Nanoencapsulated Phase-Change Materials with a Hybrid DOPO/SiO ₂ Shell for Integrated Thermal Regulation and Flame Retardancy in Building Materials

Nanoencapsulated phase-change materials (NanoPCMs) utilizing n-octadecane as the core material have been widely explored for thermal energy storage (TES) in buildings; however, the inherent flammability of this hydrocarbon-based PCM poses a significant safety concern. Herein, we developed a multifunctional core-shell NanoPCM comprising n-octadecane encapsulated within a vinyl-functionalized silicon dioxide (SiO₂) shell and then covalently grafted with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) through an in-situ interfacial P-H addition reaction. By regulating Q (SiO₄, fully condensed with four siloxane bonds)/T (RSiO₃, with one organic substituent R and three siloxane bonds) siloxane-network distribution in a hybrid DOPO/SiO₂ shell during sol-gel synthesis, the morphology of the NanoPCMs evolves from spherical to bowl-like structures and ultimately to donut-shaped nanocapsules. Pseudo-Voigt deconvolution of Si-O vibrational spectra revealed the sucking mechanism of hybrid DOPO/SiO₂ shells, which significantly affects the thermal performance of NanoPCMs. The optimized NanoPCM exhibits excellent thermal stability, flame retardancy, and a high latent heat (∼157.2 J g^-1). When incorporated into wood-plastic composites (WPCs)─promising yet flammability-prone building materials─it significantly reduces the peak heat release rate (47%) and smoke production (42%) while exhibiting effective temperature-buffering. This morphology-engineered, flame-retardant NanoPCM presents an efficient pathway toward safer, energy-efficient WPC-based construction materials.