sulfoxide (DMSO), or mixtures of isopropanol and water containing at least 25 wt%
isopropanol, though the solubility depended on the concentration of fluoroacrylate in
the copolymer. Hydrogels were obtained by soaking the copolymer films in
de-ionized water until they reached constant mass. Table 1 also lists the equilibrium
water concentration of the hydrogels, reported as swelling ratio, S, where S (mass
of hydrogel)/(mass of dry copolymer). S can be converted to the mass fraction of
water in the hydrogel, x w, by Eq. (1).
x w ¼
S À 1
S
ð1Þ
Thus, a hydrogel with a swelling ratio of 2.0 is composed of 50% water by mass.
Figure 1 also shows the SAXS data for the water-swollen hydrogels, and the
persistence of the peak in the scattering pattern indicates that the microphaseseparated morphology remains for the hydrogel. Only the hydrophilic polymer
segments are swollen by water. The hydrophobic nanodomains are the crosslink
junctions in these gels, which notably exist as a separate phase.
2.3 Hydrogel Microstructure
The hydrogels summarized in Table 1 are unique among other supramolecular and
covalent hydrogels described in the literature in that these hydrophobically associating hydrogels exhibit microphase separation. Their microstructure consists of
~2–6-nm-diameter core–shell nanodomains dispersed in a continuous phase of
water-swollen hydrophilic polymer [10, 16, 18–21, 24, 25]. The actual dimensions
of the core–shell nanodomain structure vary with the choice of monomers and the
hydrogel composition.
The details of the microphase separation in DFx, NFx, and HF hydrogels were
determined by small-angle neutron scattering (SANS) [10, 16, 20, 21, 24, 25]. The
nanodomains, shown schematically in Fig. 4, consist of a 1–5-nm-diameter hydrophobic core composed of hydrophobically bonded fluoroacrylate groups (red circles)
surrounded by a ~1-nm-thick shell of water-depleted hydrophilic segments (dark
blue area). The light blue area in Fig. 3 is the continuous phase of water-swollen
hydrophilic polymer. Since the fluoroacrylate groups in the core are covalently
bonded to the hydrophilic groups, the nanodomains act as multifunctional crosslinks,
and the hydrophilic polymer network chains are shown by the blue lines in Fig. 4 that
connect fluoroacrylate groups in separate nanodomains.
The origin of the microphase separation in the copolymers is the thermodynamic
immiscibility of the hydrophobic fluoroacrylate groups and the hydrophilic polymer.
The persistence of the nanodomain structure in the hydrogels is a consequence of the
enhancement of the unfavorable mixing of the hydrophilic and hydrophobic parts of
the copolymer due to the solubility of the hydrophilic polymer chains and the
immiscibility of the hydrophobic groups in water. Note that one might reasonably
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B. D. Vogt and R. A. Weiss
isopropanol, though the solubility depended on the concentration of fluoroacrylate in
the copolymer. Hydrogels were obtained by soaking the copolymer films in
de-ionized water until they reached constant mass. Table 1 also lists the equilibrium
water concentration of the hydrogels, reported as swelling ratio, S, where S (mass
of hydrogel)/(mass of dry copolymer). S can be converted to the mass fraction of
water in the hydrogel, x w, by Eq. (1).
x w ¼
S À 1
S
ð1Þ
Thus, a hydrogel with a swelling ratio of 2.0 is composed of 50% water by mass.
Figure 1 also shows the SAXS data for the water-swollen hydrogels, and the
persistence of the peak in the scattering pattern indicates that the microphaseseparated morphology remains for the hydrogel. Only the hydrophilic polymer
segments are swollen by water. The hydrophobic nanodomains are the crosslink
junctions in these gels, which notably exist as a separate phase.
2.3 Hydrogel Microstructure
The hydrogels summarized in Table 1 are unique among other supramolecular and
covalent hydrogels described in the literature in that these hydrophobically associating hydrogels exhibit microphase separation. Their microstructure consists of
~2–6-nm-diameter core–shell nanodomains dispersed in a continuous phase of
water-swollen hydrophilic polymer [10, 16, 18–21, 24, 25]. The actual dimensions
of the core–shell nanodomain structure vary with the choice of monomers and the
hydrogel composition.
The details of the microphase separation in DFx, NFx, and HF hydrogels were
determined by small-angle neutron scattering (SANS) [10, 16, 20, 21, 24, 25]. The
nanodomains, shown schematically in Fig. 4, consist of a 1–5-nm-diameter hydrophobic core composed of hydrophobically bonded fluoroacrylate groups (red circles)
surrounded by a ~1-nm-thick shell of water-depleted hydrophilic segments (dark
blue area). The light blue area in Fig. 3 is the continuous phase of water-swollen
hydrophilic polymer. Since the fluoroacrylate groups in the core are covalently
bonded to the hydrophilic groups, the nanodomains act as multifunctional crosslinks,
and the hydrophilic polymer network chains are shown by the blue lines in Fig. 4 that
connect fluoroacrylate groups in separate nanodomains.
The origin of the microphase separation in the copolymers is the thermodynamic
immiscibility of the hydrophobic fluoroacrylate groups and the hydrophilic polymer.
The persistence of the nanodomain structure in the hydrogels is a consequence of the
enhancement of the unfavorable mixing of the hydrophilic and hydrophobic parts of
the copolymer due to the solubility of the hydrophilic polymer chains and the
immiscibility of the hydrophobic groups in water. Note that one might reasonably
172
B. D. Vogt and R. A. Weiss