toughness. It is likely that the internal electrostatic repulsion between the polymer
chains more or less weakens the energy dissipation.
On the other hand, the intrinsic electrostatic repulsion largely determines the
volume of the hydrogels. Changes in the pH, or ionic strength in the environment
usually, alter the intrinsic electrostatic balance in the swollen network, leading to
changes in the volume. Such volume changes are reversible when the environmental
pH and ionic strength are reversed. Reversible swelling and deswelling take place for
the cationic poly(DMAEA-Q) (PDMAEA-Q, QxMy) and anionic polyAMPS
(PAMPS, SxMy) hydrogels crosslinked by F127DA micelles. Figure 12a shows
representative swelling of QxMy hydrogels at pH 3 and deswelling at pH 11. At
pH 7, the electrostatic repulsion (Fig. 12b) leads to swelling. As the pH becomes
11, the OH
À ions with negative charges shield the electrostatic repulsion (Fig. 12d)
and thus the hydrogel shrinks. The swelling and deswelling are reversible as the
hydrogels are shuttled between acidic, neutral, and alkaline solutions (Fig. 12c).
Meanwhile, reversible swelling/shrinking takes place as the environmental ionic
strength is cyclically changed. The as-prepared QxMy hydrogels swells in pH 7
solution with an ionic strength (IS) of 0.1 to equilibrium. Subsequent immersion of
the gel in a neutral solution with ionic strength of 0.05 leads to further swelling.
Then, as the swollen gel is taken back to the IS 0.1 solution, it deswells to the
previous equilibrium volume. The hydrogel undergoes cyclic swelling and
deswelling as it is repeatedly immersed in neutral solutions with different IS values
(Fig. 13). It is interesting that the equilibrium volume at each condition almost
(a)
0
4
8
12
16
20
11
9
7
5
o
i
t
a
R
g
n
i
l
l
e
w
S
pH
Q1M5
Q1M8
Q1M11
3
0 15 30 45 60 75 90105
2
4
6
8
10
12
pH=7.0
pH=7.0
pH=11.0
pH=11.0
o
i
t
a
R
g
n
i
l
l
e
w
S
Time (h)
(b)
As-prepared
hydrogel
pH=11, I=0.1
pH=11
pH=7
pH=7, I=0.1
pH=3, I=0.1
(d)
(c)
Fig. 12 (a) Equilibrium swelling ratio of QxMy hydrogels in solutions with different pH values.
(b) Reversible swelling/deswelling of the QxMy hydrogels with pH value shuttled between 7 and
11. (c) Reversible swelling/deswelling of the hydrogels at different solution conditions. (d) Schematic illustration to the electrostatic attraction changes in the hydrogel networks at pH 7 and 11.
Adapted from Ref. [45] with permission. Copyright 2015 Royal Society of Chemistry
Triblock Copolymer Micelle-Crosslinked Hydrogels
225
chains more or less weakens the energy dissipation.
On the other hand, the intrinsic electrostatic repulsion largely determines the
volume of the hydrogels. Changes in the pH, or ionic strength in the environment
usually, alter the intrinsic electrostatic balance in the swollen network, leading to
changes in the volume. Such volume changes are reversible when the environmental
pH and ionic strength are reversed. Reversible swelling and deswelling take place for
the cationic poly(DMAEA-Q) (PDMAEA-Q, QxMy) and anionic polyAMPS
(PAMPS, SxMy) hydrogels crosslinked by F127DA micelles. Figure 12a shows
representative swelling of QxMy hydrogels at pH 3 and deswelling at pH 11. At
pH 7, the electrostatic repulsion (Fig. 12b) leads to swelling. As the pH becomes
11, the OH
À ions with negative charges shield the electrostatic repulsion (Fig. 12d)
and thus the hydrogel shrinks. The swelling and deswelling are reversible as the
hydrogels are shuttled between acidic, neutral, and alkaline solutions (Fig. 12c).
Meanwhile, reversible swelling/shrinking takes place as the environmental ionic
strength is cyclically changed. The as-prepared QxMy hydrogels swells in pH 7
solution with an ionic strength (IS) of 0.1 to equilibrium. Subsequent immersion of
the gel in a neutral solution with ionic strength of 0.05 leads to further swelling.
Then, as the swollen gel is taken back to the IS 0.1 solution, it deswells to the
previous equilibrium volume. The hydrogel undergoes cyclic swelling and
deswelling as it is repeatedly immersed in neutral solutions with different IS values
(Fig. 13). It is interesting that the equilibrium volume at each condition almost
(a)
0
4
8
12
16
20
11
9
7
5
o
i
t
a
R
g
n
i
l
l
e
w
S
pH
Q1M5
Q1M8
Q1M11
3
0 15 30 45 60 75 90105
2
4
6
8
10
12
pH=7.0
pH=7.0
pH=11.0
pH=11.0
o
i
t
a
R
g
n
i
l
l
e
w
S
Time (h)
(b)
As-prepared
hydrogel
pH=11, I=0.1
pH=11
pH=7
pH=7, I=0.1
pH=3, I=0.1
(d)
(c)
Fig. 12 (a) Equilibrium swelling ratio of QxMy hydrogels in solutions with different pH values.
(b) Reversible swelling/deswelling of the QxMy hydrogels with pH value shuttled between 7 and
11. (c) Reversible swelling/deswelling of the hydrogels at different solution conditions. (d) Schematic illustration to the electrostatic attraction changes in the hydrogel networks at pH 7 and 11.
Adapted from Ref. [45] with permission. Copyright 2015 Royal Society of Chemistry
Triblock Copolymer Micelle-Crosslinked Hydrogels
225