Chemistry Reference
In-Depth Information
necessary in order to obtain high swelling capacities under pressure. The percentage
absorption capacity of the hydrogels of copolymers of mixtures of methacrylic acid
and acrylamide onto allyl-modified maize starch was determined with distilled
water and 0.9 % NaCl solution [
26
]. The highest percentage of absorption,
6,500 %, was achieved for the developed hydrogel containing allyl starch and
acrylic monomer in a 1.7:1 w/w ratio and acrylic monomer, namely, methacrylic
acid and acrylamide in a 3.2:1 w/w ratio.
The swelling of hydrogel depends on the carboxylic group content of the
hydrogel product and ionic strength of the absorbing medium. Addition of some
salts to the polymer solution leads to network contraction [
67
]. Because the
repulsive carboxylate group on the polymeric chain was shielded by the bound
cation, the osmotic pressure difference between the gel network and the external
solution decreased. Therefore, the polyelectrolyte component in the gel cannot
imbibe as much salt water as pure water. The swelling capacity of the copolymer
gel is also found to depend on the pH. The water absorbency was low in a strongly
acidic region, because the carboxylate side chains of the potassium acrylate in the
gel became the carboxylic group, thus decreasing the charge density of anions on
the network. However, the water absorbency increased with increase in pH value
when its value was lower than pH 8.0. The reason is that the anions of base-
hydrolysed acrylamide and the carboxylate group inside the network will increase
with an increase of the pH. The water absorbency decreased with a further increase
of pH value over 8.0 [
67
]. This is primarily due to the decrease of the osmotic
pressure difference between the gel and external solution.
To develop starch-based hydrogels with greater starch content than synthetic
components, synthesis of allyl starch and consequent grafting of the allyl starch
were reported [
26
]. The highest percentage absorption capacity obtained was
6,500 % for the sample which contains allyl starch and acrylic monomer in the
ratio 1.7:1 w/w and methacrylic acid and acrylamide in the ratio 3.2:1 w/w.
The swelling ratio (SR) of starch-g-PVA hydrogel increased with the increase of
the molecular weight of grafted PVA [
62
]. The SR of starch-g-PVA hydrogel
slightly decreased with increasing the number of freeze/thaw cycles, due to more
physical cross-linking forming as a result of PVA crystallisation. The water uptake
and moisture retainment values of starch-graft-copolymers were investigated by
ΒΈ elik [
33
].
The swelling and diffusion characteristics of semi-interpenetrating polymer
network (IPN) hydrogels composed of starch and random copolymer of poly
(acrylamide-
co
-sodium methacrylate) were evaluated for different semi-IPN
hydrogels prepared under various formulations [
28
]. The hydrogel networks density
improved enormously as the crosslinker (methylene bis-acrylamide) concentration
increased. The equilibrium swelling ratio (Seq) increased very slightly from 58.21
to 58.70 g/g, when the starch content increased from 0.25 to 0.50 g, but consider-
able increase in Seq value (68.23 g/g) was noticed when the starch content was
0.75 g. The percentage of hydrophilic character in graft copolymer increases with
increasing chain of pendant poly(
N
-vinyl formamide) onto pregelled starch, thereby
increasing swelling capability of graft copolymer [
42
]. The swelling of starch-g-
