Interfacial Scattering Engineering in PDMS-Modified Waterborne Polyurethane Composite Films for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) can reduce building energy use and alleviate urban overheating without external power. Yet, waterborne polymer films that integrate strong radiative cooling with environmental durability remain limited. Here, we report a waterborne polyurethane (WPU) radiative cooling film platform in which polydimethylsiloxane (PDMS) blocks are covalently incorporated into the WPU backbone and subsequently combined with micron-sized SiO 2 /TiO 2 scatterers. At the matrix level, PDMS promotes surface siloxane enrichment and microphase reorganization, increasing the solar reflectance from 27% for neat WPU to 44–45% for the WPU-PDMS matrix, while preserving strong broadband mid-infrared thermal emission, including a Planck-weighted 8–13 μm atmospheric-window emissivity of 93–95%. Relative to the optimized WPU-PDMS matrix, subsequent incorporation and loading optimization of the SiO 2 /TiO 2 fillers (S30-ST40) further increase the solar reflectance from 44% to 90%, while the visible reflectance reaches 93%, and the average atmospheric-window emissivity remains 94%. Compared to the ambient temperature, the cooling effect of S30-ST40 is approximately 2.3 °C, and the maximum temperature reduction of the cavity is approximately 7.0 °C. In addition, it remains about 10–12 °C cooler than the PDMS-free WPU control. The films also retain practical mechanical robustness, with tensile strengths of 9–11 MPa, low water absorption of 7.60–9.10%, water contact angles up to 116°, and good UV durability; S30-ST40 exhibits only a 2.5% loss in both solar and visible reflectance after accelerated UV weathering. EnergyPlus simulations across 17 representative cities predict 16–49% reductions in annual cooling energy demand. Overall, this work demonstrates that matrix–filler coengineering is an effective route to durable waterborne PDRC films for building-envelope applications.