4 Summary
Supramolecular amphiphilic hydrogels (SAHs) based on water-swollen copolymers
of a water-soluble monomer with a fluoro(meth)acrylate exhibit high stretchability,
high stiffness, high strength, and extraordinary fracture toughness as a consequence
of their microphase-separated morphology. The microstructure is composed of core–
shell nanodomains of associated fluoroacrylate dispersed in a water-swollen polymer
phase. The nanodomains serve as multifunctional crosslinks for a physically
crosslinked network. The morphology of the network is relatively independent of
the choice of the water-soluble monomer, but the specific dimensions of the core–
shell nanostructure depend on the concentration of the fluoro(meth)acrylate used.
The high modulus and strength values achieved with these hydrogels are due to
their very high crosslink density, which is a consequence of the relatively high fluoro
(meth)acrylate concentration in the copolymer (5–25 mol%). Those concentrations
are much higher than the crosslink junction concentrations in conventional, covalently crosslinked hydrogels. Despite the high crosslink density, the SAH hydrogels
can have fracture toughness values ~10
4 J/m
2 , which is comparable to some synthetic elastomers. This remarkable toughness is due to the reversible nature of the
supramolecular hydrophobic bonds that form the crosslinks. That is, unlike conventional crosslinked hydrogels that have essentially no mechanism for dissipating
strain energy, the hydrophobic bonds can break when stressed but reform once the
stress has dissipated. However, the energy dissipation mechanism in these
microphase-separated hydrogels is fundamentally different than that reported for
non-microphase-separated, supramolecular hydrogels, in that the nanodomain structure is a multifunctional crosslink, and not all of the hydrophobic bonds within the
nanodomains break simultaneously. Therefore, the nanodomain crosslink structure
under stress persists, though the crosslink density is stress-, temperature- and timedependent. Note that the toughening mechanism for other supramolecular hydrogels
may not actually be different from that of the SAHs, but no other researcher of
hydrophobically modified hydrogels, as well as most other supramolecular
hydrogels, has reported microphase-separated morphologies for their hydrogels. In
light of the known microstructure of the SAHs, the details of the microstructure of
the various other supramolecular hydrogels that have been studied are an important
issue that needs to be resolved.
Like other supramolecular hydrogels, SAHs are viscoelastic due to the finite
relaxation times of supramolecular bond. The mechanical response is timedependent, and the hydrogel exhibits considerable hysteresis in loading and
unloading experiments and a Mullins effect. That behavior is a consequence of the
changes in the microphase-separated microstructure as hydrophobic bonds break and
may be pulled out of the fluoro(meth)acrylate nanodomains. The breaking of
physical bonds produces the high-energy losses that are responsible for the material’s extraordinary toughness. However, because of the reversible nature of the
hydrophobic bonds, they may reform once the stress is dissipated or removed. The
nanodomain network persists during the hydrogel deformation, and as a result, the
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
203
Supramolecular amphiphilic hydrogels (SAHs) based on water-swollen copolymers
of a water-soluble monomer with a fluoro(meth)acrylate exhibit high stretchability,
high stiffness, high strength, and extraordinary fracture toughness as a consequence
of their microphase-separated morphology. The microstructure is composed of core–
shell nanodomains of associated fluoroacrylate dispersed in a water-swollen polymer
phase. The nanodomains serve as multifunctional crosslinks for a physically
crosslinked network. The morphology of the network is relatively independent of
the choice of the water-soluble monomer, but the specific dimensions of the core–
shell nanostructure depend on the concentration of the fluoro(meth)acrylate used.
The high modulus and strength values achieved with these hydrogels are due to
their very high crosslink density, which is a consequence of the relatively high fluoro
(meth)acrylate concentration in the copolymer (5–25 mol%). Those concentrations
are much higher than the crosslink junction concentrations in conventional, covalently crosslinked hydrogels. Despite the high crosslink density, the SAH hydrogels
can have fracture toughness values ~10
4 J/m
2 , which is comparable to some synthetic elastomers. This remarkable toughness is due to the reversible nature of the
supramolecular hydrophobic bonds that form the crosslinks. That is, unlike conventional crosslinked hydrogels that have essentially no mechanism for dissipating
strain energy, the hydrophobic bonds can break when stressed but reform once the
stress has dissipated. However, the energy dissipation mechanism in these
microphase-separated hydrogels is fundamentally different than that reported for
non-microphase-separated, supramolecular hydrogels, in that the nanodomain structure is a multifunctional crosslink, and not all of the hydrophobic bonds within the
nanodomains break simultaneously. Therefore, the nanodomain crosslink structure
under stress persists, though the crosslink density is stress-, temperature- and timedependent. Note that the toughening mechanism for other supramolecular hydrogels
may not actually be different from that of the SAHs, but no other researcher of
hydrophobically modified hydrogels, as well as most other supramolecular
hydrogels, has reported microphase-separated morphologies for their hydrogels. In
light of the known microstructure of the SAHs, the details of the microstructure of
the various other supramolecular hydrogels that have been studied are an important
issue that needs to be resolved.
Like other supramolecular hydrogels, SAHs are viscoelastic due to the finite
relaxation times of supramolecular bond. The mechanical response is timedependent, and the hydrogel exhibits considerable hysteresis in loading and
unloading experiments and a Mullins effect. That behavior is a consequence of the
changes in the microphase-separated microstructure as hydrophobic bonds break and
may be pulled out of the fluoro(meth)acrylate nanodomains. The breaking of
physical bonds produces the high-energy losses that are responsible for the material’s extraordinary toughness. However, because of the reversible nature of the
hydrophobic bonds, they may reform once the stress is dissipated or removed. The
nanodomain network persists during the hydrogel deformation, and as a result, the
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
203