velocity.Raising temperature expedites bond breaking and therefore reduces the
tearing energy of hydrogel. As the fracture energy is greatly enhanced by the viscous
dissipation around the crack tip, it is several orders of magnitude larger than the
intrinsic fracture energy of the polymer network ~50 J/m
2 [62], so the fracture
behavior of PA gels is dominated by the bulk viscoelastic energy. The previous
study shows that the reduced stress measured from tensile test at different stretch
strain rate depends only on time, insensitive to the applied strain rates and stretch
ratio. Such decoupling suggests that the breakage of weak bonds is dominated by the
thermal activation process whereupon the forced debonding effect does not play a
substantial role. This means that the fracture behavior of PA gels is governed by the
thermally activated bond dissociation processes. Thus, the fracture properties at
large deformation are performed to be correlated to the linear viscoelastic behavior
at small deformation by rheological test. Using the principle of time-temperature
superposition, all the tearing energy data measured at different temperatures and
crack velocities can be reduced to a nice master curve at a reference temperature
when the vertical and horizontal shift factors determined from the linear dynamic
measurement are used (Figs. 9 and 11c). Furthermore, the observed relationship
between tearing energy and crack velocity follows a power law as T ~ V
α
c , and the
index of this power law (α ¼ 0.21) can be well-related to the exponent k of the
relaxation modulus G(t)~t
Àk which was performed by linear rheological test, obeying the linear viscoelastic theoretic predication α ¼ k/(1 + k). This means that we can
use the linear rheological spectrum as a fingerprint to design tough viscoelastic
hydrogels.
4 Toughening Mechanism
Understanding the toughening mechanism of PA hydrogels is indispensable for
designing the tough soft materials. By using the small-angle X-ray scattering
(SAXS) and atomic force microscope (AFM), we elucidated that the PA hydrogels
P(NaSS-co-MTPC) gel 2.1–0.52–0% have a bicontinuous network structure
(Fig. 12) consisting of a hard network and a soft network, with the hard region
having a Young’s modulus of ~1.93 MPa and the soft region with a Young’s
modulus of ~0.32 MPa. The soft and hard regions are similar in size, and the average
spacing between the adjacent regions is ~500 nm (Fig. 12) [63]. More studies show
that the formed bicontinuous network structure is observed in a lot of ionic combinations. This result revealed that the PA gels have a multiscale structure: at the
monomer scale, the cationic monomer and anionic monomers are associated to form
ionic bonds; at the nanoscale, the polymer chains are associated to each other to form
the soft and hard regions; at the microscale, a bicontinuous network structure is
formed; and at the macroscale, a smooth and homogeneous gel is formed.
In situ SAXS study revealed that the toughening of PA hydrogel is from the
synergistic effect of multiscale energy dissipation. A representative stress-stretch
curve of PA hydrogels (NaSS-co-DMAEA-Q) gel 2.4–0.52–0.1% and the
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T. L. Sun and K. Cui
tearing energy of hydrogel. As the fracture energy is greatly enhanced by the viscous
dissipation around the crack tip, it is several orders of magnitude larger than the
intrinsic fracture energy of the polymer network ~50 J/m
2 [62], so the fracture
behavior of PA gels is dominated by the bulk viscoelastic energy. The previous
study shows that the reduced stress measured from tensile test at different stretch
strain rate depends only on time, insensitive to the applied strain rates and stretch
ratio. Such decoupling suggests that the breakage of weak bonds is dominated by the
thermal activation process whereupon the forced debonding effect does not play a
substantial role. This means that the fracture behavior of PA gels is governed by the
thermally activated bond dissociation processes. Thus, the fracture properties at
large deformation are performed to be correlated to the linear viscoelastic behavior
at small deformation by rheological test. Using the principle of time-temperature
superposition, all the tearing energy data measured at different temperatures and
crack velocities can be reduced to a nice master curve at a reference temperature
when the vertical and horizontal shift factors determined from the linear dynamic
measurement are used (Figs. 9 and 11c). Furthermore, the observed relationship
between tearing energy and crack velocity follows a power law as T ~ V
α
c , and the
index of this power law (α ¼ 0.21) can be well-related to the exponent k of the
relaxation modulus G(t)~t
Àk which was performed by linear rheological test, obeying the linear viscoelastic theoretic predication α ¼ k/(1 + k). This means that we can
use the linear rheological spectrum as a fingerprint to design tough viscoelastic
hydrogels.
4 Toughening Mechanism
Understanding the toughening mechanism of PA hydrogels is indispensable for
designing the tough soft materials. By using the small-angle X-ray scattering
(SAXS) and atomic force microscope (AFM), we elucidated that the PA hydrogels
P(NaSS-co-MTPC) gel 2.1–0.52–0% have a bicontinuous network structure
(Fig. 12) consisting of a hard network and a soft network, with the hard region
having a Young’s modulus of ~1.93 MPa and the soft region with a Young’s
modulus of ~0.32 MPa. The soft and hard regions are similar in size, and the average
spacing between the adjacent regions is ~500 nm (Fig. 12) [63]. More studies show
that the formed bicontinuous network structure is observed in a lot of ionic combinations. This result revealed that the PA gels have a multiscale structure: at the
monomer scale, the cationic monomer and anionic monomers are associated to form
ionic bonds; at the nanoscale, the polymer chains are associated to each other to form
the soft and hard regions; at the microscale, a bicontinuous network structure is
formed; and at the macroscale, a smooth and homogeneous gel is formed.
In situ SAXS study revealed that the toughening of PA hydrogel is from the
synergistic effect of multiscale energy dissipation. A representative stress-stretch
curve of PA hydrogels (NaSS-co-DMAEA-Q) gel 2.4–0.52–0.1% and the
308
T. L. Sun and K. Cui