Molecular dynamics study on the glass forming process of asphalt

Abstract

Abstract Glass transition temperature (Tg) is a significant parameter that determines the viscoelastic properties of asphalt. However, the understanding of glass transition process of asphalt is ambiguous, especially at the molecular level. In this paper, molecular dynamic (MD) simulations were used to help understanding the glass forming process from the viewpoint of glass forming liquid. The temperature dependence of volumetric properties, such as free volume, fractional free volume, specific volume and coefficients of volume thermal expansion etc., for AAA-1 asphalt were computed. Results show that there are two transition temperatures with temperature decreasing from 575 K to 50 K, i.e., one is Tg at ca. 250 K, the other is ca. 400 K. The non-Arrhenius type temperature dependence of viscosity and the breakdown of Stokes-Einstein relation imply that the transition at ca. 400 K is responsible to the liquid–liquid transition of asphalt from viscoelastic liquid to Newtonian fluid. Furthermore, molecular trajectory demonstrated that at ca. 400 K, AAA-1 asphalt system experienced the transition from non-ergodic local thermal vibrations and restricted movement to temperature-independent ergodic movement, and this observation verified that the liquid–liquid transition of asphalt did occur at ca. 400 K. Namely, at temperatures higher than 400 K, the rearrangement process of asphalt colloid cage is a temperature-independent ά-relaxation, and in the temperature range from 400 K to 250 K, the relaxation associated with diffusional movement is a slow α-relaxation, i.e., structure relaxation of glass transition, and at temperatures lower than 250 K, asphalt is truly frozen glassy. Moreover, the good agreement between the rheological experiments and simulation results demonstrated that the MD results are reliable. This paper helps understanding microscopic picture of glass forming asphalt and its relaxation mechanisms with temperature, which could enable new additives and new additive strategies to modify asphalt and facilitate crude oil exploitation, transportation and refining processes, etc.