Novel mini-reactor of silicone oil droplets for synthesis of morphology-controlled polymer particles

Inside spaces of emulsion droplets can be used as mini-reactors for material synthesis. The novel application of sol−gel derived silicone oil droplets as mini-reactors was examined for the case of polymerization of styrene (St) and comonomers with the oil-soluble initiator 2,2′-azobis(2,4- dimethylvaleronitrile). Polydimethylsiloxane (PDMS) droplets prepared from dimethylsiloxane were used as the minireactors, in which the polymerization of St without comonomers was first conducted. In the polymerization, the St/PDMS volume ratio was varied from 0.025 to 0.10. After the polymerization, each PDMS droplet contained a polystyrene (PSt) particle. The St/PDMS ratio of 0.05 enabled the synthesis of micrometer-sized, spherical PSt particles with low polydispsersity. Copolymerization of St with comonomers having hydrophilic groups deformed the spherical shape of particles to lens-like or disk-like morphologies that were obtained with acrylic acid or sodium 4-styrene sulfonate, respectively. In another copolymerization, with divinylbenzene used as a crosslinker, hemispherical polymer particles were formed. To diversify the particle morphologies further, the proposed mini-reactor synthesis was combined with the recently proposed silicone oil droplet templating method (Ohta et al., 2012). Around the PDMS droplets containing a polymer particle, polymeric shells with a depression were successfully formed with the proposed method. The remaining PDMS oil inside the polymeric shells was extracted with ethanol, which caused hemispherical polymeric bowl-shaped capsules having a protrusion on the inside. ■ INTRODUCTION An emulsion confines a liquid to droplets dispersed in a liquid with which it is immiscible. The small spaces can be used as mini-reactors for the synthesis of morphology-controlled particles.1−3 Inverse emulsion synthesis and mini-emulsion polymerization are typical examples that are widely applied to the production of functional materials. A problem of those synthetic methods is the size distribution of the produced particles, resulting from the coalescence and redispersion of dispersed phases in the former method and the mechanical dispersion, normally by ultrasonication, in the latter. This limitation can be resolved by droplet-based microfluidics, in which nanoparticles are synthesized by confining the reactions to the interior of droplets.4,5 Silicone emulsion prepared by the sol−gel process can also be prepared in the form of highly monodisperse silicone oil droplets dispersed in an aqueous phase.6 The droplets are stably dispersed by electrostatic repulsion between the droplet surfaces. It has been reported that the size of the droplets can be controlled from a few hundred nanometers to several micrometers with additives such as surfactants and polymer stabilizers.7,8 The silicone emulsion synthesis was further extended to the silica encapsulation of polydimethylsiloxane (