chains. Breaking a hydrophobic bond or a pullout event lowers the effective
crosslink density of the gel, at least until the structure heals itself after the stress
dissipates.
Actually, it is not necessary that fluoroacrylate groups be completely removed
from the nanodomains to dissipate the strain energy. The nanodomains in the NFx
hydrogels can accommodate $100 FOSA groups [10], so energy dissipation may
also occur by rearrangements of the FOSA groups within the nanodomains that
allow the network chains to partially relax. In that case the crosslink density would
not change appreciably during deformation. Unfortunately, it is not possible to
unambiguously measure whether or how many fluoroacrylate groups are pulled
from the nanodomains during deformation, though SANS and stress relaxation
experiments on similar hydrogels [20] indicate that at least some removal of
FOSA groups from the nanodomains probably occurs. (This is discussed further in
the analysis below of the hysteresis behavior and self-recovery of these hydrogels.)
A similar energy dissipation mechanism as described in the preceding paragraph
has been previously proposed for ionomers, which is another type of microphaseseparated supramolecular polymer with nanodomain sizes comparable to those in the
NFx hydrogels. For ionomers, toughening is attributed to ion-hopping, where
supramolecular ionic or dipolar bonds break, but are reformed by the hopping [62]
of ions (actually, ion pairs) from one nanodomain to another. The preference for the
ionic groups to be associated and not exist as isolated ions or ion pairs is of
thermodynamic origin, i.e., the maintenance of electronic neutrality and the large
enthalpic penalty that accrues by locating a lone ion pair in the nonpolar polymer
continuous phase.
By analogy, the mechanism of toughening in the NFx hydrogels may involve a
hydrophobe-hopping mechanism, whereby the hydrophobes composing a hydrophobic bond separate under stress but reform hydrophobic bonds when the stress is
removed. It is not necessary, however, that the reformed hydrophobic bond involves
both of the original hydrophobic groups. A hydrophobic group from a broken
hydrophobic bond can hop from one position to another either within a nanodomain
or from one nanodomain to another. If the hydrophobic group hopping is
interdomain, the hydrophobic group must reside temporarily in the water-swollen
polymer phase, but as with the ion-hopping, locating a hydrophobic group in the
water-rich phase accrues a large enthalpic penalty, which favors the hydrophobic
group quickly hopping back to the nanodomain from which it resided originally or to
a new nanodomain. If the hydrophobe hopping is intradomain, breaking a hydrophobic bond within a nanodomain results simply in a rearrangement of the hydrophobic groups and bonds within that nanodomain, in which case there is no enthalpic
penalty accrued during the hopping phenomenon. The question of whether intra- or
interdomain hopping occurs presents an experimental challenge that is not yet
resolved.
The moduli and fracture energies for the DFx and NFx hydrogels listed in Table 1
are among the highest values reported for hydrogels; see Fig. 9, which shows the
relationship of fracture energy and modulus for synthetic and natural hydrogels.
These microphase-separated hydrogels have modulus and fracture toughness values
182
B. D. Vogt and R. A. Weiss
crosslink density of the gel, at least until the structure heals itself after the stress
dissipates.
Actually, it is not necessary that fluoroacrylate groups be completely removed
from the nanodomains to dissipate the strain energy. The nanodomains in the NFx
hydrogels can accommodate $100 FOSA groups [10], so energy dissipation may
also occur by rearrangements of the FOSA groups within the nanodomains that
allow the network chains to partially relax. In that case the crosslink density would
not change appreciably during deformation. Unfortunately, it is not possible to
unambiguously measure whether or how many fluoroacrylate groups are pulled
from the nanodomains during deformation, though SANS and stress relaxation
experiments on similar hydrogels [20] indicate that at least some removal of
FOSA groups from the nanodomains probably occurs. (This is discussed further in
the analysis below of the hysteresis behavior and self-recovery of these hydrogels.)
A similar energy dissipation mechanism as described in the preceding paragraph
has been previously proposed for ionomers, which is another type of microphaseseparated supramolecular polymer with nanodomain sizes comparable to those in the
NFx hydrogels. For ionomers, toughening is attributed to ion-hopping, where
supramolecular ionic or dipolar bonds break, but are reformed by the hopping [62]
of ions (actually, ion pairs) from one nanodomain to another. The preference for the
ionic groups to be associated and not exist as isolated ions or ion pairs is of
thermodynamic origin, i.e., the maintenance of electronic neutrality and the large
enthalpic penalty that accrues by locating a lone ion pair in the nonpolar polymer
continuous phase.
By analogy, the mechanism of toughening in the NFx hydrogels may involve a
hydrophobe-hopping mechanism, whereby the hydrophobes composing a hydrophobic bond separate under stress but reform hydrophobic bonds when the stress is
removed. It is not necessary, however, that the reformed hydrophobic bond involves
both of the original hydrophobic groups. A hydrophobic group from a broken
hydrophobic bond can hop from one position to another either within a nanodomain
or from one nanodomain to another. If the hydrophobic group hopping is
interdomain, the hydrophobic group must reside temporarily in the water-swollen
polymer phase, but as with the ion-hopping, locating a hydrophobic group in the
water-rich phase accrues a large enthalpic penalty, which favors the hydrophobic
group quickly hopping back to the nanodomain from which it resided originally or to
a new nanodomain. If the hydrophobe hopping is intradomain, breaking a hydrophobic bond within a nanodomain results simply in a rearrangement of the hydrophobic groups and bonds within that nanodomain, in which case there is no enthalpic
penalty accrued during the hopping phenomenon. The question of whether intra- or
interdomain hopping occurs presents an experimental challenge that is not yet
resolved.
The moduli and fracture energies for the DFx and NFx hydrogels listed in Table 1
are among the highest values reported for hydrogels; see Fig. 9, which shows the
relationship of fracture energy and modulus for synthetic and natural hydrogels.
These microphase-separated hydrogels have modulus and fracture toughness values
182
B. D. Vogt and R. A. Weiss