gels crosslinked by metal-ligand coordination bonds have been reported [27–
30]. Holten-Andersen et al. developed physical gels crosslinked by catechol-Fe
3+
complexes exhibiting pH-sensitive mechanical properties [27]. Histidine-modified
star PEG polymers were synthesized to mimic the histidine-rich regions of the
mussel byssal thread collagen [28]. 4-arm-PEG-histidine (4PEG-His) crosslinked
with metal ions was synthesized to produce hydrogels with tunable relaxation times,
which follow the order Ni
2+
> Cu
2+
> Zn
2+ .
More recently various tough dual crosslink hydrogels with metal-ligand coordination bonds have been reported [31–33]. While macroscopic tests of their toughness and self-recovery properties were studied, the nonlinear mechanical properties
as a function of the transient bond relaxation time and the strain rate have not been
systematically studied. Kean et al. synthesized a series of dual crosslink organogels
with metal-ligand coordination bonds, and showed that the short-lived transient
crosslinks do not increase the modulus (thus they are invisible), while they can
improve the extensibility of the network better than the long-lived transient
crosslinks [34]. This intriguing observation is one of the motivations of the work
we report.
We investigate here a new dual crosslink hydrogel based on polyacrylamide,
another simple neutral hydrosoluble polymer, copolymerized with
N-vinylimidazole. This gel possesses a fast and tunable dynamics due to imidazole –
metal ion coordination bonds and ingredients are commercially available and readily
soluble in water. Poly(acrylamide-co-vinylimidazole) was synthesized by free radical copolymerization in the presence of physical crosslinkers (transient metal ions,
Ni
2+ or Zn
2+ ) and a chemical crosslinker (methylene bisacrylamide) (Fig. 1). With
this relatively easy and quick “one-pot” synthesis (simultaneous chemical and
physical crosslinking and polymerization), we successfully synthesized hydrogels
having both chemical and physical crosslinks. This simple fabrication process from
inexpensive readily available components sacrifices precise network and structural
control but makes it possible to produce the necessary amount of material samples to
perform systematic mechanical tests in large strain.
In this work we attempt to understand the relation between the macroscopic
mechanical properties in large strain and the dynamics of the coordination bonds
over a wide dynamic range.
2 Experimental Section
2.1 Materials
Acrylamide (AAm), 1-vinylimidazole (VIm), methylenebisacrylamide (MBA),
potassium persulfate (KPS), N,N,N
0 ,N
0 -tetramethylethylenediamine (TEMED),
nickel chloride, and zinc chloride were purchased from Sigma Aldrich and used as
received. Milli-Q water is used for the sample preparation.
4
J. Zhao et al.
30]. Holten-Andersen et al. developed physical gels crosslinked by catechol-Fe
3+
complexes exhibiting pH-sensitive mechanical properties [27]. Histidine-modified
star PEG polymers were synthesized to mimic the histidine-rich regions of the
mussel byssal thread collagen [28]. 4-arm-PEG-histidine (4PEG-His) crosslinked
with metal ions was synthesized to produce hydrogels with tunable relaxation times,
which follow the order Ni
2+
> Cu
2+
> Zn
2+ .
More recently various tough dual crosslink hydrogels with metal-ligand coordination bonds have been reported [31–33]. While macroscopic tests of their toughness and self-recovery properties were studied, the nonlinear mechanical properties
as a function of the transient bond relaxation time and the strain rate have not been
systematically studied. Kean et al. synthesized a series of dual crosslink organogels
with metal-ligand coordination bonds, and showed that the short-lived transient
crosslinks do not increase the modulus (thus they are invisible), while they can
improve the extensibility of the network better than the long-lived transient
crosslinks [34]. This intriguing observation is one of the motivations of the work
we report.
We investigate here a new dual crosslink hydrogel based on polyacrylamide,
another simple neutral hydrosoluble polymer, copolymerized with
N-vinylimidazole. This gel possesses a fast and tunable dynamics due to imidazole –
metal ion coordination bonds and ingredients are commercially available and readily
soluble in water. Poly(acrylamide-co-vinylimidazole) was synthesized by free radical copolymerization in the presence of physical crosslinkers (transient metal ions,
Ni
2+ or Zn
2+ ) and a chemical crosslinker (methylene bisacrylamide) (Fig. 1). With
this relatively easy and quick “one-pot” synthesis (simultaneous chemical and
physical crosslinking and polymerization), we successfully synthesized hydrogels
having both chemical and physical crosslinks. This simple fabrication process from
inexpensive readily available components sacrifices precise network and structural
control but makes it possible to produce the necessary amount of material samples to
perform systematic mechanical tests in large strain.
In this work we attempt to understand the relation between the macroscopic
mechanical properties in large strain and the dynamics of the coordination bonds
over a wide dynamic range.
2 Experimental Section
2.1 Materials
Acrylamide (AAm), 1-vinylimidazole (VIm), methylenebisacrylamide (MBA),
potassium persulfate (KPS), N,N,N
0 ,N
0 -tetramethylethylenediamine (TEMED),
nickel chloride, and zinc chloride were purchased from Sigma Aldrich and used as
received. Milli-Q water is used for the sample preparation.
4
J. Zhao et al.