modulus (0.01–8 MPa), a high failure strain (150–1,500%), and a failure stress
(0.1–2 MPa), together with 100% self-recovery and a high self-healing efficiency
behavior. These excellent mechanical performances are comparable to that of rubbers, most tough double-network hydrogels, and soft bio-tissues. The extensive
experimental studies show that these multiple mechanical properties are related to
the formation of dynamical features of inter- and intra-chain ionic bonds, and this
study opens a common strategy to develop tough and self-healing hydrogels.
Keywords Ionic bond · Polyampholyte hydrogel · Self-healing · Toughness ·
Viscoelastic
1 Introduction
Hydrogels are a class of cross-linked polymer network swollen with a large amount
of water. Their high permeabilities to small molecules make hydrogels undergo the
reversible volume change by imbibing or exuding water in response to change in
temperature, light, pH, ionic strength, etc. [1–7]. The reversible volume changes
have been used to develop sensors and actuators. Furthermore, hydrogels bear
some similarities to biological tissues as a result of their soft and wet nature and
have been investigated for use in medical applications, such as extracellular matrix,
drug delivery, and tissue regeneration [8–10]. However, conventional hydrogels
are often brittle and weak, and thus little attention was paid to them as structural
materials, which substantially limit the scope of their applications [11–13]. For
example, recent advances in soft device such as loudspeakers, touch pads, electroluminescent displays, electronic skins, etc., require the hydrogels with highly
stretchable, mechanical tough, and fatigue resistance behavior [14–20].
In the past two decades, many efforts have been made to synthesize strong and
tough hydrogels, such as slide-ring gels, tetra-PEG gels, nanocomposite hydrogels,
double-network hydrogel (DN) hydrogels, etc., which has largely changed these
traditional view that hydrogels are very weak and thus broadened their applications
[21–26]. Among them, the most successful work is the DN hydrogels that show
extraordinarily high strength and toughness. DN gels consist of interpenetrating
brittle and ductile networks. When hydrogels are loaded in tension, the brittle
network breaks progressively into fragments, which dissipates substantial amounts
of energy and prevents catastrophic crack propagation, while the elasticity of ductile
network forces them to return to its original configuration [22, 27]. Thus, the
chemical cross-linked brittle network by covalent bonds serve as “sacrificial bond”
that effectively toughen the sample. They show a high toughness (fracture energy
of 1,000–1,000 J/m
2 ), a high stiffness (0.1–1 MPa), a high failure tensile strain
(1,000–2,000%), and a failure tensile stress (1–10 MPa) although they have
80–90 wt% of water at an equilibrium state. The excellent mechanical behaviors
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T. L. Sun and K. Cui
(0.1–2 MPa), together with 100% self-recovery and a high self-healing efficiency
behavior. These excellent mechanical performances are comparable to that of rubbers, most tough double-network hydrogels, and soft bio-tissues. The extensive
experimental studies show that these multiple mechanical properties are related to
the formation of dynamical features of inter- and intra-chain ionic bonds, and this
study opens a common strategy to develop tough and self-healing hydrogels.
Keywords Ionic bond · Polyampholyte hydrogel · Self-healing · Toughness ·
Viscoelastic
1 Introduction
Hydrogels are a class of cross-linked polymer network swollen with a large amount
of water. Their high permeabilities to small molecules make hydrogels undergo the
reversible volume change by imbibing or exuding water in response to change in
temperature, light, pH, ionic strength, etc. [1–7]. The reversible volume changes
have been used to develop sensors and actuators. Furthermore, hydrogels bear
some similarities to biological tissues as a result of their soft and wet nature and
have been investigated for use in medical applications, such as extracellular matrix,
drug delivery, and tissue regeneration [8–10]. However, conventional hydrogels
are often brittle and weak, and thus little attention was paid to them as structural
materials, which substantially limit the scope of their applications [11–13]. For
example, recent advances in soft device such as loudspeakers, touch pads, electroluminescent displays, electronic skins, etc., require the hydrogels with highly
stretchable, mechanical tough, and fatigue resistance behavior [14–20].
In the past two decades, many efforts have been made to synthesize strong and
tough hydrogels, such as slide-ring gels, tetra-PEG gels, nanocomposite hydrogels,
double-network hydrogel (DN) hydrogels, etc., which has largely changed these
traditional view that hydrogels are very weak and thus broadened their applications
[21–26]. Among them, the most successful work is the DN hydrogels that show
extraordinarily high strength and toughness. DN gels consist of interpenetrating
brittle and ductile networks. When hydrogels are loaded in tension, the brittle
network breaks progressively into fragments, which dissipates substantial amounts
of energy and prevents catastrophic crack propagation, while the elasticity of ductile
network forces them to return to its original configuration [22, 27]. Thus, the
chemical cross-linked brittle network by covalent bonds serve as “sacrificial bond”
that effectively toughen the sample. They show a high toughness (fracture energy
of 1,000–1,000 J/m
2 ), a high stiffness (0.1–1 MPa), a high failure tensile strain
(1,000–2,000%), and a failure tensile stress (1–10 MPa) although they have
80–90 wt% of water at an equilibrium state. The excellent mechanical behaviors
296
T. L. Sun and K. Cui