also fabricated by the micellar copolymerization of acrylamide and an amphiphilic
cross-linker consisting of an acrylic head, alkyl spacer, and UPy group providing
both hydrophobic associations and H-bonds [37]. To increase the solubility of the
cross-linker in the micellar solution, salt was included into the reaction solution to
induce the micellar growth [38]. However, weak hydrogels with a modulus of
around 2 kPa could be obtained likely because of the weakening effect of surfactant
molecules on the hydrophobic interactions [38, 39]. Physical hydrogels via both
H-bonding and hydrophobic interactions were recently fabricated by free-radical
copolymerization of UPy methacrylate, n-octadecyl acrylate (C18A), and acrylic
acid [40]. The hydrophobic interactions between crystallizable alkyl side chains of
C18A and the quadruple hydrogen bonds between UPy segments act as dual crosslinks of the hydrogels. The hydrogels with a water content between 40 and 80 wt%
exhibit a high tensile strength (up to 4.6 MPa) and elongation at break (680%). It was
shown that the UPy units promote formation of alkyl crystals, while alkyl side chains
stabilize UPy-UPy dimers [40].
Incorporation of hydrophobic groups into the hydrogel network has a significant
effect on the strength of H-bonds and hence the mechanical performance of
hydrogels consisting of H-bond acceptor and donor comonomer units. For instance,
introduction of methyl motif to acrylic acid (AAc), that is, the use of methacrylic
acid (MAAc) instead of AAc, significantly improves the mechanical performance of
H-bonded hydrogels [41]. The hydrogel based on MAAc and 1-vinylimidazole
containing 50–60 wt% water exhibits a Young’s modulus up to 170 MPa, whereas
it two orders of magnitude decreases when MAAc is replaced with AAc in the gel
preparation [42, 43].
High-strength self-recovery hydrogels with a high Young’s modulus (28 MPa),
tensile strength (2 MPa), stretch at break (800%), and a good fatigue resistance were
prepared by free-radical copolymerization of N,N-dimethylacrylamide (DMAA) and
MAAc in aqueous solutions without a chemical cross-linker [41]. It was shown that
the strong H-bond acceptor carbonyl group of DMAA and H-bond donor carboxylic
group of MAAc form multiple H-bonds, leading to the formation of polymer-rich
aggregates stabilized by the hydrophobic interactions of the α-methyl groups of
MAAc units (Fig. 4). These aggregates serving as sacrificial cross-links ensure a
high energy dissipation within the gel network. If MAAc segments in the hydrogel
are replaced with AAc ones at the same concentration and water content, soft
hydrogels with a fracture strength of less than 100 kPa could be obtained [41]. However, insolubility of DMAA/MAAc hydrogels in aqueous urea solutions reveals the
existence of chemical cross-links in these self-recovery hydrogels likely due to the
chain transfer reactions during copolymerization.
High-strength self-recovery H-bonded hydrogels were also prepared by simply
heating aqueous solutions of acrylamide (AAm) and poly(N-vinylpyrrolidone)
(PVP) at 56
C for 36 h without any chemical initiator or cross-linker (Fig. 5)
[44]. The hydrogels with 60% water exhibit a high Young’s modulus (84 MPa),
tensile strength (1.2 MPa), and elongation at break (~3,000%). It was shown that
AAm polymerization occurs by self-initiation at elevated temperature, whereas the
presence of PVP provides formation of high-strength hydrogels. Insolubility of
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
29
cross-linker consisting of an acrylic head, alkyl spacer, and UPy group providing
both hydrophobic associations and H-bonds [37]. To increase the solubility of the
cross-linker in the micellar solution, salt was included into the reaction solution to
induce the micellar growth [38]. However, weak hydrogels with a modulus of
around 2 kPa could be obtained likely because of the weakening effect of surfactant
molecules on the hydrophobic interactions [38, 39]. Physical hydrogels via both
H-bonding and hydrophobic interactions were recently fabricated by free-radical
copolymerization of UPy methacrylate, n-octadecyl acrylate (C18A), and acrylic
acid [40]. The hydrophobic interactions between crystallizable alkyl side chains of
C18A and the quadruple hydrogen bonds between UPy segments act as dual crosslinks of the hydrogels. The hydrogels with a water content between 40 and 80 wt%
exhibit a high tensile strength (up to 4.6 MPa) and elongation at break (680%). It was
shown that the UPy units promote formation of alkyl crystals, while alkyl side chains
stabilize UPy-UPy dimers [40].
Incorporation of hydrophobic groups into the hydrogel network has a significant
effect on the strength of H-bonds and hence the mechanical performance of
hydrogels consisting of H-bond acceptor and donor comonomer units. For instance,
introduction of methyl motif to acrylic acid (AAc), that is, the use of methacrylic
acid (MAAc) instead of AAc, significantly improves the mechanical performance of
H-bonded hydrogels [41]. The hydrogel based on MAAc and 1-vinylimidazole
containing 50–60 wt% water exhibits a Young’s modulus up to 170 MPa, whereas
it two orders of magnitude decreases when MAAc is replaced with AAc in the gel
preparation [42, 43].
High-strength self-recovery hydrogels with a high Young’s modulus (28 MPa),
tensile strength (2 MPa), stretch at break (800%), and a good fatigue resistance were
prepared by free-radical copolymerization of N,N-dimethylacrylamide (DMAA) and
MAAc in aqueous solutions without a chemical cross-linker [41]. It was shown that
the strong H-bond acceptor carbonyl group of DMAA and H-bond donor carboxylic
group of MAAc form multiple H-bonds, leading to the formation of polymer-rich
aggregates stabilized by the hydrophobic interactions of the α-methyl groups of
MAAc units (Fig. 4). These aggregates serving as sacrificial cross-links ensure a
high energy dissipation within the gel network. If MAAc segments in the hydrogel
are replaced with AAc ones at the same concentration and water content, soft
hydrogels with a fracture strength of less than 100 kPa could be obtained [41]. However, insolubility of DMAA/MAAc hydrogels in aqueous urea solutions reveals the
existence of chemical cross-links in these self-recovery hydrogels likely due to the
chain transfer reactions during copolymerization.
High-strength self-recovery H-bonded hydrogels were also prepared by simply
heating aqueous solutions of acrylamide (AAm) and poly(N-vinylpyrrolidone)
(PVP) at 56
C for 36 h without any chemical initiator or cross-linker (Fig. 5)
[44]. The hydrogels with 60% water exhibit a high Young’s modulus (84 MPa),
tensile strength (1.2 MPa), and elongation at break (~3,000%). It was shown that
AAm polymerization occurs by self-initiation at elevated temperature, whereas the
presence of PVP provides formation of high-strength hydrogels. Insolubility of
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
29