Synthesis of non-spherical polymer particles using the activated swelling method

The preparation of particles with non-spherical shapes is a challenging endeavor, often requiring a significant ingenuity, complex experimental procedures and difficulties to obtain reproducible results. In this work we prove that monodisperse non-spherical polymer particles possessing asymmetric Janus structure can be easily produced by using an activated swelling method in combination with a control of the rate of free radical polymerization through the addition of the inhibitors 4-methoxyphenol (MEHQ) and O2. Monodisperse non cross-linked polystyrene particles, used as seeds, are activated by the addition of an initiator, which promotes their swelling ability, and then swollen with a monomers mixture (methyl methacrylate, glycidyl methacrylate and ethylene glycol dimethacrylate), before being polymerized in presence of both MEHQ and O2. Our results show that only when both MEHQ and O2 are present during the course of the polymerization, the particles shape can be controlled, from spherical to asymmetrical. A variety of particles shapes can be obtained, ranging from dimpled spheres, flattened spheres and Janus particles by varying the swelling ratio, always with excellent monodispersity and reproducibility. Finally, to provide even more complex functionalities to these non-spherical polymer particles, iron oxide nanocrystals were grown within the polymer matrix resulting in superparamagnetic particles.