# Mechanism of Rejuvenation in Aged SBS-Modified Asphalt by Density Functional Theory
## 3 Quantum Mechanical Simulation  
Asphalt and Rejuvenator Models  
Based on the AAA-1 model [27], an asphalt molecular model was constructed, as shown in Figure 1. Due to their stable and inactive nature, the models of saturate components are unchanged in the aging process [31]. Previous studies [31] had found that the aging process of asphalt could be divided into two different stages. In the first stage, the asphalt underwent rapid initial oxidation reactions within the hydrocarbon structure and \begin{table}
\begin{tabular}{c c c} \hline \hline
**Properties** & **Experiment Values** & \begin{tabular}{c} **Technical** \\ **Requirements** \\ \end{tabular} \\ \hline \begin{tabular}{c} Penetration/0.1 mm \\ Softening point/\({}^{\circ}\)C (ringball method) \\ \end{tabular} & 80.5 & \(\geq\)55 \\ \begin{tabular}{c} Ductility/cm \\ Viscosity/Pa\(\cdot\)s\({}^{-1}\) \\ Flash point/\({}^{\circ}\)C \\ Solubility/\% \\ \end{tabular} & 285 & \(\geq\)230 \\ \begin{tabular}{c} Solubility/\% \\ \end{tabular} & 99.5 & \(\geq\)99 \\ \hline \begin{tabular}{c} Residual asphalt \\ binder remaining \\ after RTFOT \\ \end{tabular} & \begin{tabular}{c} Mass loss/\% \\ Penetration ratio/\% \\ \end{tabular} & 0.56 & \(\leq\)1 \\ \begin{tabular}{c} binder remaining \\ after RTFOT \\ \end{tabular} & \begin{tabular}{c} Penetration ratio/\% \\ \end{tabular} & 78 & \(\geq\)60 \\ \
\begin{tabular}{c} after RTFOT \\ \end{tabular} &
\begin{tabular}{c} Ductility/cm \\ \end{tabular} & 44 & \(\geq\)20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: SBS-modified asphalt technical indicators. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Properties** & **Viscosity/Pa\(\cdot\)s\({}^{-1}\)** & **Flash Point\({}^{\circ}\)C** & **Density/g\(\cdot\)cm\({}^{-3}\)** & **Asphalt Binder** & **Aromatic Components/\%** & **Saturate Components/\%** \\ \hline
\begin{tabular}{c} Experiment values \\ \end{tabular} & 109 & 208 & 0.932 & \(-\)2.5 & 36 & 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Rejuvenator technical indicators. as a result, some functional groups were generated, such as sulfoxide groups, aromatized benzene rings, and hydroxyl groups. In the other stage, hydroxyl groups were further oxidized into ketones. As the rejuvenator used in this study is a softening agent, the asphaltene model and saturate model were chosen from [PERSON] and [PERSON]'s research [32], which was used to represent the chemical families found in paving asphalts. The resin model was chosen from [PERSON] et al. 's research [33], and the aromatic model was chosen from [PERSON] et al. 's research [34]. The rejuvenator components models are exhibited in Figure 2.  
Figure 1: [PERSON] twelve-component model (carbon atoms are gray, hydrogen atoms are white, oxygen atoms are red, sulfur atoms are yellow, and nitrogen atoms are blue). Figure 2: Four-component model of rejuvenator. (Carbon atoms are gray, hydrogen atoms are white, oxygen atoms are red, sulfur atoms are yellow, and nitrogen atoms are blue). The SBS molecule comprised a styrene molecular monomer (hard segment) and 1,3 butadiene molecular monomer (soft segment). To balance the rigid and flexible molecules, an SBS molecular model was constructed with a ratio of styrene to butadiene as 3:7, as shown in Figure 3. After oxidation, the SBS polymer chains were broken at the C=C bonds in polybutadiene segments. One of the aging products was oxidized to form hydroxyl groups, another was oxidized to form carboxyl groups [35], which were shown in Figure 4. Due to severe degradation and loss of modified functionality [36], the SBS molecules in long-term aged asphalt were not considered in this study. All molecular models were established in Materials Studio. In order to study the interaction between the component molecules of rejuvenator and aged SBS-modified asphalt, amorphous cells were built, containing one rejuvenator component molecule and one asphalt molecule or SBS molecule. The cell size is 12 A \(\times\) 12 A \(\times\) 12 A.