Elastic properties of crosslinked resorcinol-formaldehyde gels and aerogels

Resorcinol-formaldehyde (RF) gels with different monomer concentrations were characterized before and after supercritical drying from carbon dioxide in order to determine the influence of the drying process on elastic moduli and structure. Wet gel shear modulus, Young's modulus and Poisson ratio were determined using the beam-bending method. In addition, acoustic shear waves were used to measure the shear modulus of wet gels. The two methods are shown to agree well. During supercritical extraction of the pore liquid the elastic moduli typically increase by a factor of 1.5 to 5 depending on density. Although in general the elastic moduli do not exhibit power-law dependence on density, in the limited density range covered by our experiments scaling exponents of 6.2 for the wet gels and 4.8 for the aerogels are derived. Aging in acetic acid is shown to have no significant impact on gel elastic properties. Wet and dry gels were analyzed for their structural efficiency and the fraction of elastically effective mass increases with density from 8 to 70%.