# Mechanism of Rejuvenation in Aged SBS-Modified Asphalt by Density Functional Theory
## 1 Introduction  
Styrene-butadiene-styrene (SBS)-modified asphalt is widely used in pavement engineering due to its excellent high-temperature stability and low-temperature crack resistance [1; 2; 3]. With the changes of environment and traffic loads, SBS-modified asphalt pavement has entered the stage of maintenance and repairment in China. As a result, a large amount of old SBS-modified asphalt mixtures are generated annually [4; 5]. Casual handing could cause resource wastage and environmental pollution [6]. To restore the performance of aged asphalt and meet the road requirements, rejuvenators are usually added for components adjustment [7; 8; 9; 10]. Different from virgin asphalt, the aging of SBS-modified asphalt consisted of asphalt aging and SBS degradation [11]. Carbonyl and sulfoxides content, average molecular mass, and viscosity all increased due to asphalt aging [12]. The physical and rheological properties of asphalt were affected by the degradation of SBS [13]. In linear SBS molecules, the chains of polybutadiene were broken by thermal oxidation, making C=C bonds break and combine with oxygen to form hydroxyl groups [14; 15; 16]. [PERSON] [17] et al. investigated the aging performance of SBS-modified asphalt with various aging methods, which found that the aging of SBS-90 (virgin asphalt penetration 90) was worse than that of SBS-70 (virgin asphalt penetration 70). [PERSON] et al. [18] compared the effects on microproperties and molecular composition of aged and rejuvenated SBS-modified asphalt. It was found that based on the energy spectrum, the carbon content of aged SBS-modified asphalt decreased, while the oxygen content and sulfur content increased. Development of density functional theory (DFT) has made significant breakthroughs in addressing quantum mechanical problems in materials science. DFT pointed out that the energy of a system was determined solely by its electronic density distribution. There existed a self-consistent relationship between electronic density and energy of the system [19; 20]. Based on DFT, [PERSON] [21] calculated the molecular charge density, electron density difference, and binding energy (\(E_{\text{binding}}\)) of graphene oxide (GO). As a result, the bonding between resins and GO was the most stable, followed by aromatic components with GO. And the bonding between saturate components and GO was the weakest. In addition, GO hindered the movement and volatilization of saturate components, improving the high-temperature stability and heat aging resistance of asphalt. Based on DFT, the impact of SBS on the molecular vibrations of each component in asphalt was studied [22]. Vibration spectrums and \(E_{\text{binding}}\) between SBS systems and molecular components of asphalt were analyzed and calculated. And Fourier-Transform Infrared Spectroscopy (FTIR) testing was performed on SBS-modified asphalt. It was found that the vibrational peak intensity of SBS with the light components in asphalt was stronger than that of the heavy components. To ensure the correctness of quantum mechanical simulation results, it is necessary to establish a reasonable asphalt model. However, due to the complexity of asphalt composition, it is impossible to measure all molecular structures and add them to the model [23]. The American Society for Testing and Materials (ASTM) has proposed the four components method to classify asphalt into saturate (S), aromatic (A), resin (R), and asphaltene (A) components [24; 25]. However, [PERSON] and [PERSON] found the \"pentane effect\" on certain asphaltene by density functional theory (DFT) and quantum mechanical calculations. This means that although the asphaltene model is feasible for simulation, it is difficult to stabilize and maintain in actual chemical reactions, due to excessively high energy of the molecular system. Non-planar aromatic rings have emerged, which is inconsistent with the real experimental results [26]. Therefore, improvements made based on the four-component model and the asphalt AAA-1 model were proposed [27].