Analysis and Control of Degradation in Covalent Adaptable Networks

Hernandez, Juan J.

Graduate Thesis Or Dissertation

Analysis and Control of Degradation in Covalent Adaptable Networks Public Deposited

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Citations

Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/x346d607m

Abstract

This thesis explores polymer degradation within covalent adaptable networks (CANs), focusing on degradation and properties controlled by the thiol-thioester exchange, the thiol-thioaminal exchange, and the thioaminal scission reactions. By comparing mass loss profiles and mechanical property changes to theoretical models developed herein, this work explores the degree to which polymer network structure and reaction kinetics tune the degradation process.

The first part of this thesis focuses on the base-catalyzed thiol-thioester exchange reaction and how this reaction was used to degrade thioester-containing CANs. Statistical models quantitatively depicted the degradation process where these models incorporated the thiol-thioester exchange reaction kinetics, polymer structure, and mass gained from the exchanging thiol. A single reaction rate constant (k) fit experimental mass loss profiles to model predictions, and only varied from 0.0024 – 0.0051 M^-1min^-1 throughout all degradation conditions studied including changing crosslinking density, reactant concentration, oligomer lengths, and oligomer distributions. Using these parameters, degradation within thioesters networks could be tuned to occur on timescales from 2.5 h to near infinity, shift the degradation mechanism from surface to bulk, enable > 90 % selective recovery of fillers, and mediate controlled mass release.

The second part of this thesis focuses on how thioaminal groups enable degradation within CANs. First, the thiol-thioaminal exchange reaction was explored as a new, reversible-exchange CAN chemistry. The exchange reaction was monitored by a small molecule system, showing the choice of thiol impacted four kinetic parameters (k_f, k_r, K_eq, t1/2,eq). Meanwhile, thioaminal-containing networks were found to stress relax (10 s at 95 °C), degrade rapidly when exposed to neat excess thiol (from 4 – 380 min), and exhibit temperature-independent crosslink density. Second, the thioaminal scission reaction was used as a means to create constructing-then-destructing CANs. Exposing thioaminal small molecules to photoradicals probed polymerization and scission reactions and found the exposure resulted in thioamide formation. Increasing thiol substitution (1° – 3°) resulted in greater extent of scission (5 – 39 %), with scission occurring semi-orthogonally to the thiol-ene polymerization reaction. Constructing-then-destructing CANs that depended on total light dose exposure were created, switching between construction and destruction at a light dose of 2 J/cm² under the selected conditions.

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