Microstructural evolution of bitumen during the glass transition: An application of digital oil

Abstract

Bitumen is one of the most important resources in the world, yet its characterization remains a significant challenge because it is an incredibly complex molecular mixture and a colloidal system in nature. Bitumen becomes amorphous with decreasing temperature. However, it is not a simple glass-forming material, and its physical properties, such as viscoelastic behaviour, cannot be understood as the consequence of a single vitrification process. Remarkably, bitumen has two or more co-existing phases and three readily distinguishable phase transitions during the glass transition. Here, we show the microstructural evolution of bitumen using a digital oil model (i.e., a molecular model of natural bitumen based on analytical data) and molecular dynamics simulations. The calculated density and viscosity agree well with experiments. Three phase transitions were identified in all the models derived from the natural bitumen. Although the heat capacity changes sharply during the glass transition, the shear modulus does not, which indicates viscoelasticity at the transition state. This study highlights the gel-like structure of bitumen, in which the immobile molecules constitute the porous network and small molecules percolate through the network, providing a theoretical framework to elucidate transition states between the liquid and glass states.