Birefringence of polymer solutions in time dependent flows

• Main Author: Geffroy Aguilar, Enrique.
• Format: Others
• Language: en
• Published: 1990
• Online Access: https://thesis.library.caltech.edu/722/1/Aguilar_eg_1990.pdf; Geffroy Aguilar, Enrique. (1990) Birefringence of polymer solutions in time dependent flows. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/gpt7-3556. https://resolver.caltech.edu/CaltechETD:etd-02232007-154744 <https://resolver.caltech.edu/CaltechETD:etd-02232007-154744>

This is a study of changes of conformation of macromolecules in polymeric solutions which are subjected to time-dependent extensional flows generated by a two-roll mill flow device. The flows produced by the two-roll mill are linear, and two-dimensional. It has a stagnation point at the center of the flow field where the magnitudes of the strain-rates are greater than the vorticity. This study of conformational changes is based on data around the vicinity of the stagnation point, I for steady state flows, and several transient flow histories such as startup, cessation, and double-step flows. We also present an analytical solution for the creeping flow generated by an infinitely long two-roll mill embedded in an unbounded fluid. This solution is used as a benchmark to compare the behavior of the polymer solutions when subjected to flows with different values for the ratio of rate-of-strain to vorticity. The conformational changes are determined experimentally using the Two-color Flow-Birefringence which provides an instantaneous and point-wise measure of the anisotropy of the fluid, together with the relative orientation of the anisotropy with respect to the principal axes of the flow field. Based on relaxation of the fluid anisotropy the characteristic time-scales of the polymer have been evaluated as a function of the flow field properties and the degree of conformational change of the macromolecules. Data for two polymeric solution is presented. The first polymer system is the so-called test-fluid Ml. This polymeric solution is shown to degrade significantly, even for small values of the velocity gradient, as measured by the changes in the macroscopic relaxation time-scales. The second solution is a concentrated polystyrene solution that presents overshoots and undershoots of the polymer conformation dependent of the ratio of vorticity to rate-of-strain. When subjected to large deformations, this polystyrene solution shows not only the possibility of a reduced number of entanglements, but also a significant deformation of the macromolecule segments associated to a shorter relaxation time-scale for the dynamics of concentrated solutions, which corresponds to that predicted by the intermediate relaxation time-scale of Doi-Edwards model.