Tailormade PMMA Spheres: Synthesis and Growth Mechanism

Inverse opal structures are of interest for various applications, as they exhibit high surface areas in conjunction with unique structure-specific properties such as the possibility to create photonic band gaps, e.g., for photocatalytic applications. An established synthetic pathway to prepare these nanostructures is to infiltrate the voids of a template comprised of close-packed spheres with a metal oxide and to remove the template subsequently by pyrolysis. To this end, polymer spheres are typically used which are produced by a water-based emulsion polymerization process. In this work, we present an improved and extended approach of that kind in case of PMMA spheresfeaturing narrow size distributions and mean diameters that can be varied over a large range between 170 up to 800 nm by properly adjusting the synthesis temperature and the ionic-strength of the water phase. By using reflux conditions, advanced experimental techniques requiring protective gas atmospheres are dispensable and comparatively short synthesis times can be realized. Time-resolved experiments reveal a two-step growth process occurring at temperatures below ∼400 K. It consists of a first phase, during which initial particles are formed, followed by a time-delayed second phase, where their diameter increases by roughly a factor of 2, most likely due to coalescence processes. At higher temperatures, both processes increasingly overlap so that only a single growth phase is observed.