Theoretical and Experimental Studies of Structure Formation in Polymer Solutions Under Flow
Vrahopoulou, Elisavet Prodromos
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https://hdl.handle.net/2142/69774
Description
Title
Theoretical and Experimental Studies of Structure Formation in Polymer Solutions Under Flow
Author(s)
Vrahopoulou, Elisavet Prodromos
Issue Date
1986
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
A series of studies have been carried out with the goal of quantifying via molecular modeling the phenomenon of flow-induced structure formation in polymer solutions. Flow-induced stresses cause changes in the free energy of the polymer solutions and can eventually lead to phase separation. The thermodynamic behavior of random polymer coils under stress is treated as equivalent to that of a model system of semiflexible macromolecules in the absence of applied forces. Application of equilibrium conditions results in model predictions which are in good agreement with experimental observations i.e., changes in the binodal curve, increase in the amount of the separated concentrated phase and molecular weight fractionation with increase in the strength of the flow field.
The kinetic mechanism for structure formation has been studied with viscometric experiments in capillary devices and shear-thickening behavior above a critical shear rate has been observed in a number of high molecular weight systems. Primary attention has been given to crystallizable polymers such as ultrahigh molecular weight polyethylene, polypropylene and poly(ethylene oxide). The results are in agreement with precursor fiber formation in tubular flow-induced crystallization experiments.
Modeling of the shear-thickening effect has been performed with the use of molecular theories. An expansion of the reduced viscosity expression to include the concentration of the associated species formed during flow, shows shear-thickening. Although such a kinetic treatment uses the FENE dumbbell model to describe the quasiaggregates and can be fitted to the experimental data, it neglects intermolecular interactions. For this, the molecular network theory proposed by Yamamoto has been considered. The use of a non-Gaussian free-energy expression for the rate of entanglement creation and a constant breakage function result in material functions which depend on the number of subunits in the network segments. For short segments, the initial increase in the viscosity is a result of the increased energy of dissipation as they deform by the applied flow field. The slip coefficient causes a shear thinning behavior which becomes more pronounced for long polymer segments.
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