High-Efficiency Polymerization for Synthesizing Polymer Nanoparticles in Melamine Foam Using Ultraviolet Irradiation

Polymeric nanoparticles are primarily synthesized through emulsion polymerization, where surfactants are necessary to maintain dispersion stability. However, surfactants cause significant environmental pollution. The soap-free emulsion polymerization of styrene in melamine foam (MF) using a three-dimensional micronetwork structure as a reactor has been used to synthesize polymer nanoparticles, resulting in an extremely low polymer yield. To address this problem, the temperature of MF was reduced to prevent the diffusion of the monomer, which was driven by the difference of temperature between the bottom and top of MF. Furthermore, ultraviolet irradiation was applied to perform radical polymerization using an azo initiator, 4,4'-azobis(4-cyanovaleric acid). The resulting polymer yield was over 80%, although the reaction temperature was reduced from 70 to 40 °C. However, at an MF porosity of 0.60, the average polystyrene particle size was 49.2 nm with a polymer yield of 89.7%. The pore sizes in the MF were made homogeneous by pressing the MF to reduce the porosity, resulting in uniform nanoparticles.