Influence of the Cross-linking Agent on the Gel Structure of Starch Derivatives

Abstract

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 FZ = 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 rheologically characterized using dynamic oscillatory measurements. From measurements of the plateau region storage modulus G'P, the network parameters molar mass between two entanglement points Me (Me ranging from 9.318 (citric acid) to 281.397 g/mol (tartaric acid)), the cross-link density νe and the distance between two entanglement points ξ 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 δ for the CMS-hydrogels showed that values of tan δ = 0.2 varied only slightly with the frequency ω. Flow curves showed a pseudopIastic flow behavior for all CMS-hydrogels (the shear viscosity η declining over five decades in the range of the shear rate γ of 10−3 to 103 s−1) The different polyfunctional carboxylic acids have a strong influence on the sturdiness of the synthesized CMS-hydrogels.