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Influence of pH on Properties of Modified Waxy Corn Starch with Acrylic Monomers
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1  University of Agriculture in Krakow, Faculty of Food Technology, ul. Balicka 122, 30-149 Kraków, Poland

Abstract: In our research the influence of pH on the properties of crosslinked waxy corn starch modified with acrylic monomers has been studied. These systems are characterized with the ability to absorb and hold a large quantity of water and aqueous solutions. However, due to the presence of the starch granules in their structure they are easily biodegradable. In our study, a number of cross-linked hydrogels based on waxy maize starch and acrylic acid derivatives were synthesized. Acrylic acid used in reactions was partially neutralized with NaOH. N,N\'-methylenebisacrylamide was used as the cross-linking agent. A series of polymers with different content of ionized carboxyl groups were obtained. These polymers were used for further study of their properties, in particular swelling properties and concentration of carboxyl and carboxylate groups. Presence of those hydrophilic groups is largely responsible for the high affinity of the polymer to water. A significant increase in the gelation time with increasing of sodium acrylate content was observed. As expected, the increase in content of carboxylate groups with an increase of neutralization degree of acrylic acid used in the synthesis of the polymer was observed, while content of unionized carboxyl groups decreased. A series of experiments were carried out to determine the effect of pH on the absorption of hydrogels. It should be noted that the amount of absorbed water is generally affected by the ionic strength of the solution, therefore, to compare the individual results of swelling in the range of pH 5.0-9.0 experiments were performed in buffers solution with the same ionic strength. High rate of absorption of aqueous solutions was observed. Approximately 60-70% of the maximum value is attained during the first 10 minutes. It was found that when pH of solution increases, amount of water absorbed per gram of hydrogel increases. Hydrogel composition also has a significant impact on the swelling properties of hydrogel. A minimum of water absorption was observed for polymers in which the ratio of acrylic acid to sodium acrylate was 1:1.
Keywords: hydrogel, starch, swelling properties