## 1 Introduction
From the intermolecular force point of view, molecules in the surface layer of a material are subjected to a higher molecular attractive force, and this molecular attractive force is the surface tension. The higher the surface tension, the higher the intermolecular interaction force. From the energy point of view, the molecules in the surface layer of a material have additional potential energy due to surface tension, and this potential energy is the surface free energy. As an important characteristic of the material itself, it has a significant impact on the interfacial properties and interfacial reactions of the material, such as surface adsorption, wetting, and bonding phenomena, all of which are governed by the surface free energy to a certain extent. [PERSON] et al. [30] calculated the adhesion work and spalling work between asphalt and aggregate using the contact angle test based on surface free energy theory and compared the results with the bond strength test results, which showed that the results of the adhesion work and spalling work calculations basically verified the results of the asphalt bond strength (ABS) test. [PERSON] et al. [31] found that based asphalt has higher surface free energy compared with modified asphalt through contact angle experiments. In addition to based asphalt and modified asphalt, the surface free energy theory has also been used to evaluate the adhesion properties of reclaimed asphalt. [PERSON] et al. [32] established an adhesion model for warm mix recycled asphalt and aggregate under water and no-water conditions based on the surface free energy theory and evaluated the water stability of warm mix recycled asphalt by calculating the adhesion work under both models. This shows that the surface free energy theory has been widely used to evaluate the adhesion properties between asphalt and aggregates. Although the surface free energy theory has rarely been used to evaluate the adhesion properties of polymer mixtures, many of the above studies on the adhesion properties of asphalt mixtures can be extended to polymer mixtures as well. Molecular dynamics (MD) is a computational simulation method used to model and study the motion and interactions of atoms or molecules in a given physical environment. By establishing molecular models and microstructures through MD, it is possible to obtain information on various parameters of materials and predict the macroscopic properties of materials from the perspective of micro-mechanisms. In the field of road engineering, MD has been widely used to analyze the rheological properties, aging and reclaimed properties, compatibility, and self-healing properties of road materials [33; 34; 35; 36; 37]. In recent years, MD has been gradually used to study the interfacial adhesion properties between road materials, and by studying the interfacial interactions between materials on the nanoscale, the chemical composition of materials can be optimized, and their adhesion properties can be improved. [PERSON] et al. [38] calculated the surface free energy as well as the adhesion work of aggregates with asphalt using MD simulation and compared it with the results of the pull-out test and found that the MD results were in agreement with the pull-out test results. [PERSON] et al. [39] used MD to establish the molecular model of road epoxy resin sealant and studied its adhesion with pavement cracks. The results showed that epoxy resin sealant has better adhesion properties than based asphalt and SBS-modified asphalt. [PERSON] et al. [40] used MD to analyze the adhesion of cold reclaimed emulsified asphalt with aggregate and found that, with the increase in emulsifier, the adhesion of asphalt with aggregates was improved. Therefore, in this study, a PU binder was prepared, and the adhesive properties between PU and aggregate were investigated using surface free energy theory and molecular dynamics simulation.