Acylhydrazone-Functionalized Waterborne Polyurethane Coating: An Intelligent Protective Material Integrating High Strength and Toughness, Self-Healing and Acid-Responsive Fluorescence

Abstract

A protective coating material exhibiting high mechanical strength, efficient self-healing, and acid-responsive fluorescence was synthesized by introducing a designed diacylhydrazone-derived molecule (HDs, namely, N′4,N′′′4′-bis(4-(diethylamino)-2-hydroxybenzylidene)-[2,2′-bipyridine]-4,4′-dicarbohydrazide), which was synthesized via the nucleophilic addition reaction of [2,2′-bipyridine]-4,4′-dicarbohydrazide and 4-(N,N-diethylamino) salicylaldehyde. Fourier transform infrared spectroscopy quantitatively showed that the introduction of HDs increased the content of ordered hydrogen bonds in waterborne polyurethane (WPU) from 43.3% to 50.4%. The tensile strength, elongation at break, and toughness of WPU-HDs3 reach 69.3 MPa, 1257.8%, and 309.5 MJ/m³, respectively. The hydrogen and ionic bonds in HDs and WPU formed a dynamic cross-linked network, endowing the material with a 100% self-healing rate within 24 h at room temperature. WPU-HDs3 also showed good adhesion to a steel substrate, with a peel force of 13.28 N. The network constraint inhibited the intramolecular rotation of HDs, stabilizes its D-π-A conjugated plane, completed the radiative transition through intramolecular charge transfer (ICT), achieved luminescence, and improved fluorescence efficiency via the proton-coupled electron transfer (PCET) mechanism. Under acidic conditions, the protonation/deprotonation of diethylamino and the alteration of PCET processes promoted the dynamic reconstruction of hydrogen bond networks. This confers a reversible fluorescence response to the material, enabling its application as a coating material for acid corrosion sensing.