Influence of the cross-linking agent on the gel structure of starch derivatives

Hydrogels were synthesized through cross-linking of carboxymethyl starch (CMS; Degree of Substitution DS = 0.45) using polyfunctional carboxylic acids (malic, tartaric, citric, malonic, succinic, glutaric and adipic acid). The syntheses used a cross-linking agent ratio (ratio of the number of cross-linking agent molecules to the number of monomer units constituting the polymer) of F-Z = 0.05. After cross-linking the gels were dried, ground and then hydrogels of a polymer concentration of 4 mass-% were produced. These CMS-hydrogels were then theologically characterized using dynamic oscillatory measurements. From measurements of the plateau region storage modulus Gb, the network parameters molar mass between two entanglement points M-e (M-e ranging from 9.318 (citric acid) to 281.397 g/mol (tartaric acid)), the cross-link density v(e) and the distance between two entanglement points xi were calculated. Using carboxylic acids without other functional groups, a maximum in gel sturdiness is found at a spacer length of two CH2-groups. The evaluation of the loss factor tan delta for the CMS-hydrogels showed that values of tan delta = 0.2 varied only slightly with the frequency omega. Flow curves showed a pseudoplastic flow behavior for all CMS-hydrogels (the shear viscosity eta declining over five decades in the range of the shear rate gamma of 10(-3) to 10(3) s(-1)) The different polyfunctional carboxylic acids have a strong influence on the sturdiness of the synthesized CMS-hydrogels.