Effect of end-capping strategies for regulating multiple-dynamic-bond crosslinked networks on the mechanical self-healing and degradability of vanillin-based waterborne polyurethane

In recent years, self-healing waterborne polyurethanes (SWPUs) with stimuli-responsive properties have attracted increasing attention. It is critical that dynamic crosslinking networks are designed to respond to appropriate stimulus. In this work, the effects of disulfide bonds, acylhydrazone bonds, and intrinsic hydrogen bonding on the mechanical properties, self-healing performance, and degradation behavior of SWPUs were explored. Initially, disulfide bonds were introduced into the crosslinked networks through a simple chemical modification of cystamine dihydrochloride, providing a cost-effective alternative to typical disulfide bond monomers. Subsequently, bio-based vanillin was used to cap isocyanate groups at ratios of 0.25, 0.5, 0.75, and 1.0, creating reactive sites for the condensation of diacylhydrazine with carbonyl groups. By adjusting the end-capping ratios, varying concentrations of acylhydrazone bonds were incorporated into the networks. The results indicated that increasing the end-capping ratio led to >83.4 % self-healing efficiency of SWPUs under both thermal and acidic conditions. The disordered hydrogen bonds improved the fracture strength of SWPUs, while the ordered hydrogen bonds enhanced the stiffness during stretching. In addition, the degradation test results revealed that SWPU was fully decomposed in a THF solvent (2-mercaptoethanol/deionized water/THF = 2/2/8, volume ratio). This excellent degradability is primarily attributed to the combined effects of imine hydrolysis, dynamic disulfide bonds, and copolymer fragmentation at single bonds near the carbonyl groups. This work presents a novel approach for designing polyurethane elastomers with multiple dynamic adaptive networks, offering the potential for low-cost production and cleaner preparation.