Size-dependent nanoscale soldering of polystyrene colloidal crystals by supercritical fluids

Hypothesis: Polymer particles self-assembled into colloidal crystals have exciting applications in photon-ics, phononics, templates for nanolithography, and coatings. Cold soldering utilizing polymer plasticiza-tion by supercritical fluids enables a novel, low-cost, low-effort, chemical-free means for uniform mechanical strengthening of fragile polymer colloidal crystals at moderate temperatures. Here, we aim to elucidate the role of particle size and gas-specific response for the most efficient soldering, exploring the full potential of this method.Experiments: We investigate the elastic properties of polystyrene colloidal crystals made of nanoparticles with different diameters (143 to 830 nm) upon treatment with supercritical Ar and He at room temper-ature. By employing Brillouin light scattering, we quantify the effect of nanoparticle size on the strength-ening of interparticle contacts, evaluating the permanent change in the effective elastic modulus upon cold soldering.Findings: The relative change in the effective elastic modulus reveals nonmonotonic dependence on the particle size with the most efficient soldering for mid-sized nanoparticles (about 610 nm diameter). We attribute this behavior to the crucial role of intrinsic fabrication impurities, which reduces the nanopar-ticles' free surface exposed to plasticization by supercritical fluids. Supercritical Ar, a good solvent for polystyrene, enabled effective soldering of nanoparticles, whereas high-pressure He treatment is entirely reversible.(c) 2022 Elsevier Inc. All rights reserved.