Mechanisms Responsible for the Increase in Bee Structure of Aged SBS-Polymer Modified Asphalt

A mysterious phenomenon can be observed under atomic force microscopy (AFM) in SBS-modified asphalt (SBSMA), where the bee-structure size significantly increases after aging. Numerous studies have focused on the factors that induce changes in bee structures but have overlooked the relationship between macromolecular accumulation and bee-structural evolution during aging. The work here investigated the essential reason for the increase in the bee structure of SBSMA during aging. The geometric and topological parameters of bee structures are used to quantify microstructural characteristics, including axial length, area, interquartile range (IQR), and standard deviation (S.D.) from the Delaunay triangulation method. Nanoindentation tests quantified nanomechanical property evolution, while molecular dynamics simulation was used to obtain the contribution of macromolecular accumulation for the bee structure of SBSMA during aging. Results revealed that aging induces significant geometric and topological restructuring of bee structures, marked by a pronounced increase in large-sized features (length >3 μm, area >500 μm2) and reduced distribution uniformity. Short-term aging enhanced interfacial adhesion, whereas long-term aging slightly diminished this property. Furthermore, aging promoted the self-aggregation of SBS-polymers and asphaltene molecules, forming clusters or entangled micelles. These interactions disrupted colloidal stability and degraded the 3D network homogeneity. Critical analysis identified π–π interactions between aromatic benzene rings as the primary driver of molecular aggregation, directly correlating with increased height and length of bee- structures. This finding provides mechanistic insights into aging-induced microstructural evolution in SBS-modified asphalt, offering implications for material durability enhancement.