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Title:Self-healing and ionic interactions: a look into thermal properties of sequence-specific ionic networks and the synthesis of spiropyran monomers for triggerd ionic networks
Author(s):Coates, Anderson
Advisor(s):Moore, Jeffrey S
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):ionic network
DNA material
sequence effects
ionic crosslink
ion coordination
Abstract:Self-healing is essential in future materials for extending lifetime beyond normal limits and lowering operational cost over extended use. Of the structural approaches to self-healing, an intrinsic system provides capability for multiple healing cycles based on reversible healing processes. Of these processes, ionic interactions provide the basis for reversible self-healing systems from organic ion associations and metal-ligand coordination. In my investigation of organic ion associations, I probed the effects of oligomer sequence on the thermal stability of guanidium-phosphate associations in DNA ionic networks. Each network contained a different five monomer sequence of DNA using two monomers crosslinked by bis(guanidinium) dications. Measuring the thermal properties by differential scanning calorimetry for all 32 ionic network sequences and comparing their properties lead to structural trends that increase overall ionic network stability indicated by a higher degradation temperature. Substituting in the more flexible monomer at one chain end lead to an increase in degradation temperature in 13 pairs of sequences. However, substitution at the other oligomer chain end did not produce a similar temperature increase. This is due to the increased flexibility of one chain end providing more stabilizing interactions during the evaporation phase of sample preparation. Due to the sequence-specific and monodisperse nature of automated DNA synthesis, a small change in the oligomer structure produces measurable effects on thermal properties. Understanding the contributions of each monomer leads to total control of a material's properties for optimal material design. Investigation of metal-ligand coordination in self-healing began with a collaboration focused on synthesizing spiropyran polymers that reversibly form ionic crosslinks on stimulation by force or UV light in a self-strengthening process. Reversible crosslinking in a stimuli responsive material provides strengthening and relaxation in cycles that counteracts fatigue to prolong material lifetime. Under stimulus, spiropyran opens to zwitterionic merocyanine which coordinates to metal ions as a ligand. Modifying spiropyran with coordinating sidechains controls the ratio of spiropyran molecules to metal ions such that two or more spiropyran molecules coordinate to a single ion. In polymers containing modified spiropyran, this multiple spiropyran to ion coordination allows the formation of ionic crosslinks that trigger on force or UV light and return to the original state on removing stimuli. My collaborators and I synthesized spiropyran and spirooxazine monomers with coordinating sidechains to analyze the ion coordination stoichiometry to probe candidates for triggered and reversible ionic crosslinking. Spiropyran with a para-nitro group stabilized merocyanine such that consistent red color was observed in the solid state polymer and solution. This drove efforts toward developing new synthesis routes with less potent electron withdrawing groups. We synthesized a naphthalene spirooxazine that showed photochromism at low temperature, and we have a promising start on synthesizing spiropyran monomers with less powerful electron withdrawing groups. Developing these syntheses of other electron withdrawing groups in spiropyran and spirooxazine will lead to candidates for triggered ionic crosslinking for reversible self-strengthening systems.
Issue Date:2016-06-30
Rights Information:Copyright 2016 Anderson Coates
Date Available in IDEALS:2016-11-10
Date Deposited:2016-08

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