melting temperature T m of crystal cross-links induces almost three orders of
magnitude decrease in the modulus in that region so that, after cooling below T m ,
the released crystallizable groups reform in that area to recover the original mechanical properties [27]. Thus, independent on the mechanical properties of hydrogels,
self-healing requires a significant reversible softening in the damaged area induced
by an external stimulus providing an enhanced mobility to the network chains.
A prerequisite for the preparation of tough and self-healing hydrogels is to reduce
their water content to a moderate level, generally between 50 and 70 wt%. This is
mainly due to the decrease in the polymer concentration of the hydrogels as the water
content is increased leading to a reduced viscoelastic energy dissipation between the
polymer chains inducing a tough-to-brittle transition. In addition, at high water
contents, the polymer chains are already stretched due to the swelling pressure of
water so that their further stretchability under an external force and hence tensile
strength reduce significantly. For example, superabsorbent hydrogen (H)-bonded
hydrogels in their as-prepared state with 65% water exhibit 1,000% stretchability
and complete self-healing efficiency, whereas, in equilibrium swollen state with
99.9% water, they become brittle in tension and lack of self-healing [28]. The water
content also affects the strength of the non-covalent bonds between the polymer
chains in the hydrogel network. The strength of H-bonds between proteins, nucleic
acids, or hydrophilic polymers in an aqueous environment is known to be much
weaker than the H-bonds between water molecules. This weakening effect arises due
to the fact that the formation of a H-bond between two polymers requires disruption
of their H-bonds with water. To hinder the weakening of intermolecular interactions
between polymers, mechanically robust and self-healable hydrogels reported so far
generally have a water content between 50 and 70 wt%, which is, in fact, similar to
that in living cartilage, skin, tendons, and ligaments [2].
2 H-Bonding Interactions
Because H-bonds between polymer chains are easily disrupted by water molecules
and hence not stable in an aqueous environment, several strategies have been
developed to create mechanically strong H-bonded hydrogels. These strategies
mainly base on creation of self-complementary dual or multiple H-bonding interactions between polymer chains as well as incorporation of hydrophobic segments into
the hydrophilic polymers to amplify the H-bonding interactions. The use of H-bond
acceptor and donor comonomers in the hydrogel preparation, dual amide groups,
ureidopyrimidinone units, and diaminotriazine-diaminotriazine interactions has been
reported to create high-strength H-bonded hydrogels.
Inspired by nature such as the double and triple H-bonds in double-stranded
deoxyribonucleic acid (DNA), stronger H-bonding interactions in hydrogels can be
generated by using polymer chains having arrays of H-bonding sites. Because dual
amide H-bonds are quite stable as compared to a simple amide H-bond, Liu and
co-workers prepared hydrogels based on N-acryloyl glycinamide (NAGA), a
26
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magnitude decrease in the modulus in that region so that, after cooling below T m ,
the released crystallizable groups reform in that area to recover the original mechanical properties [27]. Thus, independent on the mechanical properties of hydrogels,
self-healing requires a significant reversible softening in the damaged area induced
by an external stimulus providing an enhanced mobility to the network chains.
A prerequisite for the preparation of tough and self-healing hydrogels is to reduce
their water content to a moderate level, generally between 50 and 70 wt%. This is
mainly due to the decrease in the polymer concentration of the hydrogels as the water
content is increased leading to a reduced viscoelastic energy dissipation between the
polymer chains inducing a tough-to-brittle transition. In addition, at high water
contents, the polymer chains are already stretched due to the swelling pressure of
water so that their further stretchability under an external force and hence tensile
strength reduce significantly. For example, superabsorbent hydrogen (H)-bonded
hydrogels in their as-prepared state with 65% water exhibit 1,000% stretchability
and complete self-healing efficiency, whereas, in equilibrium swollen state with
99.9% water, they become brittle in tension and lack of self-healing [28]. The water
content also affects the strength of the non-covalent bonds between the polymer
chains in the hydrogel network. The strength of H-bonds between proteins, nucleic
acids, or hydrophilic polymers in an aqueous environment is known to be much
weaker than the H-bonds between water molecules. This weakening effect arises due
to the fact that the formation of a H-bond between two polymers requires disruption
of their H-bonds with water. To hinder the weakening of intermolecular interactions
between polymers, mechanically robust and self-healable hydrogels reported so far
generally have a water content between 50 and 70 wt%, which is, in fact, similar to
that in living cartilage, skin, tendons, and ligaments [2].
2 H-Bonding Interactions
Because H-bonds between polymer chains are easily disrupted by water molecules
and hence not stable in an aqueous environment, several strategies have been
developed to create mechanically strong H-bonded hydrogels. These strategies
mainly base on creation of self-complementary dual or multiple H-bonding interactions between polymer chains as well as incorporation of hydrophobic segments into
the hydrophilic polymers to amplify the H-bonding interactions. The use of H-bond
acceptor and donor comonomers in the hydrogel preparation, dual amide groups,
ureidopyrimidinone units, and diaminotriazine-diaminotriazine interactions has been
reported to create high-strength H-bonded hydrogels.
Inspired by nature such as the double and triple H-bonds in double-stranded
deoxyribonucleic acid (DNA), stronger H-bonding interactions in hydrogels can be
generated by using polymer chains having arrays of H-bonding sites. Because dual
amide H-bonds are quite stable as compared to a simple amide H-bond, Liu and
co-workers prepared hydrogels based on N-acryloyl glycinamide (NAGA), a
26
O. Okay