Chemistry Reference
In-Depth Information
Graft copolymerization is one of the promising techniques for the synthesis of
chemical hydrogels [
40
]. Grafting involves covalent linkages of a monomer onto
polymer backbone. Graft copolymerization in the presence of suitable chemical
reagent or high-energy radiations results in the formation of macro-radicals which
further cross-link to form gel structure [
41
]. The presence of initiators and
radiations activates the polysaccharide backbone chain which leads to infinite
branching and cross-linking [
42
]. The use of different chemical reagents as
initiators has been reported to activate the backbone. Psyllium has been
functionalized with acrylamide in the presence of potassium persulfate (KPS)-
hexamethylenetetramine (HMTA) as an initiator-cross-linker system. After the
initial optimization of different reaction parameters, the resultant hydrogel was
used for the absorption of water from different water-oil emulsions [
43
]. Psyllium
hydrogels have also been synthesized under the influence of gamma radiations in
the presence of hexamethylenetetramine as cross-linker [
44
]. Electosensitive
hydrogels based upon gum ghatti were synthesized by graft copolymerization
with acrylamide using potassium persulfate-ascorbic acid initiator system and
N
,
N
-methylenebisacrylamide as cross-linking agent [
45
]. Thermo- and pH-sensitive
hydrogels have been prepared by graft copolymerization of chitosan and
N
-isopropyl acrylamide. Radiation-induced grafting of acrylamide onto chitosan
resulted in the synthesis of hydrogels which can act as excellent flocculants [
46
].
Carboxymethyl cellulose-based hydrogels have been synthesized using electron
beam irradiated grafting of acrylic acid onto CMC [
47
].
7.4 Characterization
The characterization of hydrogels gave idea about their structural as well as
physicochemical properties. Techniques employed for characterization depend
upon the target application. Different techniques have been developed to study
the properties like swelling behavior, release kinetics, and mechanical properties
[
48
].
Macroscopic studies have been done to understand the properties of hydrogels at
microscopic and nanoscale. SEM is widely utilized to examine the surface mor-
phology of polysaccharide hydrogels. SEM was used to study the interior morphol-
ogy of dextran hydrogels and to determine the pore characteristics [
49
]. SEM
analysis of gel networks differentiated between enzymatically and chemically
synthesized dextran hydrogels on the basis of pore size. The enzymatically
synthesized hydrogels showed more uniform pore size than chemically prepared
hydrogels [
50
]. SEM images have also been utilized to compare acetone-
dehydrated versus air-dried cellulose-based hydrogels. Acetone-dehydrated hydro-
gel showed folding and voids, whereas air-dried cellulose hydrogel showed smooth
and dense surface [
51
]. SEM images provide evidences for the occurrence of
chemical modifications like graft copolymerization onto hydrogel surface. The
graft copolymerization onto glucomannose with acrylic acid was confirmed by
