Flexible Silanized TiO₂/SBS Nanocomposite Films with Optimized Emissivity for Passive Radiative Cooling Applications

Passive radiative cooling (RC) requires materials that combine high solar reflectance with strong long wavelength infrared (LWIR) emissivity in the 8-13 μm atmospheric window (AW). In this work, freestanding nanocomposite films were fabricated by embedding MPTMS-functionalized TiO₂ nanoparticles (TiO₂NPs-MPTMS) into a styrene-butadiene-styrene (SBS) triblock copolymer matrix. The resulting composites, referred to as NPs-SBS, were designed to be flexible, scalable, and cost-effective for potential RC applications. The SBS polymer provides emissive response in the 8-13 μm AW, whereas the inorganic component governs the solar reflection properties. Surface functionalization yielded Ti-O-Si linkages and a silica-like shell that enhances nanoparticle dispersion and stability while introducing additional vibrational modes in the LWIR range. Field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy characterization confirmed grain-like aggregates composed of nanoparticles with diameters ranging between 30 and 60 nm and verified the surface functionalization. To optimize RC performance, composites containing 1, 5, and 7 wt % of TiO₂NPs-MPTMS were investigated. Fourier transform infrared spectroscopy and hemispherical optical measurements in the solar range (0.3-15 μm) revealed that all nanocomposites exhibited enhanced emissivity within the 8-13 μm atmospheric window (ε_8-13 ≈ 0.63-0.65) compared to pristine SBS, while the solar absorptance remained relatively low, although it increased with nanoparticle loading. By combining the experimentally determined emissivity and solar-weighted absorptance within a simplified energy-balance model, the radiative cooling potential of the films was assessed. Among the investigated formulations, 1NPs-SBS and 5NPs-SBS exhibited the most favorable balance between LWIR emissivity and solar absorption, highlighting the importance of nanoparticle loading optimization for passive radiative cooling applications.