Thesis (M.Sc.)--Chulalongkorn University, 1996

Native cassava starch was chemically modified into starch graft poly(acrylic acid) under a grafting copolymerization of acrylic acid onto the polysaccharide backbone via a hydrogen peroxide-ascorbic acid initiation method. Prior to the grafting reaction, starch was gelatinized at 80oc for 1 h. Reaction variables of importance included concentrations of acrylic acid, hydrogen peroxide, ascorbic acid, and starch; polymerization temperature; polymerization time; and the addition rate of acrylic acid-hydrogen peroxide mixture. The poly(acrylic acid) grafted starch was subsequently saponified with a 25% aqueous solution of NaOH at room temperature to convert carboxylic groups into carboxylate groups. The copolymers were characterized by infrared spectroscopy and nuclear magnetic resonance spectroscopy. GPC was used as a tool to determine the average molecular weights of the starch and grafted poly(acrylic acid), which had been hydrolyzed by glacial acetic acid and perchloric acid. Viscosity measurements of the graft copolymers were carried out at various concentrations(6, 8, 10, 12% w/w) of the thickeners. The high molecular weights of the hydrolyzed(Na salt) cassava starch-acrylic acid copolymer dissolve better in distilled water resulting from the salt and polarity of the molecules. The distribution of molecular weights is in the ranges of 9,599-77,459 and 101,246-651,967 for the grafted poly(acrylic acid) chains and 190,302, 1,005 for the acid hydrolyzed starch. Water absorption of the new thickener was carried out in distilled water. Discussion of the effects of the parameters on the reactions and water absorption was also given in this research. The viscosity of thickener was also studied as a function of concentration (w/w). It is evident that the concentrations of the thickener alter the viscosity and their rheological behavior known as shear thinning or pseudoplasticity.