Prof. Dr. Axel Müller

We report a versatile large-scale synthesis strategy for hybrid Janus nanoparticles with a silica core and a unilaterally attached polymer corona in a size range below 100 nm. The stimuli-responsive behavior of these nanoparticles with a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) hemicorona is investigated. The synthesis is based on a modified version of the Pickering emulsion polymerization technique in combination with surface-initiated atom transfer radical polymerization (ATRP) in a “grafting from” approach. In a first step, poly(vinyl acetate) (PVAc) latex particles are prepared via Pickering emulsion polymerization. Colloidal stability is provided by 30 nm silica nanoparticles that adhere to the surface of the growing polymer particles. This results in polymer latexes which are armored with a layer of tightly immobilized nanoparticles, one side of which is immersed in the polymer particle and thus protected. After modification of the exposed side of the particles through chemisorption of an ATRP initiator, that is (2-bromo-2-methyl)propionyl-oxyhexyltriethoxysilane, and the removal of the particles from the interface, PDMAEMA chains are grown from this modified side of the silica particles, yielding well-defined Janus nanoparticles with a stimuli–responsive PDMAEMA hemicorona. Transmission and scanning electron microscopy, dynamic light scattering, thermogravimetric analysis and turbidity measurements were used to characterize the Janus particles. Most importantly, this synthetic approach is easily scalable and can be amended to furnish a wide range of nanoscale hybrid Janus particles. Furthermore, we demonstrate reversible switching behavior upon pH and temperature changes and find a peculiar self-assembly behavior of the Janus particles into linear strings at low pH and high concentration.