Polymer Colloidal Crystals reinforced by Supercritical Fluids*

Colloidal crystals realized by self-assembled polymer nanoparticles are highly attractive for visible light photonics, GHz phononics and superhydrophobic coatings. However, their fragility remains one of the main concerns when it comes to applications. Here, uniform mechanical reinforcement and tunability of 3D polystyrene colloidal crystals, by means of supercritical nitrogen and argon, is presented. This method is a synergistic combination of nanoscale plasticization of particles surface and compressive hydrostatic pressure that leads to strengthening at temperatures well below the glass transition. It results in the formation of permanent physical bonds between the particles while maintaining their shape and periodic arrangement, which we term "soldering". We employed Brillouin light scattering to monitor in-situ the mechanical vibrations of the crystal and thereby determine preferential pressure, temperature and time ranges for soldering. This method offers a chemical-free and efficient solution for fabrication and tuning of durable devices. Moreover, plasticization of polymeric nanostructures by means of supercritical fluids offers new effects and opportunities.