Investigation of Polymer-Asphalt Compatibility Using Molecular Dynamics Simulation.

Polymer modification is a promising approach to enhancing the performance of asphalt binders. However, a major issue plaguing polymer mixing with asphalt is the phase separation of these two components post-mixing. In the field of polymer science, the Flory-Huggins theory has been long established as the way to describe the conditions under which a mixture is stable or unstable. This Flory-Huggins interaction parameter χ and the molecular weights of the components are required under this theory. This work uses molecular dynamics simulations to calculate χ between various polymers found in waste plastics and asphalt. The compatibility of these polymers with the asphalt binder is discussed as a function of binder composition, molecular weight, and temperature. Four common polymers (polyethylene, polystyrene, polypropylene, and polybutadiene) were tested. While none of the polymers showed good compatibility with asphalt, PE was the most compatible. Stable compatibilization of plastics with asphalt will require additives or chemical modification of the plastic.

Implementation of probe rheology simulation technique in atomistically detailed molecular dynamics simulations.

The probe rheology simulation technique is a technique for measuring the viscosity of a fluid by measuring the motion of an inserted probe particle. This approach has the benefit of greater potential accuracy at a lower computational cost than other conventional simulation techniques used for the calculation of mechanical properties, such as the Green-Kubo approach and nonequilibrium molecular dynamics simulations, and the potential to allow for sampling local variations of properties. This approach is implemented and demonstrated for atomistically detailed models. The viscosity of four different simple Newtonian liquids is calculated from both the Brownian motion (passive mode) and the forced motion (active mode) of an embedded probe particle. The probe particle is loosely modeled as a nano-sized diamond particle: a rough sphere cut out of an FCC lattice made of carbon atoms. The viscosities obtained from the motion of the probe particle are compared with those obtained from the periodic perturbation method, and good agreement between the two sets of values is observed once the probe-fluid interaction strength (i.e., ε ij in the pair-wise Lennard-Jones interaction) is two times higher than their original values, and the artificial hydrodynamic interactions between the probe particle and its periodic images are accounted for. The success of the proposed model opens new opportunities for applying such a technique in the rheological characterization of local mechanical properties in atomistically detailed molecular dynamics simulations, which can be directly compared with or help guide experiments of similar nature.