Self-Healing Behavior of Medium-Temperature Thermoresponsive Polyurethane-Modified Asphalt: Multiscale Structure–Property Relationships

Asphalt surface layers are prone to deterioration under long-term vehicle loads and environmental factors, making active healing binders essential for pavement durability. This study designed a polyurethane-modified asphalt (PUA) with dynamic disulfide bonds that can be thermally activated at medium temperatures (40–60 °C). Rheological tests, adhesion tests, and molecular simulations revealed the relationship between chemical composition, molecular structure, and healing performance from molecular to macroscopic scales. Results showed that asphalt cohesion and asphalt–aggregate bond strength were key intrinsic factors. Compared with the disulfide-free PUA, the DSDDP-containing formulations exhibited higher fatigue-healing indices and modest improvements in adhesion-healing indices under the tested conditions. For PUA with a DSDDP molar fraction of 10% in composite chain extender, the fitted healing curve predicted that the complex-modulus-based healing index (HI1) would reach 0.9 after approximately 680 s at 50 °C. At construction temperatures (>135 °C), dynamic exchange reduced network density and improved workability. Molecular dynamics simulations demonstrated that diffusion and movement accelerated with increasing initial spacing and rising temperature. S-S bonds reduced intermolecular forces via topological rearrangement, accelerating migration toward cracks and forming a denser structure after healing. This thermoresponsive PUA mitigates crack formation and expansion, enabling sustainable pavement applications.