Abstract:The structure-property relationships derived here permit the prediction of both the zero-shear viscosity r/0 , as well as the shear rate dependent viscosity r/o)). Using this molecular modeling it is now possible to predict r/over the whole concentration range, independently of the molecular weight, polymer concentration and imposed shear rate. However, the widely accepted concept: dilute-concentrated, is insufficient. Moreover it is necessary to take five distinct states of solution into account if the viscous behavior of polymeric liquids is to be described satisfactorily. For non-homogeneous, semi-dilute (moderately concentrated) solutions the slope in the linear region of the flow curve (r/= fo))) must be standardized against the overlap parameter c. [r/]. As with the r/o-M~o-crelationship, a r/-M~o-c-y relationship can now be formulated for the complete range of concentration and molecular weight. Furthermore, it is possible to predict the onset of shear induced degradation of polymeric liquids subjected to a laminar velocity field on the basis of molecular modeling. These theoretically obtained results lead to the previously made ad hoc conclusion (Kulicke, Porter [32]) that, experimentally, it is not possible to detect the second Newtonian region.Key words: Zero-shear viscosity, shear viscosity, non-Newtonian viscosity, semi-_dilute solution, structure-property relationship, relaxation time, shear stability, _degradation. Nomenclature Roman and Italian symbols
This work demonstrates the effectiveness of polymers in improving, especially, the high temperature properties of asphalt. The appropriate choice of asphalt, asphalt-grade, polymer type, polymer concentration, and the method of mixing determine if a network-like structure is formed. This morphology significantly improves the creep performance of the binder at elevated temperatures, i.e., the binder has the ability to store deformation energy with subsequent recoil. This is contrary to Newtonian fluids which transform the energy into viscous flow (no recoil). Within the context of dynamic mechanical measurements, the presence of a polymeric network is manifested through the appearance of a plateau modulus. In the case of binders containing block copolymers, we have repeatedly observed that such property improvement in the high-temperature range is generally accompanied by a reduction of the glassy modulus at the low-temperature range as well. It should be noted that by modifying low-viscosity asphalts (i.e., low AC-grades) with polymers, binders can be obtained which exhibit significantly lower moduli at low temperatures and higher moduli at elevated temperatures. This suggests that although using a high AC-grade asphalt may yield satisfactory results at a particular temperature (high temperature), one may instead optimize binders over the entire temperature range (high and low) by starting with a low AC-grade and adding polymer. These results indicate that careful Theological measurements can be a powerful tool in the characterization and design of viscoelastic blends.
A semiempirical approach developed to predict the viscoelastic response of a binder in repeated creep recovery tests is described. This model provides an avenue to predict the rut resistance ( R) as a function of loading (time and load) and temperature from data at a single frequency or frequency sweeps when needed. Thus, it can be used to develop a grading procedure for asphalt binders that not only accurately captures the delayed elasticity of modified binders but also accounts for the effect of traffic speed and traffic loading. The current Superpave binder specification attempts to capture the relative high-temperature performance (i.e., resistance to rut) of a binder via the inverse shear loss compliance, 1/ J” or G*/sin δ, at 10 rad/s. This parameter represents an improvement over the absolute viscosity because it is measured at a defined rate of deformation and accounts to some degree for the viscoelasticity of the binder via the phase angle. The parameter would correctly predict the relative R for an ideally viscous ( R ∞ η) material or an ideally elastic material ( R = ∞). However, there is mounting evidence that at phase angles between 40° and 75°, the parameter may not fully capture the viscoelastic nature of many modified binders. Various authors have shown that the R of mixtures can be well described by models using data from dynamic creep experiments, in which the mix is subjected to a load followed by a relaxation period. More recently, Bahia proposed to capture their high-temperature performance by using a similar technique on neat binders.
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