Chitosan-g-Copolymers: Synthesis, Properties, and Applications - Polysaccharide Based Graft Copolymers - page 116

Schiff's bases formed with aliphatic aldehydes, O -acetylation proceeded rapidly,
provided that the aldehyde contained three or more C atoms, but the complete
O -acetylation was not achieved owing to the hydrolysis of the Schiff's base [ 57 ].
However Schiff's base derivatives formed with aromatic aldehydes show little
tendency to hydrolyze under the conditions used in the O -acetylation step, and di-
O -acetyl- N -arylidene Ch is readily obtained. Some of the important examples of
modified Ch and their fields of applications are reported in details in the work of
Mourya and Inamdar [ 27 ].
4.4.2.3 Cross-Linking of Chitosan
Gupta and Jabrail [ 58 ] have prepared the glutaraldehyde cross-linked Ch
microspheres by dissolving Ch in acetic acid and, after that, transferring the
solution into methanolic solution of NaOH to form Ch microspheres. The
microspheres were then transferred to a solution of glutaraldehyde for cross-
linking. Coelho et al. [ 59 ], on the other hand, have prepared Ch microspheres by
the phase inversion method. For the preparation of the microspheres, powdered Ch
was dissolved in acetic acid solution to produce a viscous solution. The resulting Ch
solution was then pumped through a nozzle into a precipitation bath consisting of
2.0 mol L 1 NaOH. The neutralization of the acetic acid by the alkali solution
results in the precipitation of Ch with the formation of gel microspheres. The
microspheres were washed with distilled water to neutralize their pH. Cross-linking
of Ch microspheres was achieved using epichlorohydrin as cross-linking agent in
the presence of NaOH solution.
Yisong and coworkers [ 60 ] cast Ch solutions containing glutaraldehyde. The
solvent was allowed to evaporate under infrared light, where cross-linking occurred.
The swelling of the membranes was measured by comparing the surface area before
and after immersion in water at 25 C. The swelling was found to decrease with an
increase in cross-linking agent. Ch microspheres were prepared through cross-
linking of Ch with glutaraldehyde [ 61 ]. Ch is cross-linked with epichlorohydrin,
followed by heating with alkali to give an anion exchange resin [ 62 ].
4.4.2.4 Carboxymethylation of Chitosan
Chemical modification of Ch by introducing carboxymethyl groups to improve some
of its physical properties such as water insolubility (soluble in 1 % acetic acid),
where carboxymethyl chitosan (CMCh) is water soluble and has improved chemi-
cal properties as being able to form salts with heavy metal ions and not only by
chelation through amino or hydroxyl groups as in the case of Ch.
There are two types of CMCh, O -CMCh and O , N -CMCh, as represented in
Fig. 4.3 [ 63 ].