Rheology of complex glass-forming liquids

While the rheology of chemically and structurally simple glass-forming liquids (SGFLs) has been studied extensively in the literature, very little is known about the rheology of chemically and structurally complex glass-forming liquids (CGFLs). The aim of this thesis was to study the rheological properties of these CGFLs near and below the glass transition temperature (Tg). In addition, rheological characterization and analysis techniques were developed in support of this research. In the experimental part of this work, small-diameter parallel plate (SDPP) rheometry was introduced as a simple and accurate technique to measure the rheological properties of glass-forming liquids in shear. This measurement technique was applied to petroleum fluids that were chosen as model materials for CGFLs. In particular, bitumen exhibits a very broad viscoelastic glass transition in comparison to SGFLs. This is particularly accentuated in the broadening of the relaxation time spectrum at long times. Based on this observation, a new constitutive model – called the broadened power-law spectrum model – was proposed for CGFLs. This model was successfully used to describe the viscoelastic behavior of a series of petroleum fluids. The stretching parameter β of the broadened power-law spectrum model, which serves as a quantitative measure of the broadening of the relaxation time spectrum, was found to correlate with the width of the calorimetric glass transition region in a power-law fashion. The modification of bitumen with styrene-butadiene-styrene (SBS) block copolymer resulted in a distinct two-phase morphology. The morphological characteristics of SBS modified bitumen were shown to have a profound impact on the thermorheological behavior at low temperatures. In addition, special attention was paid to the temperature dependence of viscoelastic properties in the vicinity of Tg. Analytical expressions were derived for the dynamic fragility (m) and apparent activation energy (Ea) in terms of the parameters of the modified Kaelble equation, and it was observed that the linear Tg-dependences of m(Tg) and Ea(Tg) are unusually strong in bitumens in comparison to other classes of glass-forming liquids. It was demonstrated that time-resolved rheometry in combination with the time-aging time superposition principle is a powerful technique to monitor and analyze the time dependence of linear viscoelastic properties during physical aging. This technique successfully captured the effects of temperature, crystallinity and SBS polymer modification on the physical aging of bitumen.