DLS and SAXS investigations of organic gels and aerogels

Recent investigations have shown that the structure of organic aerogels can be significantly modified by changing the precursors, the solvent and the nature of the catalyst involved in the sol-gel reaction. it is therefore hi ghly desirable to investigate the sol-gel mechanism. For this purpose, dyna mic light scattering (DLS) measurements have been performed at different st ages of the reaction for base- or acid-catalyzed gelation of resorcinol for maldehyde (RF) using water or acetone as solvents. The structure of aged ge ls was investigated by small-angle X-ray scattering (SAXS) and compared to that of the aerogels obtained after exchange of solvent by supercritical CO L and drying of the aged gels. It is shown that acid-catalyzed gelation of RF in acetone can be described by percolation, which explains that this ser ies of aerogels consists of mass fractal aggregates (D-m = 2.5). The partia l collapse of this polymeric gel yielding colloidal particles in the aeroge l can be attributed to deswelling in supercritical CO2. DLS indicates that gelation of RF with a base catalyst yields a colloidal gel whose structure remains practically unchanged in the aerogel, as shown by SAXS. (C) 2001 El sevier Science B,V. All rights reserved.