Synthesis and characterization of cross-linked carboxymethyl potato starch ether gels

Abstract

Hydrogels were synthesized by cross-linking of potato starch (PS) with dichloroacetic acid (DCA) in the presence of monochloroacetic acid (MCA) to form etherified carboxymethyl starch (CMS) gels, to be used for ultrasonic medical examinations. By etherification cross-linked CMS-hydrogels can be produced, that are stable in contrast to the in the long-term unstable esterified gels, that were presented in the last paper. The rheological benchmarks for the CMS-hydrogels were set in comparison with synthetic ultrasonic gels. Gels with potato starch contents in the reaction batch ranging from 12.5% to 20% showed the best compliance with the benchmark parameters. The DS values of these CMS-hydrogels vary from 0.42 to 0.49, increasing with decreasing amount of starch in the reaction mixture. The free swelling capacities (FSC) vary between 77 g/g for the 12.5% PS-gel and 34 g/g for 20% PS-gel, the turbidities of the samples being in the range from 14.5 NTU (Nephelometric Turbidity Units) (12.5% PS) up to 20.5 NTU (20% PS). The variation of the PS fraction in the reaction mixture showed that with an increased amount of PS in the reaction batch the number of cross-links of the CMS-gels increases, too. At a higher number of cross-links the swelling capacity is reduced and the concentration needed to form stable hydrogels is greatly increased. Thus a hydrogel of a polymer concentration of 5 mass-% from a 12.5% PS batch was produced, that showed the best accordance with the rheological benchmark parameters such as gelatinization time, visco-elastic and pseudoplastic properties and long-term stability. The ultrasonic pictures taken with this CMS-gel were not different from those taken with the synthetic gels. This hydrogel was then subjected to long-term-stability measurements performed over one year and to rheological temperature cycle tests. The tests showed that the long-term stability of the gels is sufficient for their use as ultrasonic gel.