Structure-Property Relationships of Hybrid Double-Network Hydrogels

Conventional hydrogels, composed of a single polymer network in water, have been studied for over 60 years, but their mechanical weakness limits load-bearing applications. To address this, various design strategies have been proposed, most notably hybrid doublenetwork (DN) hydrogels, which combine interpenetrating transient and permanent networks to achieve enhanced toughness and stretchability. However, to date, how the structure of these hydrogels affects their properties remains speculative. In this thesis, this question is addressed by employing two orthogonal approaches: Large Amplitude Oscillatory Shear (LAOS) rheology and Design of Experiments (DoE) statistical framework to investigate a model alginate/polyacrylamide (PAAm) hydrogel. To investigate the role of the polymer networks, the precise nature of this yielding transition is examined using LAOS rheology. A novel two-step yielding process in hybrid DN hydrogels is observed where the first yielding step is controlled by hydrogen bonds between networks, while the second is governed by ionic interactions in alginate network. Using a DoE framework, statistical relationships between formulation factors and mechanical properties are established. Results show that crosslinking density primarily governs hydrogel stiffness, while features of the PAAm network dictate toughness. Notably, it is observed that toughness can be enhanced without increasing stiffness contrary to the current literature. Finally, alginate hydrogels are independently investigated as the alginate chains adopt different structural configurations depending on a stoichiometric ratio, R, with respect to the crosslinker concentration. The elastic modulus of the hydrogels increases with R, plateaus, and then decreases. Using insights from LAOS rheology, softening at high R is attributed to a soft microstructure formed by lateral bundling of crosslinked alginate chains. These findings highlight R as a key parameter for tailoring the mechanical properties of hybrid DN hydrogels. Altogether, this thesis advances the understanding of structure–property relationships in hybrid DN hydrogels, supporting their potential for practical applications.