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

Graphene-modified asphalt nanocomposite has shown an advanced self-healing ability compared to pure asphalt. In this protocol, molecular dynamics simulations have been applied in order to understand the role of graphene in the self-healing process and to explore the self-healing mechanism of asphalt components from the atomistic level.

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This method can help answer whole crack self-heals in asphalt based the nano composites, and how graphite can improve the capacity of asphalts. This technique can analyze material behaviors that cannot be assessed easily by experiments, and can provide the underlying information about the self-healing mechanism at an atomistic level. This method could provide insights into fundamental physics of dynamic evolution in various material systems including inorgi-inorganic systems and related interfaces, or even gaze at electro molecules.

If unfamiliar with this technique, one should understand the basic theory of molecule dynamic simulations and figured out the basic meaning of each command before carrying out any simulations. To begin open the material studio software, then create a three dimensional atomistic document for graphine and build the graphine model using the sketch atom option. After importing the graphite dot MSI file into material studio, in the build menu under symmetry, construct the final structure using the super cell option.

Define the size of the graphine sheet as 40 by 40 angstroms, which is larger than the asphalt chains in the crack width. Next, to build and pack the four types of asphalt molecules, create the three dimensional atomistic documents for asphaltene, polar aromatics, napthene aromatics and saturates separately. Then using the sketch atom option, draw the molecular structures of these molecules.

Next, using the calculation option from the amorphous cell menu under modules, pack these four kinds of asphalt molecules into the simulation box. Then, to build the asphalt structure with the crack, set the height of the crack zone in the X dimension, same as the height of the box of 70 angstroms, and set the depth of the crack zone in the Y dimension to half of the height of the box as 35 angstroms. Set two cases of the crack widths in the Z dimension of 15 and 35 angstroms.

Then, using the delete option, delete the redundant molecules in the crack zones of the middle down area of asphalt bulk, and keep the asphalt matrix in the middle up area unchanged. To achieve equilibrium, place the whole simulation box fully relaxed after 500 picoseconds under the isothermal isobaric ensemble with a temperature of 300 Kelvin in pressure of 1 atmosphere. Then, using the thermal command, equilibrate the asphalt bulk to the desired density value of the experimental measurements by continuously examining the temperature, pressure, density, and energy values.

Check the convergence of potential energy in the mean squared displacement in the whole system for achieving the fully relaxed state. Next, to perform the self-healing process, set the whole simulation box under the isothermal isobaric ensemble with a temperature of 300 Kelvin and pressure of 1 atmosphere. Then, remove the constraint of the asphalt molecules on the contour of the crack zone.

Track and record the size of the simulation box in the coordinates of the atoms. Then, use the dump command for post processing. Finally, average the simulation results during the self-healing process over 3 independent configurations with 3 different initial velocity seeds to decrease the random errors.

For visualizing the self-healing behaviors and simulation progress, open the open visualization tool OVITO"then, open the trajectory files in the LAMMPS TRJ format generated by LAMMPS. Record the snapshots of the self-healing process, then, using the render command, track the paths of the asphalt molecules. Next, to analyze the contour of the atom number, export the coordinates of the atoms from the trajectory files to the data analysis and graphing software.

Project the coordinates of the atoms in the whole system onto the YZ plane, then record atom numbers at different areas of the YZ plane and plot the contour with different colors. Next, analyze the atom mobility of different asphalt components by calculating the mean squared displacement using the compute MSD command. Then using the compute RDF command and LAMMPS, calculate the relative positions between the graphine and asphalt molecules by the radial distribution factor, or RDF curves, for the graphing modified asphalt systems with the 15 and 35 angstrom crack widths.

Finally, draw the RDF curves to check how the density of asphalt varies as a function of distance from the graphine sheet. For pure asphalt, complete self-healing occurs after 300 picoseconds. The crack zone sharply decreases at 50, almost disappears at 200 picoseconds.

Inserting the graphine on the left of the crack surface can significantly accelerate the self-healing process. The healing period shortens to 200 picoseconds, with the crack width significantly decreasing at 20, and almost disappearing at around 150 picoseconds. The self-healing behaviors significantly improved when the graphine sheet is at the left crack surface.

When graphine is placed at the left crack surface, the mobilities of polar aromatics, napthene aromatics, and graphine improved significantly compared to that of pure asphalt. RDF values between graphine at the left crack surface in the asphalt components for the 15 angstrom crack width model show that the aromatic molecules and asphalt move closer to the graphine sheet, especially the polar aromatic molecules and napthene aromatic molecules. Where the 35 angstrom crack width model RDF values beyond 4 angstroms are more obvious than those of the 15 angstrom crack width model because asphaltite has more space to diffuse and move toward the graphine in the larger crack zone.

It is very important to set a reasonable crack in the asphalt model while ensuring that a crack is well kept and the system is fully equilibrated. Cost grain modeling can be performed based on this procedure to embrace a broader range of length scale, and further is brought, the self healing of crack in asphalt at different scales. This technique can monitor and optimize the molecular structure of nano fillers with special design, such as defects, photo structure and functional groups for advanced improvement of asphalt based nano composites.

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

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