4 Conclusion
In order to investigate systematically the effect of the dynamic bond exchange rate
on the macroscopic properties of hydrogels containing physical and chemical
crosslinks, we designed and synthesized P(AAm-co-VIm)-M
2+ dual crosslink gels
having a tunable characteristic relaxation time in order to characterize their timedependent mechanical properties in small and large strains. These dual crosslink gels
were permanently crosslinked by methylene bisacrylamide and transiently
crosslinked by a metal ion – imidazole ligand coordination, with two different
ions, Ni
2+ and Zn
2+ , having different characteristic breaking/reforming times.
Linear rheological measurements showed that the P(AAm-co-VIm)-M
2+ dual
crosslink gels exhibit time-dependent elasticity, i.e., the G
0 (ω) increases with frequency, and G
00 (ω) shows a peak, due to the dissociation of the transient crosslinks
followed by chain relaxation. The dynamics of the dual crosslink gels can be tuned
by changing the metal ions: the P(AAm-co-VIm)-Zn
2+ dual crosslink gel exhibited a
much faster relaxation process than the P(AAm-co-VIm)-Ni
2+ dual crosslink gel,
hence more suitable to characterize the mechanical properties at very slow stretch
rate (relative to the characteristic time). In addition we observed signs of the
existence of a slower relaxation component due to a more long-lived physical
crosslinking mechanism, since the elastic modulus of the dual crosslink gels did
not reach to the value of the corresponding chemical gel even at times of the order of
1,000 s.
At intermediate strains (λ < 2), the P(AAm-co-VIm)-Zn
2+ dual crosslink gel,
having much faster association/dissociation dynamics than that of the P(AAm-coVIm)-Ni
2+ dual crosslink gel, exhibits a lower hysteresis in the tensile loops. Based
on the strain-rate-dependent tensile behavior at small strain, a master curve of the
reduced stress as a function of time was obtained for both gels; thus in that strain
range, a separability of the stress into strain-dependent and time-dependent terms
holds.
The key result at larger strain is the difference in extensibility between the dual
crosslink gels and the chemical gel even at stretch rates significantly slower than the
inverse of the main relaxation time of the gel, a situation where the stress-strain
curves are nearly identical except for the fracture point.
The stress at large strain proved to be also strongly strain-rate dependent, with in
particular a strongly rate-dependent strain hardening at large strain. Mooney plots of
the reduced stress were used to characterize this strain hardening behavior and
showed that the reduced stress at the onset of strain hardening kept increasing with
stretch rate, suggesting the existence of a second relaxation time due to a longerlived physical crosslinking mechanism.
Finally we found that the initial elastic modulus of the gel during step-cyclic tests
started to decrease at values of λ~4 close to the stretch a break of the chemical gel.
This suggests that in dual crosslink gels, molecular damage occurs at that stage, but
no macroscopic crack forms and propagates. This strongly supports a mechanism
where the physical crosslinks are actively protecting (or shielding) neighboring
18
J. Zhao et al.
In order to investigate systematically the effect of the dynamic bond exchange rate
on the macroscopic properties of hydrogels containing physical and chemical
crosslinks, we designed and synthesized P(AAm-co-VIm)-M
2+ dual crosslink gels
having a tunable characteristic relaxation time in order to characterize their timedependent mechanical properties in small and large strains. These dual crosslink gels
were permanently crosslinked by methylene bisacrylamide and transiently
crosslinked by a metal ion – imidazole ligand coordination, with two different
ions, Ni
2+ and Zn
2+ , having different characteristic breaking/reforming times.
Linear rheological measurements showed that the P(AAm-co-VIm)-M
2+ dual
crosslink gels exhibit time-dependent elasticity, i.e., the G
0 (ω) increases with frequency, and G
00 (ω) shows a peak, due to the dissociation of the transient crosslinks
followed by chain relaxation. The dynamics of the dual crosslink gels can be tuned
by changing the metal ions: the P(AAm-co-VIm)-Zn
2+ dual crosslink gel exhibited a
much faster relaxation process than the P(AAm-co-VIm)-Ni
2+ dual crosslink gel,
hence more suitable to characterize the mechanical properties at very slow stretch
rate (relative to the characteristic time). In addition we observed signs of the
existence of a slower relaxation component due to a more long-lived physical
crosslinking mechanism, since the elastic modulus of the dual crosslink gels did
not reach to the value of the corresponding chemical gel even at times of the order of
1,000 s.
At intermediate strains (λ < 2), the P(AAm-co-VIm)-Zn
2+ dual crosslink gel,
having much faster association/dissociation dynamics than that of the P(AAm-coVIm)-Ni
2+ dual crosslink gel, exhibits a lower hysteresis in the tensile loops. Based
on the strain-rate-dependent tensile behavior at small strain, a master curve of the
reduced stress as a function of time was obtained for both gels; thus in that strain
range, a separability of the stress into strain-dependent and time-dependent terms
holds.
The key result at larger strain is the difference in extensibility between the dual
crosslink gels and the chemical gel even at stretch rates significantly slower than the
inverse of the main relaxation time of the gel, a situation where the stress-strain
curves are nearly identical except for the fracture point.
The stress at large strain proved to be also strongly strain-rate dependent, with in
particular a strongly rate-dependent strain hardening at large strain. Mooney plots of
the reduced stress were used to characterize this strain hardening behavior and
showed that the reduced stress at the onset of strain hardening kept increasing with
stretch rate, suggesting the existence of a second relaxation time due to a longerlived physical crosslinking mechanism.
Finally we found that the initial elastic modulus of the gel during step-cyclic tests
started to decrease at values of λ~4 close to the stretch a break of the chemical gel.
This suggests that in dual crosslink gels, molecular damage occurs at that stage, but
no macroscopic crack forms and propagates. This strongly supports a mechanism
where the physical crosslinks are actively protecting (or shielding) neighboring
18
J. Zhao et al.