Nanovoids contained
in hollow particles render them potential drug
carriers. However, conventional methods for the synthesis of these
particlhes involve complex processes. In this study, we developed
a template-free method for the synthesis of hollow polymer and silica
particles by initially preparing polystyrene core particles containing
2,2′-azobis(2-methylbutyronitrile) (V-59) through soap-free
emulsion polymerization. The subsequent generation of nitrogen gas
inside these particles upon the decomposition of V-59 at 70 °C
led to the formation of voids. In addition, silica shells were synthesized
on the surfaces of the V-59-containing polystyrene core particles
through a sol–gel reaction at 40 °C, following which voids
were formed by the decomposition of V-59 at 70 °C. The developed
method involves straightforward steps and is environmentally friendly,
as it does not require the use of surfactants, organic solvents, or
templates.
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.
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