Brigitta M. Baugher

Alkylene-bridged polysilsesquioxane aerogels: highly porous hybrid organic-inorganic materials

Abstract Alkylene-bridged polysilsesquioxane gels were prepared by sol-gel polymerizations of α, ω-bis(triethoxysilyl)alkanes 1–5. The gels were extracted with supercritical carbon dioxide to afford a novel class of hybrid organic-inorganic aerogels. The effect of the length of the alkylene bridging group and catalyst (HCl and NaOH) on the structure was examined. The molecular structure was characterized by solid-state 13C and 29Si cross polarization magic angle spinning nuclear magnetic resonance spectroscopy. The alkylene bridging groups survived sol-gel polymerization to give materials with average degrees of condensation of 79 and 90% for the acid- and base-catalyzed aerogels, respectively. Scanning electron microscopy was used to examine the macroscopic structure of the gels and nitrogen sorption porosimetry was used to measure their surface areas and pore structures. Most of the alkylene-bridged aerogels were mesoporous, high-surface-area materials. As with alkylene-bridged polysilsesquioxane xerogels, the surface area decreased with increasing alkylene bridging group length. Only the base-catalyzed tetradecylene-bridged aerogel was found to be non-porous.

Porosity in Polysilsesquioxane Xerogels

Polymerization of organotrialkoxysilanes is a convenient method for introducing organic functionality into hybrid organic-inorganic materials. However, not much is known about the effects of the organic substituent on the porosity of the resulting xerogels. In this study, we prepared a series of polysilsesquioxane xerogels from organotrialkoxysilanes, RSi(OR′) 3 , with different organic groups (R = H, Me, Et, dodecyl, hexadecyl, octadecyl, vinyl, chloromethyl, cyanoethyl). Polymerizations of the monomers were carried out under a variety of conditions, varying monomer concentration, type of catalyst, and alkoxide substituent. The effect of the organic substituent on the sol-gel process was often dramatic. In many cases, gels were formed only at very high monomer concentration and/or with only one type of catalyst. All of the gels were processed as xerogels and characterized by scanning electron microscopy and nitrogen sorption porosimetry to evaluate their pore structure.

Preparation and characterization of phenyl-, benzyl-, and phenethyl-substituted polysilsesquioxanes

Polysilsesquioxanes are a class of siloxane polymers commonly prepared by the hydrolysis and condensation of trialkoxysilanes or trichlorosilanes. From a trifunctional monomer one would expect the organically-modified polymers to be highly crosslinked and insoluble resins. However, while some silsesquioxane monomers with R = H, CH{sub 3}, or vinyl do form crosslinked polymers capable of forming gels, the majority react to form soluble oligosilsesquioxanes, including discrete polyhedral oligomers, and polymers. Because of their solubility, ladder structures have been proposed. However, viscosity studies by Frye indicate that the polyphenylsilsesquioxane is more likely best represented by a polymer rich in both cyclic structures and branches, but without any regular stereochemistry. In this study, the authors have examined the hydrolysis and condensation polymerizations of phenyltrialkoxysilane, benzyltrialkoxysilane, and 2-phenethyltrialkoxysilane monomers under both acidic and basic conditions. The resulting phenyl, benzyl and phenethyl-substituted polysilsesquioxanes were characterized by {sup 1}H, {sup 13}C, {sup 29}Si NMR, gel permeation chromatography, and differential scanning calorimetry. The effects of the organic substituent (phenyl, benzyl, phenethyl), alkoxide group (OMe, OEt), catalyst (HCl, NaOH), monomer concentration, and polymer processing on polymer molecular weight and glass transition temperature were determined.