Effect of concentration of reactants on porosity of hydrogels, organic and carbon aerogels

Abstract Hydrogels, organic and carbon aerogels were prepared by sol–gel polymerization of phenolic resole, m -cresol, melamine and formaldehyde followed by supercritical drying and pyrolysis. The roles of concentration of reactants in determining porosity of organic and carbon aerogels were investigated under the same ratios of reactants by extracting porous structural information of hydrogels, organic aerogels and carbon aerogels from small angle X-ray scattering (SAXS) and nitrogen adsorption measurements. Two sizes of scatterers, presumably pores, are formed in hydrogels by nucleation-and-growth and spinodal decomposition. The large sized mesopores formed by spinodal decomposition are periodical, whose percolation threshold is around 15 v/v%. At low concentrations below percolation threshold, hydrogels tend to be discrete and severe volume shrinkages are found in solvent exchange process, leading to the formation of organic and carbon aerogels with low porosity. The mesopores in hydrogels are converted to macroscopic structures in supercritical drying process via coarsening, which is enhanced by concentration of reactants above percolation threshold of large sized mesopores. The pore volumes of continuous large sized mesopores formed above percolation threshold decrease in pyrolysis with intense heat as a result of sintering of network while the discrete large sized mesopores formed below percolation threshold are stable in pyrolysis, which is related to the steric hindrance for sintering.