Laccase-Assisted Gallic Acid Functionalization of Corn Straw Fibers for Reinforcing SBS-Modified Asphalt Binder: Fiber Toughening, Rheological Performance, and Molecular-Scale Mechanisms

This study investigated the regulatory effects of differently modified corn straw fibers (CSF)—a green fiber material derived from agricultural waste—on the mechanical properties, rheological characteristics, and molecular structural evolution of asphalt binders. Although plant fibers have been widely used in asphalt modification, natural CSF remains difficult to apply directly because of its insufficient toughness, poor structural stability, and weak interfacial compatibility. To address these limitations, CSF was modified using NaOH, tannic acid, and a gallic acid–laccase synergistic system. The effects and reinforcement mechanisms of different modification methods were revealed through fiber mechanical tests, abrasion tests, SEM, FTIR, XPS, dynamic shear rheological tests, and molecular dynamics simulations. The results showed that the gallic acid–laccase synergistic modification exhibited superior overall performance. Compared with unmodified fibers, the tensile strength, elastic modulus, and elongation at break of the modified fibers increased by 127.51%, 154.00%, and 46.18%, respectively, while the abrasion loss rate decreased by 56.85%. Microscopic characterization indicated that the gallic acid–laccase modification formed a functionalized layer rich in oxygen-containing functional groups on the fiber surface, enhancing fiber surface activity and interfacial bonding potential. Rheological tests showed that GA-CSF significantly improved the high-temperature stiffness, elastic recovery ability, and resistance to permanent deformation of SBS-modified asphalt. Molecular dynamics simulations further indicated that GA-CSF increased the cohesive energy density and elastic modulus of the system while reducing the free volume fraction, suggesting enhanced intermolecular interactions and restricted molecular motion in asphalt. These findings demonstrate that gallic acid–laccase synergistic modification is an effective method for improving the road applicability of corn straw fibers and the high-temperature performance of asphalt binders.