Doctor of Philosophy (PhD)
The dissertation is about the design, synthesis and characterization of materials comprised of covalent adaptable networks (CANs). There are three chapters within this dissertation.
Chapter 1 contains a short overview of conventional thermosets and thermoplastics. A new strategy of combining advantages from thermosets and thermoplastics is utilizing dynamic covalent chemistry (DCC) for material design. Dynamic covalent bonds are able to undergo dynamic exchange which allow the material to adapt to a new state when under stimuli such as stress or strain. Material comprised of such reversible networks is called covalent adaptable networks (CANs). These types of reversible covalent bonds followed two types of exchange mechanisms: dissociation and association. A review of dissociative-pathway-based materials includes retro-Diels-Alder, [2+2] and [4+4] cycloaddition, boronate ester, and alkoxyamines. CANs followed associative mechanism including radical addition fragmentation chain transfer reaction, anionic disulfide exchange, and transesterification are also reviewed. Lastly, a general consideration in designing CANs includes the importance of each experiment, the information obtained from the experiments, and general experimental procedures.
Chapter 2 is about the design, synthesis and characterization of solvent-swelled polyimine networks. The presence of reversible imine bond exchange was found to influence the dynamical properties of the polymer networks. Investigation of the imine bond exchange kinetics and characterization of the creep and stress relaxation properties of the polyimine networks have been conducted. A correlation was found between the relative imine bond exchange rate in different solvents and the relative stress relaxation rate of the solvent-swelled networks. The polyimine networks can also be re-mended and recycled by hydrolysis.
Chapter 3 is focused on CANs based on aminal linkage. The thermodynamics and kinetics of the aminal dissociation as well as transaminalation reaction have been investigated using model compounds. Temperature-dependent stress relaxation behavior of the polyamine networks have also been characterized by dynamic mechanical analysis (DMA) method, allowing for an assessment of the activation energy of the transaminalation in the network. Aminal exchange was found to occur by a dissociative mechanism that requires the presence of catalytic protic sources to facilitate the formation of iminium intermediates. The polyaminal networks can be reprocessed as a viscoelastic solid (above the gel point) rather than a viscoelastic liquid. These materials were shown to be elastomers and can be thermally reprocessed and re-mended.
Chao, Albert, "Synthesis and Characterization of Covalent Adaptable Networks Comprised of Dynamic Imine and Aminal Covalent Bonds" (2018). LSU Doctoral Dissertations. 4506.