Bitumen microstructure by modulated differential scanning calorimetry

Bitumen was analysed by modulated differential scanning calorimetry (MDSC). This method allows for the deconvolution of overlapping reversing and non-reversing thermal events and it allows for the observation of transitions not visible on the thermal curve obtained by standard DSC. The reversing thermal curve revealed two glass transition temperatures (Tg) in bitumen. One transition, at −20°C, was assigned to the maltenes, the other at 70°C to the asphaltenes. The heat capacity of these transitions was found to depend on thermal history. From the total heat capacity, it was calculated that the relative size of the bitumen repeat unit is between 36 and 91 g mol−1, which given an average molecular weight of 300–1000 g mol−1 for bitumen, translates into a “degree of polymerisation” of ∼10. After cooling from the melt and annealing at 22°C, bitumen microstructure was found to develop in three stages. Most rapid is an ordering process that occurs when bitumen is quenched from the melt. It is postulated that this first stage arises from the partial ordering of simple aromatic structures into micro- and nano-phases; a second stage, which ends within ∼3 h of annealing, relates to the ordering of somewhat larger aromatic structures; and a third stage, which ends in ∼16 h, is thought to arise from the ordering of the largest bitumen structures, the resins and the asphaltenes. The development of bitumen microstructure and the calculations of the entropy and enthalpy of transitions suggest that bitumen is a multi-phase system with a small crystalline phase, and a large mesophase, i.e. a structured amorphous phase.