Concentrated regimes of xanthan-based hydrogels crosslinked with multifunctional crosslinkers

Abstract

Samples were synthesized via solution mixing method with a concentration of 4wt.% of Xanthan and 0, 1, 2 or, 3 wt.% of bi-functional and tetra-functional crosslinkers, i.e. citric acid and Borax, respectively. The rheological studies at two different temperatures showed an unanticipated increase in the storage modulus values with increasing temperature which is due to the conformational change from ordered helix to disordered coils in Xanthan molecules. Thus, it is hypothesized that the concentration of Xanthan and the crosslinking density in the system play crucial roles in the conformational change of the molecules. Polarized optical microscopy (POM), FTIR, and SEM results showed that in the crosslinked samples conformational change is prevented stronger than the native sample. POM images revealed that higher crosslinking density in Xanthan - Borax hydrogels causes bigger regions of crystalline phase compared to Xanthan - citric acid hydrogels, pointing at the effect of the tetra-functional crosslinking agent. Also, SEM imaging showed that higher crosslinks density has led to smaller pore sizes in the freeze-dried samples. FTIR analysis proved that the stronger effect of Borax is due to higher crosslinking density in Xanthan-Borax samples. Finally, a mechanism for crosslinking of Xanthan with Borax was suggested.