deformation of sample and lead to a small fracture strain but enhance the fracture
stress (Fig. 6b). Furthermore, the creep experiment shows that an addition of
chemical cross-linker will prevent the flow of the polymer chains and thus delay
the rupture of the PA hydrogels [53].
The approach to synthesize PA hydrogels is quite general and has been found
effective to many kinds of ionic monomer combination. Experimental result shows
that tough PA gels are formed only from relatively bulky and hydrophobic ion
combinations, and the mechanical behavior of hydrogel strongly depends on the
chemical structure of monomers. For example, the hydrogel P(NaSS-co-MPTC)
formed from the more hydrophobic combinations of cationic monomer MPTC and
anionic monomer NaSS shows a much higher fracture stress (~1.8 MPa) but much
lower fracture strain (~800%) than those of the P(NaSS-co-DMAEA-Q) gels when
they have similar compositions, while no gel is formed from the more hydrophilic
combinations of anionic 2-acrylamido-2-methylpropanesulphonic acid (AMPS) and
cationic DMAEA-Q (Fig. 1b) [39].
In summary, systematically optimizing the condition for PA gels has shown that
the tough gel should be synthesized at a high monomer concentration around the
charge balance point without or with a small amount of chemical cross-linkers. By
adjusting the structure parameters, multiple mechanical properties over wide ranges
including modulus, stretchability, fracture stress, and work of extension can be tuned
to fit the choice of tough hydrogels for applications.
3 Mechanical Behavior
3.1 Hysteresis, Self-Recovery, and Self-Healing Behavior
The hydrogels show 100% self-recovery behavior and therefore have very high
fatigue resistance [39]. When the deformation is applied, the dynamic ionic bonds
break, and polymer chains are reattached to other chains to form temporary bonds.
The newly formed temporary chains carry the load and break if further deformation
is applied. Therefore, during the uniaxial loading and unloading process, the
P(NaSS-co-MTPC) gel 2.1–0.52–0% shows a significant hysteresis (Fig. 7) which
increases very strongly with the applied maximum deformation. During the recovery
process, the rubber elasticity is dominant toward the contraction of polymer chains,
which ruptures the re-formed temporary bonds and slows down the recovery
process. Thus, the recovery time of the bulk materials is the accumulation of all
these bonds’ breaking and re-forming time during this process, larger than the
bond reassociation time. Therefore, macroscopically, it takes a long waiting time
(~120 min) to fully recover the mechanical behavior of hydrogel after experiencing a
strain of 300% (Fig. 7). Such dynamical bond breaking and re-forming process can
be well-described by the proposed self-healing theories [54–56].
As the dynamic bonds can be broken and the ruptured ionic bonds are able to
re-form, the bulk self-healing behavior is observed (Fig. 8a) [57]. For example, when
Tough and Self-Healing Hydrogels from Polyampholytes
303
stress (Fig. 6b). Furthermore, the creep experiment shows that an addition of
chemical cross-linker will prevent the flow of the polymer chains and thus delay
the rupture of the PA hydrogels [53].
The approach to synthesize PA hydrogels is quite general and has been found
effective to many kinds of ionic monomer combination. Experimental result shows
that tough PA gels are formed only from relatively bulky and hydrophobic ion
combinations, and the mechanical behavior of hydrogel strongly depends on the
chemical structure of monomers. For example, the hydrogel P(NaSS-co-MPTC)
formed from the more hydrophobic combinations of cationic monomer MPTC and
anionic monomer NaSS shows a much higher fracture stress (~1.8 MPa) but much
lower fracture strain (~800%) than those of the P(NaSS-co-DMAEA-Q) gels when
they have similar compositions, while no gel is formed from the more hydrophilic
combinations of anionic 2-acrylamido-2-methylpropanesulphonic acid (AMPS) and
cationic DMAEA-Q (Fig. 1b) [39].
In summary, systematically optimizing the condition for PA gels has shown that
the tough gel should be synthesized at a high monomer concentration around the
charge balance point without or with a small amount of chemical cross-linkers. By
adjusting the structure parameters, multiple mechanical properties over wide ranges
including modulus, stretchability, fracture stress, and work of extension can be tuned
to fit the choice of tough hydrogels for applications.
3 Mechanical Behavior
3.1 Hysteresis, Self-Recovery, and Self-Healing Behavior
The hydrogels show 100% self-recovery behavior and therefore have very high
fatigue resistance [39]. When the deformation is applied, the dynamic ionic bonds
break, and polymer chains are reattached to other chains to form temporary bonds.
The newly formed temporary chains carry the load and break if further deformation
is applied. Therefore, during the uniaxial loading and unloading process, the
P(NaSS-co-MTPC) gel 2.1–0.52–0% shows a significant hysteresis (Fig. 7) which
increases very strongly with the applied maximum deformation. During the recovery
process, the rubber elasticity is dominant toward the contraction of polymer chains,
which ruptures the re-formed temporary bonds and slows down the recovery
process. Thus, the recovery time of the bulk materials is the accumulation of all
these bonds’ breaking and re-forming time during this process, larger than the
bond reassociation time. Therefore, macroscopically, it takes a long waiting time
(~120 min) to fully recover the mechanical behavior of hydrogel after experiencing a
strain of 300% (Fig. 7). Such dynamical bond breaking and re-forming process can
be well-described by the proposed self-healing theories [54–56].
As the dynamic bonds can be broken and the ruptured ionic bonds are able to
re-form, the bulk self-healing behavior is observed (Fig. 8a) [57]. For example, when
Tough and Self-Healing Hydrogels from Polyampholytes
303