Aqueous Metallo-Megasupramolecules: From Stability to Extensional Flow Properties

Citation

Tawney, Jacqueline Rose (2025) Aqueous Metallo-Megasupramolecules: From Stability to Extensional Flow Properties. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/98fw-fx90. https://resolver.caltech.edu/CaltechTHESIS:06022025-131020183

Abstract

The addition of long, flexible polymers (> 1 Mg/mol) to a fluid is known to reduce turbulent drag and control droplet behavior, which has the potential to significantly enhance the efficiency of engineering flows across various industries, from agriculture to aviation. However, hydrodynamic forces can break the polymers and diminish their effectiveness, which is presently a major roadblock to their practical utilization in both applications and research. To address this challenge, the Kornfield group developed end-associative, self-healing polymers for use in fuel and, more recently, for use in water—aqueous terpyridine-ended polyacrylamide (TPAM) supramolecules. This thesis examines the relationships between the molecular structure of TPAM, the amount of metal provided to link pairs of chain ends, and kinetic processes of the resulting supramolecules and the rheological properties and performance they provide. The most useful polymers for reducing turbulent drag, controlling mist, and tailoring droplet impact behavior combine high efficacy at low concentration (< 0.1 wt%), minimal impact on shear viscosity (< 2x), and long extensional relaxation time (> 1 ms), enabling them to stretch and resist elongational flow in turbulent eddies or fluid filaments. This thesis explores the fundamental nature of TPAM supramolecules and their potential utility as a rheological modifier, using measurements of molecular weight distributions and extensional relaxation times to illuminate the relationship between supramolecular structure and flow behavior.

First, we examine chemical degradation (desirable in the environment, but not during use), revealing that its rate can be controlled by limiting air exposure, avoiding an excess of metal ions relative to ligands, and storing samples in refrigerated conditions (4℃). Next, we assess how changes in metal-to-ligand ratios (M:L) and unimer lengths influence TPAM’s megasupramolecular size, equilibration, and decay dynamics, showing that the presence of supramolecules comprising over 10 unimers gives rise to a relaxation time around 2 ms at 0.04 wt%—long and dilute enough to cause drag reduction. In pursuit of even longer supramolecules (and thus longer relaxation times) with the same amount of TPAM, we modified the solution preparation protocol by introducing metal ions to a more concentrated TPAM solution prior to dilution. This exposed new and intriguing topologies with molecular weights extending beyond our measurable limit (10 Mg/mol), expanding the envelope of the longest accessible relaxation times (from ~2 to ~6 ms with M:L = 1:2 for Ni(II):terpyridine). We evaluated their potential as chain scission-resistant, turbulent drag-reducing agents. Initially, they reduce drag while maintaining backbone integrity; however, their supramolecular structure and extended relaxation time are not retained after multiple passes through contraction, turbulent, and expansion flows. The preservation of backbone integrity, along with the broad range of relaxation times achieved using more conventional linear topologies (up to ~3 ms), suggests that TPAM is a promising and robust rheological modifier worthy of continued investigation. Our findings enhance understanding of TPAM’s structural and rheological properties under a range of conditions and lay the groundwork for further study of aqueous megasupramolecule dynamics and applications.

Thesis Files