Microstructure Evolution in the Preparation of Carbon Aerogels by Small Angle X-Ray Scattering

Sol-gel polymerization of phenolic resole and methylolated melamine in basic aqueous solution at 85 degreesC for 5 days led to the formation of hydrogels, which, after water in the hydrogels was exchanged with acetone, were dried under supercritical carbon dioxide to produce organic aerogels. Carbon aerogels were obtained by pyrolyzing these organic aerogels at 850 degreesC, for 3 h in nitrogen. Microstructure evolution during the preparation was followed by small angle X-ray scattering. It is found that hydrogel is monodispersed, swelled by water, and has a sharp peak in the scattering intensity profile, indicating a spatial periodical nanostructure formed by nanoscale phase separation via spinodal decomposition. After solvent exchanging, the acetone gel thus obtained is also monodispersed, but is partly de-swelled, and has a shoulder-like scattering peak. This is a result of a substantial increase in scattering intensity near the lower limit of the scattering vector, q caused by domain growth of the scattering entity. After supercritical carbon dioxide drying, the organic aerogels produced are polydispersed and de-swelled completely by removal of the solvent. The peak in the scattering intensity profile disappears completely by further growth of domains as a result of the structural relaxation from an unstable to a more stable state, induced by an increase in instability of the gel formed by supercritical drying. Moreover, a diffuse interface layer characterized by negative deviation from the Porod law is found in organic aerogels. After pyrolysis, the polydispersion is retained in the carbon aerogel, however, the diffuse interface layer disappears due to migration of the layer to particles to reduce the interface energy. Also in this process, joint actions of further growth in domain size and volume reduction by mass loss and densification slightly broaden the distribution of the radius of gyration of scattering entities, and shift the most probable peak radius of gyration to the small size direction.