Thermodynamics, phase diagrams, and stability of bitumen-polymer blends

Infrared spectroscopy was used to measure the stability, segregation rates, and equilibrium concentrations of blends with bitumen and styrene-butadiene (SB)-type copolymers stored at 100-180degreesC. The effects of SB concentration, molecular weight, S:B ratio, branching, and bitumen source were investigated. The results were analyzed in light of established mechanisms for phase segregation, blend thermodynamics, and phase diagrams for binary mixtures. It is shown that the phase diagram for bitumen-SB blends is tridimensional and that it contains a lower critical solution temperature (LCST) that decreases in temperature with an increase in polymer content or molecular weight. Equilibrium blend stability is governed by the entropy and enthalpy of mixing, i.e., molecular weights and intermolecular interactions. The rate of segregation, if any, is affected by the molecular shape of the blend components and their molecular weights. It is also found that SBS branching does not affect blend stability and that the morphology of incompatible blends likely results from spinodal decomposition. Hildebrand solubility coefficients provide inaccurate predictions of interactions between bitumen and SBS, but infrared frequency shifts do reveal the nature and strength of the intermolecular interactions. The colloidal instability index, the aromatics content, and the asphaltenes content in bitumen lead to discordant stability predictions for bitumen-SBS, blends. This study provides a theoretical framework for understanding the complex interrelationship of the various parameters that affect the stability of bitumen-polymer blends in general.