Articles Structure of Arylene-Bridged Polysilsesquioxane Xerogels and Aerogels Dale W. Schaefer,* ,† Greg Beaucage, † Douglas A. Loy, ‡ Kenneth J. Shea, § and J. S. Lin | Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio, 45221-0012, Polymers and Coating Group, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, Department of Chemistry, University of California, Irvine, Irvine, California 92717, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received October 25, 2003. Revised Manuscript Received January 26, 2004 Arylene-bridged polysilsesquioxanes are an interesting class of porous materials prepared by sol-gel processing of ethoxysilane monomers in which there are two or more trialkoxysilyl groups positioned about an arylene bridging group. The majority of these materials are highly porous with surface areas as high as 1880 m 2 /g. In an effort to understand the nature of porosity in these materials, small-angle X-ray and neutron scattering were employed to characterize phenylene-, biphenylene-, and terphenylene-bridged polysilsesquioxanes. Phen- ylene-bridged polysilsesquioxane xerogels and aerogels were also compared to understand the effect of drying protocol on pore structure. The effect of catalyst concentration is also reported for the base-catalyzed system. In all cases studied here, we find evidence for domains in the nanometer range with distinct fractal character. We associate these domains with porosity rather than microphase separation of organic and inorganic moieties. The nature of this porosity depends on the bridging group in a systematic way, but is only weakly dependent on other synthetic parameters such as catalyst type, catalyst concentration, and drying protocol. Introduction Hydrocarbon-bridged polysilsesquioxanes represent an interesting class of highly cross-linked hybrid or- ganic-inorganic polymers. 1-9 These materials are net- work polymers in which the basic building block is two silicons directly attached to a hydrocarbon bridging group (Figure 1). The remaining three bonds to each silsesquioxane silicon are siloxane linkages. By connect- ing two or more silsesquioxane groups to an organic bridging group, a material with as many as six siloxane linkages (Si-O-Si) per monomer unit can be prepared, as opposed to just four in tetraalkoxy silanes. In addition, by introducing hydrocarbon spacers into the siloxane network, the properties (hydrophobicity, sur- face area, pore size, ultraviolet-visible absorption, and fluorescence, etc.) can be significantly modified. 7,10-12 Arylene-bridged polysilsesquioxanes X-1 through X-4 were prepared by sol-gel processing of bis(triethoxysi- ly)aryl monomers 1-4 (Figure 1) under either acidic or basic conditions. Hydrolysis and condensation rapidly leads to gels at concentrations as low as 0.01 M, more than an order of magnitude lower than is possible with triethoxysilylbenzene. Trifunctional aryl silanes pref- erentially form as oligosilsesquioxanes rather than gels. 13 At high monomer concentration, however, gels can be prepared from trifunctionals with heating and in the presence of strong base. 14 * To whom correspondence should be addressed. E-mail: dale. schaefer@uc.edu. † University of Cincinnati. ‡ Los Alamos National Laboratory. § University of California. | Oak Ridge National Laboratory, current affiliation University of Tennessee, Knoxville. (1) Shea, K. J.; Loy, D. A.; Webster, O. W. Chem. Mater. 1989, 1, 572-4. (2) Shea, K. J.; Loy, D. A.; Webster, O. W. Polym. Mater. Sci. Eng. 1990, 63, 281-5. (3) Loy, D. A.; Shea, K. J.; Russick, E. M. In Better Ceramics Through Chemistry V; Hampden-Smith, M. J., Klemperer, W. G., Brinker, C. J., Eds.; Mater. Res. Soc. Symp. Proc. 271, Materials Research Society: Pittsburgh, PA, 1992; pp 699-704. (4) Small, J. H.; Shea, K. J.; Loy, D. A. J. Non-Cryst. Solids 1993, 160, 234-46. (5) Corriu, R. J. P.; Leclercq, D. Angew. Chem. Int. Ed. Engl. 1996, 35, 1420-36. (6) Cerveau, G.; Corriu, R. J. P.; Lepeytre, C. J. Organomet. Chem. 1997, 548, 99-103. (7) Cerveau, G.; Corriu, R. J. P. Coord. Chem. Rev. 1998, 180, 1051- 71. (8) Corriu, R. C. R. Acad. Sci., Ser. Ii Fascicule C: Chim. 1998, 1, 83-89. (9) Boury, B.; Corriu, R. J. P.; Le Strat, V.; Delord, P.; Nobili, M. Angew. Chem. Int. Ed. 1999, 38, 3172-5. (10) Loy, D. A.; Shea, K. J. Chem. Rev. 1995, 95, 1431-42. (11) Cerveau, G.; Corriu, R. J. P.; Framery, E. Chem. Mater. 2001, 13, 3373-88. (12) Shea, K. J.; Loy, D. A. MRS Bull. 2001, 26, 368-376. (13) Voronkov, M. G.; Lavrent’yev, V. I. Top. Curr. Chem. 1982, 102, 199-236. (14) Frye, C. L.; Klosowski, J. M. J. Am. Chem. Soc. 1971, 93, 4599- 601. 1402 Chem. Mater. 2004, 16, 1402-1410 10.1021/cm0350683 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/18/2004