3 Results and Discussion
3.1 Strength of P(AAm-co-VIm)-Ni
2+ Dual Crosslink Gel
In order to confirm that the introduction of the metal-ligand coordination bonds can
improve the strength of the polyacrylamide gel, we performed uniaxial stretching
experiments and characterized the nonlinear (large strain) behavior of the P
(AAm-co-VIm)-Ni
2+ dual crosslink gels, at different Ni
2+ concentrations
(5–100 mM). The tensile stress σ was plotted as a function of the stretch ratio λ
(defined as λ ¼ L/L 0 ) at the same stretch rate _
λ ¼ 0.06 s
À1 . The results for the dual
crosslink gels with different values of [Ni
2+ ] and for the corresponding chemical gel
are shown in Fig. 2. For all values of [Ni
2+ ], the dual crosslink gels have higher
extensibilities than the chemical gel, and the strain at break dramatically increases
with the introduction of physical crosslinks: with a value of λ at break λ b varying
from less than 4 for the chemical gel to over 6 for the dual crosslink gels. λ b can reach
to about 8.5 when the concentration of the transient crosslinker [Ni
2+ ] is increased to
100 mM.
For the values of the stress, a complex [Ni
2+ ] dependence is seen. For
[Ni
2+ ] > 20 mM, the increase of [Ni
2+ ] leads to a significant increase of the stress
over the whole range. The stress vs. stretch curves resemble more those of a straincrystallizing rubber such as natural rubber [35] or a pressure-sensitive adhesive [36]:
softening at intermediate strains, indicating the breakup of a network structure, and
strain hardening at large strain indicating finite extensibility of the polymer chains.
This strain hardening behavior becomes significant at λ > 5, a stretch level which is
not easily accessible for conventional chemical gels or even with the PVA dual
80
60
40
20
0
s (kPa)
10
8
6
4
l
2
Dual crosslink gel C(Ni
2+
)
5 mM
10 mM
20 mM
50 mM
100 mM
Chemical gel
Fig. 2 Stress-strain curves
of the dual crosslink gels
with different Ni
2+
concentrations compared
with the corresponding
chemical gels at a stretch
rate of 0.06 s
À1 at room
temperature
Dual Crosslink Hydrogels with Metal-Ligand Coordination Bonds: Tunable Dynamics. . .
7
3.1 Strength of P(AAm-co-VIm)-Ni
2+ Dual Crosslink Gel
In order to confirm that the introduction of the metal-ligand coordination bonds can
improve the strength of the polyacrylamide gel, we performed uniaxial stretching
experiments and characterized the nonlinear (large strain) behavior of the P
(AAm-co-VIm)-Ni
2+ dual crosslink gels, at different Ni
2+ concentrations
(5–100 mM). The tensile stress σ was plotted as a function of the stretch ratio λ
(defined as λ ¼ L/L 0 ) at the same stretch rate _
λ ¼ 0.06 s
À1 . The results for the dual
crosslink gels with different values of [Ni
2+ ] and for the corresponding chemical gel
are shown in Fig. 2. For all values of [Ni
2+ ], the dual crosslink gels have higher
extensibilities than the chemical gel, and the strain at break dramatically increases
with the introduction of physical crosslinks: with a value of λ at break λ b varying
from less than 4 for the chemical gel to over 6 for the dual crosslink gels. λ b can reach
to about 8.5 when the concentration of the transient crosslinker [Ni
2+ ] is increased to
100 mM.
For the values of the stress, a complex [Ni
2+ ] dependence is seen. For
[Ni
2+ ] > 20 mM, the increase of [Ni
2+ ] leads to a significant increase of the stress
over the whole range. The stress vs. stretch curves resemble more those of a straincrystallizing rubber such as natural rubber [35] or a pressure-sensitive adhesive [36]:
softening at intermediate strains, indicating the breakup of a network structure, and
strain hardening at large strain indicating finite extensibility of the polymer chains.
This strain hardening behavior becomes significant at λ > 5, a stretch level which is
not easily accessible for conventional chemical gels or even with the PVA dual
80
60
40
20
0
s (kPa)
10
8
6
4
l
2
Dual crosslink gel C(Ni
2+
)
5 mM
10 mM
20 mM
50 mM
100 mM
Chemical gel
Fig. 2 Stress-strain curves
of the dual crosslink gels
with different Ni
2+
concentrations compared
with the corresponding
chemical gels at a stretch
rate of 0.06 s
À1 at room
temperature
Dual Crosslink Hydrogels with Metal-Ligand Coordination Bonds: Tunable Dynamics. . .
7