Studying different polymer modified model asphalt using molecular dynamics simulation methods.

Asphalt is a viscous liquid derived from the distillation of crude oil. It is mainly used on road pavements and roof patching. To recycle waste tires and improve the performance of asphalt, researchers have incorporated components in waste tire-polymers into asphalt mixtures and applied them on road pavement. To understand the modification effect in molecular details, different polymers were mixed with model asphalt and the major physical and mechanical properties of the polymer modified asphalt systems were predicted and compared to the original asphalt using molecular dynamics simulation methods. In total, four kinds of common polymers were selected as additives, including polyethylene (PE), polystyrene (PS), styrene-butadiene rubber (SBR), and styrene-butadiene-styrene (SBS), to mix with the model asphalt. Different asphalt systems were set up and modelled over the temperature range from the room temperature to the hot-mix asphalt temperature using all-atom LAMMPS molecular dynamic simulations. The density, diffusion coefficients of components, radial distribution function, thermal conductivity, correlation function, and viscosity of different asphalt systems were predicted, besides the radius of gyration of polymer in both the pure systems and in asphalt systems. It was found that the diffusion coefficients of components in different asphalt systems had a similar dependence on temperature, and the packing of asphalt molecules was modified by the polymer. The addition of PS, SBS or SBR increases the density of asphalt binder, while PE lowers it. The thermal conductivity of the asphalt binder system decreased upon incorporation of a polymer. These results can help to understand both the modification effects of different polymers on asphalt and the structure and properties of polymers in different media, which can help to design the optimum strategy to recycle waste tires on asphalt road pavements.