Green Synthesis of Hollow Structures through the Decomposition of Azo Compounds Incorporated inside Polystyrene Particles.

Hollow polymer particles are applied in various fields owing to their high specific surface area and inner volume. The hollow regions in such particles are generally synthesized using a template. However, chemical agents must be used to remove the templates, which is associated with a high environmental load. To address this problem, we previously established a method for synthesizing hollow polymer particles without a template. However, the mechanism underlying this synthesis was unclear, which this study aimed to rectify. First, azo compounds were dissolved in a styrene monomer phase, and soap-free emulsion polymerization was performed to produce polystyrene particles. The azo compounds were incorporated into the polystyrene particles from the monomer phase at a polymerization temperature greater than the melting point of the azo compounds. Finally, the polystyrene particles were heated at a temperature greater than the 10 h half-life temperature of the azo compounds to emit nitrogen gas, and the azo compounds were decomposed to prepare the hollow regions in the polystyrene particles. However, the resulting particles were not hollow when the azo compound was not incorporated into the polystyrene particles. By comparing the melting behavior of different azo compounds, this study elucidates the mechanism underlying our template-free method for synthesizing hollow polystyrene particles.