Dual-Scale Hollow-in-Porous PVDF Composite Films Integrating Broadband Reflectance and Fire Safety for Secure Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) has emerged as a promising zero-energy thermal management technology for building energy conservation. However, the inherent porosity of polymeric cooling materials renders them highly flammable, which facilitates rapid flame spread and poses severe safety hazards in practical applications. To address these challenges, this study proposes a design strategy of "dual-scale optical and multifunctional chemical integration", successfully fabricating a dual-scale hollow-in-porous polyvinylidene fluoride (PVDF) composite film that elegantly integrates broadband reflectance with exceptional fire safety. Specifically, hollow core-shell hybrid fillers (ZIF-8@PA@SiO2) were synthesized by etching ZIF-8 templates with phytic acid (PA), followed by uniform encapsulation within a SiO2 shell. These fillers were subsequently incorporated into a PVDF matrix via a phase-inversion process, yielding a hierarchical dual-scale hollow-in-porous architecture. Benefiting from enhanced Mie scattering induced by this structural design, alongside the synergistic molecular vibrations of polar Si-O-Si and C-F bonds, the resulting film delivers an outstanding solar reflectance of 92.3% and a high infrared emissivity of 94.8%. Under outdoor conditions, the composite achieves a sub-ambient temperature drop of 12.7 °C, while under simulated conditions, it exhibits a temperature reduction of 19 °C relative to the bare substrate. The corresponding cooling powers reach 109.3 and 130.7 W m-2 during the day and night, respectively. Crucially, the incorporation of ZIF-8@PA@SiO2 drastically bolsters the material's flame retardancy. Compared to the pristine substrate, the peak heat release rate (pHRR) and total heat release (THR) are curtailed by 52.8 and 52.4%, respectively. This facile and highly effective fabrication strategy paves the way for the secure and practical architectural deployment of PDRC materials.