Studying chemical and topological rearrangements in covalent adaptive networks Henry Nguyen, Matthew Unthank Northumbria University, UK Covalent adaptive networks (CANs) are thermosetting polymers that contain exchangeable covalent bonds to allow the topology of the network to be rearranged in response to a stimulus. This phenomenon brings elasticity into an infinite 3D polymeric structure, which introduces the self-healing, stress relaxation, and reprocessibility properties to a mechanically strong and chemically resistant material. Generally, the mechanism of the bond exchange reactions occurs in either associative or dissociative pathways, resulting in differences in the material’s bulk properties. In this study, two dynamic materials were synthesised based on identical monomer mixtures, cross-linked by bismaleimide and diboronic acid, resulting in a dissociative covalent network (Diels-Alder) and associative covalent network (boronate ester), respectively. In the latter, the macromolecular structure during the bond exchange process remains almost unchanged due to approximately constant linkage density. Whereas in the Diels-Alder material, there is a dramatic drop in the total linkages and significant rearrangement in the network’s topology due to the activation of the bond breakage (retro Diels-Alder reaction). As a result, the rheological data of the two materials showed significant differences in storage and loss modulus, relaxation time, and activation energy during and after the network’s rearrangement. From the Arrhenius plot of relaxation time, the Diels-Alder polymer showed two activation energy values, corresponding to below and above the retro Diels- Alder temperature, whilst the boronate ester polymer only illustrated one activation energy value. Furthermore, the relationship between Tg and viscosity of the two series of materials is also investigated in this study. The viscosity drop temperature of the boronate polymers depends significantly on the Tg and the cross-linking density of the network, whereas the Diels-Alder polymers showed a decrease in viscosity around the retro Diels-Alder temperature, regardless of the Tg of the materials.
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© The Author(s), 2023
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