Scalable All‐Polymer Photonic Crystals for Daytime Radiative Cooling

ABSTRACT Photonic crystals hold significant promise for passive daytime radiative cooling (PDRC) yet face inherent scalability–durability–performance trade‐offs. Although all‐polymer photonic crystals (APPCs) offer solutions, their development is constrained by limited refractive index contrasts ( Δn < 0.2), sub‐100‐nm layer fabrication challenges, interfacial delamination, and insufficient mechanical robustness. Here, we fabricated a scalable, high‐performance all‐polymer photonic crystal film via the self‐assembled gradient nanolayer coextrusion of poly(methyl methacrylate) and poly(ethylene naphthalate), followed by biaxial stretching. The resulting 1500‐layer hierarchical architecture, featuring gradient layer thicknesses ranging from 50 to 400 nm, achieved a solar reflectance of 95.4% and mid‐infrared emissivity of 93.4%, enabling sub‐ambient cooling of 11°C under 980 W/m 2 solar irradiance. The dense nanolayer structure also imparted exceptional mechanical properties, including a tensile strength of ∼103.8 m Pa, toughness of ∼54.9 m J/m 3 , and Young's modulus of ∼2.9 GPa, substantially exceeding those of existing polymer‐based radiative coolers. This solvent‐free, continuous fabrication process bridged nanophotonic design with industrial‐scale manufacturing, offering a practical fabrication route for durable, high‐performance polymeric cooling films.