Swelling behaviour and glass transition in genipin-crosslinked chitosan systems

High-solid chitosan matrices were prepared to investigate the effect of their swelling on structural relaxation and glass transition. Degree of crosslinking in genipin-crosslinked chitosan networks was measured with a ninhydrin assay and a suitable crosslinker concentration was determined for gels used in swelling and thermomechanical analysis. Fourier transform infrared spectroscopy and wide angle X-ray diffraction examined the intermolecular interactions, crystallinity and amorphicity of the biopolymer networks. Swelling characteristics following immersion in water included approximate equilibrium values of the average molecular weight between crosslinks and network mesh size, e.g. 902 g mol^-1 and 110 nm for the preparation with an initial solids content of 60% (w/w) after 80 min of swelling, which were quantified with the modified Flory-Rehner theory. Thermal glass transition temperature was observed in the condensed crosslinked networks (≥ 70% w/w total solids) during differential scanning calorimetry experiments. Time-temperature superposition of rheological measurements, obtained with dynamic oscillation in-shear, generated a master curve describing the viscoelastic behaviour of the system, moving through the rubbery, glass transition region and glassy state. Combined analysis using the modified Arrhenius and William-Landel-Ferry theories determined mechanical glass transition temperatures in the range of -68 to -8 °C for the crosslinked biopolymer at intermediate-solid concentrations (40 to 60% w/w solids). An understanding of the effect of swelling on molecular network characteristics was achieved, which is crucial for greater control in the design of systems for the targeted delivery of bioactive compounds.