Formation and Structure of Porous Gel Networks from Si(OMe)4 in the Presence of A(CH2)nSi(OR)3 (A = Functional Group)

Abstract

Monolithic silica aerogels modified by functional organic groups were prepared by sol−gel processing of Si(OMe)4/A(CH2)nSi(OR)3 mixtures under identical experimental conditions, followed by drying of the wet gels with supercritical CO2. The employed functional groups A were SH (n = 3), (n = 3), OC(O)C(Me)CH2 (n = 3), NCO (n = 3), Cl (n = 3), NHC(O)OMe (n = 3), and PPh2 (n = 2). These groups were retained in the aerogels except the isocyanate groups, which reacted with methanol to the corresponding carbamate. The properties of the obtained aerogels are rather independent of the kind of functional group, but strongly depend on the Si(OMe)4/A(CH2)nSi(OR)3 ratio, which was varied between 9:1 and 6:4. The density of the aerogels was 0.2−0.3 g cm-3; some aerogels with lower densities were also prepared for comparison. Gelling of the precursor mixtures is drastically slowed with an increasing portion of A(CH2)nSi(OR)3, and the water consumption is retarded. During supercritical drying, shrinkage of ∼10% was observed for the aerogels prepared from the 9:1 precursor mixtures. Increasing the portion of A(CH2)nSi(OR)3 or decreasing the aerogel density lead to a larger shrinkage and an incomplete incorporation of the functional organic groups. The chemical composition of the resulting aerogels was investigated by infrared (IR) and Raman spectroscopy, elemental analysis, and titration of the functional groups, and their structure by nitrogen sorption and small-angle X-ray scattering (SAXS). The Brunauer−Emmett−Teller (BET) surface areas generally decreased with an increasing portion of A(CH2)nSi(OR)3, whereas the C parameter showed a saturation behavior. An analysis of the pore volumes indicated that with an increasing portion of A(CH2)nSi(OR)3 or a decreasing bulk density, the gel skeleton is increasingly compressed during the N2 sorption experiments. SAXS measurements showed larger particles upon increasing the A(CH2)nSi(OR)3/Si(OMe)4 ratio, which correlates with the observed decrease of the specific surface areas. The results were interpreted that an increasing portion of A(CH2)nSi(OR)3 has the same kinetic effects on the hydrolysis and condensation reactions and the same structural consequences for the network formation as decreasing the bulk density of an aerogel obtained from the one-component Si(OMe)4 system. The fractal dimension increased with an increasing portion of A(CH2)nSi(OR)3; it was significantly larger for A = NCO and NHC(O)OMe than for A = SH or OC(O)C(Me)CH2.