Properties of polymer modified asphalt by polyphosphoric acid through molecular dynamics simulation and experimental analysis

Polyphosphoric acid (PPA) plays an important role in improving the performance of polymer modified asphalt binders. This study aims to understand the modification mechanism of PPA in styrene-butadiene rubber (SBR) modified asphalt and investigate the rheological and chemical properties after modification based on molecular dynamics (MD) simulation and experimental analysis. Unaged and aged SBR asphalt binders with six PPA contents (0, 0.3%, 0.6%, 0.9%, 1.2%, and 1.5%) were prepared and tested. Four modified asphalt system models in different modifying and aging states were constructed in the molecular dynamics process. The calculated molecular dynamics parameters were used to observe intermolecular behaviors and system phenomena. Dynamic shear rheometry (DSR) and Fourier transform infrared (FTIR) spectroscopy were used to characterize rheological and chemical properties respectively. The results showed that the addition of PPA increased the complex modulus (G*) to promote elasticity and stiffness, and then enhanced the rutting resistance of the SBR asphalt binder. Multiple stress creep recovery (MSCR) results revealed that PPA/SBR asphalt had a better high-temperature performance than SBR asphalt due to its excellent creep recovery capability. Besides, the interaction between PPA and asphalt introduced phosphorus-containing functional groups into asphalt. In addition, analysis of the free volume and mean square displacement (MSD) displayed larger and higher molecular weight in PPA/SBR asphalt hindered the diffusion and mobility of the system. The radial distribution function (RDF) and radius of gyration (Rg) were calculated to observe that PPA provided a more ordered and more stable molecular structure. The electron density and potential were estimated to explore the adsorption mechanism of PPA to asphalt or SBR molecules and explain the source of stability and orderliness of the unaged and aged PPA/SBR/asphalt system.