Effect of SBS Copolymer Molecular Structure on the Long-Term Aging and Resilience of Asphalt Binders: A Multiscale Morphological Study

Abstract

The long-term performance and durability of polymer-modified bitumen (PMB) are strongly influenced by the molecular architecture and vinyl content of styrene–butadiene–styrene (SBS) copolymers. This study investigates the effect of different SBS structures—linear (L-SBS), high-vinyl (HV-SBS), branched (B-SBS), and di-block (DB-SB)—on the morphological, micromechanical, and chemical characteristics of PMBs after Pressure Aging Vessel (PAV) ageing. Atomic Force Microscopy (AFM), Fluorescent Microscopy (FM), Scanning Electron Microscopy (SEM), and Fourier Transform Infrared Spectroscopy (FTIR) were employed to evaluate ageing-induced degradation. AFM results revealed a pronounced increase in bee-phase proportion for L-SBS and B-SBS binders (30% and 19%, respectively), whereas HV-SBS and DB-SB exhibited comparatively smaller changes (8% and 7%). Similarly, Derjaguin–Muller–Toporov (DMT) modulus values increased markedly for L-SBS and B-SBS, indicating significant oxidation-induced hardening. FM and SEM analyses confirmed severe fragmentation of the polymer network in these binders, while HV-SBS and DB-SB maintained greater structural integrity after ageing. FTIR analysis further revealed more pronounced chemical degradation in L-SBS and B-SBS, whereas HV-SBS and DB-SB exhibited enhanced chemical stability. Overall, SBS copolymers with lower vinyl content (≤15%) were more susceptible to ageing-induced degradation, while high-vinyl SBS demonstrated superior resistance to thermal-oxidative ageing, highlighting the critical role of molecular architecture in improving the long-term durability of PMBs.