Multiscale Simulation Reveals Mechanism of Water Resistance in Long-Chain-Modified Polyurethanes and Their Potential for Pavement Engineering Applications

Polyurethane (PU) has been extensively investigated for road engineering applications due to its low production energy demand and excellent mechanical performance. However, insufficient water resistance severely limits its long-term durability under service conditions. This study proposes a strategy to simultaneously enhance the water resistance and mechanical properties of PU through the incorporation of long-chain alkane side chains (MG). The underlying mechanisms were systematically investigated using atomic force microscopy (AFM), molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. The results indicate that optimal comprehensive performance was achieved at MG contents of 20% for PTMEG-PU and 30% for PPG-PU at both binder and mixture levels. AFM characterization reveals that an appropriate MG dosage promotes the formation of a more distinct microphase-separated morphology in PU. MD simulations indicate that moderate MG incorporation effectively suppresses water–PU interactions by masking polar hard segments, thereby reducing the number of available hydrogen-bonding sites and enhancing surface hydrophobicity. In contrast, excessive MG accumulation induces steric hindrance, accelerates water molecular mobility, and weakens hydrophobic stabilization. From a bulk structural perspective, MG incorporation strengthens intermolecular dispersion forces while weakening dipole–dipole interactions. At low MG contents, enhanced dispersion interactions dominate, resulting in improved hydrophobicity. However, excessive MG coverage of polar regions leads to reduced electrostatic interaction energy, increased relative free volume, and consequent deterioration of hydrophobicity. DFT calculations further reveal that MG acts as an electron donor, reducing the molecular dipole moments of PTMEG-PU and PPG-PU by 18.0 and 23.3%, respectively, accompanied by reductions in water adsorption energies of 53.6 and 57.4%. Pearson correlation analysis further validates the reliability of the simulation descriptors. Overall, this study provides mechanistic insights into water-resistance enhancement in PU systems and establishes a theoretical basis for their broader application in road engineering.