3.1 Reversible Covalent Bonds
3.1.1 Imine Bonds (Schiff Base)
Imine bonds, commonly termed Schiff base, are considered as the comparably strong
reversible covalent bonds, which feature exchange with only very few side reactions
through a dynamic imine formation between aldehyde groups and amino groups
[112]. Intriguingly, imine bonds can automatically occur cleavage and regeneration
in the hydrogel network without any external stimuli, which are recognized as an
attractive strategy to create self-healing cellulose-based hydrogels [113–120].
For example, Huang et al. [118] developed a typical self-healing hydrogel from
naturally occurring polymers combining water-soluble carboxymethyl chitosan
(CMC) and rigid rodlike dialdehyde-modified cellulose nanocrystal (DACNCs)
(Fig. 5). The abundant amine groups from CMC can react with aldehyde groups
from DACNC to form dynamic and reversible Schiff base linkages, which can be
readily broken and reformed, allowing the hydrogels to self-heal rapidly. Taking
advantage of the improved reinforcing effect from rigid DACNCs and their restriction for the “soft” CMC chains’ motion via reversible covalent bonds, the hydrogels
exhibit high self-healing efficiency, good mechanical strength, injectability, and a
high equilibrium swelling ratio of 350% while maintaining the cross-linked network
integrity. Besides, the hydrogel’s mechanical strength and self-healing efficiency
could be tailored by adjusting the substitution degree of aldehyde of the modified
cellulose nanocrystal and the molar ratio (MR) of the amine to aldehyde. With these
considerations in mind, this group stepped forward and integrated the
CMC/DACNC reversible network into covalent cross-linked polyacrylamide
(PAAm), forming a novel double network (DN) hydrogel with self-recoverable
and self-healable properties in favor of reforming the structure of hydrogel after
deformation and restoring their functions [119]. Specifically, reversible Schiff base
linkages between the CMC and DACNC in the first network serve as the sacrificial
bonds to dissipate energy, endowing the good recovery and fatigue resistance. At the
same time, the dynamic Schiff base cross-linked networks are responsible for the
self-healable functionalities at ambient temperature without any external stimuli.
Despite self-healing polysaccharide hydrogel based on Schiff base has been
reported to be an attractive method [114, 121–123], most studies transforming
adjacent hydroxyl groups into a pair of aldehyde groups using the sodium periodate
may lead to the severe degradation of polysaccharide polymer and struggle to control
complicated synthesis process. Hence, Mao’s group [120] designed a self-healing
cellulose hydrogel derived from two natural polymers of cellulose and chitosan
based on dynamic covalent enamine bonds. In their work, cellulose acetoacetate
(CAA) was successfully synthesized by transesterification of hydroxyl with tertbutyl acetoacetate (t-BAA) in an ionic liquid 1-allyl-3-methylimidazoliumchloride
(AMIMCl). The CAA showed tunable degree of substitution (DS ¼ 0.34–1.35) and
high water solubility with a DS about 0.58–1.11. By just simply mixing CAA and
chitosan aqueous solutions, a self-healing hydrogel can be quickly formed by the
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
331
3.1.1 Imine Bonds (Schiff Base)
Imine bonds, commonly termed Schiff base, are considered as the comparably strong
reversible covalent bonds, which feature exchange with only very few side reactions
through a dynamic imine formation between aldehyde groups and amino groups
[112]. Intriguingly, imine bonds can automatically occur cleavage and regeneration
in the hydrogel network without any external stimuli, which are recognized as an
attractive strategy to create self-healing cellulose-based hydrogels [113–120].
For example, Huang et al. [118] developed a typical self-healing hydrogel from
naturally occurring polymers combining water-soluble carboxymethyl chitosan
(CMC) and rigid rodlike dialdehyde-modified cellulose nanocrystal (DACNCs)
(Fig. 5). The abundant amine groups from CMC can react with aldehyde groups
from DACNC to form dynamic and reversible Schiff base linkages, which can be
readily broken and reformed, allowing the hydrogels to self-heal rapidly. Taking
advantage of the improved reinforcing effect from rigid DACNCs and their restriction for the “soft” CMC chains’ motion via reversible covalent bonds, the hydrogels
exhibit high self-healing efficiency, good mechanical strength, injectability, and a
high equilibrium swelling ratio of 350% while maintaining the cross-linked network
integrity. Besides, the hydrogel’s mechanical strength and self-healing efficiency
could be tailored by adjusting the substitution degree of aldehyde of the modified
cellulose nanocrystal and the molar ratio (MR) of the amine to aldehyde. With these
considerations in mind, this group stepped forward and integrated the
CMC/DACNC reversible network into covalent cross-linked polyacrylamide
(PAAm), forming a novel double network (DN) hydrogel with self-recoverable
and self-healable properties in favor of reforming the structure of hydrogel after
deformation and restoring their functions [119]. Specifically, reversible Schiff base
linkages between the CMC and DACNC in the first network serve as the sacrificial
bonds to dissipate energy, endowing the good recovery and fatigue resistance. At the
same time, the dynamic Schiff base cross-linked networks are responsible for the
self-healable functionalities at ambient temperature without any external stimuli.
Despite self-healing polysaccharide hydrogel based on Schiff base has been
reported to be an attractive method [114, 121–123], most studies transforming
adjacent hydroxyl groups into a pair of aldehyde groups using the sodium periodate
may lead to the severe degradation of polysaccharide polymer and struggle to control
complicated synthesis process. Hence, Mao’s group [120] designed a self-healing
cellulose hydrogel derived from two natural polymers of cellulose and chitosan
based on dynamic covalent enamine bonds. In their work, cellulose acetoacetate
(CAA) was successfully synthesized by transesterification of hydroxyl with tertbutyl acetoacetate (t-BAA) in an ionic liquid 1-allyl-3-methylimidazoliumchloride
(AMIMCl). The CAA showed tunable degree of substitution (DS ¼ 0.34–1.35) and
high water solubility with a DS about 0.58–1.11. By just simply mixing CAA and
chitosan aqueous solutions, a self-healing hydrogel can be quickly formed by the
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
331