WORK-HARDENING EFFECTS IN GELLAN GUM GELS

Gellan gum is a bacterial polysaccharide that is marketed as a broad s pectrum gelling agent. The shear and Young's moduli, and the fracture strength of gellan gels Increase with increasing ionic strength, pass through maxima and then decrease to zero at higher ionic strength. The position of the maxima depend on the valency of added counterions and are virtually independent of polymer concentration. At low ionic stre ngth the measured theological parameters show small variability but th ese values become increasingly variable with increasing ionic strength . Stress relaxation experiments were carried out in order to examine t he mechanical behaviour of gellan gels and to explain these effects. A t low ionic strength the gels approximate to elastic solids whereas at high ionic strength the increased stress relaxation is colloid-like i n behaviour. However, unlike colloidal dispersions, the magnitude of t he stress relaxation decreases on successive applications of stress an d the relaxation modulus increases. The gels work harden. Such behavio ur is attributed to a heterogeneous structure within the gel.