## 2 Materials and Methods
### Measurement
#### 3.1.1 LSCM Test  
Laser confocal microscopy experiments were conducted on pre-cut SBS-modified asphalt samples mixed for different amounts of time. The microscopic morphology was observed, and the experimental results are presented in Figure 2.  
The process of morphological change of the SBS modifier in asphalt after mixing was observed. After 15 min of mixing, the particle size of the SBS-modified asphalt was still large, exceeding the display range of the laser confocal microscope. The areas in which SBS came into contact with the asphalt exhibited reticular, filamentous, and punctate patterns. After 30 min of mixing, the particle size of the SBS significantly decreased, and the morphology of the SBS appeared filamentous and punctate within the observation range of the laser confocal microscope. As the mixing continued, at 60 min, the SBS morphology remained filamentous and punctate. However, compared to the 30 min mark, the filaments were narrower and the punctate particles were smaller. Additionally, some filamentous SBS particles exhibited a trend towards becoming punctate. After 120 min of mixing, the SBS morphology had transformed completely into a punctate form.  
The glass transition temperature of polystyrene ranges from 80 \({}^{\circ}\)C to 105 \({}^{\circ}\)C, while the glass transition temperature of polybutadiene is around \(-\)15 \({}^{\circ}\)C. In star-shaped SBS, the polystyrene phase is dispersed as spherical domains within the continuous polybutadiene phase and the polystyrene domains are interconnected directly [40]. It can be assumed  
Figure 2: Laser confocal micrographs of pre-cut SBS-modified asphalt mixed for different amounts of time.  
that during the heat absorption process of SBS, a phase transition will occur first in the portions with higher contents of polybutadiene, while the parts with higher contents of polystyrene will not undergo a phase transition at this point. Based on these considerations and the morphological changes of the SBS during the mixing process, this study produced speculations about the changes in SBS during the preparation process, as illustrated in Figure 3.  
Reticulation of SBS edges: After coming into contact with asphalt, the edges of the SBS, being the first parts to make contact with the high-temperature asphalt, absorb heat initially. When a certain amount of heat has been absorbed, the portions with higher contents of polybutadiene undergo a phase transition, leading to the formation of voids along the edges of the SBS. However, the parts with higher contents of polystyrene do not yet undergo a phase transition, resulting in the formation of a reticular structure.  
Reticular filamentation: The reticular structure continues to absorb heat from the high-temperature asphalt. In the connecting region between two areas with higher polystyrene contents, the glass transition temperature is lower than that of the high-polystyrene-content part. This causes a phase transition in the connecting regions, leading to the disruption of the reticular structure and its transformation into a filamentous one.  
Filamentous granulation: The filamentous structure absorbs heat from the high-temperature asphalt. In the edges and some internal areas of the filamentous structure, where the polystyrene content is relatively lower, phase transitions are more likely to occur. As a result, filaments break off from the edges or interior, forming granules with relatively higher polystyrene contents.  
Granular atomization: The granular SBS structure continues to absorb heat, repeating the aforementioned morphological changes. With each iteration of the reticular, filamentous, and granular transformations, the particle size of the SBS significantly reduces. After undergoing several cycles of these changes, the diameter of the SBS particles approaches the resolution of the laser confocal microscope, visually presenting an atomization effect.  
Figure 3: Morphological transformation process of shredded modifier during mixing.