Dielectric properties of chitosan membranes cross-linked with glutaraldehyde or genipin

This study investigates the impact of covalent cross-linking using glutaraldehyde (GA) and the biobased alternative genipin (GP) on the dielectric and conductive properties of chitosan membranes. For protonation, the solvent-casted membranes were immersed in a 1.0 M sulfuric acid solution (S). The resulting property changes were assessed using Fourier-Transform Infrared (FTIR) spectroscopy, water uptake studies, Differential Scanning Calorimetry (DSC), and Dielectric Thermal Analysis (DETA). The genipin-cross-linked membrane demonstrated higher water uptake than the glutaraldehyde-cross-linked one, attributed to less effective cross-linking and greater free volume between polymer chains. The membranes immersed in the sulfuric acid solution exhibited reduced water uptake and swelling due to a denser structure formed by short ionic bridges. All the chitosan-based materials showed dielectric spectra containing the same relaxation mechanisms (beta-, beta wet-, alpha-relaxation, and MWS polarisation). Covalent cross-linking and sulfuric acid immersion increased the activation energy of the beta-relaxation, due to the formation of strong bonds that limit the mobility of glycosidic linkages. The covalent bonds restrict the segmental dynamics of the chitosan backbone, leading to an increase in the Vogel temperature of the associated alpha-relaxation, particularly with glutaraldehyde. The beta wet-relaxation temperature decreased following sulfuric acid solution treatment, due to greater retention of bound water resulting from the incorporation of charged sulfate groups.