Reversible molecular structuring and thixotropy in bitumen

Chemical composition influences inter-molecular interaction and structuring within bitumen. Additionally, microstructural properties affect physical properties like viscosity and more complex rheological properties like thixotropy. Thixotropy is a reversible phenomenon that is caused by the breakdown of microstructures as a result of deformations from the applied stress. In this study, a manifestation of the loss and recovery of microstructure as observed by atomic force microscopy is interpreted as thixotropy. The nature and extent of reversibility as a function of time and temperature have been presented. The application of tensile load on a thin film of bitumen resulted in stress concentrations at the interfaces due to the stiffness difference between the micro-phases. Damage is observed as micro-cracks and subsequent fragmentation of the dispersed phase. The crack morphology and damage levels vary in different penetration grades of bitumen, as they exhibit different microstructural properties. By introducing thermal energy or rest periods, the molecular structures reform and rearrange and create new configurations. Hence, in response to a change in thermodynamic condition, the ‘microstructure buildup’ can be related to successive recovery in mechanical properties. This study manifests the reversible nature of the formation of the bitumen microstructure and postulated to be the fundamental mechanism of thixotropy.