Solution Rheology: Scaling Theory and Membrane Applications

Solution Rheology: Scaling Theory and Membrane Applications

Alexander T. Bridge, Dr. Matthew D. Green, PhD

Polymer engineering is a rapidly expanding field that stands to offer a range of material-based solutions to everyday problems. Rheology, the study of flow characteristics of materials, is a discipline that is a fundamental staple to advancements in polymer research. As such, this project explored fundamental rheological scaling theory in the context of standard polymer and polyelectrolyte solutions. Using M_w~75,000 g/mol polystyrene, a synthetic aromatic polymer, analysis of viscosity versus concentration was performed to model existing scaling relationships. Samples were tested in toluene and tetrahydrofuran with concentrations ranging from 0.0001 to 0.002 M in the dilute and semi-dilute unentangled regimes. Analysis of specific viscosity versus molar concentration on log-log plots revealed power law relationships of 1.01 and 1.08 in dilute solutions and 1.62 and 1.48 in semi-dilute unentangled solutions in toluene and tetrahydrofuran, respectively. Polyelectrolyte rheology was performed with M_w~70,000 g/mol poly(sodium 4-styrenosulfonate), or PSS, with similar success. PSS is fundamentally identical to polystyrene in structure, differing only with an added sulfonate to the aromatic side group to induce dissociation in solution. Samples residing in the semi-dilute unentangled and semi-dilute entangled regimes were prepared with concentrations ranging from 0.0001 to 0.006 M in water. Solubility limitations prevented the formation of concentrated PSS solutions. Testing of PSS in water revealed specific viscosity versus molar concentration power law relationships of 0.67 in the semi-dilute unentangled regime and 1.64 in the semi-dilute entangled regime on log-log plots. The entanglement concentration of PSS was roughly 0.0011 M in water. In comparison to polystyrene, it was observed that the added ionizing group in PSS considerably lowered viscosity in the semi-dilute unentangled regime, likely a result of ionic repulsions between the negatively charged chains acting to prevent overlap. Applications of rheology and polymer engineering to the optimization of medical devices are to be additionally explored through the production and operation of a bench-scale hollow fiber membrane spinning apparatus. This device is set to be used for continuous extrusion of hollow fiber membrane strands using novel polymer and polyelectrolyte candidates as membrane materials. Polymer types to be explored include segmented ionenes, ion-containing block copolymers, and a range of neutral structures. Rheological analysis based on the above experiments will be employed to better understand solution behavior, determine optimal solution compositions, and confirm applicability for intended applications.