Chemistry Reference
In-Depth Information
give graft copolymer. The following reaction scheme was proposed on the basis of
experimental results [
42
] (Scheme
3.9
).
The optimum conditions for grafting of
N
-vinyl formamide onto pregelled starch
were
N
-vinyl formamide 50 % based on weight of substrate, cyclohexanone
15 mmol/L, bromate ion, 30 mmol/L, liquor ratio 10, pH 6, time 120 min and
temperature 40
C[
42
].
3.2.6
Initiation by Irradiation
Employing high-energy
-radiation is an efficient method for initiating radical graft
polymerisation onto polysaccharides [
43
]. It was reported that although the radia-
tion-based grafting is cleaner and more efficient in this regard than chemical
initiation methods, they are harder to handle under technical conditions. Hence,
research reports available for the synthesis of graft copolymers using radiation
initiation system are few.
Gamma-rays were used as initiator for the graft copolymerisation of acrylamide
and/or acrylic acid onto cassava starch and the effect of various parameters such as
monomer-starch ratio, total dose of gamma rays (kGy), dose rate (kGy/h) and
acrylamide-acrylic acid ratio and the addition of nitric acid and maleic acid as the
additives were studied [
44
]. The porosity of the saponified starch-graft-copolymers
prepared by the acrylamide/acrylic acid ratios of 70:30 and 50:50 was much higher
than the porosity of copolymers in terms of fine networks.
Cassava starch-g-poly(acrylic acid) copolymers were prepared by a simulta-
neous irradiation technique with
γ
-ray irradiation from a
60
Co source [
45
]. The
graft copolymer was prepared using
γ
-ray irradiation at a dose rate of 2 kGy/h to a
total dose of 10 kGy, and a ratio of acrylic acid to cassava starch of 1:1. The
resulting graft copolymer contained 2.7 % homopolymer, 24.9 % add-on, 40 %
conversion, 90 % grafting efficiency and 33.2 % grafting ratio. Graft
copolymerisation of acrylonitrile onto maize starch by a simultaneous irradiation
technique using gamma-rays as the initiator was studied with regard to the
monomer-to-maize starch ratio and total dose (kGy) [
46
].
Potato starch-graft-poly(acrylonitrile) could be efficiently synthesised using
small concentration of ammonium peroxydisulphate (0.0014 M) in aqueous
medium under microwave irradiation [
47
]. Maximum grafting ratio and efficiency
of 225 % and 98 %, respectively, were obtained at 0.17 M acrylonitrile, 0.0014 M
(NH
4
)
2
S
2
O
8
, 0.1 g potato starch and 70 s microwave exposure at a power of
1,200 W keeping total reaction volume fixed at 25 mL. The temperature of the
reaction mixture just after the microwave exposure was 98
C.
Microwaves are reported to have the special effect of lowering of Gibbs energy
of activation of the reactions [
48
]. Along with this effect, the dielectric heating of
the reaction medium causes quick decomposition of peroxydisulphate into sulphate
ion radicals as shown below (Schemes
3.10
and
3.11
).
The primary radicals SO
4
-•
and OH
•
initiate the vinyl polymerisation as the vinyl
polymerisation is reported to be faster
γ
than the H abstraction from the
