To generate self-healing in semicrystalline hydrogels, they were recently
prepared in the absence of a chemical cross-linker by micellar, bulk, and solution
polymerizations using several hydrophobic and hydrophilic comonomer pairs
[27, 128, 135, 136]. Because of the cooperative H-bonding and hydrophobic interactions, AAc/C18A comonomer pair leads to the formation of hydrogels with the
highest melting temperature (48–56
C), degree of crystallinity (10–33%), Young’s
modulus (up to 308 Æ 16 MPa), and tensile strength (up to 5.1 Æ 0.1 MPa)
[135]. Compressive mechanical tests conducted on virgin and cut-repaired hydrogels
reveal extraordinary self-healing capability of the hydrogels. The healing of the
hydrogels was induced by heating locally the cut region above T m followed by
pressing the cut surfaces together and finally cooling below T m to reform alkyl
crystals bridging the cut surfaces (Fig. 16b). Healed DMAA/C18A hydrogels with
50 mol% C18A exhibit a compressive strength of 138 Æ 10 MPa, which is around
87% of the virgin ones [135]. Moreover, the healing can also be induced by internal
heating using laser light if the hydrogel contains gold nanoparticles that generate
heat due to the surface plasmon resonance [137]. On/off switching of the laser light
provides melting and recrystallization of alkyl crystals in the damaged area resulting
in healing of the hydrogel. Instead of the temperature-induced healing, treatment of
the cut surfaces with ethanol was also reported which provides solubilization of the
cut surfaces to allow merging the surfaces together [136].
One disadvantage of highly crystalline hydrogels prepared from hydrophilic and
hydrophobic monomers is their low stretchability due to the existence of stiff and
strong alkyl crystals. One may apparently overcome this drawback by changing the
tensile testing parameters, i.e., by reducing the strain rate to provide enough time for
the relaxation processes in the physical network. For instance, the toughness and
stretch at break of DMAA/C18A hydrogels increase by five- and sevenfold, respectively, when the strain rate is reduced from 7.8 Â 10
À3 to 4 Â 10
À3 s
À1 (Fig. 18a)
[135]. However, a versatile alternative strategy to induce such a brittle-to-tough
transition without changing the testing parameters is to incorporate hydrophobic
monomers with relatively short alkyl side chains creating mobility in the gel network
[128]. Figure 18b shows the effect of the non-crystallizable, weak hydrophobe
C12M on the stress-strain curves of DMAA/C18A hydrogels at a fixed strain rate.
The hydrogels were prepared at a fixed content of the hydrophobes (30 mol%) but at
various fractions of C12M between 0.1 and 20 mol%. In the absence of C12M, the
hydrogel ruptures in a brittle fashion, as seen by the blue thick curve labeled by “0”
in the inset to the figure. However, incorporation of a small amount of C12M induces
significant yielding accompanied with a brittle-to-ductile transition. For instance,
without and with 0.2 mol% C12M, the hydrogels exhibit almost identical Young’s
moduli E, i.e., 71 Æ 3 and 70 Æ 5 MPa, respectively, indicating that the cross-link
density is not effected from the added amount of C12M. However, this tiny amount
of C12M affects considerably the ultimate mechanical properties of the hydrogels.
The hydrogel without C12M ruptures at 20% stretch, while that with 0.2 mol%
C12M sustains eightfold larger stretches (167%) and exhibits tenfold larger toughness (9.6 Æ 0.3 vs 1.0 Æ 0.2 MJ m
À3 ) [128]. Moreover, the highest yield stress σ y
of 7.3 MPa was observed after incorporation of the smallest amount of C12M
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
49
prepared in the absence of a chemical cross-linker by micellar, bulk, and solution
polymerizations using several hydrophobic and hydrophilic comonomer pairs
[27, 128, 135, 136]. Because of the cooperative H-bonding and hydrophobic interactions, AAc/C18A comonomer pair leads to the formation of hydrogels with the
highest melting temperature (48–56
C), degree of crystallinity (10–33%), Young’s
modulus (up to 308 Æ 16 MPa), and tensile strength (up to 5.1 Æ 0.1 MPa)
[135]. Compressive mechanical tests conducted on virgin and cut-repaired hydrogels
reveal extraordinary self-healing capability of the hydrogels. The healing of the
hydrogels was induced by heating locally the cut region above T m followed by
pressing the cut surfaces together and finally cooling below T m to reform alkyl
crystals bridging the cut surfaces (Fig. 16b). Healed DMAA/C18A hydrogels with
50 mol% C18A exhibit a compressive strength of 138 Æ 10 MPa, which is around
87% of the virgin ones [135]. Moreover, the healing can also be induced by internal
heating using laser light if the hydrogel contains gold nanoparticles that generate
heat due to the surface plasmon resonance [137]. On/off switching of the laser light
provides melting and recrystallization of alkyl crystals in the damaged area resulting
in healing of the hydrogel. Instead of the temperature-induced healing, treatment of
the cut surfaces with ethanol was also reported which provides solubilization of the
cut surfaces to allow merging the surfaces together [136].
One disadvantage of highly crystalline hydrogels prepared from hydrophilic and
hydrophobic monomers is their low stretchability due to the existence of stiff and
strong alkyl crystals. One may apparently overcome this drawback by changing the
tensile testing parameters, i.e., by reducing the strain rate to provide enough time for
the relaxation processes in the physical network. For instance, the toughness and
stretch at break of DMAA/C18A hydrogels increase by five- and sevenfold, respectively, when the strain rate is reduced from 7.8 Â 10
À3 to 4 Â 10
À3 s
À1 (Fig. 18a)
[135]. However, a versatile alternative strategy to induce such a brittle-to-tough
transition without changing the testing parameters is to incorporate hydrophobic
monomers with relatively short alkyl side chains creating mobility in the gel network
[128]. Figure 18b shows the effect of the non-crystallizable, weak hydrophobe
C12M on the stress-strain curves of DMAA/C18A hydrogels at a fixed strain rate.
The hydrogels were prepared at a fixed content of the hydrophobes (30 mol%) but at
various fractions of C12M between 0.1 and 20 mol%. In the absence of C12M, the
hydrogel ruptures in a brittle fashion, as seen by the blue thick curve labeled by “0”
in the inset to the figure. However, incorporation of a small amount of C12M induces
significant yielding accompanied with a brittle-to-ductile transition. For instance,
without and with 0.2 mol% C12M, the hydrogels exhibit almost identical Young’s
moduli E, i.e., 71 Æ 3 and 70 Æ 5 MPa, respectively, indicating that the cross-link
density is not effected from the added amount of C12M. However, this tiny amount
of C12M affects considerably the ultimate mechanical properties of the hydrogels.
The hydrogel without C12M ruptures at 20% stretch, while that with 0.2 mol%
C12M sustains eightfold larger stretches (167%) and exhibits tenfold larger toughness (9.6 Æ 0.3 vs 1.0 Æ 0.2 MJ m
À3 ) [128]. Moreover, the highest yield stress σ y
of 7.3 MPa was observed after incorporation of the smallest amount of C12M
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
49