Green Deep Eutectic Solvents for Functionalizing Chitosan–Dialdehyde Materials with Varied Crosslinker Content

A series of chitosan-based films was obtained by combining the covalent crosslinking of chitosan with dialdehyde starch (DAS) and plasticization using a choline chloride–malonic acid deep eutectic solvent (DES), thereby engineering their structural, mechanical, and surface properties for advanced packaging applications. DAS was synthesized via periodate oxidation of potato starch and characterized by FTIR and quantification of aldehyde groups through acid–base titration, enabling precise control of the –NH2 (chitosan) to –CHO (DAS) molar ratios (40:1, 20:1, 10:1) used for film formation. Chitosan films (neat, DAS-crosslinked, DES-plasticized, and DES-plasticized/DAS-crosslinked) were obtained by solution casting, with constant total chitosan and/or Ch+DES mass across formulations, and subsequently examined in terms of molecular structure, density, mechanical characteristics, micro- and nanoscale morphology, color, wettability, and surface free energy. The most significant changes relevant to potential applications were observed in mechanical properties and surface free energy. The incorporation of DAS and DES into chitosan resulted in a significant reduction in Young’s modulus from 1150 MPa to 130 MPa, accompanied by a significant increase in elongation at break—from 10% to almost 90%. Moreover, it should be noticed that the addition of DAS and DES led to a nearly twofold increase in surface free energy, from 32.5 to 59.9 mJ m−2. While previous studies have predominantly focused on single modifications of chitosan—either covalent crosslinking with dialdehyde starch (DAS) or plasticization with deep eutectic solvents (DES)—this work introduces a pioneering dual-modification strategy that simultaneously integrates both techniques, representing the first systematic investigation of their synergistic effects unattainable through individual approaches.