BEHAVIOR OF STARCH BEARING NITROGEN-CONTAINING MOIETIES TOWARDS ACID DEGRADATION

Chemical modification of starch (substrate I) was effected through int roduction of acrylonitrile or acrylamide moieties. The chemical modifi cation reactions were carried out in such a way that each of these moi eties is in the form of a group or a polymer. While cyanoethylated sta rch (substrate II) and carbamoylethylated starch (substrate IV) repres ent the moieties in the form of a group, poly (AN)starch graft copolym er (substrate III) and poly (Aam)-starch graft copolymer (substrate V) represent them in the form of a polymer. The five substrates I-V were subjected to acid treatment (0.5 N and 1 NHCl) at 50 degrees C for di fferent periods of time (15-60 min). Acid degradation was assessed by monitoring carboxyl and carbonyl contents, nitrogen content, apparent viscosity and him-forming properties. The carboxyl content, carbonyl c ontent and apparent viscosity of the polyacrylonitrile and polyacrylam ide starch copolymers as well as cyanoethylated and carbamoylethylated starches differ considerably among themselves as well as when compare d with native starch. The latter seems to undergo hydrolytic/oxidative degradation during the chemical modifications via grafting, carbamoyl ethylation and cyanoethylation. Polyacrylamide starch copolymer acquir es the highest resistance to acid hydrolysis whereas polyacrylonitrile starch copolymer acquires the least. On the other hand, cyanoethylate d starch displays much greater resistance to acid hydrolysis than does carbamoylethylated starch. Meanwhile the resistance of polyacrylamide starch copolymer exceeds that of cyanoethylated starch when the decre ase in nitrogen content of these modified starches after acid hydrolys is is taken as a measure. A different situation is encountered when th e resistance to acid hydrolysis was judged from decrease in apparent v iscosity. This reflects the contribution of the nature of the moiety b eside its form in determining the stability of the modified starch to acid attach. (C) 1998 Elsevier Science Limited. All rights reserved.