Rheology of a simultaneously phase-separating and gelling biopolymer mixture

The dynamic shear modulus of aqueous solutions of the biopolymers gelatin and dextran was found to be highly sensitive to thermal history even for samples of identical composition. Rheological differences could be correlated with differences in morphology, visible by optical microscopy, arising from the interaction of gelation with phase separation. Quenching a single-phased morphology to a temperature at which phase separation and gelation occurred simultaneously, gave a relatively high shear modulus-reflecting the maintenance of a connected gelatin rich phase. A sample which before its final quench was annealed at an intermediate temperature, in the two-phase region but above the gelation temperature, developed a much lower modulus. In both cases, the mechanism of phase separation at early stages was spinodal decomposition, but in the latter case the much lower modulus reflects a transition to a morphology of disconnected gelatin rich droplets during the late stage of phase separation.