Evolution of Porosity and Morphology in Alkylene-Bridged Polysilsesquioxane Xerogels as a Function of Gel Aging Time
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EE10.3.1
EVOLUTION OF POROSITY AND MORPHOLOGY IN ALKYLENE-BRIDGED POLYSILSESQUIOXANE XEROGELS AS A FUNCTION OF GEL AGING TIME
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 Aging of silica gels before drying is known to result in significant changes in xerogel morphology, porosity and properties. In this study, the influence of aging gels on the porosity and morphology of alkylene-bridged polysilsesquioxane xerogels was examined. Gels of hexylene-, heptylene, octylene, nonylene, and decylene-bridged polysilsesquioxanes were prepared by the sol-gel polymerization of the respective bis(trimethoxysilyl)alkane monomers under acidic or basic conditions in methanol and in tetrahydrofuran. The gels were aged 3,7,14, 28, 35, 42, 49, and 56 days before drying to afford xerogels. The xerogels were characterized by nitrogen sorption porosimetry. Xerogels prepared in THF were non-porous. Those prepared and aged under basic conditions in methanol or tetrahydrofuran exhibited coarsening of porosity with aging time. With the exception of the hexylene-bridged gels, those prepared and aged in acidic methanol showed little change with aging. The surface area of the hexylene-bridged xerogels nearly tripled with aging times of up to several weeks, then decreased, for the gels aged for more than two weeks, to around 100 meters squared per gram. INTRODUCTION Sol-gel processing of hybrid organic-inorganic materials, such as bridged polysilsesquioxanes (Scheme 1) is a versatile, mild method for preparing porous materials, protective coatings, optical materials and membranes [1-3]. As with most sol-gel systems, there are numerous reaction and processing variables that can have dramatic effects on the morphologies and properties of the ultimate materials. In bridged polysilsesquioxanes, the nature of the bridging group seems to have paramount influence on the porosity of air-dried xerogels or supercritically dried aerogels. Historically, to investigate these bridging groupstructure property relationships, we have taken care to keep reaction and processing parameters as constant as possible. The gel aging time (the time between gelation and work-up) was arbitrarily designated as two weeks in nearly all of our studies. Because other groups used different aging times and because silica gel aging times have been shown to have profound influences on final xerogel morphologies [4], we embarked on a study of the effects of gel aging on the porosities of alkylene-bridged polysilsesquioxane xerogels prepared from the sol-gel polymerization of α,ω-bis(trimethoxysilyl)alkanes.
EE10.3.2
Scheme 1. Sol-gel polymerization of bridged monomers yields highly cross-linked
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