Synthesis, characterisation and properties of α,β-poly(N-2-hydroxyethyl)-dl-aspartamide-graft-maleic anhydride precursors and their stimuli-responsive hydrogels

A family of poly(amino acid)-maleic anhydride hydrogels were designed and synthesized. Water soluble polymeric precursors were prepared by partially substituting the hydroxyl groups of the a,b-poly(N-2- hydroxyethyl)-DL-aspartamide backbone with maleic anhydride, so as to provide double bonds for crosslinking and carboxylic acid groups for pH and electric field responsiveness. Reaction conditions (reactive mixture composition and catalysis) were systematically varied in order to obtain PHEA–MA precursors with different and reliable graft-maleic anhydride levels. PHEA–MA precursors were characterised by titration, Nuclear Magnetic Resonance (1H NMR), Fourier-Transformed Infrared Spectroscopy (FTIR) and Size Exclusion Chromatography (SEC) for structural and molecular determination. Aqueous solutions of selected PHEA–MA precursors were subjected to gamma-irradiation at different irradiation doses and polymer to water concentrations in order to induce chemical crosslinking without the addition of crosslinking agents. The yield of crosslinking reactions was evaluated by solubility tests as well as the effect of ammonium persulphate, as assistant radical initiator for gamma crosslinking. Selected hydrogels were tested through swelling measurements to prove their pH and electric field responsiveness. Structural features of the different variants produced were related to the swelling behaviour.