Molecular simulation study on the high-temperature viscoelasticity of PU–CR composite modified asphalt sealants

To investigate the high-temperature viscoelastic performance of polyurethane-crumb rubber modified asphalt sealants under typical high summer pavement temperatures, this study established an experimental system with a temperature gradient of 46-76 °C. Macroscopic rheological tests, including multiple stress creep and recovery, frequency sweep, and temperature sweep, were combined with molecular dynamics simulation parameters-mean squared displacement, shear modulus, and radius of gyration recovery-to systematically analyze the effects of PU content on sealant viscoelasticity and microstructure. Meanwhile, the temperature sensitivity index and Fourier transform infrared spectroscopy were employed to quantitatively evaluate the thermal susceptibility and internal chemical interactions of the sealant. These analyses provide complementary insights into the modification mechanism from both macroscopic performance responsiveness and microstructural chemical evolution perspectives. The macroscopic test results indicate that under low-stress conditions, increasing PU content significantly enhances the creep recovery rate, thereby improving elastic recovery performance. Under high-stress conditions, PU incorporation distinctly minimizes the non-recoverable creep compliance, successfully shifting the deformation mechanism from low-stiffness viscous flow to a robust elastic-dominated behavior. Frequency sweep tests reveal a synergistic enhancement effect between PU and CR, which not only markedly increases the complex shear modulus but also reduces the phase angle, thereby strengthening resistance to deformation and elastic response under dynamic loading. Temperature sweep results further confirm that PU incorporation effectively increases the proportion of elastic components, mitigating high-temperature-induced reductions in overall modulus, with modification performance superior to CR-only systems. Fourier transform infrared spectroscopy and molecular dynamics simulations elucidate the concentration-dependent microstructural regulation mechanism. At low PU dosages of 2% and 4%, the crosslinking network remains relatively sparse with limited chain interactions. Conversely, medium-to-high PU contents from 6% to 8% enhance chemical crosslinking density, chain traction, and inter-chain hydrogen bonding. This triggers an exponential growth of elastic stiffness constants and shear modulus at high temperatures, successfully building a multidimensional support network formed by crosslinked PU chains. Analysis of the radius of gyration recovery shows that these optimal formulations form a thermally stable PU-asphalt-CR interlocking skeleton, optimizing chain folding density, restricting thermal chain cleavage, and improving structural recovery, thereby maintaining superior structural integrity and viscoelastic balance across a wide high-temperature range up to 76 °C. This study, through the integration of macroscopic experiments and microscopic simulations, reveals the regulatory mechanism of PU on sealant performance, providing a theoretical basis for the optimized design of pavement sealing materials in high-temperature regions.