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 similar or equal to 2 with a bulk density of 160 kg/m(3) and a Brunauer, Emmett and Teller (BET) surface area of 970 m(2)/gm. MF #2 was an opaque aerogel formed at pH similar or equal to 0.5 with a bulk density of 130 kg/m(3) and a surface area of 850 m(2)/gm. Both MF aerogels have a polymeric nanostructure. Melamine and formaldehyde formed single polymeric chains of similar or equal to 0.95 nm in diameter. The MF #1 aerogel connectors were frequently composed of two or three polymeric chains with the connector diameter averaging similar or equal to 2.0 and similar or equal to 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.