Revealing the rejuvenation mechanism of SBS modified asphalt through combined rheology-AFM-MD-DFT analysis

This study investigates the changes in microstructure and rheological properties of SBS modified asphalt during aging and regeneration processes. The high-temperature rutting resistance and low-temperature crack resistance at different ageing stages were characterized using a dynamic shear rheometer (DSR) and a bending beam rheometer (BBR). The effects of aging time and rejuvenator on the surface topography characteristics of asphalt were analyzed using atomic force microscopy (AFM). Molecular dynamics and density functional theory were employed to analyze the interactions between the rejuvenator and SBS modified asphalt. The results show that aging improves the resistance to high temperature deformation but reduces the temperature sensitivity, which can be effectively mitigated by rejuvenator. Aging deteriorates stress relaxation performance and low-temperature crack resistance, while rejuvenators significantly restore low-temperature properties. Aging action and rejuvenator changed the surface microstructure of SBS modified asphalt. Molecular dynamics simulations reveal that rejuvenator molecules disperse among SBS and polar molecules, reducing intermolecular forces. The density functional theory analysis indicates that the rejuvenator exhibits strong electrostatic interactions with polar molecules in aged asphalt, particularly showing significant interactions with the -COH and -COOH functional groups in the SB and BS components. These findings provide a theoretical basis for performance evaluation and optimized design of recycled asphalt.

1 Introduction

Styrene-butadiene-styrene (SBS) polymer modified asphalt has been widely adopted in high performance pavement construction due to its outstanding viscoelastic behavior, thermal stability, and mechanical properties (Zhang et al., 2019; Hong et al., 2022; Cao et al., 2023). However, during long-term service, asphalt binders inevitably undergo aging under the combined effects of thermal oxidative reactions, ultraviolet radiation, and repeated loading, which leads to the hardening of the binder, embrittlement of the pavement surface, and progressive deterioration of overall pavement performance (Shah and Mir, 2020; Khalighi et al., 2024; Li et al., 2022; Yang et al., 2022). The maintenance and rehabilitation of these aged pavements generate large quantities of reclaimed asphalt pavement (RAP), which, if not effectively reused, results in resource waste and environmental burdens through stockpiling or landfilling (Mariyappan et al., 2023; Rout et al., 2023; Antunes et al., 2019). Consequently, enhancing the recycling and rejuvenation of aged SBS modified asphalt has become a key strategy for promoting sustainability and extending the service life of asphalt pavements.

Hot recycling technology is recognized as one of the most effective approaches for the reutilization of reclaimed asphalt pavement (RAP) materials (Li M. et al., 2023; Ma et al., 2022; Ai et al., 2022; Wang et al., 2024). Numerous studies have focused on understanding the rejuvenation mechanisms of aged asphalt and optimizing rejuvenator formulations to restore performance. Haghshenas et al. (2022) demonstrated that the degree of aged asphalt recovery is closely governed by its colloidal index, identifying this parameter as a critical indicator for evaluating rejuvenator effectiveness. Lin et al. (2022) showed that the changes in intermolecular adhesive forces are closely related to the rheological properties of asphalt. Similarly, Li et al. (2024) conducted systematic chemical and rheological analyses on four rejuvenators, concluding that the proportions of asphaltene, saturates, and aromatics within rejuvenator formulations are positively correlated with the high-temperature performance of rejuvenated asphalt. Fan et al. (2022) quantitatively linked chemical indices to rheological parameters using grey-entropy correlation analysis, achieving a correlation coefficient greater than 0.96 and developing a binary linear model based on a composite aging index (AI). Collectively, these studies have advanced the understanding of the macroscopic rheological and chemical behaviors of SBS modified asphalt during aging and rejuvenation processes (Li B. et al., 2023; Cui et al., 2023; Zhu et al., 2023). However, most of them have primarily focused on material performance, while the microscopic interaction mechanisms among rejuvenators, SBS polymers, and asphalt molecular structures remain insufficiently clarified.

However, the regeneration process of SBS-modified asphalt is governed by micro-scale physicochemical phenomena such as molecular diffusion, interfacial bonding, and polymer network restructuring. In recent years, researchers have progressively elucidated its regeneration mechanism from the perspectives of microstructural evolution and the interaction mechanisms between components. Zhu et al. (2023) discovered that the rejuvenator can mitigate the ageing behavior of SBS-modified asphalt at the molecular level and enhance the compatibility between the SBS modifier and the asphalt. Li and Greenfield (2014) observed that the rejuvenator re-interacts with aged SBS-modified bitumen, thereby modulating the composition of the aged bituminous binder. Nian (Ji et al., 2023) has discovered that rejuvenator molecules enhance the dispersibility of aged SBS-modified bitumen, suppressing bituminous aggregate formation and thereby restoring the colloidal structure. Moreover, scholars such as Wen (Yu et al., 2023), Gou (Li D. et al., 2023), and Dong (Wang et al., 2022) have also elucidated the regeneration mechanism of aged SBS-modified asphalt from different perspectives. Despite considerable progress, the molecular level mechanism governing the interaction between rejuvenators and aged SBS modified bitumen remains incompletely elucidated. To further elucidate this problem, this study employs a combined approach involving rheological property evaluation, nanoscale morphological analysis, and molecular simulation methods.

To address this gap, the present study systematically evaluates the high-temperature rutting resistance and low-temperature cracking resistance of SBS modified asphalt at multiple aging stages, both before and after rejuvenation. Atomic force microscopy (AFM) was employed to quantitatively characterize nanoscale morphological changes induced by aging and regeneration. Furthermore, molecular dynamics (MD) simulations and density functional theory (DFT) calculations were conducted to investigate the interfacial interaction mechanisms among rejuvenator molecules, SBS copolymers, and asphalt components at the atomic scale. The findings of this study provide a multiscale mechanistic understanding of SBS modified asphalt aging and rejuvenation, offering theoretical guidance for the design of high performance and environmentally sustainable recycled asphalt materials.

2 Materials and methods

2.1 SBS modified asphalt

The asphalt selected for this study is SBS modified asphalt produced by the Gansu Provincial Transportation Materials and Commerce Group, with a modifier content of 4.5 wt. % by mass of the asphalt. The basic physical properties of the selected SBS modified asphalt are presented in Table 1.

Table 1

Table 1. Basic physical properties of SBS modified asphalt.

2.2 Rejuvenator

The rejuvenator used in this study was developed in-house by the research team. The basic physical properties of the rejuvenator are shown in Table 2. The content of rejuvenator in recycled asphalt is 10% of the mass of aged asphalt.

Table 2

Table 2. Basic physical properties of the rejuvenator.

2.3 Simulated ageing method

According to JTG E20-2011, the short-term aging of asphalt was conducted using the Rolling Thin Film Oven Test (RTFOT). The long-term aging test of the original SBS modified asphalt was conducted using the Pressure Aging Vessel (PAV) method. The temperature of the pressure aging vessel was set at 100 °C ± 0.5 °C, the pressure was maintained at 2.1 ± 0.1 MPa, and the aging durations were 20 h and 40 h.

2.4 Temperature scanning test

In this study, a TA-type dynamic shear rheometer (DSR) was used to perform temperature scanning tests on each asphalt sample. All asphalt samples were set to test at temperatures of 46 °C, 52 °C, 58 °C, 64 °C, 70 °C, and 76 °C, with a strain level of 1% and a loading frequency of 10 rad/s, and always ensuring that the loading range was within the linear viscoelastic interval of the asphalt.

2.5 Bending beam rheometer (BBR) test

In this paper, TE-BBR type low temperature bending beam rheology tester was used to test the low temperature rheological properties of each asphalt. The test temperatures were −12 °C, −18 °C and −24 °C, the test contact load was 35 mN ± 10 mN and the test load was 980 mN ± 50 mN.

2.6 AFM test

The microscopic surface morphology of the asphalt samples was investigated using a Bruker Dimension Icon AFM. Surface topography characterization was performed in light-tapping mode under controlled ambient conditions (25 °C). The AFM measurements were conducted with a scanning area of 10 × 10 μm² at a scanning rate of 2 Hz.

2.7 Molecular model of asphalt

2.7.1 Molecular model of virgin asphalt

In this paper, the 12 components proposed by Li and Greenfield were used as the molecular model of virgin asphalt, as shown in Figure 1. The AAA-1 model represents the molecular model of asphalt (Yu et al., 2023).

Figure 1

Figure 1. 12-component molecular of virgin asphalt.

2.7.2 Molecular model of aged asphalt

It has been shown that many oxygenated polar functional groups (C=O and S=O) are generated when asphalt is aged. The molecular model of aged asphalt used in this paper is shown in Figure 2.

Figure 2

Figure 2. 12-component molecular of aged asphalt (gray for carbon atoms, white for hydrogen atoms, red for oxygen atoms and blue for nitrogen atoms).

2.7.3 Molecular model of SBS modifier

SBS is a block copolymer with a specific ratio of styrene and butadiene monomers. The SBS modifier molecular model used in this paper is composed of 2 styrene, 18 butadiene and 2 styrene as shown in Figure 3.

Figure 3

Figure 3. Molecular modeling of SBS modifiers (gray for carbon atoms and white for hydrogen atoms). (a) Styrene. (b) Butadiene. (c) SBS.

It was shown that the aging products of SBS are SB- and BS- obtained by oxidative breakage of double bonds, as shown in Figure 4.

Figure 4

Figure 4. Molecular model of aged SBS modifier (gray for carbon atoms, white for hydrogen atoms and red for oxygen atoms). (a) SB-. (b) BS-.

2.7.4 Molecular model of rejuvenator

Rejuvenator as a mixture of aromatic and saturated fractions, one aromatic and two saturated fractions are selected as molecular models of rejuvenator in this paper, as shown in Figure 5. It should be noted that these molecular models are considered as typical molecular models of regenerators. In addition, to minimize the influence of the rejuvenator molecular structure on the asphalt molecular system, the number of the three rejuvenator molecules in this paper is 1:1:1.

Figure 5

Figure 5. Molecular model of rejuvenator. (a) Naphthene aromatic. (b) Cyclic saturate. (c) Straight saturate.

2.7.5 Construction of asphalt molecular model

This paper utilized the amorphous cell module of MS software to construct molecular models of asphalt based on the number of molecules listed in Tables 3, 4. The number of SBS modifier and rejuvenator molecular models were determined to be 4, 6, and 9 according to the previous content of 4% and10% for SBS modifier and rejuvenator.

Table 3

Table 3. Number of molecules in the virgin asphalt (VA).

Table 4

Table 4. Number of molecules in the aged asphalt (AA) and recycled asphalt (RA).

The molecular models of VA, AA, and RA were built based on the number of molecules in each asphalt model in Tables 3, 4. The initial density of the asphalt molecular model was set to 0.8 g/cm³ to reduce the cross overlap between asphalt molecular chains. The molecular model of asphalt is shown in Figure 6.

Figure 6

Figure 6. Molecular model of asphalt. (a) VA. (b) AA. (c) RA.

3 Results and discussion

3.1 High-temperature rheological properties of SBS modified asphalt at different aging levels

This study examines how different aging degrees affect the high-temperature rheological properties of SBS modified asphalt. The complex shear modulus and rutting factor were obtained from temperature sweep tests. Test results for various asphalt samples are shown in Figures 7, 8. The samples include virgin asphalt (VA), short-term aged asphalt (SA), 20-hour PAV-aged asphalt (CA20), 40-hour PAV-aged asphalt (CA40), rejuvenated short-term aged asphalt (RSA), rejuvenated 20-hour PAV-aged asphalt (RCA20), and rejuvenated 40-hour PAV-aged asphalt (RCA40). The curves clearly show the changes in complex shear modulus and rutting factor under different aging and rejuvenation conditions.

Figure 7

Figure 7. Complex shear modulus.

Figure 8

Figure 8. Rutting factor.

The rheological behavior of SBS modified asphalt under shear loading is characterized by its complex shear modulus (G*), which quantifies the material’s overall resistance to deformation. Elevated G* magnitudes directly correlate with enhanced high-temperature performance, as demonstrated in Figure 7. With increasing temperature, G* decreases significantly, as the thermally activated molecular motion reduces the material’s resistance to deformation.

The aging effect significantly changed the high temperature rheological properties of asphalt. For example, at 64 °C, the G* values of SA, CA20, and CA40 increased by 0.025 kPa, 1.129 kPa, and 1.865 kPa, respectively, compared to VA. This is because aging alters the chemical composition of SBS-modified asphalt, causing the light components in the asphalt to transform into heavy components. While this transformation enhances stiffness, it simultaneously reduces the material’s capacity for deformation recovery. This phenomenon is attributed to a chemical reaction between asphaltenes and resin molecules. During this process, lighter components undergo oxidation, condensation, and polymerization reactions, ultimately forming polar molecules. Saturates and aromatics are primarily low-molecular-weight hydrocarbons with weak intermolecular forces, whereas resins and asphaltenes contain oxygenated functional groups such as carbonyl and sulfoxide, which increase molecular polarity and promote aggregation. This shift strengthens the internal molecular structure but restricts molecular mobility, thereby limiting the reversible deformation behavior of the aged asphalt binder.

Rejuvenation treatment produces intermediate rheological properties, with rejuvenated asphalt exhibiting G* values between those of virgin and aged materials. Notably, the temperature dependence of G* in rejuvenated specimens shows reduced sensitivity compared to aged counterparts, indicating restored flexibility. This improvement stems from the rebalancing of maltene fractions by rejuvenators, though the final composition retains higher asphaltene content than virgin asphalt. Consequently, rejuvenated asphalt achieves superior high-temperature stability relative to unaged material while maintaining improved deformation characteristics compared to aged samples. The compositional adjustments induced by rejuvenation agents effectively mitigate the excessive stiffness caused by aging while preserving beneficial high-temperature performance attributes.

It is noteworthy that the G* value of recycled asphalt has been significantly reduced. As demonstrated in Figure 8, SBS modified asphalt exhibits progressively higher rutting factors with advancing aging levels. Specifically at 64 °C, the rutting factors for SA, CA20, and CA40 increased by 0.018 kPa, 1.329 kPa, and 2.101 kPa respectively relative to virgin asphalt (VA). The results indicate that the deformation resistance of SBS modified asphalt increases with the aging degree. This phenomenon can be attributed to the physicochemical changes induced by aging. The observed enhancement stems primarily from increased asphalt hardness and reduced viscosity during aging, which collectively improve the binder’s high-temperature stability against deformation.

With increasing temperature, the G*/sinδ of SBS modified asphalt decreases sharply. This change reflects the transition of the material from solid-like to more viscous behavior and the corresponding loss of rutting resistance at high temperatures. As shown in Figure 8, the rutting factor decreases more significantly with increasing aging degree. This result indicates that asphalt becomes more sensitive to temperature deformation as aging becomes more severe. Rejuvenation reduces the rutting factor markedly, as also illustrated in Figure 8. This is primarily due to the softening effect of the rejuvenator, which reduces the viscosity and deformation resistance of the asphalt. Additionally, at the same aging degree, the magnitude of G*/sinδ variation with temperature is smaller for recycled asphalt than aged asphalt. This finding indicates that temperature has a weaker effect on the high temperature properties of recycled asphalt compared to aged asphalt.

3.2 Temperature sensitivity analysis of asphalt

The findings reveal that G*/sinδ decreases progressively with rising temperature, and the extent of this reduction becomes more pronounced as the aging level of SBS modified asphalt intensifies. Furthermore, the temperature sensitivity of asphalt is determined by the rate at which G*/sinδ changes in response to temperature variations. To quantify this relationship, a linear fitting approach was applied to the data distribution between points lg(G*/sinδ) and lg(T). The slope (A) of the fitted line serves as an indicator of temperature sensitivity, with a smaller slope value corresponding to reduced sensitivity. Using this method, the temperature sensitivity of the aforementioned asphalt samples was evaluated, as illustrated in Figure 9. For enhanced clarity and analysis, Table 5 provides the slope (A), intercept (B), and coefficient of determination (R²) of the fitting function.

Figure 9

Figure 9. Fitting relationship between rutting factor and temperature for SBS modified asphalt at different aging levels.

Table 5

Table 5. Fitting results of rutting factors for SBS modified asphalt with different degrees of aging.

By analyzing the variations in the G*/sinδand temperature data, the parameters in Table 5 correspond to the fitting relationship described in Equation 1.

$\lg \frac{G*}{\sin \delta }=A\lg T+B(1)$

Where, G* is the complex shear modulus in kPa; δ is the phase angle in degrees (°); T is the temperature in °C; A is the slope, representing the temperature sensitivity parameter; and B is the fitting parameter, representing the intercept.

From Table 3, it can be observed that the R² values for the functional relationship between the G*/sinδ and T, as fitted by Equation 1, exceed 0.99 for SBS modified asphalt at various aging levels. This demonstrates that Equation 1 precisely characterizes the temperature-dependent variation of G*/sinδ.

The parameter A represents the rate of change of G*/sinδ with respect to temperature and serves as an indicator of the temperature sensitivity of SBS modified asphalt. Table 3 clearly shows that the temperature sensitivity coefficient of SBS modified asphalt progressively decreases with advancing aging degree, indicating a gradual reduction in temperature sensitivity as the asphalt undergoes aging. Furthermore, for SBS modified asphalt at equivalent aging levels, the temperature sensitivity coefficient A of rejuvenated asphalt is consistently lower than that of aged asphalt, confirming that the incorporation of rejuvenators further diminishes the temperature sensitivity of the material.

3.3 Low-temperature rheological behavior of SBS modified asphalt at different aging levels

In order to investigate the effect of the aging degree on the low temperature rheological properties of SBS modified asphalt, this paper uses the low temperature bending creep strength test to test the creep strength modulus S and creep rate m of VA, SA, CA20, CA40, RSA, RCA20 and RCA40, and the test results are shown in Figures 10–11.

Figure 10

Figure 10. Creep strength modulus versus temperature for SBS modified bitumen with different degrees of ageing.

Figure 11

Figure 11. Creep rate versus temperature for SBS modified asphalt with different degree of aging.

Figure 10 presents the temperature-dependent creep stiffness modulus (S) characteristics of SBS modified asphalt across various aging states. The S value is a key indicator to characterize the resistance of asphalt to permanent deformation, with lower values indicating stronger deformation resistance. The results showed that as the aging degree increased, the S value exhibited a continuous upward trend. Specifically, at −18 °C, the SA, CA20, and CA40 samples demonstrated 27.6%, 55.1%, and 60.6% higher S values respectively compared to the VA, confirming the gradual deterioration of low-temperature deformation resistance with aging progression.

The thermal dependence analysis further indicates an exponential growth pattern in creep stiffness as temperature decreases, with particularly notable increments observed in subzero conditions. For the CA20 specimen, recorded S values escalated from 78.8 MPa at −12 °C to 197 MPa (−18 °C) and 360 MPa (−24 °C), highlighting the material’s increasing propensity for brittle fracture at cryogenic temperatures. These findings collectively demonstrate the significant impact of aging on the low-temperature performance of SBS modified asphalt, showing both reduced crack resistance and enhanced susceptibility to thermal cracking with extended aging duration.

Figure 11 presents the temperature-dependent variation of creep rate (m) for SBS modified asphalt at different aging levels, where m serves as an indicator of the material’s time sensitivity and stress relaxation capacity. Higher m values correspond to better resistance against low-temperature cracking. The experimental results demonstrate a clear trend of decreasing creep rate with reducing temperature, as evidenced by the CA20 sample showing m values of 0.357, 0.325, and 0.214 at −12 °C, −18 °C, and −24 °C respectively. This temperature-dependent behavior indicates progressively impaired stress relaxation capability at lower temperatures, resulting in increased susceptibility to stress-induced cracking.

Furthermore, the results showed that more severe aging led to a further decrease in creep rate. At −18 °C, the m values of SA, CA20, and CA40 were 6%, 12%, and 20% lower than VA, respectively. The combined analysis of Figures 10, 11 shows that rejuvenation treatment effectively counteracts these aging effects by reducing S while increasing the m. Although rejuvenators significantly improve the low-temperature performance of aged asphalt, both aging degree and temperature remain critical factors influencing the material’s relaxation characteristics. These findings provide valuable insights into the relationship between aging, temperature, and the mechanical behavior of SBS modified asphalt.

3.4 Microstructure analysis of asphalt surface

3.4.1 Qualitative analysis

To investigate the effects of ageing action and rejuvenator on the surface micro-morphology of SBS modified asphalt, the surface micro-morphology of VA, SA, CA20, CA40, RSA, RCA20 and RCA40 was investigated in this paper using AFM. The test results are shown in Figure 12.

Figure 12

Figure 12. Micro-morphology of asphalt surface. (a) VA. (b) SA. (c) CA20. (d) CA40. (e) RSA. (f) RCA20. (g) RCA40.

The SBS modified asphalt (VA) exhibits a typical honeycomb-like microstructure, with surface height distribution ranging from −64.2 to 47.9 nm, as shown in Figure 12a. This is mainly attributed to the physical crosslinking points formed by polystyrene (PS) in the SBS modifier through π-π stacking interactions, while the polybutadiene (PB) segments exhibit good compatibility with the aromatic components in asphalt, which results in microphase separation behavior.

During short-term thermal-oxidative aging (SA), the microstructure of the asphalt undergoes significant changes, as shown in Figure 12b. During the aging process, oxidative cracking occurs in the PB segment, compromising the integrity of the primary phase interface, while the volatilization of lightweight components leads to volume shrinkage. Additionally, oxidation products continuously accumulate on the surface. These changes collectively cause the bee structure to gradually deteriorate, reducing the surface roughness to 42.9 nm.

As aging progresses, the long-term aged samples (CA20/CA40) exhibit more severe structural deterioration, with complete degradation of the SBS network and the oxidation-condensation of asphaltenes forming aggregates, significantly worsening component compatibility. It is worth noting that a dip at −102.6 nm was observed in the CA40 sample, which is closely associated with a significant decrease in low temperature ductility.

For RSA, the micromorphology still shows some aging characteristics. The bee-like structure is not fully recovered, and irregular surface depressions with depths of about −84.6 nm are observed. The phase interfaces appear blurred, and localized agglomeration occurs.

In contrast, RCA20 and RCA40 show almost complete elimination of extreme phase separation. The surface becomes smoother, and bee-like structures reappear in certain regions. This improvement results from the effective penetration of small rejuvenator molecules, which dissolve asphaltene aggregates. Aromatic fractions reorganize the broken polymer networks, and component rebalancing reconstructs phase interface compatibility.

3.4.2 Quantitative analysis

In order to further analyses in depth the effect of aging effect and rejuvenator on the micro-morphology of SBS modified asphalt, in this paper, based on the obtained micro-morphology of asphalt surface, the average roughness and root mean square roughness can be calculated according to Equations 2, 3, which can be used to characterize the micro-structure of asphalt binder. The results of R_a and R_q calculations are shown in Table 6.

$R_{\mathrm{a}}=\frac{1}{A}\underset{A}{\iint }\left|Z\left(\mathrm{x},y\right)\right|dxdy(2)$

$R_{\mathrm{q}}=\sqrt{\frac{1}{A}\underset{A}{\iint }\left|Z^2\left(\mathrm{x},y\right)\right|dxdy}(3)$

Table 6

Table 6. Calculation results of asphalt surface roughness.

Where, A is the average area of the measurement area and Z(x, y) is the morphometric height function with regard to the x and y variables.

The aging process exhibits stage-dependent effects on the surface morphology of asphalt. Short-term aging leads to a significant reduction in roughness (R_q decreased by 37.5%, R_a decreased by 47.8%), primarily due to the surface smoothing effect caused by light component volatilization and initial oxidation. In contrast, long-term aging shows a different trend: the Ra value of CA20 increases by 18.9% compared to VA, reflecting the formation of SBS phase separation. Meanwhile, the R_q value of CA40 recovers to near VA levels, indicating that the surface morphology stabilizes after CA40. Evaluation of regeneration treatment effectiveness reveals that RCA40 demonstrates the best recovery performance, with its R_a value decreasing from 9.67 nm to 4.83 nm, whereas RSA shows a 20.5% increase in R_a value.

3.5 Analysis of molecular dynamics results

3.5.1 Density of asphalt models

Density is a physical property of asphalt binders. It is often used to check the plausibility of asphalt models. Figure 13 illustrates the density variation curves of the five asphalts during the NPT simulation at 298.15 K.

Figure 13

Figure 13. Density variation curves for asphalt binders.

From Figure 13, it was observed that the densities of the original SBS modified asphalt, aged SBS modified asphalt, and regenerated SBS modified asphalt at kinetic equilibrium were 0.982 g/cm³, 1.042 g/cm³, and 1.040 g/cm³, respectively. These results are in good agreement with previous experimental data and literature (Ding et al., 2021; Luo et al., 2021). The findings indicate that the density of aged asphalt is higher than that of the virgin asphalt, while the addition of the rejuvenator only slightly decreases the density by 0.002 g/cm³. Structural analysis of asphalt four-component.

The radial distribution function (RDF) can accurately analyze the surroundings of the particles in the asphalt molecular system and can also represent the interactions between the particles, as shown in Equation 4.

$\mathrm{g}\left(r\right)=\frac{1}{4\rho \pi r^2\delta r}=\frac{{\displaystyle {\sum }_{t=1}^T}{\displaystyle {\sum }_{j=1}^N}\Delta N\left(r\to r+\delta r\right)}{N\times T}(4)$

Where, $\rho$ is the system’s density (g/cm³), $r$ is the distance of a given particle(Å), and N is the number of molecules in the system.

To investigate the changes in the colloidal structure of SBS modified asphalt due to aging effects, and rejuvenator, this paper used RDF to analyze the interactions between the SBS modifier and asphaltene (AS), as shown in Figure 14.

Figure 14

Figure 14. Radial distribution function between SBS-AS of asphalt binders.

Figure 14 displays the RDF (Radial Distribution Function) variation curves of SBS-AS in the original, aged, and regenerated asphalt, respectively. The Fig. shows that as the distance increases, the RDF values all converge to 1, which is typical since asphalt generally has an amorphous structure.

In Figure 14, the first RDF peaks for SB-SAS in the original, aged, and regenerated asphalt are 0.85, 0.94, and 0.93, respectively. Compared to the original asphalt, the RDF value of the aged asphalt increases by 0.09, indicating that the aging of SBS modified asphalt leads to aggregation behavior between asphaltenes (AS) and SBS. The terminal oxygen atoms introduced in BS- and SB- enhance the polarity of SBS molecular chains. Furthermore, aged AS contains a large number of polar functional groups. The adsorption of aged SBS molecules with AS molecules disrupts the original colloidal structure, leading to deteriorated rheological properties.

The RDF value of the regenerated asphalt is slightly lower than that of the aged asphalt, with a reduction of 0.01 compared to AA 4. This may be attributed to the high polarity of the cyclic saturated phenol rejuvenator, which exhibits strong attraction toward asphalt molecules, thereby weakening the interaction forces between the SBS modifier and asphalt molecules.

To further explain the above phenomena, the last frame after the equilibrium of NVT kinetic simulation is visualized in this paper. RE molecules, AR molecules, and SA molecules are hidden in the MS. The AS is represented by Corey-Pauling-Koltun (CPK), SBS molecules by ball and stick, rejuvenator by line. Figure 15 shows the final structure of VA, AA, RA.

Figure 15

Figure 15. Aggregation of SBS and AS. (a) VA. (b) AA. (c) RA.

Figures 15a–c respectively illustrate the aggregation structures of the SBS modifier and AS (asphaltene) molecules in the VA, AA, and RA systems. In Figure 15a, it can be clearly observed that SBS and AS are uniformly distributed, contributing to the superior rheological properties of the virgin asphalt. In the aged asphalt (Figure 15b), aggregation between the SBS modifier and AS is evident. This phenomenon is primarily attributed to the high polarity of the fragmented SBS, which enhances the intermolecular interactions between AS and SBS chains. Figure 15c depicts the aggregation structure among SBS, AS, and rejuvenator molecules. The image reveals that some cyclic saturated phenol rejuvenator molecules are adsorbed around AS, weakening the interaction between the SBS modifier and AS. As a result, partial depolymerization occurs between the SBS modifier and AS.

3.5.2 Molecular diffusion analysis

The diffusion coefficient (MDC) can be used to evaluate the mobility of molecules. In this paper, the diffusion coefficients of each molecule in asphalt are obtained by plotting the MSD scatter plots for the NVT kinetic equilibrium process from 100 ps to 200 ps. The calculation procedure is shown in Equation 5 (Yan et al., 2024).

$MDC=\underset{t\to \infty }{\lim }\frac{{〈r_i\left(t\right)-r_i\left(0\right)〉}^2}{6t}=\underset{t\to \infty }{\lim }\frac{MSD\left(t\right)}{6t}=\frac{\mathrm{a}}{6}(5)$

Where MDC is the diffusion coefficient, r is the distance, t is the time, MSD(t) is the mean square displacement function, and a is the slope of MSD(t).

Figure 16 illustrates the MSD fitting results for the five asphalts VA, AA, and RA. From the Fig., it can be seen that the fitting results of MSD curves are linearly correlated and the decidability coefficients R² are all higher than 0.95. This indicates that the MSD segments selected in the article are reliable. To visualize the diffusion coefficients (MDC) of the components in the asphalt, the diffusion coefficients of each component were calculated according to Equation 2, as shown in Table 7.

Figure 16

Figure 16. MSD curves of asphalt binders. (a) VA. (b) AA. (c) RA.

Table 7

Table 7. Diffusion coefficients of asphalt binders.

The diffusion coefficients of asphaltenes, resins, saturates, and aromatics in aged asphalt were 0.000973, 0.00151, 0.00244, and 0.00129, respectively, as shown in Table 7. Compared to the original asphalt, these values increased by 146%, 19%, 43%, and 47%, respectively. This is just the opposite of previous studies The diffusion coefficient of the SBS modifier decreased significantly. This is attributed to the broken SBS molecular chains enhancing the interaction forces between SBS and the four asphalt components, thereby restricting the movement of SBS molecules. After adding the rejuvenator, the most notable changes occurred in the diffusion coefficients of asphaltene and SBS molecules. Compared to aged asphalt, the diffusion coefficient of asphaltenes in regenerated asphalt increased by 21%, while that of SBS molecules decreased by 79%. Additionally, the diffusion coefficients of resins, saturates, and aromatics decreased to varying degrees. The small-molecule rejuvenator readily diffused between AS-SBS aggregates, weakening the forces between SBS and AS molecules and increasing the mobility of AS. Notably, the cyclic saturated phenol rejuvenator molecules possess moderate polarity, and their interaction with SBS molecules further restricted SBS mobility.

3.5.3 Fractional free volume analysis

Free volume theory is widely used to analyze the molecular motion and physical properties of materials. The procedure for calculating the fractional free volume is shown in Equation 6.

$FFV=\frac{V_{\mathrm{f}}}{V_{\mathrm{f}}+V_0}\times 100\%(6)$

Where, $FFV$ is the fraction of free volume, $V_{\mathrm{f}}$ is the free volume (FV), and $V_0$ is the occupied volume (OV) by molecules.

In this study, a probe radius of 1.1 Å (for H atoms) was employed to determine the distribution of free volume within asphalt, as depicted in Figure 17. The Fig. clearly shows that the free volume is uniformly distributed within the gaps of the occupied volume. Table 8 presents the occupied volume (OV), free volume (FV), and fractional free volume (FFV) in the original, aged, and regenerated asphalt systems. The results reveal that the diffusion coefficients of asphalt correlate with the trends in FFV variation.

Figure 17

Figure 17. Free volume distribution of asphalt (OV: blue; FV: red; Connelly surface: grey). (a) VA. (b) AA. (c) RA.

Table 8

Table 8. Results of OV, FV, and FFV of asphalt.

Notably, the FFV of aged asphalt increased from 14.07% to 14.10%, which may be attributed to the strong polarity of fragmented SBS molecular chains. This polarity reduces electrostatic and van der Waals interactions among aged asphalt molecules.

Furthermore, the FFV of regenerated asphalt was measured at 13.67%. Compared to aged asphalt, the FFV of regenerated asphalt decreased significantly. This reduction primarily stems from the smaller molecular structure of the rejuvenator, which enhances diffusion between asphalt and SBS molecules. By occupying most of the available voids, the rejuvenator substantially diminishes the FFV.

3.6 Electronic properties

This study aims to investigate the interaction mechanisms between SBS modifier molecules and asphalt molecules, with a focus on systematically analyzing the electronic properties of the relevant molecules. Based on the DMol³ module in Materials Studio software, the geometric configurations of SBS modifier molecules, asphaltene molecules, resin molecules, and aromatic phenol molecules were optimized using dispersion-corrected density functional theory (DFT-D). During the calculations, electron density and electrostatic forces were selected as key parameters for energy computation, and simulations were performed using the Becke-Lee-Yang-Parr (BLYP) functional within the generalized gradient approximation (GGA) framework. The charge density distribution characteristics of these molecules were obtained through computation, and the visualization results are presented in Figures s 18–25.

Figure 18

Figure 18. Charge density map of asphaltene (before aging). (a) AS-PH. (b) AS-PY. (c) AS-TH.

Figure 19

Figure 19. Charge density map of asphaltene (after aging). (a) AS-PH. (b) AS-PY. (c) AS-TH.

Figure 20

Figure 20. Charge density map of resin (before aging). (a) RE-PV. (b) RE-TR. (c) RE-BE. (d) RE-QU. (e) RE-TH.

Figure 21

Figure 21. Charge density map of resin (after aging). (a) RE-PV. (b) RE-TR. (c) RE-BE. (d) RE-QU. (e) RE-TH.

Figure 22

Figure 22. Charge density map of aromatic (before aging). (a) AR-PH. (b) AR-DO.

Figure 23

Figure 23. Charge density map of aromatic (after aging). (a) AR-PH. (b) AR-DO.

Figure 24

Figure 24. Charge density map of SBS modifiers. (a) BS-. (b) SB-. (c) SBS.

Figure 25

Figure 25. Charge density diagram of rejuvenator.

The changes in charge distribution of asphaltene molecules during the aging process can be visually observed in Figures 18, 19. From the charge distribution characteristics, the aromatic ring region of unaged asphaltene molecules exhibits significant electronegativity, primarily due to the coexistence of C-C single bonds and C=C double bonds within the aromatic rings. The C=C double bonds contain both σ and π bonds, with π electrons conferring strong electronegativity to this region. The peripheral side chains of the aromatic rings predominantly display electro positivity, but when these chains extend beyond a certain length, their terminal regions regain electronegativity. This phenomenon arises from the stable chemical nature of aromatic rings and the diminished hydrogen adsorption capacity of long carbon chains, causing hydrogen atoms to detach under the attraction of highly polar atoms.

Aging significantly alters the charge distribution pattern of asphaltene molecules, as illustrated in Figure 19. The electronegativity of the aromatic ring region notably weakens after aging, primarily because oxygen atoms introduced during oxidation possess stronger electron affinity, concentrating charge distribution around oxygen-rich areas while carbon chains farther from oxygen exhibit enhanced electro positivity. Furthermore, Figure 19 reveals a marked migration of negative potential regions before and after aging. In unaged molecules, negative potentials are uniformly distributed across aromatic rings, whereas in aged molecules, they predominantly localize near carbonyl groups. This shift stems from the strong polarity of carbonyl groups, which exhibit both electron-withdrawing inductive and conjugative effects, thereby reshaping the molecular charge distribution landscape.

From Figures 20, 21, it can be seen that resin molecules and asphaltene molecules exhibit similar characteristics in negative potential distribution, with aromatic ring regions showing significant electronegativity. This is mainly due to carbon atoms in the aromatic rings being connected through σ and π bonds, where the delocalization and conjugation effects of π-electron clouds substantially increase the electron density in these regions. Notably, sulfur (S) and nitrogen (N) atoms in the aromatic rings also display electronegative properties, primarily because nitrogen atoms adopt sp² hybridization while sulfur atoms exist in an sp³ hybridized state.

The aging process significantly alters the chemical composition and charge distribution characteristics of resin molecules. The introduction of carbonyl and sulfoxide groups leads to the formation of a more complex conjugated system within the molecule, markedly enhancing its electron distribution properties. These structural changes increase both the polarity and electronegativity of the functional groups.

The polarity of the carbonyl group stems from its unique electronic structure. The carbon atom undergoes sp² hybridization, forming three sp² hybrid orbitals from its s orbital and two p orbitals, while the unhybridized p orbitals participate in the formation of the π bond. Due to the high electronegativity of oxygen, the carbonyl group exhibits significant polarity, with the oxygen atom carrying a partial negative charge and the carbon atom carrying a partial positive charge.

In sulfoxide groups, the sulfur atom adopts sp² hybridization, forming three symmetrically arranged hybrid orbitals while retaining one unhybridized p orbital for π-bond formation. The strong electronegativity of oxygen gives the sulfoxide group distinct polarity, with the oxygen atom bearing a negative charge and the sulfur atom showing partial positive character.

Figures 22, 23 illustrate the changes in charge density distribution of aromatic phenols during the aging process. The introduction of oxygen atoms during aging significantly alters the molecular polarity characteristics. The electronegativity of oxygen containing regions increases, while that of aromatic ring regions relatively decreases. This modification in electronic structure leads to a marked enhancement in the polarity of the aromatic fraction.

Electrostatic potential analysis provides quantitative characterization of the aging effects on aromatic fraction properties. For the PH aromatic component, the maximum electrostatic potential increases from 0.03 kcal/mol to 0.05 kcal/mol after aging, while the minimum electrostatic potential decreases from −0.04 kcal/mol to −0.06 kcal/mol. The DO aromatic component demonstrates a similar trend, with its maximum electrostatic potential rising from 0.0231 kcal/mol to 0.04 kcal/mol and the minimum electrostatic potential declining from −0.0343 kcal/mol to −0.06 kcal/mol.

These changes are primarily attributed to the distinct polarity of the carbon-oxygen double bond (C=O) in carbonyl groups, where the oxygen atom exhibits strong electronegativity and the carbon atom carries partial positive charge. This polar characteristic not only modifies local charge distribution but also influences the overall electronic structure of the molecule through inductive effects, ultimately resulting in significant alterations in electrostatic potential.

Figure 24 displays the charge density distribution characteristics of SBS molecules and their cleavage products (BS- and SB- molecules). The electrostatic potential ranges are −0.0661 to 0.0360 kcal/mol for BS- molecules, −0.0731–0.112 kcal/mol for SB- molecules, and −0.0490–0.0728 kcal/mol for SBS molecules. Comparative analysis reveals that SB- molecules exhibit stronger electrical characteristics than BS- molecules, primarily due to the unique electronic structure of their terminal aldehyde group (-COH). In SB- molecules, the carbon atom in the aldehyde group forms a double bond with the highly electronegative oxygen atom while maintaining a single bond with the less electronegative hydrogen atom. This asymmetric electron distribution results in high electron density at the carbon-oxygen double bond region (showing strong negative polarity), while the carbon-hydrogen single bond region exhibits partial positive polarity due to lower electron density.

In contrast, the terminal carboxyl group (-COOH) of BS- molecules also contains a carbon–oxygen double bond. However, its electron distribution is different because a hydroxyl group (-OH) is attached. The oxygen atom in the hydroxyl group forms a single bond with hydrogen and introduces partial positive polarity. At the same time, the electron-withdrawing effect of the oxygen atom offsets part of this polarity. As a result, the overall electrical characteristics of BS- molecules are relatively weak.

Both the carboxyl and aldehyde groups formed after the cleavage of BS- and SB- molecules show strong polarity. These polar functional groups can interact with polar groups in AS, RE, and aromatic hydrocarbon components through hydrogen bonding or dipole-dipole forces. Such interactions promote the formation of AS–SBS aggregated structures.

The high electrostatic potential difference in rejuvenator molecules induces significant electrical interactions between their active components and the polar constituents of aged asphalt (including resins RE, aromatic fractions, as well as SB- and BS- fragments). These interactions primarily manifest as charge attraction effects and molecular dipole interactions. Such distinctive electrical effects can substantially enhance intermolecular binding forces, thereby promoting the formation of more stable molecular aggregates.

4 Conclusion

This study employed a dynamic shear rheometer (DSR) and bending beam rheometer (BBR) to investigate the high-temperature rutting resistance and low-temperature cracking resistance of SBS modified asphalt before and after rejuvenation at different aging levels. Atomic force microscopy (AFM) was used to analyze the effects of aging and rejuvenators on the surface micromorphology. The key findings and broader implications are summarized as follows:

1. Aging enhanced the rutting resistance of SBS modified asphalt, with G* and G*/Sinδ at 64 °C increasing by up to 2.101 kPa compared with virgin asphalt. However, it simultaneously deteriorated the low-temperature performance, as S increased by 60.6% and m decreased by 20% at −18 °C. Rejuvenation mitigated these effects by reducing stiffness and increasing creep rate, thereby restoring flexibility at low temperatures while maintaining acceptable high-temperature stability.

2. As the degree of aging increased, the stress relaxation capacity of SBS modified asphalt decreased and its crack resistance weakened, making it more prone to brittle fracture under low-temperature loading. The rejuvenator effectively reversed these trends, improving flexibility and delaying crack initiation through enhanced molecular mobility and matrix compatibility.

3. AFM observations revealed distinct surface morphologies: VA exhibited a honeycomb structure. CA20/40 degraded the SBS network, forming −102.6 nm depressions. RCA40 improved Ra from 9.67 to 4.83 nm via dissolution–reorganization processes.

4. The small molecules of the rejuvenator penetrate the aged asphalt matrix, weaken intermolecular interactions among polar components, and modify the colloidal structure from an asphaltene-dominated network toward a more balanced maltene–asphaltene system, thereby improving overall performance characteristics.

5. The strong electrostatic potential difference between rejuvenators and fractured SBS molecules promotes charge attraction and dipole–dipole interactions. These interactions enable rejuvenator molecules to bond with the polar functional groups (-CHO and -COOH) of broken SBS chains, reducing undesired associations between SBS fragments and other polar molecules. This molecular-level reorganization underpins the rejuvenator’s ability to restore elasticity and compatibility in aged binders.

The present findings provide a comprehensive micro-to-macro understanding of how aging alters the rheological and structural characteristics of SBS modified asphalt, and how rejuvenators counteract these effects through molecular interactions and colloidal reconstruction. This work establishes a mechanistic basis for designing rejuvenator formulations that balance high-temperature stability and low-temperature flexibility.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

MZ: Methodology, Writing – original draft, Visualization. JZ: Writing – review and editing, Validation, Investigation. XW: Writing – review and editing, Data curation, Visualization.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Conflict of interest

Authors JZ and XW were employed by Gansu Provincial Transportation Planning Survey & Design Institute Co., Ltd.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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