Small-angle X-ray scattering of a new series of organic aerogels

During the last decade, several new precursors have been proposed for the synthesis of organic aerogels. In a large majority of cases, the sol-gel reaction is base catalyzed and water is used as the solvent. Because of the poor solubility of liquid CO2 in water, a time consuming two-step exchange is necessary (exchange of water by acetone followed by exchange of acetone by liquid CO2). To eliminate this step, a new process with the solvent acetone was developed. Also in order to reduce gelation time, acid catalysis is used. The aim of the present work is to compare the solid and the porous texture of resorcinol-formaldehyde (RF) aerogels prepared by the conventional method (in water and base catalysis) to that of samples prepared by the new method (in acetone and acid catalysis), all other parameters (mass ratio, catalyst concentration R/C, supercritical extraction, pyrolysis conditions) remaining the same. Determination of pore size distribution (PSD) in different series of organic and pyrolyzed aerogels by thermoporometry and characterization of the solid structure by small-angle X-ray scattering (SAXS) measurements over a wide range of length are reported. The main difference between the two series of aerogels lies in the aggregation of the primary particles: the new series prepared in acetone and acid catalysis displays fractal scaling over more than a decade in length, while the conventional one does not. It is also shown that thermoporometry yields reliable information for both series of aerogels. (C) 2001 Elsevier Science B.V. All rights reserved.