Facile synthesis of acryloxy polyhedral oligomeric silsesquioxane (AC-POSS) for simultaneous improvement of flame retardancy and transparency in vinyl ester resins

This study reports a facile synthesis of acryloxy-functionalized polyhedral oligomeric silsesquioxane (AC-POSS) through hydrolytic condensation of methacryloxypropyltrimethoxysilane (MPS) and phenyltrimethoxysilane (PTMS). Structural analysis via FTIR, NMR, and MALDI-TOF MS confirmed the formation of a T₁₀ cage structure with coexisting methacryloxypropyl (R₁) and phenyl (R₂) substituents, where R₁-Si groups predominated. When integrated into vinyl ester resin (VER), AC-POSS significantly enhanced thermal stability and flame retardancy. Thermal analysis demonstrated char residues of 45.9 wt% (VER/50AC-POSS) and 65.7 wt% (AC-POSS-pure) at 900 °C, surpassing pure VER (17.3 wt%). DSC revealed a 15 % increase in glass transition temperature (T_g) for AC-POSS-pure, attributed to restricted chain mobility from crosslinked POSS cages. Flame retardancy tests showed UL-94 V-0 ratings with limiting oxygen indices (LOI) of 29.0 % (VER/50AC-POSS) and 36.5 % (AC-POSS-pure), alongside 38.5 % and 58.8 % reductions in peak heat release rate (p-HRR). Remarkably, VER/50AC-POSS and AC-POSS-pure retained high optical transparency (83.4 % and 86.3 % at 550 nm) with minimal haze (10.5 % and 4.2 %), underscoring their compatibility and suitability for transparent applications. This work highlights AC-POSS as a multifunctional additive for developing high-performance, fire-safe, and optically clear thermosetting composites.