Flexibilisation of resorcinol–formaldehyde aerogels

Ambient pressure dried resorcinol–formaldehyde (RF) organic aerogels are usually hard and brittle with Young's moduli in the range of 1–2 MPa, strengths of about 100 kPa and densities in the range of 0.2 to 0.4 g cm−3. Modifications of the classical sol–gel synthesis route transform these brittle materials into rubber- or cork-like flexible aerogels. We observed that in a small window of process parameters, the aerogel density decreases by an order of magnitude as well as the Young's moduli and the compression strengths. These new types of RF aerogels are elastically deformable by more than 40% in an almost reversible manner. In this paper we describe the effects of various sol–gel parameters on the flexibility, such as resorcinol to water and catalyst molar ratio and the pH of the initial solution. The aerogels are characterized with respect to the envelope density, stress–strain behavior and microstructure as observed from SEM fractographs. The chemical structure and structural differences arising between brittle and flexible RF aerogels were studied by recording 13C-NMR spectra.