Macroscopic shear alignment of bulk transparent mesostructured silica.

Transparent aligned composite or porous silica structures are desirable for use as membranes or as host materials for optical dyes, conducting polymers, and large molecules. However, few bulk aligned structures with orientational order have been produced.1-4 Moreover, those that have been produced are generally not transparent and thus are not useful for optical applications. Creating orientational order within materials generally requires that they be processed in an externally applied field. For example, bulk aligned silica-surfactant mesophases have been prepared by using strong magnetic fields to orient low-molecular-weight liquid crystal precursors,2,5 though these materials are unfortunately not transparent. A number of transparent mesostructured silica thin films and fibers with macroscopic alignment have been synthesized by using interactions at air-(water, oil, or mica) interfaces,6-9 surface effects,10-12 or extensional flow.13 However, these techniques are not well suited to the formation of aligned bulk materials. Surface interactions, for example, generally do not extend far from the interface into the bulk, and practical considerations limit the use of extensional flow to alignment within thin fibers. Monolithic materials with large domain sizes have been reported, though the domains are not uniformly aligned with respect to one another and require lengthy annealing times.1,14 Shear alignment is not possible for these materials, because mesophase formation occurs after gelation of the silica. In this Communication, we describe a procedure for producing bulk mesoscopically ordered silica with high degrees of macroscopic alignment and transparency using capillary flow or parallel plate shearing. Typically, silica/block-copolymer mesophase syntheses require evaporation of alcohol species produced by hydrolysis of inorganic alkoxide precursors before the structure-directing agents can selfassemble.10,11,15,16 As noted above, sample fluidity is lost, because self-assembly occurs after gelation, thus precluding effective shear alignment. Feng et al. recently demonstrated that by hydrolyzing the alkoxide precursors and removing the alcohols under vacuum prior to mixing with the structure-directing block-copolymer solution, a viscous but still fluid mesophase with hexagonal or lamellar order can be produced before gelation occurs.17 Mesoscopically ordered silica-block copolymer composites were formed under acidic solution conditions with amphiphilic poly(ethyleneoxide)-poly(propyleneoxide)-poly(ethyleneoxide) [EO106PO70EO106] (Pluronic F127) triblock copolymer mixed with butanol and cyclohexane, which act as hydrophobic swelling agents. The concentrations of the swelling agents are important variables in establishing the final structure: 0.0, 0.16, and 0.20 g of swelling agent/g of F127 led to cubic, hexagonal, and lamellar mesophases accordingly. A typical hexagonal mesophase was prepared as follows: 2.0 mL of tetraethoxysilane (TEOS) was mixed with 2.0 mL of HCl (pH 1.4) and the mixture stirred until homogeneous. The sample was placed under vacuum until ∼80% of the ethanol produced by the hydrolysis of the TEOS was removed. This solution was added directly to a mixture of 0.75 g of F127 polymer/0.15 mL of butanol/0.15 mL of cyclohexane and mixed manually for several minutes until the triblock copolymer had completely dissolved. While the resulting material was highly viscous, centrifugation for 10 min at 3000 rpm was sufficient to remove any bubbles, yielding a colorless transparent gel with strong birefringence (except for the cubic phase). Highly aligned bulk hexagonal or lamellar mesophases were synthesized by using the silica/block copolymer mixture described above and a simple capillary shear-flow technique demonstrated by Davidson et al.18 The procedure involved using a vacuum to draw the viscous silica mixture 2-4 cm into a capillary tube, aligning the silica/block-copolymer mesophase by shearing. A variety of capillary tube diameters and geometries were evaluated, including 0.5, 0.7, and 1.0 mm cylindrical quartz X-ray diffraction (XRD) capillaries, as well as 0.3, 0.5, 0.75, and 1.0 mm rectangular optical glass capillaries (Vitrocom). In all cases, the transparent as-synthesized samples showed highly oriented birefringence along the capillary axes. When annealed for 3 days and calcined in air at 450 °C for 6 h, the samples prepared within the 0.5 mm cylindrical capillaries yielded large transparent pieces ∼1-2 cm long. Large transparent, crack-free pieces were also obtained in better yield via calcination at 350 °C for 36 h with a 0.25 °C/min temperature ramp. Thinner samples were less prone to cracking, and cylindrical capillaries appeared to produce more robust materials than the flat optical capillaries. Structure identification and quantitative analyses of the orientational ordering of the silica-EO106PO70EO106 mesophases with respect to the shear direction were provided by transmission X-ray diffraction patterns recorded on two-dimensional image plates.14 The degree of mesophase alignment was determined from the full-width-at-half-maximum (fwhm) line width of each two-spot pattern, measured around the azimuthal angle and deconvolved for 5° finite-beam-size effects. For hexagonal phases, the fwhm refers to the distribution of lengthwise orientations of the cylindrical aggregates, and for lamellar phases, the distribution of layer alignments. ‡ Department of Chemical Engineering, University of California. † Department of Materials, University of California. § Université Paris-Sud. (1) Ryoo, R.; Ko, C. H.; Cho, S. J.; Kim, J. M. J. Phys. Chem. B 1997, 101, 10610-10613. (2) Firouzi, A.; Schaefer, D. J.; Tolbert, S. H.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 1997, 119, 9466-9477. (3) Zhao, D.; Yang, P.; Huo, Q.; Chmelka, B. F.; Stucky, G. D. Curr. Opin. Solid State Mater. Sci. 1998, 3, 111-121. (4) Wu, J. J.; Gross, A. F.; Tolbert, S. H. J. Phys. Chem. B 1999, 103, 2374-2384. (5) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264-268. (6) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589-592. (7) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703-705. (8) Raimondi, M. E.; Maschmeyer, T.; Templer, R. H.; Seddon, J. M. Chem. Commun. 1997, 1843-1844. (9) Miyata, H.; Kuroda, K. Chem. Mater. 1999, 11, 1609-1614. (10) 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, 364-368. (11) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 1998, 10, 1380-1385. (12) Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Lavergne, M.; Mazerolles, L.; Babonneau, F. J. Mater. Chem. 2000, 10, 2085-2089. Klotz, M.; et al. Chem. Mater. 2000, 12, 1721-1728. (13) Yang, P.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033-2036. (14) Melosh, N. A.; Davidson, P.; Chmelka, B. F. J. Am. Chem. Soc. 2000, 122, 823-829. (15) Attard, G. S.; Glyde, J. C.; Göltner, C. G. Nature 1995, 378, 366-368. (16) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredrickson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332-4342. (17) Feng, P.; Bu, X.; Pine, D. J. Langmuir 2000, 16, 5304-5310. (18) Imperor-Clerc, M.; Davidson, P. Eur. Phys. J. B 1999, 9, 93-104. 1240 J. Am. Chem. Soc. 2001, 123, 1240-1241