Hyperbranched Polyborophosphate towards Transparent Epoxy Resin with Ultrahigh Toughness and Fire Safety

Inherent transparency makes epoxy resins ideal for aircraft windows, yet their brittleness and flammability remain challenges. Existing strategies for these issues often compromise transparency, with limited research on the mechanisms involved. Herein, a novel strategy is proposed for fabricating transparent epoxy resin by tuning the electrostatic potential distribution via hyperbranched polyborophosphate. Electron-deficient boron and relatively electron-rich phosphorus atoms work synergistically to increase the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap, preventing visible light absorption. Meanwhile, the hyperbranched structure facilitates polymer network interpenetration to reduce porosity for decreased light scattering. This synergy results in a nearly colorless material with over 80% transmittance at 550 nm even at 4 mm thickness, along with full-band UV shielding. Notably, the material demonstrates a 114.7% increase in impact toughness (45.2 kJ m^-2) due to dual dynamic B─O and P─O linkages. Besides, it yields a limiting oxygen index of 33% and a V0 rating in the underwriter laboratories vertical burning test, along with significant reductions in heat, smoke, and toxic gas release. The outstanding performance makes it stand out compared to reported advanced transparent epoxy resins, highlighting the significance of this work.