The dynamic fragility and apparent activation energy of bitumens as expressed by a modified Kaelble equation

The temperature dependence of the dynamics of glass-forming liquids can be characterized by the dynamic fragility (m) and apparent activation energy (E_a) at the glass transition temperature T_g. In this study, we derive analytical expressions that allow the calculation of these parameters from a modified Kaelble equation which divides the temperature dependence into two regimes above and below a characteristic temperature T_d. Special emphasis is given to the analysis of the T_d parameter that can be considered as the rheological glass transition temperature. Rheological characterization is performed on twenty-seven bitumens originating from various crude oil sources and refining processes. Their dynamic fragilities and apparent activation energies are calculated at the calorimetric T_g and at T_d. Bitumen can be classified as a strong glass-forming liquid, dynamic fragilities varying in the range of m(T_g) = 26 … 52 for the individual bitumen samples. The results indicate that m(T_g) and E_a(T_g) are linearly correlated with T_g, and these T_g-dependences are unusually strong in comparison to other classes of glass-forming liquids. However, dynamic fragilities and apparent activation energies evaluated at T_d are nearly independent of the type of bitumen and show only a weak dependence on T_d.