High-resolution transmission electron microscopy of the nanostructure of melamine-formaldehyde aerogels

The transparency of melamine-formaldehyde (MF) aerogels strongly depends on the solution pH during polymerization. Two MF aerogels were studied. MF #1 was transparent and was formed at pH ⋍ 2 with a bulk density of 160 kg/m3 and a Brunauer, Emmett and Teller (BET) surface area of 970 m2/gm. MF #2 was an opaque aerogel formed at pH ⋍ 0.5 with a bulk density of 130 kg/m3 and a surface area of 850 m2/gm. Both MF aerogels have a polymeric nanostructure. Melamine and formaldehyde formed single polymeric chains of ⋍ 0.95 nm in diameter. The MF #1 aerogel connectors were frequently composed of two or three polymeric chains with the connector diameter averaging ⋍ 2.0 and ⋍ 2.8 nm which usually showed branching every 12.6 ± 4.5 nm thereby defining the connector length. The side chains along the connectors in MF #1 formed a reticular MF matrix. The MF #1 matrix pores averaged 10.4 ± 5.5 nm and pores between sections of the MF matrix averaged 28.8 ± 19 nm. In MF #2, the connectors also formed side chains but these side chains did not form a reticular matrix. As a consequence, the connector diameter, 8.8 ± 5.8 nm, includes the side chains projecting from a core chain of 1.9 ± 0.7 nm. In addition to the average pore size of 47.7 ± 30 nm, MF #2 contained cavities or voids within the aerogel with length dimensions averaging 328 ± 130 nm, responsible for its opacity. The high BET surface areas of both MF aerogels correlate with their polymeric structure.