Structure-Property Relationships of
Hybrid Double-Network Hydrogels

Abstract

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.