main factors determining the mechanical performance and self-healing efficiency of
the hydrogels. The hydrogels with a tensile strength of $0.5 MPa exhibit a selfhealing efficiency of around 90% with respect to the tensile strength, elongation at
break, and toughness. Elastin-like polypeptides consisting of a long hydrophobic
block with hydrophilic ends also form self-healing hydrogels via hydrophobic and
ionic interactions [79]. Heating polypeptide solution triggers the self-assembly of
polypeptides to form micelles which were then cross-linked using zinc ions via metal
coordination. At 10% polypeptide concentration, the hydrogels exhibit a storage
modulus of ~1 MPa and an effective self-healing behavior within minutes [79].
3 Hydrophobic Interactions
Segregation tendency of water-fearing (hydrophobic) molecules and water, which is
called hydrophobic interactions, is important in many self-assembly processes such
as formation of micelles, protein folding, and molecular recognition [80]. The
driving force for the hydrophobic interactions arises to reduce the hydrophobic
moieties of their exposure to water leading to hydrophobic associations and crystalline regions. Fu et al. used the amphiphilic triblock copolymer F127 for preparing
hydrophobically cross-linked micellar hydrogels [81], which are reviewed in this
volume [82]. Weiss et al. used fluoroacrylate monomers in the preparation of
hydrophobically modified fluorocarbon-based hydrogels [83]. The strong hydrophobic interactions between fluoroacrylate segments lead to core-shell nanodomains
within the hydrogels providing a MPa level modulus, as also detailed in this volume
[84]. In the following, we first discuss the studies conducted on self-healing hydrocarbon-based hydrogels formed by hydrophobic associations exhibiting a modulus
in the kPa range and a high stretchability. In the second section, the order-to-disorder
transition from association to alkyl crystals and formation of high-strength selfhealing semicrystalline hydrogels with a modulus and tensile strength in the range of
MPa are discussed.
3.1 Hydrophobically Modified Associative Hydrogels
Creton and co-workers were the first to report that hydrophobic modification of
chemically cross-linked polyelectrolyte hydrogels creates variable dissipative properties at almost identical cross-link densities [85]. They showed that the incorporation of hydrophobic side groups into polyelectrolyte hydrogels significantly
increases their loss moduli and hence generates energy dissipation because of the
formation of hydrophobic associations serving as reversible cross-links. However, a
complicated three-step synthetic approach was used for the synthesis of such hybrid
cross-linked hydrogels due to the solubility mismatches between hydrophilic and
hydrophobic monomers [85]. The micellar polymerization technique is a simple and
36
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the hydrogels. The hydrogels with a tensile strength of $0.5 MPa exhibit a selfhealing efficiency of around 90% with respect to the tensile strength, elongation at
break, and toughness. Elastin-like polypeptides consisting of a long hydrophobic
block with hydrophilic ends also form self-healing hydrogels via hydrophobic and
ionic interactions [79]. Heating polypeptide solution triggers the self-assembly of
polypeptides to form micelles which were then cross-linked using zinc ions via metal
coordination. At 10% polypeptide concentration, the hydrogels exhibit a storage
modulus of ~1 MPa and an effective self-healing behavior within minutes [79].
3 Hydrophobic Interactions
Segregation tendency of water-fearing (hydrophobic) molecules and water, which is
called hydrophobic interactions, is important in many self-assembly processes such
as formation of micelles, protein folding, and molecular recognition [80]. The
driving force for the hydrophobic interactions arises to reduce the hydrophobic
moieties of their exposure to water leading to hydrophobic associations and crystalline regions. Fu et al. used the amphiphilic triblock copolymer F127 for preparing
hydrophobically cross-linked micellar hydrogels [81], which are reviewed in this
volume [82]. Weiss et al. used fluoroacrylate monomers in the preparation of
hydrophobically modified fluorocarbon-based hydrogels [83]. The strong hydrophobic interactions between fluoroacrylate segments lead to core-shell nanodomains
within the hydrogels providing a MPa level modulus, as also detailed in this volume
[84]. In the following, we first discuss the studies conducted on self-healing hydrocarbon-based hydrogels formed by hydrophobic associations exhibiting a modulus
in the kPa range and a high stretchability. In the second section, the order-to-disorder
transition from association to alkyl crystals and formation of high-strength selfhealing semicrystalline hydrogels with a modulus and tensile strength in the range of
MPa are discussed.
3.1 Hydrophobically Modified Associative Hydrogels
Creton and co-workers were the first to report that hydrophobic modification of
chemically cross-linked polyelectrolyte hydrogels creates variable dissipative properties at almost identical cross-link densities [85]. They showed that the incorporation of hydrophobic side groups into polyelectrolyte hydrogels significantly
increases their loss moduli and hence generates energy dissipation because of the
formation of hydrophobic associations serving as reversible cross-links. However, a
complicated three-step synthetic approach was used for the synthesis of such hybrid
cross-linked hydrogels due to the solubility mismatches between hydrophilic and
hydrophobic monomers [85]. The micellar polymerization technique is a simple and
36
O. Okay