Chitosan Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 77

methacrylate (HEMA) onto chitosan in the presence of redox initiators, in this case TCPB.
Here, the total conversion of HEMA monomer was found to be up to 75%. The resulting
material was found to increase the hydrophilicity and therefore may be used as textile
finishes enhancing the hydrophilicity of synthetic fibers.
A novel redox system, PDC [Cu(III)-chitosan], was employed to initiate the graft copoly-
merization of methyl acrylate onto chitosan in an alkali aqueous solution [262]. In this work,
Cu(III) was employed as an oxidant and chitosan as a reductant in the redox system used to
initiate the grafting reaction. The result showed that there is a high grafting efficiency and
percentage when using PDC as an initiator. Since the activation energy of the reaction
employing Cu(III)-chitosan as an initiator is low, graft copolymerization is carried out at a
mild temperature of 35°C and in an alkali aqueous medium, which makes it superior to
other initiators. Graft copolymerization onto chitosan has also been attempted by using
AIBN. Some vinyl monomers such as acrylonitrile, methyl methacrylate, methyl acrylate,
and vinyl acetate were grafted onto chitosan with AIBN in aqueous acetic acid solutions or
in aqueous suspensions [230]. Here, the grafting percentages were generally low [230].
Fenton's reagent (Fe 2+ /H 2 O 2 ) was also successfully used as a redox initiator for grafting
methyl methacrylate onto chitosan [256]. Although chitosan is an effective flocculating
agent only in acidic media, the derivatives having side-chain carboxyl groups showed
zwitterionic characteristics with high flocculation abilities in both acidic and basic media.
2.11.2 grafting by using radiation
Recently, great effort has been made to graft natural polymers using the radiation method.
Grafting of polystyrene onto chitin and chitosan using 60 Co γ-irradiation at room tempera-
ture was investigated [265]. Grafting yield was controlled by changing the grafting condi-
tions. It was found that grafting yield increased with an increase in adsorbed dose. Singh
and Ray [266] have also reported the radiation grafting of chitosan with N , N ′-
dimethylaminoethylmethacrylate (DMAEMA). Parameters such as solvent composition,
monomer concentration, radiation dose rate, and total dose/time were found to affect the
rate of grafting and homopolymerization. The degree of swelling, crystallinity, and tensile
strength decreased by 51%, 43%, and 37%, respectively, at a 54% graft level of DMAEMA,
whereas modified films showed improved thermal stability.
Yu et al. [268] have reported the graft copolymerization of butyl acrylate onto chitosan by
using γ-irradiation. In this study, grafting percentage was observed to increase when
monomer concentration and total dose were increased or when chitosan concentration and
reaction temperature were decreased. Under lower dose rates, grafting percentage has no
significant change, whereas above 35 Gy/min (dose rate), grafting percentage exhibits a
sharp decrease. Compared with pure chitosan film, the chitosan graft poly(butyl acrylate)
films have enhanced hydrophobic and impact strength.
Similar work has also been reported on grafting poly(hydroxyethyl methacrylate) with
chitosan in the presence of UV light [271]. Here, the sulfite oxidase enzyme was covalently
immobilized onto the matrix of the grafted polymer. After the completion of photo-induced
polymerization reaction, p -benzoquinone (an electron transfer mediator) was coupled onto
the polymer network for activation of the chitosan-poly(hydroxyethyl methacrylate) copo-
lymer. This study demonstrated the feasibility of using chitosan in electrochemical biosen-
sor fabrication.
Singh et al. [296] grafted PAN onto chitosan using the microwave irradiation technique
under homogeneous conditions. They obtained 170% grafting yield within 1.5 min. The
effects of reaction variables such as monomer or chitosan concentration, microwave power,