Dynamic polymer hydrogels based on the orthogonal combination of reversible C=N bonds and metal-ligand coordination bonds
(2021)
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- Dynamic polymer hydrogels crosslinked by dynamic bonds are capable of a controlled lifetime, enhanced toughness, self-healing properties or responsiveness to external stimuli. Dynamic bonds generally fall into two categories: supramolecular interactions and dynamic covalent bonds, which can break and reform spontaneously or in response to external stimuli. Supramolecular interactions (e.g., hydrogen bonds, metal-ligand coordination bonds, host-guest interactions) are labile and reversible, and usually under (fast) continuous equilibrium between associated and dissociated forms. In comparison with supramolecular interactions, dynamic covalent bonds (e.g., C=N bonds, disulfide bonds, boronic ester bonds) are much more robust but also exhibit slower exchange. The pros and cons of these two types of linkages make them a promising complementary toolbox for the design and fine-tuning of the properties of hydrogels. The focus of this dissertation is the development of dynamic polymer hydrogels by the orthogonal combination of dynamic covalent bonds and supramolecular interactions, and the investigation of the contribution of the two types of dynamic bonds to the properties of the hydrogels. Metal-terpyridine coordination bonds (zinc(II)-terpyridine and iron(II)-terpyridine bis-complexes) and reversible C=N bonds (acylhydrazone and oxime) were selected and employed as crosslinks to form hydrogels. First, we sought to design four interpenetrating polymer network (IPN) hydrogels with tunable combinations by leveraging the differences in lifetime and stability of the two types of dynamic bonds. The dynamic, oscillatory mechanical properties and pH responsiveness of the four hydrogels were systematically investigated using oscillatory rheological methods. Second, we prepared hydrogels with three topological strategies, namely, IPN, dual-crosslinked network (DCN) and double network (DN), using a combination of zinc(II)-terpyridine bis-complexes and oxime bonds. The dynamic and linear rheological behaviors of hydrogels prepared by these three strategies that allow the integration of two dynamic bonds into one system were explored and compared. Finally, we prepared and compared the rheological properties of two DN hydrogels made from a combination of labile zinc(II)-terpyridine bis-complexes and stable oxime bonds, as well as a combination of less stable acylhydrazone bonds and long-lifetime iron(II)-terpyridine bis-complexes. Overall, this work has demonstrated the feasibility and interest in combining dynamic covalent bonds and supramolecular interactions for the preparation of dynamic polymer hydrogels, and has explored the pros and cons of three network topologies for combining the two crosslinks into a polymer hydrogel.
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Yang, H. (2021). Dynamic polymer hydrogels based on the orthogonal combination of reversible C=N bonds and metal-ligand coordination bonds. https://hdl.handle.net/2078.5/27367