such hydrogels. A major question here is what actually constitutes healing for a
multi-phase morphology. One criterion may be the restoration of the nanodomain
structure, i.e., the dimensions and spacing. In that case, healing involves diffusion
and translation of chain segments to reform the hydrophobic bonds and restore the
nanodomain structure, though it is unlikely that one can unambiguously determine,
or expect, that each hydrophobic group from a broken hydrophobic bond that pulls
out of a nanodomain by the retractive force in the network chains returns to its
original nanodomain and spatial position within the nanodomain.
Although it is not possible to directly observe the changes and reformation of the
nanodomain structure due to deformation, scattering experiments, SAXS and SANS,
can provide that information. That approach for assessing healing has the caveat that
quantitative interpretation of scattering data requires a morphological model, and
there is no unique model for describing the microstructure of these hydrogels. SANS
is a particularly useful technique for evaluating the microstructure of hydrogels,
because the large differences in the scattering length density (SLD) between hydrogen and deuterium allow one to contrast-match different parts of the microstructure,
e.g., the hydrophilic polymer or hydrophobic nanodomain phases by using mixtures
of H 2 O and D 2 O with different compositions [39]. The details of contrast-matching
SANS experiments of the DFx hydrogels are provided in Refs. [16, 20, 25] and are
not reproduced here.
Simultaneous deformation and scattering measurements (i.e., rheo-scattering) for
hydrogels can be performed at a variety of international laboratories with SAXS or
SANS capabilities. For example, for the work discussed in this chapter, simultaneous tensile stretching and SAXS experiments [24] were conducted at Brookhaven
National Laboratory (BNL) in Upton, NY (USA), and stress relaxation–SANS
experiments [16] were conducted at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) in Gaithersburg, MD (USA).
In general, rheo-scattering experiments showed that the nanodomain microstructure
became anisotropic during large strain deformations [20], and although the structure
relaxed following a large amplitude strain, it did so with different kinetics than the
stress relaxation. Although the microstructure anisotropy decreased during relaxation, it did not appear to be on a trajectory to recover to the original dimensions in an
accessible time scale [20].
An alternative and simpler criterion for healing is the restoration of the mechanical properties of the hydrogel. That is a less rigorous definition that allows the
establishment of a new equilibrium distribution of the hydrophobic groups in the
healed multifunctional nanodomain crosslinks. It is also a more practical criterion,
since the experiments are easier to perform, and with regard to healing, the most
important outcome is the restoration of the mechanical integrity of the material.
In this case, dynamic mechanical testing of the viscoelastic properties of the hydrogel undergoing nonlinear deformation, followed by relaxation, was used to judge the
reversibility of the properties of these hydrogels [12]. As described in the next two
paragraphs, those experiments revealed that self-healing occurred in that the viscoelastic properties of the hydrogel were reversible, even when the strain amplitudes
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
177
multi-phase morphology. One criterion may be the restoration of the nanodomain
structure, i.e., the dimensions and spacing. In that case, healing involves diffusion
and translation of chain segments to reform the hydrophobic bonds and restore the
nanodomain structure, though it is unlikely that one can unambiguously determine,
or expect, that each hydrophobic group from a broken hydrophobic bond that pulls
out of a nanodomain by the retractive force in the network chains returns to its
original nanodomain and spatial position within the nanodomain.
Although it is not possible to directly observe the changes and reformation of the
nanodomain structure due to deformation, scattering experiments, SAXS and SANS,
can provide that information. That approach for assessing healing has the caveat that
quantitative interpretation of scattering data requires a morphological model, and
there is no unique model for describing the microstructure of these hydrogels. SANS
is a particularly useful technique for evaluating the microstructure of hydrogels,
because the large differences in the scattering length density (SLD) between hydrogen and deuterium allow one to contrast-match different parts of the microstructure,
e.g., the hydrophilic polymer or hydrophobic nanodomain phases by using mixtures
of H 2 O and D 2 O with different compositions [39]. The details of contrast-matching
SANS experiments of the DFx hydrogels are provided in Refs. [16, 20, 25] and are
not reproduced here.
Simultaneous deformation and scattering measurements (i.e., rheo-scattering) for
hydrogels can be performed at a variety of international laboratories with SAXS or
SANS capabilities. For example, for the work discussed in this chapter, simultaneous tensile stretching and SAXS experiments [24] were conducted at Brookhaven
National Laboratory (BNL) in Upton, NY (USA), and stress relaxation–SANS
experiments [16] were conducted at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) in Gaithersburg, MD (USA).
In general, rheo-scattering experiments showed that the nanodomain microstructure
became anisotropic during large strain deformations [20], and although the structure
relaxed following a large amplitude strain, it did so with different kinetics than the
stress relaxation. Although the microstructure anisotropy decreased during relaxation, it did not appear to be on a trajectory to recover to the original dimensions in an
accessible time scale [20].
An alternative and simpler criterion for healing is the restoration of the mechanical properties of the hydrogel. That is a less rigorous definition that allows the
establishment of a new equilibrium distribution of the hydrophobic groups in the
healed multifunctional nanodomain crosslinks. It is also a more practical criterion,
since the experiments are easier to perform, and with regard to healing, the most
important outcome is the restoration of the mechanical integrity of the material.
In this case, dynamic mechanical testing of the viscoelastic properties of the hydrogel undergoing nonlinear deformation, followed by relaxation, was used to judge the
reversibility of the properties of these hydrogels [12]. As described in the next two
paragraphs, those experiments revealed that self-healing occurred in that the viscoelastic properties of the hydrogel were reversible, even when the strain amplitudes
Hydrophobically Associating Hydrogels with Microphase-Separated Morphologies
177