Abstract Introducing additional physical and reversible crosslinks to a chemically
crosslinked hydrogel is an interesting and viable alternative to increase the toughness
of a hydrogel. Yet while in general the physical crosslink points provide dissipative
mechanisms, there are still many details that are unknown in particular on the role
that physical crosslinks play on the large strain behavior. We explore the mechanical
properties in small and large strain of two dual crosslink gels made from a random
copolymer of poly(acrylamide-co-vinylimidazole) with a range of elastic moduli in
the tens of kPa. The interaction between vinylimidazole groups and metal ions (Zn
2+
and Ni
2+ ) results in physical crosslink points and in a markedly stretch-rate-dependent mechanical behavior. While a main relaxation process is clearly visible in linear
rheology and controls the small and intermediate strain properties, we find that the
strain hardening behavior at stretches of λ > 4 and the stretch at break λ b are
controlled by an additional longer-lived physical crosslinking mechanism that
could be due to a clustering of physical crosslinks.
Keywords Mechanical properties · Metal-ligand coordination bonds · Tough
hydrogel · Transient crosslink
1 Introduction
Hydrogels are promising candidates for biomedical applications such as artificial
organs or tissue engineering thanks to their liquid-like and solid-like properties
[1]. However, contrary to biological hydrogels such as cartilage, conventional
synthetic hydrogels made by free radical polymerization suffer from mechanical
fragility, due to the heterogeneous network structures and the lack of dissipative
mechanisms [2]. Mechanical reinforcement has become one of the hottest topics of
gel science in the last decades [3–9]. Among the different reinforcement strategies,
the introduction of sacrificial bonds inside the gel in order to dissipate energy near
the crack tip has proved promising. The pioneering work of Gong [9] in 2003
provided a good solution by creating two interpenetrated networks having different
properties: a low volume fraction of highly crosslinked and stretched network and a
high volume fraction of loosely crosslinked second network. This hydrogel has a
much better fracture toughness than either network on its own. However, this
network design strategy based on irreversible bond breaking results in permanent
damage in the network [10]; thus the same mechanical behaviors of the virgin gels
cannot be recovered after loading cycles.
A successful alternative can be the incorporation of reversible crosslinks into the
network. Because the reversible crosslinks serve as sacrificial bonds they can break
and reform under strain and hence make the propagation of a crack more energetically costly [11]. Although the detailed mechanism by which these dynamic bonds
delay crack propagation may be complex, it has been proposed that the presence of
2
J. Zhao et al.
crosslinked hydrogel is an interesting and viable alternative to increase the toughness
of a hydrogel. Yet while in general the physical crosslink points provide dissipative
mechanisms, there are still many details that are unknown in particular on the role
that physical crosslinks play on the large strain behavior. We explore the mechanical
properties in small and large strain of two dual crosslink gels made from a random
copolymer of poly(acrylamide-co-vinylimidazole) with a range of elastic moduli in
the tens of kPa. The interaction between vinylimidazole groups and metal ions (Zn
2+
and Ni
2+ ) results in physical crosslink points and in a markedly stretch-rate-dependent mechanical behavior. While a main relaxation process is clearly visible in linear
rheology and controls the small and intermediate strain properties, we find that the
strain hardening behavior at stretches of λ > 4 and the stretch at break λ b are
controlled by an additional longer-lived physical crosslinking mechanism that
could be due to a clustering of physical crosslinks.
Keywords Mechanical properties · Metal-ligand coordination bonds · Tough
hydrogel · Transient crosslink
1 Introduction
Hydrogels are promising candidates for biomedical applications such as artificial
organs or tissue engineering thanks to their liquid-like and solid-like properties
[1]. However, contrary to biological hydrogels such as cartilage, conventional
synthetic hydrogels made by free radical polymerization suffer from mechanical
fragility, due to the heterogeneous network structures and the lack of dissipative
mechanisms [2]. Mechanical reinforcement has become one of the hottest topics of
gel science in the last decades [3–9]. Among the different reinforcement strategies,
the introduction of sacrificial bonds inside the gel in order to dissipate energy near
the crack tip has proved promising. The pioneering work of Gong [9] in 2003
provided a good solution by creating two interpenetrated networks having different
properties: a low volume fraction of highly crosslinked and stretched network and a
high volume fraction of loosely crosslinked second network. This hydrogel has a
much better fracture toughness than either network on its own. However, this
network design strategy based on irreversible bond breaking results in permanent
damage in the network [10]; thus the same mechanical behaviors of the virgin gels
cannot be recovered after loading cycles.
A successful alternative can be the incorporation of reversible crosslinks into the
network. Because the reversible crosslinks serve as sacrificial bonds they can break
and reform under strain and hence make the propagation of a crack more energetically costly [11]. Although the detailed mechanism by which these dynamic bonds
delay crack propagation may be complex, it has been proposed that the presence of
2
J. Zhao et al.