Stability of mesopores in organic aerogels produced from melamine, phenolic resole and formaldehyde in pyrolysis

Organic aerogels were synthesized by the solution-sol-gel polymerization of melamine (M), phenolic resole (PR), m-cresol (m-C) and formaldehyde in a basic aqueous solution followed by supercritical petroleum ether drying. The stability of the nanostructure of the organic aerogels in pyrolysis was investigated. It was found that bimodal mesopores are formed in organic aerogels and the sintering of a nanonetwork that forms the mesopores occurs during the pyrolysis of the aerogels, leading to a decrease of mesopore size and volume. The sizes and volumes of the large mesopores (5-50 nm) decrease with pyrolysis temperature and/or time for the same reactant formulation. In all cases the small mesopores (2-5 nm) are more stable during pyrolysis than the large mesopores. The large mesopores prepared at PR concentration of 7.5 g/100 mL are more stable during pyrolysis than those at 10 g/100 mL. The stability of the large mesopores during pyrolysis for organic aerogels increases with M/PR ratio or decreases with m-C/PR ratio, which can be ascribed to an increase of intermolecular interaction in the nanonetwork of the organic aerogels with increasing M/PR ratios or with decreasing m-C/PR ratios.