Nonlinear Mechanics and Applications of Bipyridine-Based Associative Polymer Networks
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Authors
Locke, Cameron Keegan
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
associative polymer networks , bipyridine , nonlinear mechanics , polymer
Alternative Title
Abstract
Associative polymer networks are inspired by the dynamic nature of biomaterials and have broad applications as stimuli-responsive materials. This dissertation investigates the fundamental structure-property relationships in associative networks and explores applications of polymers and networks containing bipyridine ligands in the backbone for photochromism and electrocatalysis.In Chapter 2, we investigated the nonlinear extensional response of associative polymer networks, with a focus on tensile strain softening. Biological systems often exhibit strain stiffening to prevent deformation and softening to avoid catastrophic failure, yet the mechanisms underlying their nonlinear mechanics remain poorly understood. Metallo-supramolecular networks formed by pendant group crosslinking with different metal ions and with a low degree of entanglements were used to access different network temporal parameters. While all networks exhibited shear-thinning, the ones with fast metal-ligand crosslinks showed tensile strain softening with a yield point, while networks with slow crosslinks did not. The mechanism of yielding was explained by comparing the changing instantaneous extension rate at different stages of the tensile test with the network relaxation times probed by sticky Rouse model and KWW fitting.
Chapter 3 expands on the work of strain softening associative networks by implementing dual-crosslinked networks to investigate the effect of mixing metals on yielding and softening behavior. Ideally, the yield stress and recoverable strain of softening materials can both be increased. This chapter discusses the approaches used towards advancing the complex and dynamic mechanical properties of associative networks.
Chapter 4 reports two applications of the bipyridine-containing polymers with the first being a novel and tunable photochromic small molecule complex formed between 4,4’-disubstituted-2,2’-bipyridine and sulfur oxides via a light-activated interaction. Later, a photochromic bipyridine polymer was also developed. The photoactivated N-oxide-SO2 complex absorbs visible light and exhibits reversible photochromism. Second, in collaboration with the Barile Group, we explored the utilization of bipyridine polymers for electrochemical catalysis, namely oxygen reduction reactions (ORR). ORR catalysis is essential for energy conversion applications; however, the reactions are challenged by sluggish kinetics and large overpotentials required for activation. Therefore, we probed structure-activity relationships of copper-coordinated bipyridine small molecules and bipyridine-containing polymers for potential applications in electrocatalysis for ORR.
Lastly, in collaboration with the Urban Group at Clemson University, Chapter 5 analyzes the rheological behavior of poly(ionic liquid)s. Poly(ionic liquid)s are of importance because they possess the properties of high conductivity, high thermal stability, low vapor pressure, and tunable chemical structure. The structural advantages of polymers result in material properties that can be tuned by manipulation of the polymer backbone for various applications.