Advanced Self-Healing Asphalt Reinforced by Graphene Structures: An Atomistic Insight

The contour of atom number
The contours of the atom number of pure asphalt and graphene-modified asphalt models in the yz plane are shown in Figure 3, where the color bar from blue to red exhibits atom numbers varying from 0 to 28. Figure 3a–c illustrates the contour of the atom number of the structures with 15 Å crack width in pure asphalt and asphalt nanocomposites modified by graphene at the crack tip and the crack surface. For pure asphalt, complete healing occurs after about 300 ps. The self-healing behavior starts from the area of the crack tip, as the area around the crack tip becomes a melted blunt shape with a blue color after 50 ps, and several asphalt molecules bridge the two crack surfaces at the middle of the crack tip. The green color in the contour presents the bulk asphalt, which is the stage of the crack zone getting full self-healing. At around 100 ps, the crack zone is almost closed with a small void left, and the colors of the initial crack surfaces change to green, which indicates that the self-healing process is finished in these areas; however, there are still some blue and white areas remaining to be self-healed. After about 300 ps, most of the color of the crack zone has changed to green, which is the same as that of the asphalt bulk, indicating that the self-healing process is complete. As shown in Figure 3b, the self-healing process is not significantly changed after adding the graphene sheet on the top of the crack. The self-healing process takes about 500 ps to complete, and the crack zone is sharply decreased at 50 ps and almost disappears at 200 ps. The graphene sheet on the top of the crack tip seems to have little influence on the self-healing process of the crack surface. However, inserting the graphene on the left of the crack surface can significantly accelerate the self-healing process, as shown in Figure 3c, where the red line in the contour is the graphene sheet. The self-healing period is shortened to around 200 ps, which is half as much as that of pure asphalt. The crack width is significantly decreased at 20 ps, and the asphalt molecules from the bulk tend to move to the graphene area and fill up the crack area. The crack zone almost disappears at around 150 ps, although some of the areas at the bottom remain blue. After another 50 ps of the self-healing process, the crack area is full of blue color, which indicates the end of the process.

The self-healing process of the models with 35 Å crack width takes almost twice as long as that of the models with 15 Å crack width, while the self-healing process of pure asphalt lasts around 1,000 ps. The self-healing behavior starts in the crack tip area, and the crack shape becomes shrunken and irregular at 100 ps. Most of the crack zone is healed by 500 ps, with a small void left in the middle of the crack zone. After performing the self-healing process for another 500 ps, the crack zone is filled with asphalt molecules until the self-healing process is completed. The graphene sheet is located on the top of the crack tip, as shown by the red line of Figure 3e. The self-healing period is about 1,100 ps, which is close to that of pure asphalt. However, the crack shape changes differently. There are some asphalt molecules that bridge the crack area at around 400 ps, which can advance the self-healing process. As shown in Figure 3f, the self-healing behaviors can be significantly improved when the graphene sheet is located at the left crack surface. A phenomenon can be observed similar to the model with 15 Å crack width: some of the asphalt molecules in the asphalt bulk tend to move to the graphene area and wrap around the graphene sheet, which can decrease the crack area significantly and help the self-healing process. The width of the crack is decreased to around half of the initial crack width by only 50 ps, and most of the crack area is healed at around 300 ps. The whole self-healing process lasts about 600 ps and most of the crack zone disappears; this takes only half the length of time taken by pure asphalt.

Figure 3: The contour of the atom number during the self-healing process. The contour of the atom number during the self-healing process for the models with 15 Å crack width for (a) pure asphalt, (b) graphene on the crack tip, and (c) graphene at the left surface of the crack tip, and the models with 35 Å crack width for (d) pure asphalt, (e) graphene on the crack tip, and (f) graphene at the left surface of the crack tip. The dotted black boxes refer to the locations of graphene. The color bar from blue to red stands for the atom numbers varying from 0 to 28 in the contour. Please click here to view a larger version of this figure.

Molecular interactions
To explore the difference in self-healing behaviors between pure asphalt and graphene-modified asphalt composites, the molecular interactions and movement during the self-healing process are captured and analyzed, as shown in Figure 4. From Figure 4a, it can be observed that aromatic molecules like asphaltene, polar aromatics and naphthene aromatics are attracted by the graphene sheet through π-π stacking when graphene is placed on the top area of the crack tip. These asphalt molecules are captured tightly by the graphene sheet and cannot readily diffuse into the neighborhood of the crack zone or fill up the crack, which hinders the self-healing process to a certain extent. However, the self-healing behaviors mainly arise from the asphalt molecules near the crack surface, and the influence of these molecules in the top area needs further exploration. From Figure 4b, it is observed that the polar aromatic molecule at the crack surface is attracted by the graphene sheet at the other crack surface, which can further increase the nearby naphthene aromatic molecule's likelihood of moving onto the crack area. The gathered asphalt molecules attracted by the graphene sheet can pack the crack zone with a higher speed than that of pure asphalt and the self-healing ability of the graphene-modified asphalt nanocomposite can be improved. The self-healing process of the model with 35 Å crack width modified by graphene at the left crack surface is shown in Figure 4c. The polar aromatic molecules are attracted by the graphene sheet through π-π stacking when the self-healing begins, and these asphalt molecules can rapidly wrap around the graphene sheet and reduce the space of the crack zone, as shown in Figure 3f. This indicates that graphene plays an important role in the initial stage of self-healing when it is located around the crack surface. A self-healing snapshot of pure asphalt with 15 Å crack width is shown in Figure 4d. It can be clearly observed that the chain structure of saturate is important for the self-healing process since the molecules can become entangled with each other and bridge the crack surface. This bridging effect between saturate molecules and the side chains of asphaltene molecules can significantly increase the packing efficiency and decrease the self-healing period. It is also observed that asphalt molecules with polyaromatic rings, such as asphaltene, polar aromatics, and naphthene aromatics, reorientate themselves at the crack surface by π-π stacking. This reorientation allows the asphalt molecules to move in a parallel direction and contributes to crack wetting, which further closes the crack surfaces.

Figure 4: Details of non-bond interaction of pure asphalt and graphene-modified asphalt nanocomposites during the self-healing process. For the model with (a) 15 Å crack width and graphene located on the top of the crack tip, aromatic molecules in asphalt are attracted by the graphene sheet through π-π stacking. For the model with (b) 15 Å crack width and graphene at the left side of the crack surface, the polar aromatic molecules at the other crack surface move onto the graphene surface due to strong aromatic interactions. For the model with (c) 35 Å crack width and graphene at the left side of the crack surface, the polar aromatic molecules are attracted by the graphene sheet and thus protrude from the crack surface. For the model with (d) 15 Å crack width and pure asphalt, there is a reorientation of aromatic molecules at the crack surface and a chain bridging and entanglement of saturate molecules during the self-healing process. The blue dotted boxes and purple dotted boxes in the figure indicate the π-π stacking and reorientation behaviors, respectively. Please click here to view a larger version of this figure.

The reorientation of aromatic molecules including asphaltene, polar aromatics, and naphthene aromatics around the crack surface during the self-healing process, is shown in Figure 5. Figure 5a shows that the tracked molecules before self-healing are almost perpendicular between naphthene aromatics and polar aromatics and between asphaltene and polar aromatics. The distance between asphaltene and the other two aromatic molecules is 13.3 Å, which is larger than the distance between the aromatic molecules. After self-healing for 40 ps, the naphthene aromatic molecule diffuses to the space between asphaltene and polar aromatics and plays an important role in interacting with the other two molecules. In Figure 5b, it can be observed that the distance and angle between the polar aromatic molecule and naphthene aromatic molecule are 4.6 Å and 89°, which indicates a T-shaped π-π stacking interaction between the two aromatic molecules. The angle and distance between the naphthene aromatic and asphaltene decrease to 32° and 4.6 Å, respectively. This indicates that the non-bond interactions between naphthene aromatics and asphaltene cause them to rotate and adjust the orientation gradually, contributing to the wetting of the crack surface. The orientations of the three molecules are almost parallel after 50 ps, as the angles between them are 26° and 35°, as shown in Figure 5c. The distance between them declines below 4.0 Å, which indicates that the π-π stacking facilitates the parallel structure and brings the aromatic molecules closer together. Overall, the reorientation at the crack surface promotes the interaction of asphalt molecules, which shortens the intermolecular distance and increases the attraction among them. The reorientation and diffusion of asphalt molecules further aid the filling up of the crack zone and speed up the self-healing process.

Figure 5: The reorientation of asphalt molecules during the self-healing process. Angles and distances among asphalt molecules (a) before self-healing, (b) after 40 ps, and (c) at 50 ps. Please click here to view a larger version of this figure.

Mobility of asphalt components
To understand the roles of different components quantitatively in the self-healing behaviors of asphalt, the MSD of the center mass for asphalt composite is calculated to represent the transitional mobility during the self-healing process, which is expressed by:

where r_i(t) is the position vector of particle i at time t, and the angular bracket indicates the average value of the traveling distance. The MSD values of pure asphalt and graphene-modified asphalt are tracked and shown in Figure 6. Figure 6a–c shows the MSD of asphalt composite with a 15 Å width crack, while those with a 35 Å width crack are shown in Figure 6d–f. It can be observed that saturates are the most active component in the self-healing behaviors of asphalt, while asphaltene is the least active. There are two possible reasons: one relates to the molecular mass, as asphaltenes have the highest molecular mass in asphalt, and they are less able to move and fill the crack zone. The other is the chain-like structure of saturates, which have higher mobility than other components and are more likely to become entangled and stretch out at the crack surface. The mobility of polar aromatics is higher than that of naphthene aromatics; this is because of the higher molecular mass and polarity of polar aromatics. The polar atoms on polar aromatics, such as sulfur atoms, can form an H-bond with asphaltenes, and the mobility can be impeded. The MSD figures for graphene-modified asphalt on the top area of the crack tip and the left crack surface are shown in Figure 6b and Figure 6c. It can be seen in Figure 6b that the MSD of graphene is lower than those of asphalt components since graphene occupies the largest volume and has the highest molecular mass in the asphalt nanocomposites. The MSD values of asphalt components are relatively lower than those of pure asphalt; this is because the interactions among these molecules and graphene hinder the mobility of the asphalt molecules and slow the self-healing process. However, when graphene is placed at the left crack surface, the mobilities of polar aromatics, naphthene aromatics, and graphene improve significantly compared to that of pure asphalt. This indicates that graphene plays an important role in the self-healing process and that its interactions with aromatic molecules in asphalt contribute to the self-healing process of asphalt. For the 35 Å width crack case in Figure 6d, the MSD of pure asphalt follows a similar trend to that of the case with 15 Å crack width, as the MSD of asphaltene, polar aromatics, naphthene aromatics, and saturates vary in an increasing manner. When inserting graphene into the top area of the crack tip, the MSD of saturates decreases by about 15 Ų. The existence of graphene sheets in asphalt bulk vertically influences the mobile space of the saturate molecules and prevents the routes of self-healing. From Figure 6f, it can be observed that the MSD values of asphaltene, polar aromatics and naphthene aromatics are all improved compared to pure asphalt, while the MSD of saturates decreases slightly. Graphene is highly responsible for the improvement of the self-healing process, especially with molecules containing aromatics. The π-π stacking interactions among graphene and asphaltene, polar aromatics, and naphthene aromatics improve the mobility of these asphalt molecules and help to form a stable packing structure in the crack zone, which accelerates the self-healing process of asphalt.

Figure 6: MSD of pure asphalt and graphene-modified asphalt molecules during the self-healing process. For the models with 15 Å crack width, MSD of (a) pure asphalt and graphene-modified asphalt on (b) the top of the crack tip and (c) the left surface is presented. For the models with 35 Å crack width, MSD of (d) pure asphalt and graphene-modified asphalt on (e) the top of the crack tip and (f) the left crack surface is presented. The X-axis represents the time of the simulation and the Y-axis represents the MSD values of asphalt components and the graphene molecule during the self-healing process. Please click here to view a larger version of this figure.

Molecular locations after self-healing
To explore the relative locations between graphene and asphalt molecules during the self-healing process, the radial distribution functions between graphene and aromatic molecules in asphalt are calculated and shown in Figure 7. Figure 7a–c shows the RDF of the model with 15 Å crack width before and after the self-healing process. It can be seen that the aromatic molecules in asphalt move closer to the graphene sheet following the self-healing process, especially the polar aromatic molecules and naphthene aromatic molecules. As indicated in Figure 4, there are strong π-π stacking interactions between graphene and aromatic molecules such as asphaltene, polar aromatics, and naphthene aromatics, which cause the graphene sheet to attract these molecules toward the crack surface. However, the difference in the g(r) values of asphaltene before and after self-healing is not as significant as those of polar aromatics and naphthene aromatics. This is because the asphaltene molecules obtain a higher molecular mass and volume than the polar aromatic and naphthene aromatic molecules, making it harder for them to rotate and diffuse to the graphene area and fill the crack zone. The increased g(r) values between graphene and polar aromatic or naphthene aromatic molecules within 4.0 Å are within the typical interaction distance for π-π stacking, and the increased g(r) values beyond 4.0 Å are due to the combination of molecular interactions and the elimination of the crack zone. An RDF of the model with 35 Å crack width before and after the self-healing process is shown in Figure 7d–f. The g(r) values between graphene and asphaltene beyond 4.0 Å through the self-healing process are more obvious than those of the 15 Å crack width; this is because asphaltene has more space to diffuse and move toward the graphene in the larger crack zone. The g(r) values within 4.0 Å are more significant for naphthene aromatics than those for polar aromatics; this is due to the smaller molecular mass and better diffusion ability of naphthene aromatic molecules.

Figure 7: The RDF values between graphene at the left crack surface and the asphalt components. The RDF values between graphene at the left crack surface and the asphalt components of (a) asphaltene, (b) polar aromatics, (c) naphthene aromatics from the model with 15 Å crack width, (d) asphaltene, (e) polar aromatics, and (f) naphthene aromatics from the models with 35 Å crack width. The X-axis represents the distance of the two molecules and the Y-axis represents the RDF values. Please click here to view a larger version of this figure.