed for the systems with n ) 16 and 18. The variations in the XRD patterns of the films in the C16- and C18TES-TEOS systems are shown in Figure S3 (see Supporting Information) and Figure 5, respectively. Well-ordered films with sharp and intense peaks were formed at around 40-45 and 55-60 °C, respectively. The improved optical quality

at higher temperature is also evident, as shown by the optical micrographs of the films (n ) 18) deposited at 40 and 55 °C (Figure 6). Figure 7 shows the SEM images for the cross sections of the well-ordered films derived from the C10- and C18TES-TEOS systems (prepared at 25 and 55 °C, respectively). For both of the systems, platelike morphologies oriented parallel to the substrate surface are observed throughout the sample (∼300 nm). Such a specific morphology was observed for neither the conventional sol-gel-derived hybrid coatings nor the disordered films in the present system, reflecting the multilayered nature of these films. Temperature Dependence of the Self-Assembly. The variations of the nanostructure and the macroscopic morphology of the resulting hybrids depending on the solution temperature after the reaction at 40 °C for 90 min are summarized in Figure 8. Transparent and ordered films are formed at higher temperatures with increasing chain length. This result appears to be associated with the fact described above that the self-assembly of alkylsiloxane oligomers depends essentially on the solution temperature. During the withdrawing process, the temperature of the solutions should not be lowered so rapidly in as-formed liquid films on the substrates in a short period. A similar temperature dependence was also reported on the formation of self-assembled monolayers from alkyltrichlorosilanes.21 It has been demonstrated that the structural order of the monolayer is strongly affected by the deposition temperature. When a substrate is withdrawn from the reaction mixtures, silicate species in the solutions are concentrated (21) (a) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367-4373. (b) Parikh, A. N.; Allara, D. L. J. Phys. Chem. 1994, 98, 7577-7590. (c) Rye, R. R. Langmuir 1997, 13, 2588-2590.

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Figure 9. Variation in the d values as a function of the number of carbon atoms in the alkyl chain: the layered hybrid films derived from the CnTES-TEOS systems (open circles) and the layered hybrids derived from CnTES alone (filled circles, taken from ref 16).

Figure 7. Cross-sectional SEM images of well-ordered hybrid films derived from the (a) C10TES-TEOS and (b) C18TESTEOS systems (deposited from the solutions at 25 and 55 °C, respectively).

Figure 8. Variation of the products derived from the CnTESTEOS systems (n ) 8-18) depending on the solution temperatures.

by the evaporation of the solvent and finally polycondensed to form siloxane networks. The formation of oriented multilayered films requires uniform and continuous organizations, presumably from solid-liquid and liquidvapor interfaces, during this coating process. In the case of CnTES with longer chains (n g 14), the films deposited at lower temperature exhibited a less-ordered structure.

This behavior is probably attributed to the random nucleation of the layered aggregates caused by a slight decrease in the temperature as well as the evaporation of the solvent during the film formation. The suppression of the segregation of the layered aggregate appears to play an important role in the formation of oriented films. In all the systems, further increase in the deposition temperature caused the structural disordering and eventually led to amorphous films. This is explained by the general behavior of amphiphilic assemblies that become isotropic states at higher temperature. The films may be solidified by the siloxane formation prior to the self-organization. Variation of the Interlayer Structure with the Chain Length. Figure 9 represents the basal spacings of the well-ordered films as a function of the alkyl chain length. For comparison, the d values for the layered hybrids derived from CnTES (n ) 12-18) alone are also shown. As we reported previously, the products derived from CnTES alone are composed of a bilayer structure with all-trans chains almost perpendicular to the siloxane layers.16 In contrast to the linear relationship in these products, the d spacing of the films increases continuously in the range of n ) 8-14 but decreases between n ) 14 and 16. It is reasonable to assume that the thickness of the siloxane layer is almost constant independent of the chain lengths, because the TEOS/CnTES ratio is identical in all the systems. Therefore, the above behavior should be attributed to the variation in the conformation and/or the packing arrangement of the interlayer alkyl chains depending on the alkyl chain length. The conformations of the alkyl chains in the films were examined by 13C CP/MAS NMR.22 As shown in Figure 10, the 13C signals ascribed to the interior methylene carbons appear at 30 ppm for n ) 8-14, suggesting the presence of gauche conformers. However, the relative intensity of the signal at 33 ppm, indicative of the chains in all-trans conformations, increases as the chain length increases to n ) 16 and 18. This result is further supported by IR data. It is well-known for long alkyl chain compounds that the absorption bands due to methylene stretching vibrations (νas(CH2) and νs(CH2)) are shifted to lower frequencies as the chains become an ordered state and appear at 2918 and 2849 cm-1, respectively, for the crystalline state with (22) (a) Gao, W.; Reven, L. Langmuir 1995, 11, 1860-1863. (b) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1997, 13, 115-118.

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the C18TES-TEOS system is considerably smaller than that of a bilayer structure derived from C18TES, although there is no significant conformational difference in both of the products. It is therefore concluded that the alkyl chains are arranged in an interdigitated monolayer with a rather ordered state. Such interlayer structures are also reported for the alkylsilylated derivatives of a crystalline layered polysilicate24 and may arise from the increases in the distance between alkylsilyl groups due to the presence of co-condensed Q units. Conclusions

Figure 10. 13C CP/MAS NMR spectra of well-ordered hybrid films prepared at optimum solution temperatures: (a) n ) 10 (25 °C), (b) n ) 12 (27.5 °C), (c) n ) 14 (35 °C), (d) n ) 16 (45 °C), and (e) n ) 18 (55 °C).

all-trans chains.22 The IR spectra of the films showed the νas(CH2) band at 2927, 2926, 2925, 2925, 2923, and 2920 cm-1 for n ) 8, 10, 12, 14, 16, and 18, respectively. The band frequency for C18 chains is relatively low, corresponding to the higher population of chains in all-trans conformations. The packing arrangements of the interlayer chains can be divided into two types, that is, interdigitated monolayer and bilayer structures, according to the representative models for amphiphilic lamellar phases.23 In the cases of n ) 8-12, the alkyl chains are supposed to adopt a bilayer structure with disordered state, based on our recent study on the layered hybrid films derived from alkyltrimethoxysilane (n ) 8-12)-tetramethoxysilane systems.19 However, in the case of n ) 16 and 18, the observed d spacings appear to be too small for the similar bilayer structures, considering that the chains are longer and more extended states. Indeed, the d value of the film in (23) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1-46.

The sol-gel processing of alkyltriethoxysilane-tetraethoxysilane binary systems under well-regulated conditions affords multilayered hybrids consisting of alternating organic and siloxane layers with Si-C bonds at the interface. The introduction of tetraalkoxysilane in the selfassembly process of alkyltrialkoxysilane contributes to the morphological variation and increases the thermal stability that could not be attained by hydrolysis and condensation of alkyltrialkoxysilane alone. The selfassembly of oligomeric species is strongly dependent on the temperature of the precursor solutions, and highly ordered and transparent films are obtained from the mixtures of TEOS and CnTES with various chain lengths by controlling the deposition temperature. The hybrid films thus obtained represent a variety of interlayer structures depending on the alkyl chain lengths. The overall results provide an access to the generalized synthesis of ordered hybrid films utilizing various organoalkoxysilanes with hydrophobic organic groups. Acknowledgment. The authors are grateful to Professor Y. Sugahara (Waseda University) for helpful discussion and also to Mr. T. Goto (Material Characterization Central Laboratory, Waseda University) for FE-SEM measurements. This work was financially supported by the Grants-in-Aid for COE research and for JSPS Fellows from the Ministry of Education, Culture, Sports, Science and Technology. Supporting Information Available: The XRD patterns of the hybrid films for the C8-, C12-, and C16TES-TEOS systems. This material is available free of charge via the Internet at http://pubs.acs.org. LA011016L (24) (a) Ogawa, M.; Okutomo, S.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 7361-7362. (b) Shimojima, A.; Mochizuki, D.; Kuroda, K. Chem. Mater. 2001, 13, 3603-3609.