Nanoporous Melamine−Formaldehyde Gels by Microemulsion Templating

Abstract

The polycondensation behavior of water-soluble melamine formaldehyde (MF) resins under acidic polymerization conditions was studied within a bicontinuous microemulsion comprising an oil phase (an alkane), a water phase (containing the reactive resin), and surfactant. The intrinsic nanoscopic feature size of such microemulsions was used as a structural template. Nonionic surfactants were found to be highly compatible with MF resin chemistry. Polycondensation using unmodified MF resins lead to gels showing large spherical voids arising from the separation of the oil phase. In contrast, by incorporating hydrophobic monomers such as benzoguanamine and caprinoguanamine into the resin (amphiphilic resin approach), a gel was formed consisting of aggregated nanoparticles confined within a continuous pore network. In this way, the effects of phase separation were successfully diminished, and porosities of 86 vol % (ρ = 240 g/L) were achieved with 65 nm pore sizes. Characterization of the materials was performed via scanning electron microscopy, nitrogen sorption at 77 K, and mercury intrusion.