Effect Of Graphene On Modified Asphalt Microstructures Based On Atomic Force Microscopy Ⅱ

3.4. Discussion of Asphalt Microstructure Based on Liquid-Solid Phase Transformation Theory

During cooling, some components of molten asphalt can undergo liquid–solid phase transition, which results in phase separation in asphalt. The “bee structure” is the result of phase separation in asphalt. According to Gibbs, the phase change process can be categorized into two, i.e., nucleation-growth phase change and continuous phase change.

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Figure 6.Height distribution of asphalt micromorphology

3.4. Discussion of Asphalt Microstructure Based on Liquid-Solid Phase Transformation Theory 

In our opinion, the formation of “bee structures” and the effect of graphene on the “bee structures” can be elucidated using the basic theory of “liquid–solid phase transition”. According to thermodynamic equilibrium theory, a phase transition can occur to form a new phase when the material is cooled to the phase transition temperature. AsFigure 6. Height distribution of asphalt micromorphology. 3.4.1. Analysis of “Bee Structure” Formation The formation of the “bee structure” has been debated, and in our opinion, the “bee structures” of asphalt are due to wax crystallization or associations involving wax and other concretes (such as asphaltenes and modififiers). During the freezing of asphalt, crude and residue oil—hydrocarbon mixtures with high melting points capable of precipitation via crystallization—are collectively known as wax. Wax is vital to the formation of the “bee structure.” The appearance of needle-flake crystals after cooling signifies the presence of microcrystalline wax in asphalt [37].

Wax crystallization involves the transition from a nuclear embryo to a crystal nucleus, followed by a crystal. During cooling, the alkane molecules that were distributed randomly in molten asphalt change from a high free-energy state (liquid state) to a low free-energy state (crystalline state), in which the alkane molecules within the short-range are arranged orderly to form nucleus embryos of the “nuclear embryo”, which facilitates the further formation of a stable crystal nucleus. A nuclear embryo is a prerequisite for the construction of a nucleus. However, the nuclear embryo will disintegrate if the temperature increases, whereas it will form a stable nucleus that will enlarge and develop crystals if the melt continues to be cooled. The crystallization process comprises nucleation formation and grain growth, both of which require the appropriate supercooling degree. As the temperature decreases, these molecules will undergo a continuous connect–fracture and fracture–connect the process to form ordered lattice points until a critical size is attained (a new stable state), i.e., a crystal nucleus [38]. Finally, other surrounding molecules will always cover the crystal lattice points and gradually form a thin slice structure that approaches the crystal nucleus and causes the crystal nucleus to develop into a needle-shaped crystal. 

The growth of crystal occurs in the region where the polymerization energy between the crystal and free paraffin is the greatest, resulting in the fastest growth of the sheet structure located on the side of the crystal nucleus. In the other components of the microcrystalline wax and asphalt, asphaltene molecules serve as the core for accumulating crystal clusters and then further develop them into “bee structures.” When the asphalt system is cooled to below the crystallization temperature, the oil in the asphalt on both sides of the peak sheet will ascend along the peak, which can be considered a capillary phenomenon in molten asphalt [39]. Figure 7 shows a schematic diagram of the precipitation of the “bee structures.” In the molten state, asphaltic components (saturates, aromatics, resins, and asphaltenes) are mixed to a homogeneous state. Subsequently, through modification, the graphene modifier is uniformly dispersed in the molten asphalt to form a homogeneous system. During cooling, graphene and asphaltenes will become nucleation sites, and the wax will crystallize easily, thereby resulting in “bee structures.” 

3.4.2. Effect of Graphene on the Bee Structures of Asphalt

Previous conclusions indicated that the modified graphene “bee structures” appeared in a greater quantity and were smaller than those of the base asphalt. The formation of a crystal nucleus is the first step in crystallization, and the nucleation process can be categorized into inhomogeneous and homogeneous nucleation based on crystal nucleation theory. Homogeneous nucleation refers to the same probability of nucleus generation in undercooled melts. Meanwhile, inhomogeneous nucleation refers to a formation process facilitated by various catalytic positions such as the surface, interface, cracks, and walls.

The body of the stable graphene modifier will become the catalytic site for nucleus formation in modified asphalt, and this is classified under inhomogeneous nucleation.The incorporation of graphene provides numerous nucleation sites, and interfaces provide regular (spherical) templates on which wax molecules can be deposited [40].The barrier of inhomogeneous nucleation (∆Gk*) is less than that of homogeneous nucleation (∆Gk), and the relationship exists in the asphalt, as shown in Equation (3), where θ represents the contact angle between a cap-shaped nucleus and a flat substrate, as depicted in the classical nucleation theory. Figure 8 shows the cap-shaped model of inhomogeneous nucleation. Cos θ can be calculated from Young’s equation (Equation (4)). In Equation 4,γnl, γsl, and γsn refer to the interfacial free energies between the nucleus and liquid, substrate and substrate and nucleus, respectively. f(0) can be obtained from Equation (5) of the geometric relationship of the cap model, and its value is less than or equal to 1.

When the crystal nucleus is formed on the nucleating agent, the nucleation barrier decreases with the contact angle (θ), and the inhomogeneous nucleation barrier is lower than the homogeneous nucleation barrier, which facilitates crystallization. In the base asphalt, asphaltene can serve as a nucleating agent. By contrast, in the graphene-modified asphalt, uniformly dispersed graphene in asphalt will share its role as a nucleating agent with asphaltene. Although wax can be detached via inhomogeneous nucleation in both the base and modified asphalt, the number of nucleating agent particles in both cases will differ. In our opinion, the graphene modifier can serve as an additional dispersed nucleation center that facilitates the formation of a large number of smaller wax crystals [41]. Therefore, the number of “bee structures” in the graphene-modified asphalt will be higher than that of the base asphalt. Additionally, graphene can result in a lower “bee structure” volume owing primarily to the formation of a relatively compact gel network in the modified asphalt. In our opinion, the formation of the “bee structures” is categorized under diffusive phase transition, and the increased viscosity of modified asphalt hinders the diffusion and transfer of wax molecules

The growth stage of the “bee structure” can be explained based on diffusion theory. A detailed explanation has been presented in our previous study [27]. In asphalt, a lower viscosity results in fewer intermolecular interactions, whereas a smaller resistant force toward migration results in a higher molecular migration rate, which facilitates the migration of asphalt components. The viscosity of the base asphalt is less than that of the graphene-modified asphalt. The wax components can migrate quickly in the base asphalt, thereby facilitating the development of the “bee structure.”

Meanwhile, the number of nucleation sites in the base asphalt is less than that of the graphene-modified asphalt. Therefore, the asphaltic “bee structures” of the base asphalt enlarge, and their distributions are scattered. Meanwhile, the viscosity of the modified asphalt is high, resulting in the low migration speed of the wax molecules. Furthermore, graphene as an asphaltene can serve as a nucleation site and impede particle migration. The factors above can result in the abundant quantity and smaller size of “bee structures” in modified asphalt

The micromorphology of unaged and aged base asphalt and graphene-modified asphalt were investigated via AFM. The micrograph variations were compared and analyzed. The formation mechanism of asphaltic “bee structures” and the effect of graphene on the“bee structures” were discussed. The main conclusions obtained are as follows: