Achieving Ultrahigh Transparency and Superior Mechanical Properties in Flexible Polyimide Nanofoams Through CO₂ Foaming for Thermal Insulation

Abstract Transparent polymer nanofoams offer significant potential for thermal insulation, particularly for energy‐saving windows, where both transparency and low thermal conductivity are crucial. However, grand challenges remain in preparing polymer nanofoam monoliths with pore sizes smaller than 40 nm to achieve high optical transparency via minimized light scattering. Herein, the preparation of highly transparent nanofoam monoliths based on fluorinated polyimide (FPI) using CO 2 foaming is reported for the first time. The high CO 2 absorption capacity and robust melt strength of the FPI matrix significantly promote bubble nucleation while minimizing cell coalescence during CO 2 foaming process, resulting in ultra‐small pore sizes down to sub‐10 nm. These FPI nanofoams, with a thickness of ≈0.2 mm, can exhibit high optical transmittance (>85% in the wavelength range of 500–700 nm) and low haze (≈18%), which is unprecedented for polymer foams prepared using foaming methods. Furthermore, these FPI nanofoams demonstrate superior mechanical and thermal insulation properties compared to micron‐sized FPI foams and maintain remarkable mechanical flexibility and structural stability even under harsh conditions, including extreme temperatures and water exposure. Transparent nanofoams prepared using a scalable CO 2 foaming method can serve as the next‐generation thermal insulation materials, with broad potential applications in energy‐efficient buildings, and aerospace.