Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Christopher N. Bowman

Second Advisor

Jeffrey W. Stansbury

Third Advisor

Kristi S. Anseth

Fourth Advisor

Robert R. McLeod

Fifth Advisor

Mark P. Stoykovich

Abstract

This thesis aimed to expand upon the chemistry and potential applications of covalent adaptable networks (CANs), focusing on three types of reversible reactions: Diels-Alder, transesterification, and thiol-thioester exchange. The thermoreversible Diels-Alder (DA) reaction was synergistically combined with two other polymerization types to create novel hybrid materials. In the first aspect, a second-stage acrylate photopolymerization was used to "fix" the DA network in a spatially controlled manner. During light exposure, a sharp increase in acrylate conversion and a 60°C jump in the polymer glass transition temperature were observed. More importantly, the network could no longer be depolymerized into a soluble material by the retro-DA reaction. This dual-cure network polymer was applied as a solid-state photoresist to create 3D microstructures. In the second aspect, DA and polyurethane polymerizations were performed simultaneously to form a pseudo-interpenetrating network structure. The two thermally-driven reactions were shown to be orthogonal by real-time FTIR studies. Coatings of the hybrid polymer were repeatedly damaged and healed, as confirmed by tensile fracture testing and electrochemical conductivity experiments, while retaining their overall shape, in contrast to pure DA networks.

In addition, two types of exchange-based covalent adaptable networks (CANs) were studied in the context of thiol-ene “click” polymerizations. The transesterification reaction was implemented in a photocurable thiol-ene polymer network. Between 145 and 175°C, a strong Arrhenius dependence of the characteristic relaxation time with temperature was observed, which provided confirmation that the material behaved as a vitrimer. Nanoimprint lithography (NIL) was performed at 175°C to create optically active, yet reversible nanoscale topologies. Micro-scale photopatterning was also applied in combination with NIL to create hierarchical, mixed-scale patterns. Finally, the thiol-thioester exchange reaction was demonstrated for ambient temperature CANs with unprecedented ambient temperature exchange kinetics. These crosslinked polymers were shown to undergo significant creep and stress relaxation at ambient temperature, provided that free thiol, base, and thioesters were all present. Up to 90% tensile stress relaxation was achieved within 15 minutes, using a 2:1 thiol:ene ratio and a mild aliphatic tertiary amine catalyst. Several techniques were investigated to afford greater spatiotemporal control over the occurrence of bond exchange reactions, including a two-stage cure of a stoichiometric formulation as an “off switch” to consume free thiol groups, as well as the inclusion of a photo-protected base to serve as a latent “on switch” to generate thiolate anions within the network.

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