Size-controlled polymer nanoparticles are useful materials in various applications. In this work, we have successfully synthesized polymer nanoparticles which reflect precisely the size of monomer droplets formed by tandem acoustic emulsification. This synthetic method involves sequential ultrasonic irradiation (20 kHz→500 kHz→1.6 MHz→2.4 MHz→5 MHz) for acoustic emulsification of a water-insoluble monomer in an aqueous medium without any surfactants, followed by emulsion polymerization in the obtained solution.
Among the nanoparticles, hollow particles are particularly attractive materials due to their unique structure. Therefore, we also tried to prepare polymer hollow particles to demonstrate a further application of tandem acoustic emulsification. In this process, the acoustic emulsification is used to produce fluorous solvent droplets which are templates for polymer capsule synthesis. TEM-EDS analysis revealed that the core of the obtained polymer capsules filled with a fluorous solvent. Furthermore, by heating treatment, the core solvent could be removed, and consequently size-controlled polymer hollow nanoparticles were obtained.