2 Cellulose-Based Hydrogels
2.1 Hydrogels Prepared Directly from Native Cellulose
Cellulose hydrogels can be prepared directly from a native cellulose solution through
physical cross-linking due to the presence of numerous hydroxyl groups, which can
form hydrogen bonding linked network easily. However, cellulose displays relatively poor solubility characteristic due to its highly extended inter- and intramolecular hydrogen bonds between cellulose chains [57]; thus the appropriate solvents are
required to dissolve native cellulose. To date, only a few solvent systems have been
developed for the dissolution of native cellulose, such as conventional polar solvent
systems [58], ionic liquids (ILs) [59–63], and alkali or alkali/(thio)urea aqueous
systems [64–68], providing great opportunities for the preparation of cellulose
hydrogels by a straightforward strategy. Bacterial cellulose (BC) is also a good
candidate for the fabrication of cellulose-based hydrogels, since certain bacterial
species possesses the ability to create pure cellulose hydrogel.
2.1.1 Polar Solvent Systems
A series of polar solvent systems have allowed the exploration of hydrogel production from native cellulose, including N-methylmorpholine oxide (NMMO) [69],
lithiumchloride/dimethylacetamide (LiCl/DMAc) [70, 71], paraformaldehyde/
dimethylsulfoxide (PF/DMSO), triethylammonium chloride/dimethylsulfoxide
(TEAC/DMSO) [72], tetrabutylammonium fluoride/dimethylsulfoxide (TBAF/
DMSO) [73], lithium chloride/N-methyl-2-pyrrolidone (LiCl/NMP) [58], and calcium chloride dihydrate/methanol (CaCl 2 •2H 2 O/MeOH) [31]. Although the insolubility of cellulose can be alleviated using these polar solvents, the toxicity or
corrosivity of these organic components can inhibit batch production and potential
applications of the resulting gels.
2.1.2 Ionic Liquids (ILs) Systems
Ionic liquids (ILs) are a group of new organic salts that exist as liquids at a relatively
low temperature (<100
C), which possess many attractive properties, such as
chemical and thermal stability, non-flammability, and immeasurably low vapor
pressure. Cellulose can be dissolved in some hydrophilic ILs that consist of an
imidazolium, pyridinium, ammonium, or phosphonium cation paired with a strongly
basic, hydrogen bond accepting anion, such as 1-butyl-3-methylimidazolium chloride (BMIMCl) and 1-allyl-3-methylimidazolium chloride (AMIMCl) [62, 63]. The
regenerated cellulose can be obtained by subsequent addition of water, ethanol, or
acetone, which show almost the same degree of polymerization and polydispersity as
the initial one. However, the morphology of generated cellulose is significantly
324
C. Shao and J. Yang
2.1 Hydrogels Prepared Directly from Native Cellulose
Cellulose hydrogels can be prepared directly from a native cellulose solution through
physical cross-linking due to the presence of numerous hydroxyl groups, which can
form hydrogen bonding linked network easily. However, cellulose displays relatively poor solubility characteristic due to its highly extended inter- and intramolecular hydrogen bonds between cellulose chains [57]; thus the appropriate solvents are
required to dissolve native cellulose. To date, only a few solvent systems have been
developed for the dissolution of native cellulose, such as conventional polar solvent
systems [58], ionic liquids (ILs) [59–63], and alkali or alkali/(thio)urea aqueous
systems [64–68], providing great opportunities for the preparation of cellulose
hydrogels by a straightforward strategy. Bacterial cellulose (BC) is also a good
candidate for the fabrication of cellulose-based hydrogels, since certain bacterial
species possesses the ability to create pure cellulose hydrogel.
2.1.1 Polar Solvent Systems
A series of polar solvent systems have allowed the exploration of hydrogel production from native cellulose, including N-methylmorpholine oxide (NMMO) [69],
lithiumchloride/dimethylacetamide (LiCl/DMAc) [70, 71], paraformaldehyde/
dimethylsulfoxide (PF/DMSO), triethylammonium chloride/dimethylsulfoxide
(TEAC/DMSO) [72], tetrabutylammonium fluoride/dimethylsulfoxide (TBAF/
DMSO) [73], lithium chloride/N-methyl-2-pyrrolidone (LiCl/NMP) [58], and calcium chloride dihydrate/methanol (CaCl 2 •2H 2 O/MeOH) [31]. Although the insolubility of cellulose can be alleviated using these polar solvents, the toxicity or
corrosivity of these organic components can inhibit batch production and potential
applications of the resulting gels.
2.1.2 Ionic Liquids (ILs) Systems
Ionic liquids (ILs) are a group of new organic salts that exist as liquids at a relatively
low temperature (<100
C), which possess many attractive properties, such as
chemical and thermal stability, non-flammability, and immeasurably low vapor
pressure. Cellulose can be dissolved in some hydrophilic ILs that consist of an
imidazolium, pyridinium, ammonium, or phosphonium cation paired with a strongly
basic, hydrogen bond accepting anion, such as 1-butyl-3-methylimidazolium chloride (BMIMCl) and 1-allyl-3-methylimidazolium chloride (AMIMCl) [62, 63]. The
regenerated cellulose can be obtained by subsequent addition of water, ethanol, or
acetone, which show almost the same degree of polymerization and polydispersity as
the initial one. However, the morphology of generated cellulose is significantly
324
C. Shao and J. Yang