(704a) Mechanical Properties of Ionically and Covalently Crosslinked Alginate Hydrogels (Invited Talk)

Polysaccharide networks are widely used in biomedical and food applications. Polysaccharides are semiflexible polymers and the networks consisting of these molecules can display interesting mechanical properties. In this presentation, results on alginate hydrogels, a model polysaccharide network, will be presented. Effect of two different types of crosslinking, ionic and covalent, on the mechanical properties will be considered. Calcium salts have been used to obtain ionically crosslinked gels and these gels consist of junction zones with finite dimensions. Diamines have been used for synthesizing covalently crosslinked gels. Shear and cavitation rheology techniques were utilized to investigate the mechanical properties of these gels. The large amplitude oscillatory shear (LAOS) rheology captures the strain-stiffening behavior for these gels. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large-strain. Deformation mechanism of alginate gels involves stretching and bending of the alginate chains. In addition, for ionically crosslinked gels, bending of the junction zones play an important role. Cavitation rheology results indicate that the pressurization of defect inside these gels resulted in fracture. The results presented here provide new understanding of the deformation behavior of alginate hydrogels.