Chemistry Reference
In-Depth Information
extent of grafting was found to increase with total gamma radiation dose [
68
].
Chauhan and co-workers [
69
] carried out the graft copolymerisation of styrene onto
cellulose and studied the effect of some additives on grafting parameters in limited
aqueous medium.
Microwave Radiation-Induced Grafting
Microwave radiation is another important energy source and is emerging as an
effective tool for graft copolymerisation. Microwave radiations have an advantage
of instantaneous in core processing of reactants in homogeneous manner. Microwave
radiations can rapidly transfer the energy into bulk of reaction mixture which results
in the rapid interaction of the material in the reaction mixture [
70
]. Polyacrylamide
was graft copolymerised onto chitosan under microwave radiations with reaction rate
eight times higher than conventional heating [
71
]. Grafting of
-caprolactone [
72
]
and acrylic acid [
73
] onto chitosan has been studied under microwave radiations.
Grafting of hydroxymethylacrylate onto wool fibres [
74
], methyl methacrylate onto
flax fibres [
75
], acrylic acid onto Artemisia seed gum [
76
] and polyacrylamide onto
xanthan [
77
] and guar gum [
78
] has been reported under microwave radiations.
Cassia siamea
seed gum has been found to undergo graft copolymerisation under
microwave radiations without the use of radical initiator [
79
].
Microwave irradiation significantly reduces the use of toxic solvents, as well as
the reaction time for almost all the grafting reactions of interest here, ensuring high
yields, product selectivity and clean product formations. Microwave-synthesised
polysaccharide copolymers exhibit better properties for commercial exploitation
than their conventionally synthesised counterparts [
80
]. A comparison of
advantages of carrying out grafting reactions under microwaves and conventional
methods is explained in Fig.
1.1
.
Radiation-induced grafting can proceed in three different manners: (a) mutual or
direct method, (b) pre-oxidation method and (c) pre-irradiation method. Mutual
method involves the simultaneous irradiation of backbone and monomer to gener-
ate free radicals. The product formed is a mixture of homopolymer and graft
copolymer [
81
].
Pre-irradiation technique involves the exposure of the polymer backbone to
radiations in an inert environment to form free radicals which when treated with
monomers result in the graft copolymerisation. Gamma radiation-induced graft
copolymerisation of methyl methacrylate onto jute fibres was carried out by the
pre-irradiation method in an aqueous medium by using octylphenoxy polyethox-
yethanol as an emulsifier. The different factors that influenced the graft copolymer
reaction process were investigated [
82
]. Pre-oxidation method involves the irradia-
tion of polymer in the presence of oxygen. The peroxides and hydroperoxides are
formed which in subsequent steps initiate the polymerisation of monomer resulting
in the formation of graft copolymers. The rate of formation of peroxide and
hydroperoxide is dependent upon the total dose and which in turn influences the
rate of grafting [
83
].
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