Microstructural Dependence Of Aerogel Mechanical Properties

Aerogels are highly porous solids derived from the hypercritical drying of covalently crosslinked gels. SEM, TEM and scattering studies reveal that these materials have an open-cell morphology with a solid matrix composed of interconnected colloidal-like particles (30–200 A) Processing variables and synthetic conditions largely determine the microstructure and morphology. In this study, the structure-mechanical property relationships of silica, resorcinolformaldehyde, and carbon aerogels are examined.

Parameters Affecting Elastic Properties of Silica Aerogels

The Young's modulus of silica aerogels is measured using the three points flexural technique. Various parameters have been investigated such as the catalyting conditions used to elaborate the initial alcogel and the heat treatment performed to density the material. In a given sample porosity the elastic behaviour of aerogels depends on the conditions of gel preparation. This different elastic behaviour is related to the different spatial arrangement of the particles pointed out by fractal features.

A Comparison of Mechanical Properties and Scaling Law Relationships for Silica Aerogels and their Organic Counterparts

Aerogels are a unique type of ultrafine cell size, low density foam. Traditional aerogels are inorganic, but the synthesis of organic aerogels has also been reported. In all cases, solution chemistry can be used to tailor the structure and properties of the resultant aerogels. This study examines the microstructural dependence of the compressive mechanical properties of silica, resorcinol-formaldehyde, carbon, and melamine-formaldehyde aerogels.

Semimicroscopic theory of elasticity near the vulcanization transition

Randomly cross-linked macromolecules undergo a liquid to amorphous-solid phase transition at a critical cross-link concentration. This transition has two main signatures: the random localization of a fraction of the monomers and the emergence of a nonzero static shear modulus. In this paper, a semimicroscopic statistical mechanical theory of the elastic properties of the amorphous solid state is developed. This theory takes into account both quenched disorder and thermal fluctuations, and allows for the direct computation of the free energy change of the sample due to a given macroscopic shear strain. This leads to an unambiguous determination of the static shear modulus. At the level of mean field theory, it is found (i) that the shear modulus grows continuously from zero at the transition, and does so with the classical exponent, i.e., with the third power of the excess cross-link density and, quite surprisingly, (ii) that near the transition the external stresses do not spoil the spherical symmetry of the localization clouds of the particles.