Proper selection of silane precursors and polymer reinforcements yields more durable and stronger silica aerogels. This paper focuses on the use of silane-end-capped urethane prepolymer and chain-extended polyurethane for reinforcement of silica aerogels. The silane end groups were expected to participate in silica network formation and uniquely determine the amounts of urethanes incorporated into the aerogel network as reinforcement. The aerogels were prepared by one-step sol-gel process from mixed silane precursors tetraethoxysilane, aminopropyltriethoxysilane (APTES), and APTES-end-capped polyurethanes. The morphology and mechanical and surface properties of the resultant aerogels were investigated in addition to elucidation of chemical structures by solid-state (13)C and (29)Si nuclear magnetic resonance. Modification by 10 wt % APTES-end-capped chain-extended polyurethane yielded a 5-fold increase in compressive modulus and 60% increase in density. APTES-end-capped chain-extended polyurethane was found to be more effective in enhancement of mechanical properties and reduction of polarity.

This study evaluated polyhedral oligomeric silsesquioxane (POSS) molecules as useful, multifunctional reinforcing agents of silica aerogels. Silica aerogels have low-density and high surface area, although their durability is often compromised by the inherent fragility and strong moisture absorption behavior of the silica networks. POSS molecules carrying phenyl, iso-butyl, and cyclohexyl organic side groups, and several Si-OH functionalities were incorporated into silica networks via reactions between Si-OH functionalities in POSS molecules and silanes. Solid state (13)C and (29)Si NMR spectra established that greater than 90% of POSS molecules grafted onto silica networks and led to an increase in fractal dimensions. An almost 6-fold increase in compressive modulus was achieved with less than 5 wt % trisilanol phenyl POSS, and a 50-fold decrease in polarity with negligible changes in density were seen in aerogels modified with less than 5 wt % trisilanol isobutyl POSS.

Mechanically reinforced organic-inorganic hybrid silica aerogels are produced from simultaneous hydrolysis and condensation reactions of silane precursorstetraethoxy silane (TEOS) and aminopropyltriethoxysilane (APTES)and silane-modified polyurethane urea molecules each carrying multiple ($3) reactive silane groups. In this manner, the post-gelation crosslinking reactions are avoided, the amount of polymer introduced into the aerogel structures is controlled, and the chain length between two crosslink points is tailored. The long chain polymer molecules introduce a certain degree of flexibility to the hybrid aerogel structures. The morphology, compressive properties, and surface area are obtained respectively using scanning electron microscopy, Instron tensile testers, and Brunauer-Emmett-Teller (BET) surface area analysis. The data on solid state 13 C and 29 Si NMR spectra reveal chemical reactions of the silane-modified polymers with the silica particle networks. Small angle X-ray scattering (SAXS) data are used to determine the fractal dimension of the silica networks. It is found that the self-crosslinkable multifunctional polyurethane urea chains form coatings on the silica networks and produce large enhancements in compressive modulus although with increases in shrinkage and bulk density.

In this work, silica gel networks were modified with a silylating agent dimethoxy-methyl (3,3,3-trifluoropropyl) silane (SiF3) to obtain hydrophobic aerogels of various surface energy values. The baseline aerogels were synthesized from tetraethoxy silane (TEOS) using a twostep sol-gel process followed by supercritical drying in liquid carbon dioxide. The resultant aerogels were characterized using scanning electron microscopy, Instron tensile tester, contact angle goniometry, and nitrogen adsorption-desorption isotherms. Three modification methods were studied. In method 1, TEOS and SiF3 were combined before gelation; in method 2, SiF3 was added after TEOS was hydrolysed and before its condensation, and in method 3, SiF3 was added after the gels were produced from TEOS. It was found that method 3 produced the best results in terms of achieving high values of hydrophobicity and compressive properties. The data on solid state 13 C and 29 Si NMR spectra revealed chemical reactions between the silylating agents and the silanol groups on silica surface. The bulk density and the fractal dimensions of silica networks gleaned from small angle X-ray scattering (SAXS) data showed weak dependence on the degree of silylation. The silylation process rendered the aerogels strongly hydrophobic and also doubled its compressive modulus.