A dynamic chitosan-based self-healing hydrogel with tunable morphology and its application as an isolating agent

Abstract

A biopolymer chitosan based 3D hydrogel framework (ChF) with good transparency and rapid self-healing activity has been successfully synthesized and utilized to get high purity separation of long-lived fission products ¹⁵²Eu (T_1/2 = 13.33 a) and ¹³⁷Cs (T_1/2 = 30.17 a) employing solid liquid extraction (SLX) technique. The unique properties of covalently cross-linked polymeric ChF have been analyzed through FT-IR, solid-state ¹³CP/MAS NMR, rheological measurement, SEM, AFM, TG-DTG, DSC, XRD and swelling study. Rheological analysis reveals that the maximum mechanical strength of a hydrogel is achieved upon synthesis using 1 : 0.43 chitosan to formaldehyde stoichiometry (ChF3). A plateau of storage modulus (G′), over a wide range of angular frequency of ChF reveals its visco elastic nature. On varying the cross linking of the hydrogel network, we found a precise tuning of morphology and properties of ChF. This ChF was utilized to separate ¹⁵²Eu from a mixture of ¹⁵²Eu and ¹³⁷Cs in aqueous HCl medium (pH 5) using SLX technique and it was observed that ChF specifically adsorbs ¹⁵²Eu without any contamination from ¹³⁷Cs. ¹⁵²Eu was back extracted easily using 1 M aqueous HCl from ¹⁵²Eu-adsorped-ChF. The adsorption of ¹⁵²Eu on ChF is influenced by pH, amount of hydrogel, time of contact, temperature, cross-linking density and dosage of γ-radiation. To establish the self-healing property of the gels, rheological analysis through step-strain measurements (strain = 0.1 to 100%) at 25 °C was monitored. The polymeric hydrogel network displays a reversible sol–gel transition for several cycles due to the dynamic equilibrium between the Schiff base linkage and the aldehyde and amine functional groups.