Engineering Dual Dynamic Polymer Networks with Tunable Elasticity and Diffusive Permeability
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Abstract
Hydrogels are employed in everyday life products such as contact lenses, diapers, and cosmetic creams. In addition, their aqueous composition, soft mechanics, and high permeability, makes them appealing materials in the biomedical field. However, these diverse applications have specific requirements in terms of mechanics and functionality that can often not be fulfilled by a single polymer. To overcome this limitation, multiple crosslinking types can be embedded in the same polymer matrix, thereby, realizing a dual dynamic network (DDN). However, the determination of structure–property relationships for these multi-component systems is often not straightforward and therefore, comprehensive studies are needed for a rational materials design. This thesis aims to contribute to this field with the development and methodological exploration of elasticity and permeability of a novel DDN as well as the investigation of the effect of defects on the targeted properties.
The synthesis of this novel dual dynamic network is presented in Chapter 3. To a model 4-arm poly(ethylene glycol) (pEG) main network, two dynamic motifs are attached. The first dynamic motif is a terpyridine ligand which is capable of forming bis-terpyridine coordination complexes with different metal ions, whereas the second dynamic motif is a linear poly(N-isopropylacrylamide) (pNIPAAm) chain. The combination of these dynamic motifs makes it possible to customize the properties of the DDN such as elasticity and diffusive permeability by changing the metal ion or the molar mass of the thermo-responsive polymer. In this work, it is particularly shown, by oscillatory shear rheology and fluorescence recovery after photobleaching, how the elasticity and permeability of the DDN can be switched on demand upon change of the temperature. Furthermore, this system is reversibly degradable due to the supramolecular nature of the bonds, allowing the recyclability of the DDN.
Chapter 4, constitutes a methodological follow-up study of this tunable dual dynamic network. Starting from the different building blocks that constitute the system, it is shown how the elastic properties of the hydrogel can be optimized by adjusting the molar mass of the pEG and pNIPAAm blocks and with the choice of a different metal ion. This systematic investigation constitutes an excellent toolkit for dual dynamic networks and allows to tailor the properties of these networks on demand thereby, enlarging the applicability spectrum of these multi-responsive hydrogels.
In the engineering of new materials, the role of defects cannot be underestimated as in supramolecular systems especially connectivity defects can always occur. It is thus crucial to understand their impact on the properties of hydrogels starting from model systems with the perspective of transferring the knowledge on more complex systems such as DDN. This issue is tackled in Chapter 5, where connectivity defects are systematically introduced into a 4-arm pEG-terpyridine model network by doping such network with different amounts of 8-arm pEG-terpyridine precursors. The origin and effect of this connectivity mismatch on the hydrogel’s properties, such as elasticity and permeability, are then investigated by DQ-NMR, rheological and fluorescence recovery after photobleaching experiments.