2.2.2 Chemical Cross-Linking
In order to achieve the stable structure and effective swelling of cellulose-based
hydrogels, the chemical cross-linkers are usually employed to anchor the covalently
cross-linked hydrophilic cellulose network. According to the mechanism of crosslinking reactions, chemical cross-linkers for cellulose can be classified into esterifying agents and etherifying agents. Esterifying agents include carboxylic acids and
carboxylic anhydrides, i.e., citric acid (CA) [86], 1,2,3,4-butanetetracarboxylic
dianhydride (BTCA) [87], succinic anhydride (SA) [88], and etherifying agents
include organochlorine, epoxide, and vinyl compounds, i.e., epichlorohydrin
(ECH) [89], ethylene glycol diglycidyl ether (EGDE) [90], and divinyl sulfone
(DVS) [91]. For instance, a superabsorbent cellulose hydrogel based on the crosslinking CMC and HEC by divinylsulfone (DVS) was reported by Rodrı́ guez et al.
[90]. The obtained hydrogels displayed high sensitivity in sorption capacity to
variations of the ionic strength and pH of the external solution. Besides, Demitri
et al. [86] demonstrated the HEC/CMC hydrogels could be also fabricated with citric
acid as a cross-linker, providing a green method to design cellulose hydrogels in
contrast to the former chemical reagents with toxicity and high cost.
Irradiation is a useful method for the formation of covalent bonding between
polymer chains. This method possesses the advantages of the high purity of the
hydrogel product without use of toxic cross-linkers, thus enlarging the applications
in food and pharmaceutical industries. The concentrated aqueous solutions of
cellulose derivatives, including CMC, HPC, and MC, can be cross-linked under
γ-ray irradiation to prepare cellulose-based hydrogels. However, only a low gel
fraction (17–30%) could be obtained by γ-ray irradiation at a dose of 20 kGy from
20 wt% biopolymer solutions (such as cellulose/IL/water) with the assistance of
generated hydroxyl radicals [92, 93]. Electron beam (EB) irradiation in vacuum
seems to be able to increase the gel fraction up to 90–95%, and the gel fraction
increased with the radiation dose. It should be noted that the presence of air during
the irradiation diminishes the maximum gel fraction, and the ratio of cross-linking is
the lowest for highly concentrated solutions irradiated by an EB. EB irradiation can
be utilized to prepare hydrogels with cellulose derivatives and functional monomers.
For example, Ibrahim et al. developed a superabsorbent hydrogel via radiation crosslinking of CMC and acrylamide (AAm) monomers with an EB accelerator [94]. A
novel cellulose-based hydrogel by inserting acrylic onto cellulose backbone has
been heterogeneously synthesized under radical copolymerizing in the presence of
N,N-dimethylacrylamide (MBA) [95].
2.3 Cellulose-Based Composite Hydrogels
Cellulose-based composite hydrogels are made by mixing natural biopolymer,
synthetic polymers, or inorganics with cellulose or its derivatives to achieve a new
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
327
In order to achieve the stable structure and effective swelling of cellulose-based
hydrogels, the chemical cross-linkers are usually employed to anchor the covalently
cross-linked hydrophilic cellulose network. According to the mechanism of crosslinking reactions, chemical cross-linkers for cellulose can be classified into esterifying agents and etherifying agents. Esterifying agents include carboxylic acids and
carboxylic anhydrides, i.e., citric acid (CA) [86], 1,2,3,4-butanetetracarboxylic
dianhydride (BTCA) [87], succinic anhydride (SA) [88], and etherifying agents
include organochlorine, epoxide, and vinyl compounds, i.e., epichlorohydrin
(ECH) [89], ethylene glycol diglycidyl ether (EGDE) [90], and divinyl sulfone
(DVS) [91]. For instance, a superabsorbent cellulose hydrogel based on the crosslinking CMC and HEC by divinylsulfone (DVS) was reported by Rodrı́ guez et al.
[90]. The obtained hydrogels displayed high sensitivity in sorption capacity to
variations of the ionic strength and pH of the external solution. Besides, Demitri
et al. [86] demonstrated the HEC/CMC hydrogels could be also fabricated with citric
acid as a cross-linker, providing a green method to design cellulose hydrogels in
contrast to the former chemical reagents with toxicity and high cost.
Irradiation is a useful method for the formation of covalent bonding between
polymer chains. This method possesses the advantages of the high purity of the
hydrogel product without use of toxic cross-linkers, thus enlarging the applications
in food and pharmaceutical industries. The concentrated aqueous solutions of
cellulose derivatives, including CMC, HPC, and MC, can be cross-linked under
γ-ray irradiation to prepare cellulose-based hydrogels. However, only a low gel
fraction (17–30%) could be obtained by γ-ray irradiation at a dose of 20 kGy from
20 wt% biopolymer solutions (such as cellulose/IL/water) with the assistance of
generated hydroxyl radicals [92, 93]. Electron beam (EB) irradiation in vacuum
seems to be able to increase the gel fraction up to 90–95%, and the gel fraction
increased with the radiation dose. It should be noted that the presence of air during
the irradiation diminishes the maximum gel fraction, and the ratio of cross-linking is
the lowest for highly concentrated solutions irradiated by an EB. EB irradiation can
be utilized to prepare hydrogels with cellulose derivatives and functional monomers.
For example, Ibrahim et al. developed a superabsorbent hydrogel via radiation crosslinking of CMC and acrylamide (AAm) monomers with an EB accelerator [94]. A
novel cellulose-based hydrogel by inserting acrylic onto cellulose backbone has
been heterogeneously synthesized under radical copolymerizing in the presence of
N,N-dimethylacrylamide (MBA) [95].
2.3 Cellulose-Based Composite Hydrogels
Cellulose-based composite hydrogels are made by mixing natural biopolymer,
synthetic polymers, or inorganics with cellulose or its derivatives to achieve a new
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
327