# Molecular Dynamics-Based Study of Graphene/Asphal Mechanism of Interaction
## 3 Results and Discussion
### Interaction between Graphene and Asphalt Components
#### 3.2.4 Relative Concentration of Asphalt Molecules  
The healing temperatures of the asphalt were selected as the corresponding optimal healing temperatures, i.e., the test temperatures of 70# and PGR-modified 70# asphalt were set to 318 K (45 \({}^{\circ}\)C), the test temperatures of SBS and PGR/SBS composite-modified asphalt were fixed to 353 K (80 \({}^{\circ}\)C), and the test temperatures of ACR and PGR/ACR composite-modified asphalt were set to 333 K (60 \({}^{\circ}\)C), respectively. For the 70# and SBS-modified asphalt systems, the concentration of asphalt molecules along the OZ direction of the asphalt self-healing model at the moments of 0 ps, 30 ps, 60 ps, 90 ps, 120 ps, and 150 ps were calculated, respectively. For the ACR-modified asphalt system, the asphalt molecular concentration of the asphalt self-healing model along the OZ direction at 0 ps, 20 ps, 40 ps, 60 ps, 80 ps, and 100 ps were calculated, respectively.  
The relative concentration changes of the PGR-modified different types of asphalt molecules along the OZ direction are shown in Figure 15, where the initial asphalt model shows high-concentration peaks on both sides of the crack and a 20 A crack with zero concentration in the middle. The addition of PGR resulted in a slight decrease in the concentration peaks on both sides of the asphalt.  
For 70# asphalt, the concentration peaks decreased, the cracks narrowed after 30 ps, and the cracks healed further after 90 ps and approached complete healing at 150 ps. The addition of PGR accelerated the healing process of 70# asphalt. The healing process was completed at 90 ps, which was 60 ps faster than that of the unmodified asphalt, and this accelerated healing process was attributed to the \(\pi\)-\(\pi\) force between the PGR and asphalt molecules (aromatic structural substances) and the bridging effect of the PGR mesh structure, which promoted the migration of small molecules (saturated fractions) to increase the healing efficiency, which is in line with the result of the heat-induced self-healing test. This is consistent with the results of the heat-induced self-healing test.  
For SBS and ACR-modified asphalt, the presence of long-chain polymers hinders the mobile diffusion of asphalt molecules. Therefore, higher temperatures and longer diffusion times are needed to heal the microcracks gradually. For the 150 ps diffusion of SBS-modified asphalt, the relative concentration of asphalt molecules is close to 1, indicating that the molecules of SBS-modified asphalt are generally self-healing at this time. After 120 ps kinetic calculations, composite-modified asphalt can be realized after the relative concentration of 1, so 30 ps shortens the healing time.  
Figure 14: Glass transition temperature of graphene-modified asphalt Tg.  
ACR-modified asphalt after 100 ps diffusion can repair most of the microcracks; at this time, the relative concentration curve in the middle of the depression area still exists; for PGR/ACR composite-modified asphalt after 100 ps diffusion, the concentration curve in the middle of the depression area almost disappeared, the minimum value of the relative concentration of the microcracked area was more than 0.92, and the concentration of the two sides of the molecules was in the range of 1.2-1.4. The above analysis confirms that PGR can also promote the polymer-modified asphalt healing time, which the presence of  
Figure 15: Graphene-modified asphalt healing model relative concentration versus time curve.  
PGR can achieve. This presence can also encourage the self-healing of polymer-modified asphalt molecules.