shows the stress-strain curves of the samples mended at different healing time under
room temperature. For a relatively short healing time, the healing sample can stretch
to a strain of ~500% without breakage. Longer healing time leads to better healing.
When a healing time of 24 h was performed, the failure strain of healing sample
reached ~3,000% which is very close to the failure strain of the virgin sample. The
self-healing efficiency, the ratio of work of extension for mended sample to the
virgin sample, increases with the healing time and saturates to a value of
82.3% Æ 12.7% after 24-h healing time.
The self-healing behavior depends not only on healing time but also on healing
temperature and chemical structures. Mending temperature has two positive effects
on the self-healing behavior. One is that high mending temperature will reduce the
relative ionic bond strength and increase the bond exchange rate, leading to good
self-healing behavior. The second effect is that the mending temperature will change
the modulus of the sample. The gel becomes soft at high mending temperature,
favoring the formation of a full contact between the cut surfaces and therefore good
self-healing behavior. Furthermore, the PA gel formed from more hydrophobic ion
combinations, such as P(NaSS-co-MPTC) 2.0–0.52–0, shows lower self-healing
efficiency of 64.1 Æ 23.1% even if the healing experiment was performed at high
temperature, ~55
C. Extensive experiments and model calculation studies show that
the self-healing is due to re-forming of dynamic weak ionic bonds [57, 58]. For soft
hydrogels that can form a good contact at the interface, the self-healing efficiency is
related to the fraction of the weak ionic bonds in the total ionic bonds (Fig. 8c).
3.2 Time-Dependent of Tensile and Fracture Behavior
In kinetically breaking and re-forming of ionic bonds, PA gels exhibit timedependent mechanical behavior. The linear rheological spectrum for small strain
behavior is constructed over a wide range of frequency ranging from 10
À5 to 10
5
rad/s, following the principle of time-temperature superposition (Fig. 9). The
dynamic modulus of PA gel (NaSS-co-MTPC) gel 2.1–0.52–0% increases gradually
with the frequency, and a very broad loss factor tanδ peak was observed, indicating
that the hydrogel has a wide strength distribution of ion complex structure in the
dynamic rheological spectrum, weak ionic bonds (dynamic bonds), and strong ionic
bonds. The gel possesses weak ionic bonds with a relaxation time ~3 s and strong
ionic bonds with a relaxation time longer than ~10
5 s. The strong ionic bonds form
the primary network, while the weak ionic bonds form the sacrificial network that
can rupture during deformation. This relaxation time is considered as the average
bond exchange time of ionic bonds. When the applied frequency is close to the
inverse of bond exchange time, most ionic bonds are relaxed, and the gel shows the
viscoelastic features [59].
The time-dependent mechanical behavior is observed not only at small deformation by rheological test but also at large deformation by uniaxial tensile test. The
mechanical tensile behaviors are strongly influenced by the stretch rates, and
Tough and Self-Healing Hydrogels from Polyampholytes
305
room temperature. For a relatively short healing time, the healing sample can stretch
to a strain of ~500% without breakage. Longer healing time leads to better healing.
When a healing time of 24 h was performed, the failure strain of healing sample
reached ~3,000% which is very close to the failure strain of the virgin sample. The
self-healing efficiency, the ratio of work of extension for mended sample to the
virgin sample, increases with the healing time and saturates to a value of
82.3% Æ 12.7% after 24-h healing time.
The self-healing behavior depends not only on healing time but also on healing
temperature and chemical structures. Mending temperature has two positive effects
on the self-healing behavior. One is that high mending temperature will reduce the
relative ionic bond strength and increase the bond exchange rate, leading to good
self-healing behavior. The second effect is that the mending temperature will change
the modulus of the sample. The gel becomes soft at high mending temperature,
favoring the formation of a full contact between the cut surfaces and therefore good
self-healing behavior. Furthermore, the PA gel formed from more hydrophobic ion
combinations, such as P(NaSS-co-MPTC) 2.0–0.52–0, shows lower self-healing
efficiency of 64.1 Æ 23.1% even if the healing experiment was performed at high
temperature, ~55
C. Extensive experiments and model calculation studies show that
the self-healing is due to re-forming of dynamic weak ionic bonds [57, 58]. For soft
hydrogels that can form a good contact at the interface, the self-healing efficiency is
related to the fraction of the weak ionic bonds in the total ionic bonds (Fig. 8c).
3.2 Time-Dependent of Tensile and Fracture Behavior
In kinetically breaking and re-forming of ionic bonds, PA gels exhibit timedependent mechanical behavior. The linear rheological spectrum for small strain
behavior is constructed over a wide range of frequency ranging from 10
À5 to 10
5
rad/s, following the principle of time-temperature superposition (Fig. 9). The
dynamic modulus of PA gel (NaSS-co-MTPC) gel 2.1–0.52–0% increases gradually
with the frequency, and a very broad loss factor tanδ peak was observed, indicating
that the hydrogel has a wide strength distribution of ion complex structure in the
dynamic rheological spectrum, weak ionic bonds (dynamic bonds), and strong ionic
bonds. The gel possesses weak ionic bonds with a relaxation time ~3 s and strong
ionic bonds with a relaxation time longer than ~10
5 s. The strong ionic bonds form
the primary network, while the weak ionic bonds form the sacrificial network that
can rupture during deformation. This relaxation time is considered as the average
bond exchange time of ionic bonds. When the applied frequency is close to the
inverse of bond exchange time, most ionic bonds are relaxed, and the gel shows the
viscoelastic features [59].
The time-dependent mechanical behavior is observed not only at small deformation by rheological test but also at large deformation by uniaxial tensile test. The
mechanical tensile behaviors are strongly influenced by the stretch rates, and
Tough and Self-Healing Hydrogels from Polyampholytes
305