"RHEOLOGICAL PROPERTIES OF SALICYL-IMINE-CHITOSAN HYDROGELS: EFFECT OF CROSSLINKING DENSITY "

Abstract

The present paper focuses on the rheological behaviour of a series of hydrogels prepared from chitosan and salicylaldehyde. The unusual crosslinking of chitosan with this monoaldehyde was assessed by 1H NMR spectroscopy, which demonstrated the formation of covalent imine bonds. The hydrogels exhibited a super-porous morphology, evidenced by SEM measurements, and the layered supramolecular structure of the hydrogels was sustained by the birefringence texture of the hydrogels, observed by polarized light microscopy (POM). The hydrogel-like behaviour was confirmed by rheologic measurements for the sample containing the highest salicylaldehyde amount. The dynamic flow properties of salicyl-imine-chitosan hydrogels with different crosslinking degrees (NH2/CHO ratios between 2 and 4) were investigated at temperatures in the range of 20–40 °C. The rheological moduli were determined over a wide range of oscillatory frequencies and the experimental results were presented using master curves. In addition, the thixotropic behaviour of the hydrogels based on chitosan and salicylaldehyde was determined and discussed. The measurements of thixotropy were performed by increasing the shear rate to 400 s-1 in an upward sweep, followed by its decreasing in a downward sweep. It was noticed that the hydrogels’ thixotropy increases with an increasing crosslinking degree. In line with this rheological behaviour, the self-healing ability was tested, and it was proved that the hydrogels were able to reshape after applying deformation stress.