Effect of shrinkage on the modulus of silica gel

The shear modulus of alkoxide-derived silica gels was measured during aging and drying; base- and acid-catalyzed gels were examined, and gave qualitatively similar results. There is a continual increase in shear modulus (G) during syneresis, which is most rapid for the first two weeks after gelation. If the gel is soaked in a liquid containing more water than the pore liquor, it shrinks and stiffens more rapidly; base-catalyzed gels exhibit phase separation (apparently from segregation of residual partially condensed TEOS). If evaporation is prevented, G only increases by a factor of ∼ 2 upon aging from 1 to 12 months. When the liquid is allowed to evaporate, G increases by about 3 orders of magnitude as the gel contracts by ∼ 50% linearly. There is an approximate exponential relation between G and the radial strain that applies about as well for samples aged in various solutions as for samples dried in air. The relation between G and strain (contraction) is nearly independent of drying rate when the linear strain exceeds ∼ 10%. The modulus of rupture rises by about 2.5 orders of magnitude during drying.