Dual Role of Coupling Agents at Rubber–Silica Interfaces: Adsorption-Induced Mobilization versus Bonding-Driven Stabilization

The molecular mechanism for coupling agents in regulating the rubber–filler interface remains unclear, which plays an important role in enhancing the mechanical properties. Herein, molecular dynamics simulations are used to elucidate how silane coupling agents affect the structure and dynamics in the interface between styrene-butadiene rubber and silica. To mimic different local environments in describing physical adsorption and chemical bonding, five cases are constructed by varying the grafting density and reaction ratio of the coupling agents. It is found that the coupling agents play dual roles at the interface. Physical adsorption of coupling agents creates inhomogeneity for surface interaction sites, which greatly reduces the relaxation time of polymer chains by at least 2 orders of magnitude and enhances their mobility in the interface. The interface becomes less stable under the physical adsorption effects of coupling agents, which would suppress load transfer. In contrast, chemical bonding effects always increase the relaxation time and restrict the chain mobility, giving a more stable interfacial structure with improved load transfer. The introduction of coupling agents decreases the train conformation length and increases the tail conformation length for polymer chains in the first adsorption layer, whereas chemical bonding promotes the formation of loop conformations. To enhance the overall mechanical properties, it is important to balance the effects of physical adsorption and chemical bonding for coupling agents and polymer chains. These results provide important insights for the rational design of high-performance rubber materials.