Small angle X-ray scattering study of microstructure changes of organic hydrogels from supercritical carbon dioxide drying

Hydrogels (HG) were prepared by the sol–gel polymerization of phenolic resole, m-cresol resole and methylolated melamine mixtures with different recipes in basic aqueous solution at 85 °C for 5 days. Water in the HG was displaced by acetone to obtain acetone gels (AG). Organic aerogels (OA) were obtained by drying of AG under supercritical carbon dioxide. Small angle X-ray scattering (SAXS) using synchrotron radiation as X-ray source was employed to investigate microstructure of HG, AG and OA. It is found that physical cross-linking is dominant in HG and supercritical drying of the physically dominant cross-linked gels cause the following microstructure changes: (1) random growth of large scatterers that “bury” the scattering peak; (2) shrinking of gels; (3) deswelling of gel network and (4) evolution of gel structure from tangled or rough interface to smooth or flat interface.