Chemistry Reference
In-Depth Information
copolymers like random, block, or alternative with respect to main polymeric chain.
The properties of copolymer hydrogels depend upon the varied combination of
polymeric units and respective arrangements of these polymeric networks. Poly-
saccharide copolymer hydrogels may involve two different polysaccharide units or
combination of a polysaccharide with synthetic polymer. Blend hydrogels based
upon carboxymethylcellulose and carboxymethyl chitosan were reported for metal
absorption of Pb and Au [
15
,
16
].
7.2.2.3
Interpenetrating Polymer Network Hydrogels
These hydrogels involve two independent cross-linked networks. These cross-
linked networks intermesh into each other in the presence of cross-linker. One
polymeric network swells in the network of another polymer results in the forma-
tion of interpenetrating polymeric network. These are called semi-IPN when one of
the components is cross-linked, while the other component remains uncross-linked.
The properties of these interpenetrating polymer network hydrogels (IPNs) depend
upon relative hydrophilicity and hydrophobicity of two polymeric networks. IPN
hydrogels of alginate and hydrophobically modified ethyl hydroxyl ether cellulose
have been studied for gelation behavior [
17
]. pH and temperature-sensitive semi-
interpenetrating polymer network hydrogel based on linear carboxymethyl chitosan
and poly(
N
-iso-polyacrylamide) cross-linked with organic clay were reported [
18
].
IPN hydrogel based on chitosan and
N
-vinylpyrrolidone was prepared with
photopolymerization technique [
19
].
7.2.3 On the Basis of Ionic Charge
On the basis of ionic charge, hydrogels are divided into three categories: neutral,
ionic, and ampholytic hydrogels.
7.2.3.1 Neutral Hydrogels
These hydrogels have neutral monomeric units which cross-link to form three-
dimensional networks. These nonionic hydrogels may be homopolymeric or copol-
ymeric in nature. The copolymeric polysaccharide hydrogels with monomers
like acrylamide, hydroxyalkyl methacrylates, and
N
-vinyl pyrrolidone result in
neutral hydrogels. These hydrogels are temperature sensitive resulting in
swelling-deswelling behavior with change in temperature. Swelling behavior
depends upon the equilibrium between sub-chain stretching and osmotic pressure
of solvent. Neutral polysaccharide hydrogels mainly involve covalent cross-
linking. These hydrogels have permanent polymer networks as linkages formed
are irreversible. This type of linkage allows absorption of water without dissolution
and thus allows drug release by diffusion [
20
].
