Composition-Structure Relations in Organically Modified Silica Gels
- PDF / 2,120,550 Bytes
- 11 Pages / 417.6 x 639 pts Page_size
- 76 Downloads / 225 Views
ABSTRACT Monolithic silica aerogels modified by functional organic groups were prepared by basecatalyzed sol-gel processing of Si(OR) 4 / R'Si(OMe) 3 mixtures (R' = organofunctional group), followed by drying of the wet gels with supercritical CO 2. When the functional organic group has only weakly or no basic properties, the microstructure of the obtained aerogels is similar to that of an unmodified silica aerogel prepared under the same conditions and quite independent of the kind of functional group. The experimental findings are explained by a two-stage process in which the R'Si-- units condense to a pre-formed gel network obtained by hydrolysis and condensation of Si(OR) 4 . An increasing portion of R'Si(OMe) 3 has the same effects on the hydrolysis and condensation reactions as decreasing the bulk density of an unmodified silica aerogel and the same structural consequences as increasing the water / silane ratio and the catalyst concentration acting on Si(OR) 4 . This leads to larger primary particles and, associated with that, smaller specific surface areas. The two-stage process is not observed when R' contains a strongly basic substituent such as NH 2 or NHCH 2CH 2NH 2. The structural parameters indicate that in these cases both R'Si(OMe) 3 and Si(OR) 4 are involved in the built-up of the gel network. This can be explained by extensive hydrogen bonding between the amino groups and silanol groups. [NTRODUCTION A great number of hybrid inorganic-organic materials has been prepared by sol-gel processing of R'Si(OMe) 3 / Si(OR) 4 mixtures, with a variety of functional and non-functional organotrialkoxysilanes. However, surprisingly little attention was paid to how the microstructure and particularly the mutual arrangement of the molecular building blocks in such materials is related to the chemical processing parameters. Aerogels [1] are materials in which the structure of the wet gel can be largely preserved if supercritical drying is performed with liquid carbon dioxide as a fluid. The underlying assumption for the discussion of structural trends in this article is that the structural characteristics of the gel network are largely preserved when the aged wet gel is supercritically dried by CO 2. While supercritical drying with alcohols can lead to network rearrangements due to the harsher physical conditions and the cleavage of Si-O-Si bonds by the alcohol, such reactions are less likely with CO 2 [2]. Obviously, C0 2-dried aerogels are particularly well suited to correlate the chemistry during sol-gel processing with the structural characteristics of the gel network. We have prepared inorganic-organic hybrid silica aerogels starting from R'Si(OMe) 3 / Si(OR) 4 mixtures, where R' is a functional or non-functional organic group [3]. In this paper, we mainly address the question how the silicon-bonded organic groups R' influence the built-up and structure of the gel network-
117
Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
EXPERIMENTAL Sol-Gel Processing of R'Si(OMe)_/Si(0)4_ Mixture
Data Loading...
