| To provide rational predictions of industrial polymer processes such as extrusion, fiber spinning, or blow molding, the constitutive model relating stresses to deformation history must provide adequate representation of fluid response to complex deformations involving mixtures of shear and/or extension. It therefore becomes imperative to provide an experimental platform on which rigorous examination of models---those currently used in commercial computational fluid dynamics software and those presently in the developmental stages in academia---can be made. This thesis provides such a platform with a comprehensive set of flow birefringence measurements of the stress response of two polystyrene solutions with different entanglement densities in time-dependent axisymmetric flows exhibiting regions of reversing complex deformations. To reveal subtleties of constitutive models, this experiment is designed to probe predetermined regions of nonlinear viscoelasticity by independently adjusting the characteristic strain and time constant of the flow. Using this experimental platform, detailed examinations of the Giesekus model were performed.; For the lightly entangled fluid, simulations of a multi-mode Giesekus model (fit to shear rheology of the fluid) were obtained for all flow conditions studied. In regions of strong reversing extensional flows, the Giesekus model underpredicted the normal stresses in regions of strong uniaxial extension. The discrepancy reveals the inherent difficulty of the Giesekus model to simultaneously predict both strong shear thinning and extension hardening behavior. Otherwise, the model performed very well, particularly in predicting the steady-periodic response following the first oscillation cycle.; For the more highly entangled fluid, simulations of an abbreviated multi-mode Giesekus model (fit to shear rheology of the fluid) were obtained for three flow conditions of increasing severity. In contrast to the lightly entangled fluid, the Giesekus model overpredicted the periodic normal stresses in regions of strong uniaxial extension, providing evidence that the more highly entangled fluid is less extension hardening than the lightly entangled fluid.; Attempts have been made to fit newer reptation-based constitutive models to steady shear rheology of the highly entangled test fluid. While these models employ more sophisticated molecular concepts, it is not yet clear whether they offer improved promise over more empirical models for accurate flow simulation. |
