Surface Engineering of Polymeric Colloidal Crystals by Temperature - Pressure Annealing.

Polymer colloidal crystals (PCCs) have been widely explored as acoustic and optical metamaterials and as templates for nanolithography. However, fabrication impurities and fragility of the self-assembled structures are critical bottlenecks for the device's efficiency and applications. We have demonstrated that temperature-assisted pressure [ $T,p]$ annealing results in the mechanical strengthening of PCCs, which improves with the annealing temperature. Here, the enhancement of elastic properties and morphological features of self-assembled PCC's is evaluated using Brillouin light scattering and scanning electron microscopy. The pressure-induced effects on the vibrational modes of PCCs are also illustrated at temperatures well below the polymer glass transition. While the PCCs colloid constituents display reversibility, the PCC material is strongly irreversible in the performed thermodynamic cycle. The effective elastic modulus enhances from 0.7 GPa for the pristine sample to 0.8 GPa, solely by pressure annealing at room temperature. [ $T,p]$ annealing at higher temperatures leads to a maximum effective elastic modulus of 1.7 GPa, more than twice the value in the pristine sample. Above a cross-over pressure, $p_c(\approx$ 725 bar at 348 K), the PCCs respond elastically and, hence, reversibly to pressure changes.