Linear PEI-based responsive hydrogels: Synthesis and characterization

Two responsive PEI hydrogels (P1.5E and P2E) were synthesized through a click reaction using linear PEI polymers with different cross-linker contents (ethylene glycol diglycidyl ether-EGDE) and avoiding organic solvents. Through C-13 HRMAS NMR and DSC studies the presence of residual epoxy groups and different protonation states of the linear PEI segments in the hydrogel were detected. The swelling and rheological behavior was dependent on the cross-linking degree. The hydrogel with the lower cross-linking degree (P1.5E) presented higher swelling values, but it was less resistant to the deformation. Besides, the swelling capacity and the resistance to deformation were found to be dependent on the interchain repulsions and charge of the amine groups, which endowed the hydrogels with stimuli-responsive behavior to pH or ionic strength changes. For both hydrogels, the swelling capacity and rheological properties were inversely proportional and directly proportional to the pH increase, respectively. Both materials presented adsorption capacity for the azo dye methyl orange (MO) and Cu2+ ions. The interaction of the materials with the pollutants was also dependent on the cross-linking degree, being P1.5E the one that presented higher sorption capacities. Furthermore, the hydrogels did not release toxic molecules, as assessed by lettuce seeds germination experiments.