Name: advanced self-healing asphalt reinforced by graphene structures: an atomistic insight
Uploaded: 2022-05-31T13:00:00.000+00:00
Duration: 8 min 3 s
Description: Watch this Scientific Journal Video about Advanced Self-Healing Asphalt Reinforced by Graphene Structures: An Atomistic Insight at JoVE.com

The authors are grateful for the support from City University of Hong Kong Strategic Research Grant with the Project No. 7005547, the support from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region, China, with the&#160;Project No. R5007-18, and the support from Shenzhen Science and Technology Innovation Committee under the grant JCYJ20170818103206501.

1. Build the atomistic models
<ol>
	<li>Open the Materials Studio software to create five 3D atomistic documents and rename these documents as graphene, asphaltene, polar aromatics, naphthene aromatics, and saturates, respectively.</li>
	<li>Build the graphene model by creating the unit cell of graphene sheet in the 3D atomistic document using the Sketch Atom option.</li>
	<li>Construct the final structure using the Supercell option in the Build &#62; Symmetry menu. Define the size of the graphene sheet as 40 &#197; x 40 &#197;, which is larger than the asphalt chains and the crack width.</li>
	<li>Build and pack the four types of asphalt molecules.
	<ol>
		<li>Use the Sketch Atom option to draw the molecular structures of asphaltene, polar aromatics, naphthene aromatics, and saturates separately.</li>
		<li>Pack the four kinds of asphalt molecules into the simulation box using the Calculation option in the Modules &#62; Amorphous Cell menu.</li>
	</ol>
	</li>
	<li>Build the asphalt structure with the crack.
	<ol>
		<li>Set the height of the crack zone in the x dimension the same as the height of the box of 70 &#197; and the depth of the crack zone in the y dimension is half of the height of the box as 35 &#197;.</li>
		<li>Set two cases of the crack widths in the z dimension of 15 &#197; and 35 &#197;. Delete the redundant molecules in the crack zones of the middle-down area of asphalt bulk using the Delete option and keep the asphalt matrix in the middle-up area unchanged.</li>
	</ol>
	</li>
	<li>Build the graphene-modified asphalt structure with the crack. Incorporate the graphene sheet into the top area of the crack tip and the left crack surface separately before the packing step using the Copy + Paste command.</li>
	<li>Pack the asphalt molecules into the simulation box based on the final compositions listed in Table 1 to construct the graphene-modified asphalt structure.</li>
	<li>Convert the structure file to a data file. Save the structure files as the molecule files with structure information (*.car and *.mdf) from Materials Studio. Convert the molecule files (*.car and *.mdf) to data files using the msi2lmp tool in large-scale atomic/molecular massively parallel simulator (LAMMPS)55 package. Read the data file by the read_data command in LAMMPS.</li>
</ol>
2. Perform the simulations
<ol>
	<li>Define the parameters of the simulations.
	<ol>
		<li>Set the timestep as 1 fs in the input file considering the balance of accuracy and efficiency of the carried simulations.</li>
		<li>Set the cutoff distance of non-bonded interactions as 12 &#197;, which is less than half the length of the simulation box in consideration of the periodic boundary condition and the calculational efficiency.</li>
		<li>Employ the particle-particle particle-mesh (PPPM) algorithm to describe the long-range Coulombic interactions and set the relative error in per-atom forces calculated by the long-range solver as 10-5 for high accuracy.</li>
	</ol>
	</li>
	<li>Fix the profile of crack. Select the asphalt molecules on the profile by the Group Molecules command in LAMMPS. Apply the constraints on the asphalt molecules using the Fix Spring/Self command in LAMMPS to avoid the movements of asphalt molecules.</li>
	<li>Achieve the equilibrium
	<ol>
		<li>Keep the whole simulation box fully relaxed after 500 ps under the isothermal-isobaric (NPT) ensemble with a temperature of 300 K and pressure of 1 atm.</li>
		<li>Make the asphalt bulk equilibrated to the desired density value of the experimental measurements41 of 0.95-1.05 g/cm3&#160;by continuously examining the temperature, pressure, density, and energy values using the Thermal command.</li>
		<li>Check the convergence of potential energy and the mean-squared displacement (MSD) in the whole system for achieving the fully relaxed state.</li>
	</ol>
	</li>
	<li>Perform the self-healing process.
	<ol>
		<li>Set the whole simulation box under the NPT ensemble with a temperature of 300 K and pressure of 1 atm.</li>
		<li>Remove the constraint of the asphalt molecules on the contour of the crack zone.</li>
		<li>Track and record the size of the simulation box and the coordinates of atoms and use the Dump command for postprocessing.</li>
		<li>Average the simulation results during the self-healing process over three independent configurations with three different initial velocity seeds in order to decrease the random errors.</li>
	</ol>
	</li>
</ol>
3. Postprocessing
<ol>
	<li>Visualize the self-healing behaviors. Open the Open Visualization Tool OVITO56 to visualize the simulation progress, and then open the trajectory files in the lammpstrj format generated by LAMMPS55. Record the snapshots of the self-healing process and track the paths of asphalt molecules using the Render command.</li>
	<li>Analyze the contour of the atom number. Export the coordinates of the atoms to data analysis and graphing software from the trajectory files outputted from LAMMPS. Project the coordinates of atoms in the whole system onto the yz plane. Record atom numbers at different areas of the yz plane and plot the contour with different colors.</li>
	<li>Analyze the atom mobility and relative position.
	<ol>
		<li>Analyze the atom mobility of different asphalt components by the mean-squared displacement (MSD) using the Compute msd command.</li>
		<li>Calculate the relative positions between graphene and asphalt molecules by the radial distribution functions (RDF) curves for the system of graphene-modified asphalt systems with the 15 &#197; and 35 &#197; crack widths using the Compute rdf command in LAMMPS.</li>
		<li>Draw the RDF curves to check how the density of asphalt varies as a function of distance from the graphene sheet.</li>
	</ol>
	</li>
</ol>

The authors have no conflicts of interest to declare.

The critical steps within the Protocol part are as follows: step 1.4 - Build and pack the four types of asphalt molecules; step 1.5 - Build the asphalt structure with the crack; step 2.3 - Achieve the equilibrium; step 2.4 - Perform the self-healing process. These steps indicate the most cohesive and important contents of the protocol. To create the desired shapes of the inserted crack, the packing process is modified compared to the normal packing in Materials Studio. The crack shape is created and filled inside the simulation box, and then the asphalt molecules are packed into the other part of the simulation box. After that, the redundant asphalt molecules are deleted around the created crack contour. The limitation of MD simulations is that the time scale and length scale are relatively small in the order of nanosecond and nanometer compared to traditional methods such as the finite element method, in which the simulations can be analyzed up to seconds and meters57. The significance of this method is that it can reveal the self-healing mechanism of asphalt and graphene-modified asphalt at the atomistic level by capturing nanostructure evolution, molecular interactions, and motions, which are hard to be accessed by traditional approaches58. The self-healing mechanism can help researchers and engineers to apply nanomaterials at the appropriate site and improve asphalt in an efficient way. The future application of this technique is that it can monitor the molecular structure in a decent manner and help to investigate the effect of other variables of nanomaterials such as defects, folded structures, and functional groups. This technique can also be combined with other approaches to observe the self-healing behaviors of asphalt nanocomposites from a multiscale aspect. The self-healing properties of asphalt can be understood thoroughly and get improved significantly in the future.
Graphene is critical in the changes and migration of the interface and components during the self-healing process. Without inserting the graphene sheet, saturate plays an important role in the self-healing process, since the chain structure of saturates can entangle with each other and bridge the crack surface. The bridging effect between saturate molecules and the side chains of asphaltene molecules can strongly increase the packing density and decrease the time of the self-healing process. Besides, asphalt molecules with polyaromatic rings, such as asphaltene, polar aromatics, and naphthene aromatics, reorientate themselves at the crack surface by &#960;-&#960; stacking, which makes the asphalt molecules move in a parallel direction and contributes to crack wetting and closes the crack surfaces. With the insertion of graphene, the polar aromatic molecules at one side of the crack surface are attracted by the graphene sheet at the other side of the crack surface, which can further increase the possibility of nearby naphthene aromatic molecules moving into the crack area. The gathered asphalt molecules attracted by the graphene sheet can fill in the crack zone with a higher speed than that in pure asphalt, and the self-healing ability is significantly improved in the graphene-modified asphalt nanocomposite. Asphaltene molecules have higher molecular mass and volume in the asphalt matrix, which makes them hard to diffuse to the graphene part and fill in the crack zone. Naphthene aromatics have faster movement than polar aromatics, which is due to the smaller molecular mass and better diffusion ability of naphthene aromatic molecules39.
In this study, the self-healing properties of pure asphalt and graphene-modified asphalt nanocomposites are investigated in consideration of different crack widths and graphene locations using MD simulations. It is observed that the self-healing behavior starts from the crack tip area, with the sharp tip becoming blunt and fuzzy. Asphalt molecules at the crack boundary can diffuse to decrease the width of the crack and continue filling the gap. The complete self-healing process is confirmed when the atom density of the crack area is the same as that of the asphalt bulk. MD simulations can help to reveal the molecular interactions and the chain movement in the asphalt matrix during the self-healing process. The entanglement and reorientation of asphalt molecules play an important role in self-healing behaviors. The self-healing rate with the incorporation of the graphene sheet is determined by its location. For the graphene sheet located at the crack tip area, the movement of the asphalt molecules is hindered and cannot easily diffuse into the crack zone. For the graphene sheet at the side of the crack zone, the asphalt molecules are attracted by the graphene sheet due to &#960;-&#960; stacking interaction and easily gather at the crack zone, indicating an increasing self-healing rate. The simulation results show that the modification of asphalt by nanomaterials can improve both thermomechanical and self-healing properties, which has great potential for the development of smart asphalt pavements. The fundamental understanding of the self-healing mechanism in asphalt nanocomposites based on MD simulations can facilitate efficient manipulation of nanomaterials at the optimal site, which is beneficial to the advanced design of asphalt nanocomposites with desired properties and functions.

Deterioration under daily vehicle loadings and variant environmental conditions, and the aging of asphalt during service result in degradation or even structural failures, i.e., cracking and raveling, which can further weaken the durability of asphalt pavements. The inherent response of asphalt to repair micro-cracks and voids automatically helps it recover from damages and restore strength1. This self-healing capability can considerably extend the service life of asphalt, save costs on maintenance, and reduce the emission of greenhouse gases2,3. The self-healing behavior of asphalt generally depends on several influencing factors, including its chemical composition, the degree of damage, and environmental conditions4. The improved self-healing capability of asphalt that can fully heal damage within a short period is desired; this has attracted extensive research interest in better mechanical performance and durability for asphalt pavements within civil engineering.
Novel methods to improve the self-healing capability of asphalt mainly include three approaches - inducing heating, encapsulation healing, and incorporating nanomaterials - which can be applied individually or simultaneously5,6. Inducing heating can significantly improve the mobility of asphalt and activate its self-healing for recovery7. The self-healing technology of asphalt by inducing heating can be ascribed to the assisted self-healing technique, which indicates that the self-healing properties of asphalt are improved by external stimuli. The objective of adding the steel wool fibers is to enhance the electrical conductivity so as to increase the healing capacity of the asphalt binder8. The approach to induce heat is to expose these electrically conductive fibers to the high-frequency alternating electromagnetic field, which can induce eddy currents, and the heat energy can diffuse into the asphalt binder by the conductive fibers9. The steel wool fibers enhance not just the&#160;electrical conductivity&#160;but also the thermal conductivity, both of which can positively affect the self-healing properties of asphalt. However, it is challenging to select the proper mixing time for fibers10. The length of fibers decreases with increased mixing time and influences thermal conductivity, while the decreased mixing time leads to clusters of fibers and impedes the mechanical properties of asphalt9. The encapsulation method can supply light components of aged asphalt such as aromatics and saturates and refresh the self-healing capability of asphalt11,12. However, this is a once-only treatment, and the healing materials cannot be replenished after the release. With the development of nanotechnology, nanomaterials have become promising modifiers for enhancing asphalt-based materials. Asphalt binders incorporated with nanomaterials present better thermal conductivity and mechanical properties13. Graphene with excellent mechanical performance and high thermal performance is regarded as an excellent candidate to improve the self-healing ability of asphalt14,15,16,17. The increased healing properties of graphene-modified asphalt can be attributed to the fact that graphene increases the capacity of the asphalt binder to be heated and produce heat transfer inside the asphalt binder, which means that graphene-modified asphalt can be heated more rapidly and reach up to higher temperature than pure asphalt18. The generated heat can be transferred throughout the graphene-modified asphalt at a faster speed than that through pure asphalt. The crack region of the asphalt binder can be influenced easily and healed faster by the heat flow with higher temperature and higher heating capacity. The self-healing reaction will begin if the energy that is equal to or larger than the healing activation energy exists at the crack surface of the asphalt19. Graphene can improve the thermal activation healing performance and accelerate the healing rate of asphalt19,20. Besides, graphene can save heating energy&#160;up to 50% during the healing process, which can benefit energy efficiency and reduce maintenance costs21.&#160;As a microwave-absorbent material, graphene is reported to improve the healing ability of asphalt during the rest period of microwave heating22. It is expected that the addition of graphene into asphalt will improve not only the mechanical performance but also the self-healing and energy-saving capacity, which requires in-depth knowledge of the self-healing mechanism.
Self-healing at the nanoscale is mainly due to the wetting and diffusion of asphalt molecules at the fractured faces23. As asphalt consists of various polar and non-polar molecules, its self-healing capability is strongly related to molecular interactions and movements of&#160;asphalt molecules of different components1. However, current research mainly relies on experimental techniques to quantify macroscopic mechanical properties, which causes missing information in the change of microstructures and the interactions among asphalt molecules when trying to understand the healing mechanism. The reinforcing mechanism of graphene in the self-healing capability of asphalt is also unclear at this stage. Molecular dynamics (MD) simulations play an influential role in investigating molecular interactions and motions of nanocomposite systems, and link microstructural deformation with molecular interactions and movements24,25,26,27,28,29,30,31. MD simulations have become more and more popular for analyzing material behaviors that cannot be accessed easily by experiments32,33. Existing studies have shown the feasibility and availability of MD simulations in asphalt systems; the cohesion, adhesion, aging, and thermomechanical properties of asphalt and asphalt composites can be explored by MD simulations34,35,36,37. The self-healing behaviors of asphalt can also be predicted by MD simulations38,39,40. Therefore, it is believed that the investigation using MD simulations is an effective way to understand both the self-healing and reinforcing mechanisms.
The objectives of this study are to investigate the self-healing behaviors of pure asphalt and graphene-modified asphalt nanocomposites and to understand the role of graphene in improving the healing capacity of asphalt through MD simulations. The self-healing simulations of pure asphalt and graphene-modified asphalt composites are carried out by introducing cracks into the initial structures. The self-healing capabilities are characterized by the contour of atom numbers, the reorientation and entanglement&#160;of molecules at the fractured face, and the mobility of asphalt components during the self-healing processes. By investigating the healing efficiency of graphene at different sites, the reinforcing mechanism of graphene contributing to the self-healing abilities of asphalt is unveiled, which can help with the monitoring of nanofillers in an optimal way and thus enable the life extension of asphalt pavements. An investigation of the self-healing capacity at the atomistic scale can provide an efficient way to develop advanced asphalt-based materials for future research.
According to asphalt chemistry, asphalt consists of various types of hydrocarbons and non-hydrocarbons with different polarity and shapes, which can mainly be divided into the four components of asphaltene, polar aromatics, naphthene aromatics, and saturates41,42. Asphaltene molecules are relatively larger and heavier than other molecules in asphalt, with a mean atomic mass of roughly 750 g/mol and a molecular diameter in the range of 10-20 &#197;. It has been widely accepted that asphaltene is composed of large aromatic cores that contain heteroatoms and are surrounded by different lengths of alkyl groups43. A modified asphaltene molecule is constructed, as shown in Figure 1a. The molecules of polar aromatics and naphthene aromatics are constructed based on the polarity and the element ratio of asphalt molecules, with benzobisbenzothiophene (C18H10S2) representing the polar aromatic molecule and 1,7-dimethylnaphthalene (C12H12) chosen as the representative naphthene aromatic molecule, as shown in Figure 1b-c. N-docosane (n-C22H46) is constructed as shown in Figure 1d. The parameters listed in Table 1 for asphalt molecules are selected and used to meet the desired criteria, including the elemental mass fraction, the atom ratio, and the aromatic/aliphatic ratio, of real asphalt from experiments41. The same mass ratio has been defined in our previous studies, and the other thermomechanical properties like density, glass transition temperature, and viscosity are in good agreement with experimental data of real asphalt36. The molecular structure of graphene applied in this study is shown in Figure 1e. The adopted graphene sheet in this study has no defect and no fold compared to that of the real case, while the real graphene sheet usually has several defects such as&#160;atomic vacancies and Stone-Wales defects44, and some of the graphene sheets can be folded during the mixing process in the asphalt matrix45. These imperfect situations are not considered in this study, since we focus on the effect of the site of the graphene sheet on the self-healing properties and choose it as the only variable. The variables of graphene sheets in terms of the defects and folded cases will be the focus of our future studies. The mass ratio of graphene to asphalt in this study is 4.75%, which is the normal situation (&#60;5%) for graphene modified asphalt in the experiment46,47.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/63303/63303fig01.jpg" /> 
Figure 1: Chemical structure. The atomistic models of (a) asphaltene molecule (C53H55NOS), (b) naphthene aromatic molecule (C12H12), (c) polar aromatic molecule (C18H10S2), (d) saturate molecule (C22H46), (e) graphene, and (f) pure asphalt. For the atomistic asphalt model, the carbon, oxygen, nitrogen, sulfur, and hydrogen atoms are shown in gray, red, blue, yellow, and white, respectively. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/63303/63303fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Asphalt model</td>
			<td>Mass (g/mol)</td>
			<td>Chemical formula</td>
			<td>Numbers of molecules</td>
			<td>Total mass (g/mol)</td>
			<td>Mass fraction (%)</td>
		</tr>
		<tr>
			<td>Asphaltene</td>
			<td>754.04</td>
			<td>C53H55NOS</td>
			<td>43</td>
			<td>32423.72</td>
			<td>26</td>
		</tr>
		<tr>
			<td>Naphthene aromatic</td>
			<td>156.22</td>
			<td>C12H12</td>
			<td>65</td>
			<td>10154.3</td>
			<td>8</td>
		</tr>
		<tr>
			<td>Polar aromatic</td>
			<td>290.38</td>
			<td>C18H10S2</td>
			<td>74</td>
			<td>21485.16</td>
			<td>17</td>
		</tr>
		<tr>
			<td>Saturate</td>
			<td>310.59</td>
			<td>C22H46</td>
			<td>205</td>
			<td>63670.95</td>
			<td>49</td>
		</tr>
		<tr>
			<td>Asphalt binder</td>
			<td></td>
			<td></td>
			<td>387</td>
			<td>127734.13</td>
			<td>100</td>
		</tr>
		<tr>
			<td>Graphene</td>
			<td>6369.28</td>
			<td>C525H63</td>
			<td>1</td>
			<td>6369.28</td>
			<td></td>
		</tr>
	</tbody>
</table>
Table 1: Overall components of pure asphalt model and graphene-modified asphalt model.
With respect to the protocol described below, two types of wedge-like cracks with different sizes are inserted into the middle of the asphalt model with a blunt crack tip and two parallel crack surfaces, while the middle-top area of the asphalt bulk remains intact. Two crack widths are chosen as 15 &#197; and 35 &#197;, as shown in Figure 2a-b. The reason for selecting 15 &#197; is that the crack width should be wider than the cutoff of 12 &#197; to avoid the early self-healing of asphalt molecules during the equilibrium process while investigating an extreme case for a small crack. The reason for selecting 35 &#197; is that the crack width should be wider than the length of the saturate molecules of 34 &#197; in order to prevent the bridging effect. The height of the crack is 35 &#197;, the same as the box width, and the depth of the crack is 70 &#197;, the same as the box length. In the real situation, the observed micro-crack sizes can be varied in the range from several micrometers to several millimeters, which is far larger than the length scale we are modeling here. Normally, the length scale in MD simulation is limited to the scale of 100 nm, which is still several orders of magnitude smaller than the real crack size. However, the cracks initiate at the nanoscale and grow into macroscale cracks with continuous deformation48. The understanding of the self-healing mechanism at the nanoscale can help to prevent the growth and further propagation of the crack at the macroscale. Even though the selected crack sizes are in the range of nanometer, the results can still be influential and applicable to explore the self-healing behaviors of asphalt molecules. There are two locations for the graphene sheets in the crack areas: one is on top of the crack tip and the other is perpendicular to the left crack surface. It has been found that these are the most common positions for graphene in graphene-modified nanocomposites with cracks49.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/63303/63303fig02.jpg" /> 
Figure 2: The self-healing schemes for pure asphalt and graphene-modified asphalt. The self-healing model of pure asphalt with a crack width of (a) 15&#160;&#197; and (b) 35 &#197;. The self-healing model of graphene-modified asphalt with the graphene sheet is located (c) at the top of the crack tip and (d) perpendicular to the crack surface. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/63303/63303fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In MD simulations, the intramolecular and intermolecular interactions in the asphalt nanocomposites are described by the Consistent Valence Forcefield (CVFF)50, which works well with asphalt and graphene-based materials. The functional form of CVFF is expressed as&#160;the following expression:
<img alt="Equation 1" src="/files/ftp_upload/63303/63303eq01v2.jpg" /> 1
Here, the total energy Etotal is composed of the bonded energy terms and the non-bonded energy terms. The bonded interactions consist of the covalent bond stretching, the bond angle bending energy, the torsion angle rotation, and the improper energies as expressed in the first four terms. The non-bonded energy includes an LJ-12-6 function for the van der Waals (vdW) term and a Coulombic function for the electrostatic interactions. CVFF has been widely employed in simulating asphalt materials51,52. The simulated physical and mechanical properties such as density, viscosity, and bulk modulus are in good agreement with the experimental data, which demonstrates the reliability of CVFF51. CVFF is not only suitable for inorganic materials, but it has also been successfully employed in structures consisting of organic and inorganic phases such as asphalt-silica52 and the system of epoxy-graphene53. In addition, the interfacial interactions between graphene and asphalt can be characterized by CVFF36,54. Since the major part in selecting forcefield is to determine the asphalt-graphene interface, the non-bonded interactions described by CVFF are more reliable, which is also considered in our previous study36. Overall, the forcefield CVFF is adopted in this study.&#160;The partial charges for different kinds of atoms are calculated by the forcefield-assigned method.

<table><tbody><tr><td>Atomistic models of asphalt and graphene/Materials Studio</td><td>BIOVIA</td><td>Materials Studio 8.0</td><td>The atomistic models are built for molecular dynamics simulations.</td></tr><tr><td>Large-scale Atomic/Molecular Massively Parallel Simulator Package</td><td>Sandia National Laboratories</td><td>lammps-stable20</td><td>The equilibrium is achieved under NPT ensemble, and the atomistic models get self-healed.</td></tr><tr><td>OVITO</td><td>Materials Science Department of Technische Universit&amp;auml;t Darmstadt, Germany</td><td>ovito-basic-3.1.0-win64</td><td>The self-healing behaviors of the atomistic models are visualized.</td></tr><tr><td>Origin</td><td>OriginLab</td><td>Origin 2018 64Bit</td><td>The contours of the atom numbers of the trajectory are drawn and analyzed.</td></tr></tbody></table>

advanced self-healing asphalt reinforced by graphene structures: an atomistic insight

Graphene can improve the self-healing properties of asphalt with high durability. However, the self-healing behaviors of graphene-modified asphalt nanocomposite and the role of incorporated graphene are still unclear at this stage. In this study, the self-healing properties of pure asphalt and graphene-modified asphalt are investigated through molecular dynamics simulations. Asphalt bulks with two crack widths and locations for graphene are introduced, and the molecular interactions among asphalt components and the graphene sheet are analyzed. The results show that the location of graphene significantly affects the self-healing behaviors of asphalt. Graphene near the crack surface can greatly accelerate the self-healing process by interacting with the aromatic molecules through &#960;-&#960; stacking, while graphene at the top area of the crack tip has a minor impact on the process. The self-healing process of asphalt goes through the reorientation of asphaltene, polar aromatic, and naphthene aromatic molecules, and the bridging of saturate molecules between crack surfaces. This in-depth understanding of the self-healing mechanism contributes to the knowledge of the enhancement for self-healing properties, which will help to develop durable asphalt pavements.

Watch this Scientific Journal Video about Advanced Self-Healing Asphalt Reinforced by Graphene Structures: An Atomistic Insight at JoVE.com

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.

Graphene can improve the self-healing properties of asphalt with high durability. However, the self-healing behaviors of graphene-modified asphalt ...

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

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Bioengineering

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

The integration of conductive nanoparticles, such as graphene nanoplatelets, into glass fiber composite materials creates an intrinsic electrical network susceptible to strain. Here, different methods to obtain strain sensors based on the addition of graphene nanoplatelets into the epoxy matrix or as a coating on glass fabrics are proposed.

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are ...

Grafting Multiwalled Carbon Nanotubes with Polystyrene to Enable Self-Assembly and Anisotropic Patchiness

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a free-radical polymerization strategy to enable the modulation of the nanotube surface properties and produce supramolecular self-assembly of the nanostructures. First, a selective hydroxylation of the pristine nanotubes through a biphasic catalytically mediated oxidation reaction creates superficially distributed reactive sites at the sidewalls. The latter reactive sites are subsequently modified with methacrylic moieties using a silylated methacrylic precursor to create polymerizable sites. Those polymerizable groups can address further polymerization of styrene to produce a hybrid nanomaterial containing PS chains grafted to the nanotube sidewalls. The polymer-graft content, amount of silylated methacrylic moieties introduced and hydroxylation modification of the nanotubes are identified and quantified by Thermogravimetric Analysis (TGA). The presence of reactive functional groups hydroxyl and silylated methacrylate are confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Polystyrene-grafted carbon nanotube solutions in tetrahydrofuran (THF) provide wall-to-wall collinearly self-assembled nanotubes when cast samples are analyzed by transmission electron microscopy (TEM). Those self-assemblies are not obtained when suitable blanks are similarly cast from analogous solutions containing non-grafted counterparts. Therefore, this method enables the modification of the nanotube anisotropic patchiness at the sidewalls which results into spontaneous auto-organization at the nanoscale.

A procedure for the synthesis of polystyrene-grafted multiwalled carbon nanotubes using successive chemical modification steps to selectively introduce the polymer chains to the sidewalls and their self-assembly via anisotropic patchiness is presented.

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a ...

Chemistry

Preparation and Characterization of C60/Graphene Hybrid Nanostructures

Physical thermal deposition in a high vacuum environment is a clean and controllable method for fabricating novel molecular nanostructures on graphene. We present methods for depositing and passively manipulating C60 molecules on rippled graphene that advance the pursuit of realizing applications involving 1D C60/graphene hybrid structures. The techniques applied in this exposition are geared towards high vacuum systems with preparation areas capable of supporting molecular deposition as well as thermal annealing of the samples. We focus on C60 deposition at low pressure using a homemade Knudsen cell connected to a scanning tunneling microscopy (STM) system. The number of molecules deposited is regulated by controlling the temperature of the Knudsen cell and the deposition time. One-dimensional (1D) C60 chain structures with widths of two to three molecules can be prepared via tuning of the experimental conditions. The surface mobility of the C60 molecules increases with annealing temperature allowing them to move within the periodic potential of the rippled graphene. Using this mechanism, it is possible to control the transition of 1D C60 chain structures to a hexagonal close packed quasi-1D stripe structure.

Preparation and Characterization of C60/Graphene Hybrid Nanostructures

Here we present a protocol for the fabrication of C60/graphene hybrid nanostructures by physical thermal evaporation. Particularly, the proper manipulation of deposition and annealing conditions allow the control over the creation of 1D and quasi 1D C60 structures on rippled graphene.

Physical thermal deposition in a high vacuum environment is a clean and controllable method for fabricating novel molecular nanostructures on graphene. We ...

Fabrication of Three-Dimensional Graphene-Based Polyhedrons via Origami-Like Self-Folding

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent properties has been of great interest for the development of novel device applications. Here, fabrication of 3D, microscale, hollow polyhedrons (cubes) consisting of a few layers of 2D graphene or graphene oxide sheets via an origami-like self-folding process is described. This method involves the use of polymer frames and hinges, and aluminum oxide/chromium protection layers that reduce tensile, spatial, and surface tension stresses on the graphene-based membranes when the 2D nets are transformed into 3D cubes. The process offers control of the size and shape of the structures as well as parallel production. In addition, this approach allows the creation of surface modifications by metal patterning on each face of the 3D cubes. Raman spectroscopy studies show the method allows the preservation of the intrinsic properties of the graphene-based membranes, demonstrating the robustness of our method.

Fabrication of Three-Dimensional Graphene-Based Polyhedrons via Origami-Like Self-Folding

Here, we present a protocol for fabrication of 3D graphene-based polyhedrons via origami-like self-folding.

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent ...

A Facile and Eco-friendly Route to Fabricate Poly(Lactic Acid) Scaffolds with Graded Pore Size

Over the recent years, functionally graded scaffolds (FGS) gaineda crucial role for manufacturing of devices for tissue engineering. The importance of this new field of biomaterials research is due to the necessity to develop implants capable of mimicking the complex functionality of the various tissues, including a continuous change from one structure or composition to another. In this latter context, one topic of main interest concerns the design of appropriate scaffolds for bone-cartilage interface tissue. In this study, three-layered scaffolds with graded pore size were achieved by melt mixing poly(lactic acid) (PLA), sodium chloride (NaCl) and polyethylene glycol (PEG). Pore size distributions were controlled by NaCl granulometry and PEG solvation. Scaffolds were characterized from a morphological and mechanical point of view. A correlation between the preparation method, the pore architecture and compressive mechanical behavior was found. The interface adhesion strength was quantitatively evaluated by using a custom-designed interfacial strength test. Furthermore, in order to imitate the human physiology, mechanical tests were also performed in phosphate buffered saline (PBS) solution at 37 &#176;C. The method herein presented provides a high control of porosity, pore size distribution and mechanical performance, thus offering the possibility to fabricate three-layered scaffolds with tailored properties by following a simple and eco-friendly route.

A Facile and Eco-friendly Route to Fabricate PolyLactic Acid Scaffolds with Graded Pore Size

In this work, poly(lactic acid)/polyethylene glycol (PLA/PEG) scaffolds were prepared by using a combination of melt mixing and selective leaching. The method herein discussed permitted to develop three-layer scaffolds by highly controlling both porosity and pore size. The mechanical properties were also evaluated in a physiological environment.

Over the recent years, functionally graded scaffolds (FGS) gaineda crucial role for manufacturing of devices for tissue engineering. The importance of ...

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

A combination of solution and surface assisted synthesis opens new directions in the atomically precise synthesis of nanostructures. Scanning tunneling microscopy (STM) supplemented by non-contact atomic force microscopy (nc-AFM) enables detailed characterization of newly designed and generated carbon-based nano-objects.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly ...

Preparation and Characterization of Graphene-Based 3D Biohybrid Hydrogel Bioink for Peripheral Neuroengineering

Peripheral neuropathies can occur as a result of axonal damage, and occasionally due to demyelinating diseases. Peripheral nerve damage is a global problem that occurs in 1.5%-5% of emergency patients and may lead to significant job losses. Today, tissue engineering-based approaches, consisting of scaffolds, appropriate cell lines, and biosignals, have become more applicable with the development of three-dimensional (3D) bioprinting technologies. The combination of various hydrogel biomaterials with stem cells, exosomes, or bio-signaling molecules is frequently studied to overcome the existing problems in peripheral nerve regeneration. Accordingly, the production of injectable systems, such as hydrogels, or implantable conduit structures formed by various bioprinting methods has gained importance in peripheral neuro-engineering. Under normal conditions, stem cells are the regenerative cells of the body, and their number and functions do not decrease with time to protect their populations; these are not specialized cells but can differentiate upon appropriate stimulation in response to injury. The stem cell system is under the influence of its microenvironment, called the stem cell niche. In peripheral nerve injuries, especially in neurotmesis, this microenvironment cannot be fully rescued even after surgically binding severed nerve endings together. The composite biomaterials and combined cellular therapies approach increases the functionality and applicability of materials in terms of various properties such as biodegradability, biocompatibility, and processability. Accordingly, this study aims to demonstrate the preparation and use of graphene-based biohybrid hydrogel patterning and to examine the differentiation efficiency of stem cells into nerve cells, which can be an effective solution in nerve regeneration.

In this manuscript, we demonstrate the preparation of a biohybrid hydrogel bioink containing graphene for use in peripheral tissue engineering. Using this 3D biohybrid material, the neural differentiation protocol of stem cells is performed. This can be an important step in bringing similar biomaterials to the clinic.

Peripheral neuropathies can occur as a result of axonal damage, and occasionally due to demyelinating diseases. Peripheral nerve damage is a global ...

Synthesis of Graphene-Hydroxyapatite Nanocomposites for Potential Use in Bone Tissue Engineering

Developing novel materials for bone tissue engineering is one of the most important thrust areas of nanomedicine. Several nanocomposites have been fabricated with hydroxyapatite to facilitate cell adherence, proliferation, and osteogenesis. In this study, hybrid nanocomposites were successfully developed using graphene nanoribbons (GNRs) and nanoparticles of hydroxyapatite (nHAPs), that when employed in bioactive scaffolds may potentially improve bone tissue regeneration. These nanostructures can be biocompatible. Here, two approaches were used for preparing the novel materials. In one approach, a co-functionalization strategy was used where nHAP was synthesized and conjugated to GNRs simultaneously, resulting in nanohybrids of nHAP on GNR surfaces (denoted as nHAP/GNR). High-resolution transmission electron microscopy (HRTEM) confirmed that the nHAP/GNR composite is comprised of slender, thin structures of GNRs (maximum length of 1.8 &#181;m) with discrete patches (150-250 nm) of needle-like nHAP (40-50 nm in length). In the other approach, commercially available nHAP was conjugated with GNRs forming GNR-coated nHAP (denoted as GNR/nHAP) (i.e., with an opposite orientation relative to the nHAP/GNR nanohybrid). The nanohybrid formed using the latter method exhibited nHAP nanospheres with a diameter ranging from 50 nm to 70 nm covered with a network of GNRs on the surface. Energy dispersive spectra, elemental mapping, and Fourier transform infrared (FTIR) spectra confirmed the successful integration of nHAP and GNRs in both nanohybrids. Thermogravimetric analysis (TGA) indicated that the loss at elevated heating temperatures due to the presence of GNRs was 0.5% and 0.98% for GNR/nHAP and nHAP/GNR, respectively. The nHAP-GNR nanohybrids with opposite orientations represent significant materials for use in bioactive scaffolds to potentially promote cellular functions for improving bone tissue engineering applications.

Novel nanocomposites of graphene nanoribbons and hydroxyapatite nanoparticles were prepared using solution-phase synthesis. These hybrids when employed in bioactive scaffolds can exhibit potential applications in tissue engineering and bone regeneration.

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

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