Triblock-Copolymer-Directed Syntheses of Large-Pore Mesoporous Silica Fibers

There is currently intense interest in the topological construction of mesoporous materials both at the molecular and macroscale levels1 because of the need for selective processing of mesoporous materials into forms suitable for applications in catalysis,2 separations,3,4 chemical sensing, low-dielectric coatings,5 and optical communications.6 Mesoporous silica films have been grown under acidic conditions,7,8 at solid-liquid and liquid-vapor interfaces through an interfacial silicasurfactant self-assembly process,9-11 and more recently by sol-gel dip coating12 and polymer templating.13 Liquid crystalline templating14 has been used for the fabrication of monolithic mesoporous silica with different macroscopic shapes.15,16 Oil-in-water emulsion chemistry has been employed to selectively create mesoporous hard and hollow spheres of controllable diameter.17 Highly ordered, optically transparent mesoporous silica fibers have been grown in a two-phase reaction system at room temperature.6 These fibers have been demonstrated to have potential as high-surface-area optical waveguides. Recently, it was reported that mesoporous spun fibers can be fabricated by a solvent-evaporation technique using cetyltrimethylammonium chloride (CTAC) as the structure-directing agent18 in acidic media.7,8 This technique involved the addition of polymers, such as poly(ethylene oxide), to increase the viscosity for fiber spinning and is an extension of the traditional method of fabricating nonporous silica gel fibers.19 Under such circumstances, the polymers do not act as structure-directing agents and are incorporated homogeneously into the framework during synthesis. Since these macroscale syntheses of mesoporous silica use low molecular weight surfactants as the structuredirecting agents, the resulting materials normally have small mesopore sizes (∼2 nm), which impose limitations on many of their potential applications. Herein we report a simple procedure to synthesize mesoporous silica fibers with large pores using amphiphilic block copolymers of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)20,21 as the structure-directing agents. The fibers have three-dimensional (3D) hexagonal cage and simple two-dimensional (2D) hexagonal channel structures depending on the choice of the copolymer surfactants. These fibers have a uniform mesoporous structure and the pore channels are uniaxially aligned. The pore sizes are as large as 63 Å and the diameter of these fibers can be varied from submicron to several hundred microns. Mesoporous silica fibers were prepared by drawing a gel strand from a highly viscous amphiphilic polymer/ silicate solution. Tetraethoxysilane (TEOS, Aldrich) was used as the silica source. Poly(alkylene oxide) block copolymers HO(CH2CH2O)106(CH2CH(CH3)O)70(CH2CH2O)106H (designated EO106PO70EO106, BASF, Pluronic F-127) and HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H (designated EO20PO70EO20, BASF, Pluronic P-123) were used as structure-directing agents. The gel solution was prepared using the compositions (in moles): 1 Si(OEt)4, 3-4 EtOH, 1-2 H2O, 0.005-0.015 HCl, 0.006-0.011 polymer. A typical preparation was as follows: 2 g of polymer was dissolved in 4.69 g of ethanol (EtOH), then 0.94 g H2O and 0.15 g HCl (2M) were added with stirring, resulting in a clear solution. To this solution was added 6.25 g of TEOS slowly with stirring. The mixture was aged without stirring for 4 h at 80 °C in a 50 mL beaker. Longer aging times are necessary at lower temperatures. The viscosity of the solution increases during aging, because of partial hydrolysis and condensation of the silica and solvent evaporation. A glass rod or pipet tip was then dipped into the highly viscous gel solution and withdrawn to form the silica/copolymer fibers. The fibers were air-dried for 1 week. The polymer structure-directing agents were removed by calcination at 450 °C for 4 h in air. Figure 1 shows several images of the as-synthesized and calcined silica fibers. The fibers can be either free† Department of Chemistry. ‡ Materials Research Laboratory. § Department of Materials. | Department of Chemical Engineering. (1) Zhao, D.; Yang, P.; Huo, Q.; Chmelka, B. F.; Stucky, G. D. Curr. Opin. Solid State Mater. 1998, 3, 111. (2) Corma, A. Chem. Rev. 1997, 97, 2373. (3) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (4) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500. (5) Bruinsma, P. J.; Hess, N. J.; Bontha, J. R.; Liu, J.; Baskaran, S. Mater. Res. Soc. Symp. Proc. 1997, 443, 105. (6) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schuth, F. Adv. Mater. 1997, 9, 974. (7) Huo, Q.; Magolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schüth, F.; Stucky, G. D. Nature 1994, 368, 317. (8) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Chmelka, B. F.; Schüth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (9) McGrath, K. M.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Gruner, S. M. Science 1997, 277, 552. (10) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (11) Tolbert, S.; Schäffer, T. E.; Feng, J.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1997, 9, 1962. (12) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, J. C.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (13) Zhao, D.; Yang, P.; Melosh, N.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. In press. (14) Attard, G. S.; Glyde, J. C.; Göltner, C. G. Nature 1995, 378, 366. (15) Göltner, C. G.; Antonietti, M. Adv. 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