Compressing some sol-gel materials reduces their stiffness: a textural analysis

Abstract

The mechanical behaviour of two series of silica and of resorcinol xerogels is analyzed by mercury porosimetry. The data are expressed as pressure-density curves, which enables textural information to be obtained. In particular, it is shown that some of the analyzed samples exhibit a marked lowering of their mechanical stiffness upon compression. This observation is analyzed in terms of the collapse of the sample's porosity and of the heterogeneity of the microstructure.

Evolution of mechanical properties and final textural properties of resorcinol–formaldehyde xerogels during ambient air drying

“…Sound propagation experiments showed a decrease of RF aerogel stiffness when the samples are uniaxially compressed [27]. Recently, the same behaviour was found when RF samples are submitted to mercury porosimetry [28].…”

“…This behaviour was firstly put in evidence by Gross et al [27] using the velocity of sound propagation on SiO 2 and RF aerogels for R/C=200. More recently, Gommes et al [28] reported similar results using mercury porosimetry measurements on RF xerogels with 5.8<pH<7.3, which correspond roughly to 50<R/C<1200 using mercury porosimetry measurements. They interpreted this behaviour as caused by the heterogeneity of the xerogel porous structure.…”

“…Mercury porosimetry, which is usually used to determine the pore size distribution [30] and [31], is affected by a peculiar behaviour in the case of RF samples [13], [17] and [28]. For this material, the pore size distribution cannot be obtained in a straightforward manner due to the collapse of the sample under mercury pressure [32].…”

“…Nevertheless the data allow to determine the compression modulus of the network according to Scherer's methodology [33]. This parameter has been shown to be related to the microstructure [28].…”

Characterization of gels via solvent desorption measurements

“…The extrapolation is based on (5) and (6) that do not necessarily apply. For instance, some gels exhibit a lowering of their compression modulus at the beginning of their compression (Gross et al 1992;Gommes et al 2005;Chaudhuri et al 2007). This might be the reason why K 0 underestimates K g for gels ET10 to ET20.…”