2.6 Mechanical Behavior
In general, conventional single network, covalently crosslinked hydrogels are soft
and flexible but are relatively weak and brittle elastic solids with an elastic modulus
between 0.01 and 10 kPa [40]. In accordance with the theory of rubber elasticity
[36], the modulus can be increased by increasing the crosslink density, but that also
further embrittles the gel. The network chains in a hydrogel are already highly
stretched in their swollen, unperturbed state, so further stretching of the gel is
limited, and, in general, covalent hydrogels have poor mechanical strength
(<100 kPa) and poor fracture toughness (<10 J/m
2 ) [41].
Supramolecular hydrogels, however, can have much high tensile strengths,
$1 MPa, and high fracture toughness, $10
3
–10
4 J/m
2 [41], comparable to or
exceeding that of human knee cartilage, $10
3 J/m
2 [42]. The tensile and compressive properties of the DFx and NFx hydrogels are summarized in Table 1. As
indicted earlier, the crosslink densities of the supramolecular hydrogels are much
higher than is typical for covalently crosslinked hydrogels. As a result, the tensile
modulus for these hydrogels ranged from 100 kPa to 14 MPa, and the tensile strength
varied from 200 to 800 kPa, depending on the fluoroacrylate concentration in the
amphiphilic copolymer. Even with their high crosslink density and high modulus
and strength values, the fluoroacrylate amphiphilic hydrogels were stretchable to
over 100% and to as much as 1,600%.
Covalently bonded hydrogels are brittle materials. With the exception of defects
in the network structure, such as dangling chains that are not part of the network, the
only energy dissipation mechanism for covalently crosslinked hydrogels are the
conformational fluctuations of the chain segments between crosslinks. As a result,
any additional deformation to the already highly extended network chains in the
swollen gel fractures bonds and produces failure of the hydrogel. This effect is
clearly observed when a covalent hydrogel is frozen [43]. Since water expands when
frozen, additional stress is developed in the network chains, leading to fracture of the
hydrogel, Fig. 7 [44]. This is also the case with double network hydrogels, where the
breaking of a sacrificial network that provides toughness in those materials is
irreversible. In contrast, however, for supramolecular hydrogels, the physical
bonds can rupture without breaking polymer chains. In that case, when the water
freezes and expands, the additional stresses are dissipated by breaking the reversible
physical crosslinks, which reform once the stress dissipates. The physical gel is able
to accommodate additional stress and failure does not occur [15, 16].
Toughness of materials is defined in two different ways: (1) the amount of energy
a material can absorb without breaking and (2) the amount of energy required for
propagating a crack in the material. The former quantity, which is often referred to as
the strain energy to break, is measured from the area under a stress–strain curve, and
the latter quantity is determined from a fracture mechanics experiment that measures
the energy required to break a material containing a preformed crack [45, 46]. A
common method for measuring fracture toughness of hydrogels is a pure shear test
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
179
In general, conventional single network, covalently crosslinked hydrogels are soft
and flexible but are relatively weak and brittle elastic solids with an elastic modulus
between 0.01 and 10 kPa [40]. In accordance with the theory of rubber elasticity
[36], the modulus can be increased by increasing the crosslink density, but that also
further embrittles the gel. The network chains in a hydrogel are already highly
stretched in their swollen, unperturbed state, so further stretching of the gel is
limited, and, in general, covalent hydrogels have poor mechanical strength
(<100 kPa) and poor fracture toughness (<10 J/m
2 ) [41].
Supramolecular hydrogels, however, can have much high tensile strengths,
$1 MPa, and high fracture toughness, $10
3
–10
4 J/m
2 [41], comparable to or
exceeding that of human knee cartilage, $10
3 J/m
2 [42]. The tensile and compressive properties of the DFx and NFx hydrogels are summarized in Table 1. As
indicted earlier, the crosslink densities of the supramolecular hydrogels are much
higher than is typical for covalently crosslinked hydrogels. As a result, the tensile
modulus for these hydrogels ranged from 100 kPa to 14 MPa, and the tensile strength
varied from 200 to 800 kPa, depending on the fluoroacrylate concentration in the
amphiphilic copolymer. Even with their high crosslink density and high modulus
and strength values, the fluoroacrylate amphiphilic hydrogels were stretchable to
over 100% and to as much as 1,600%.
Covalently bonded hydrogels are brittle materials. With the exception of defects
in the network structure, such as dangling chains that are not part of the network, the
only energy dissipation mechanism for covalently crosslinked hydrogels are the
conformational fluctuations of the chain segments between crosslinks. As a result,
any additional deformation to the already highly extended network chains in the
swollen gel fractures bonds and produces failure of the hydrogel. This effect is
clearly observed when a covalent hydrogel is frozen [43]. Since water expands when
frozen, additional stress is developed in the network chains, leading to fracture of the
hydrogel, Fig. 7 [44]. This is also the case with double network hydrogels, where the
breaking of a sacrificial network that provides toughness in those materials is
irreversible. In contrast, however, for supramolecular hydrogels, the physical
bonds can rupture without breaking polymer chains. In that case, when the water
freezes and expands, the additional stresses are dissipated by breaking the reversible
physical crosslinks, which reform once the stress dissipates. The physical gel is able
to accommodate additional stress and failure does not occur [15, 16].
Toughness of materials is defined in two different ways: (1) the amount of energy
a material can absorb without breaking and (2) the amount of energy required for
propagating a crack in the material. The former quantity, which is often referred to as
the strain energy to break, is measured from the area under a stress–strain curve, and
the latter quantity is determined from a fracture mechanics experiment that measures
the energy required to break a material containing a preformed crack [45, 46]. A
common method for measuring fracture toughness of hydrogels is a pure shear test
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
179