Synthesis of PMMA-SiO2 Microstructures

The PMMA-SiO₂ microstructures were synthesized by mixing SiO₂ aerogel microparticles (4–10 μm in size, Sigma-Aldrich) and 20 wt% PMMA solution in anisole (950 A2, Kayaku) at a mass ratio of 1:5 using a central mixer (ARE-310, Thinky) at 2000 rpm for 3 min. Then, the mixture was baked at 100 °C for 1 min to remove anisole. Then, c-PI (TPI-100, PNS tech) was then mixed with the prepared PMMA-SiO₂ microstructures at the desired mass ratio (0–24 wt% (i.e., 0–16 vol%) PMMA-SiO₂ microstructures in c-PI). The prepared mixture was spin-coated onto an aluminum substrate at 700 rpm for 5 min. The samples were cured at 100 °C for 30 min, and 150 °C for 30 min in successive manner. Finally, the cured film was peeled off from the substrate, and the film thickness was approximately 50 μm.