Size-controlled spherical polymer nanoparticles: Synthesis with tandem acoustic emulsification followed by soap-free emulsion polymerization and one-step fabrication of colloidal crystal films of various colors

•Tandem acoustic emulsification gives clear and stable nanoemulsions of methylmethacrylate (MMA) monomer.•Tandem acoustic emulsification made soap-free emulsion polymerization possible.•Colloidal crystal films were prepared from the as-polymerized PMMA nanoparticle solution using the fluidic-cell method.•Addition of small amounts of organic solvent to the as-polymerized nanoparticles solution tune the structural color.

We have developed a novel synthesis method for size-controlled polymer nanoparticles using soap-free emulsion polymerization. This new synthetic method involves sequential ultrasonic irradiation (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz) for acoustic emulsification of a water-insoluble monomer such as methylmethacrylate (MMA) in an aqueous medium, followed by emulsion polymerization in the obtained solution without using any surfactants. The sequential ultrasonication (tandem acoustic emulsification) could provide a clear and stable emulsified solution containing monomer droplets with relatively narrow size distribution in the nanometer range. The subsequent polymerization in this solution yielded size-controlled polymethylmethacrylate (PMMA) nanoparticles and monodisperse PMMA nanoparticles of different sizes. Furthermore, colloidal crystal films could be easily prepared from the as-polymerized nanoparticle solution using the fluidic-cell method. Moreover, we succeeded to modify the structural color of colloidal crystal films by the addition of a small amount of organic solvent to the as-polymerized nanoparticle solution for the fluidic-cell method.