Molecular dynamics simulation of diffusion coefficients between different types of rejuvenator and aged asphalt binder

This study developed molecular dynamics (MD) models to investigate the diffusion behaviour of rejuvenator with aged binder. Four types of rejuvenators, straight saturate, cyclic saturate, naphthene aromatic, and polar aromatic, were proposed with different molecular structures. Both short-term and long-term aged asphalt binder models were established based on oxidation aging mechanism. Bi-layered models were built to study the inter-diffusion process between rejuvenators and aged asphalt binder. Results show that the diffusion of rejuvenators into asphalt binder can be described with Fick's second diffusion law. The calculated diffusion coefficients indicated that with different molecular structures and functional groups, the diffusion ability of rejuvenators varied. It was found that long-term aging had a negative impact on diffusion behaviour. Moreover, polar aromatic performed the worst for diffusivity for both short-term and long-term aged binders, while naphthene aromatic achieved superior diffusion ability than saturate-based rejuvenators. The diffusion coefficient was proven to be significantly dependent on the chemical characteristics of rejuvenators, such as heteroatoms, molecular structures. The findings can provide microscopic insights on a selection of rejuvenators for better use of reclaimed asphalt pavement (RAP) binder.