[17]. Therefore, it is likely that the overall hydrophobic association in the gel
networks almost remains unchanged, despite the different DMSO fractions.
In order to explore the structures and mechanisms underneath the observations,
SEM imaging of freeze-dried gels and in situ small-angle X-ray scattering measurements of the gels are performed.
Figure 8 shows representative SEM images of fractured and freeze-dried surfaces
of MFD 0 , MFD 0.5 , MFD 0.9 , and MFD 1 gels. The MFD 0 hydrogels swollen with
water show typical porous structures, with smooth walls (Fig. 8a). With increasing
DMSO content, no porous structures are observed at the fracture surface. Instead,
the surface becomes rough, with small particles gradually appearing (Fig. 8b–d). The
particle size increases with DMSO content. In the MFD 1 gels, a lot of particles with
dimensions ranging from tens of nanometers to about 100 nm are observed (Fig. 8d).
These particles are smaller than the ξ H of F127DA micelles formed in DMSO
solutions. Similar formation of particles has been observed in other micellecrosslinked hydrogels and recognized as microphase separation structures [25–
27]. Here, the presence of DMSO promotes microphase separation of micelles
from PAAm networks. This will be discussed later.
0
500 1000 1500 2000 2500 3000
0.00
0.03
0.06
0.09
0.12
0.15
f=1
f=0.9
f=0.75
f=0.5
f=0
Strain (%)
)
a
P
M
(
s
s
e
r
t
S
0.0 0.2 0.4 0.6 0.8 1.0
500
1000
1500
2000
2500
3000
DMSO Volume Fraction
)
%
(
k
a
e
r
B
t
a
n
o
i
t
a
g
n
o
l
E
0.0
0.5
0.75
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
y
g
r
e
n
E
e
r
u
t
c
a
r
F
(
m
/
J
M
3
)
DMSO Volume Fraction
(c)
(d)
(b)
(a)
0.0 0.2 0.4 0.6 0.8 1.0
0.03
0.06
0.09
0.12
0.15
0.18
DMSO Volume Fraction
)
a
P
M
(
h
t
g
n
e
r
t
S
g
n
i
k
a
e
r
B
Fig. 7 (a) Representative tensile stress-strain curves of micelle-crosslinked gels containing solvents with different DMSO volume fractions ( f ) and the corresponding (b) breaking strength, (c)
breaking elongation, and (c) fracture energy. Adapted from Ref. [24] with permission. Copyright
2019 John Wiley and Sons
Triblock Copolymer Micelle-Crosslinked Hydrogels
219
networks almost remains unchanged, despite the different DMSO fractions.
In order to explore the structures and mechanisms underneath the observations,
SEM imaging of freeze-dried gels and in situ small-angle X-ray scattering measurements of the gels are performed.
Figure 8 shows representative SEM images of fractured and freeze-dried surfaces
of MFD 0 , MFD 0.5 , MFD 0.9 , and MFD 1 gels. The MFD 0 hydrogels swollen with
water show typical porous structures, with smooth walls (Fig. 8a). With increasing
DMSO content, no porous structures are observed at the fracture surface. Instead,
the surface becomes rough, with small particles gradually appearing (Fig. 8b–d). The
particle size increases with DMSO content. In the MFD 1 gels, a lot of particles with
dimensions ranging from tens of nanometers to about 100 nm are observed (Fig. 8d).
These particles are smaller than the ξ H of F127DA micelles formed in DMSO
solutions. Similar formation of particles has been observed in other micellecrosslinked hydrogels and recognized as microphase separation structures [25–
27]. Here, the presence of DMSO promotes microphase separation of micelles
from PAAm networks. This will be discussed later.
0
500 1000 1500 2000 2500 3000
0.00
0.03
0.06
0.09
0.12
0.15
f=1
f=0.9
f=0.75
f=0.5
f=0
Strain (%)
)
a
P
M
(
s
s
e
r
t
S
0.0 0.2 0.4 0.6 0.8 1.0
500
1000
1500
2000
2500
3000
DMSO Volume Fraction
)
%
(
k
a
e
r
B
t
a
n
o
i
t
a
g
n
o
l
E
0.0
0.5
0.75
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
y
g
r
e
n
E
e
r
u
t
c
a
r
F
(
m
/
J
M
3
)
DMSO Volume Fraction
(c)
(d)
(b)
(a)
0.0 0.2 0.4 0.6 0.8 1.0
0.03
0.06
0.09
0.12
0.15
0.18
DMSO Volume Fraction
)
a
P
M
(
h
t
g
n
e
r
t
S
g
n
i
k
a
e
r
B
Fig. 7 (a) Representative tensile stress-strain curves of micelle-crosslinked gels containing solvents with different DMSO volume fractions ( f ) and the corresponding (b) breaking strength, (c)
breaking elongation, and (c) fracture energy. Adapted from Ref. [24] with permission. Copyright
2019 John Wiley and Sons
Triblock Copolymer Micelle-Crosslinked Hydrogels
219