Multiscale Interfacial Engineering of Ferric‐Ion‐Coordinated Polydopamine‐Coated Geopolymer Reinforced Epoxy/Polyurethane Adhesives for High Strength and Freeze–Thaw Durability

ABSTRACT

Structural adhesives used in wet–cold environments are vulnerable to moisture ingress, ice expansion, and thermal mismatch. Here, a low‐loading ferric‐ion (Fe ³⁺ )‐coordinated polydopamine (PDA)‐coated geopolymer (GP), denoted Fe ³⁺ ‐PDA@GP, was introduced into an epoxy/polyurethane (EP/PU) adhesive to construct a multiscale hybrid interphase. The geopolymer core provides physical reinforcement and crack‐pinning effects, the PDA shell improves organic–inorganic interfacial compatibility, and Fe ³⁺ –catechol coordination introduces additional coordination‐mediated interfacial interactions that are associated with improved stress redistribution and fracture resistance. At an optimized loading of only 0.75 wt%, the adhesive achieved a lap‐shear strength of 29.2 MPa and a work of debonding of 14.5 kN m ⁻¹ , representing increases of 47.5% and 202.1% over neat EP/PU, respectively. Strong adhesion was maintained across multiple metallic substrates. After 100 freeze–thaw cycles between −40°C and 20°C, the adhesive retained 20.8 MPa, while two‐dimensional correlation analysis of Fourier transform infrared spectra indicated reduced molecular rearrangement. The modified adhesive also exhibited improved storage modulus and slightly increased glass‐transition temperature without compromising insulation. This work demonstrates a bioinspired, low‐filler strategy for developing strong, tough, and freeze–thaw‐resistant structural adhesives.