Collapse of Porosity During Drying of Alkylene-Bridged Polysilsesquioxane Gels. Influence of the Bridging Group Length
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COLLAPSE OF POROSITY DURING DRYING OF ALKYLENE-BRIDGED POLYSILSESQUIOXANE GELS. INFLUENCE OF THE BRIDGING GROUP LENGTH Douglas A. Loy,* James H. Small, Kimberly A. DeFriend, Kennard V. Wilson, Jr., McKenzie Minke,1 Brigitta M. Baugher,1 Colleen R. Baugher,1 Duane A. Schneider,1 and Kenneth J. Shea2 Polymers and Coatings Department, Los Alamos National Laboratories, PO Box 1662, MS E549, Los Alamos, NM 87545, U.S.A. 1 Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. 2 Department of Chemistry, University of California Irvine, U.S.A. ABSTRACT The introduction of organic substituents into sol-gel materials can often result in networks that collapse during drying to afford non-porous xerogels. This can prove useful if non-porous coatings or membranes are the ultimate objectives. Collapse of porosity is also manifested in bridged polysilsesquioxanes with flexible bridging groups. Alkylene-bridged polysilsesquioxanes are hybrid xerogels whose organic bridging group is an integral constituent of the network polymer that can be systematically varied to probe the influence of its length on the xerogels’ porosity and morphology. Our previous studies have shown that hexylene-bridged polysilsesquioxane xerogels prepared from 1,6-bis(triethoxysilyl)hexane under acidic conditions are nonporous while the pentylene-bridged polysilsesquioxanes prepared under the same conditions are porous. We also discovered that the more reactive 1,6-bis(trimethoxysilyl)hexane monomer could polymerize under acidic conditions to afford porous xerogels. Here, we have extended our study of bis(trimethoxysilyl)alkanes to include the heptylene (C7), octylene (C8), nonylene(C9) and decylene (C10) bridges so as to ascertain at what bridging group length the porosity collapses. The morphology of the resulting xerogels was characterized by nitrogen sorption porosimetry and electron microscopy. Solid state NMR was used to structurally characterize the materials. INTRODUCTION A useful group of materials within the hybrid organic-inorganic materials family, bridged polysilsesquioxanes is being used for a number of applications from optical materials to coupling agents [1-3]. Most commonly prepared by the sol-gel polymerization of monomers with two or more trialkoxysilyl groups attached to an organic bridging group (Scheme 1), these materials provide an excellent vehicle for integrating organic moieties into a highly cross-linked, glassy matrix. Most fundamental efforts to date have focused on systematically varying the nature of the organic group to clarify the structure property relationships, though some investigations of the influence of reaction and processing conditions on the resulting xerogels’ or aerogels’ properties have been conducted. These materials’ capacity for very high surface areas and relatively narrow pore size distributions has been of particular interest. Prior investigations of alkylene-bridged polysilsesquioxanes, prepared from bis(triethoxysilyl)alkane monomers, revealed that long, flexible bridging groups could resul
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