VISCOELASTIC BEHAVIOR AT THE THERMAL SOL-GEL TRANSITION OF GELATIN

Rheological studies of gelatin solutions in the concentration range 4-14% were performed through the sol-gel transition as a function of concentration, pH and ionic strength. At the gel point, a power-law frequency dependence of the viscoelastic functions G'(omega), G''(omega) and eta*(omega) was observed. The power-law exponents n obtained from these dynamic measurements were confirmed by creep experiments and are in agreement with a previous study in a lower concentration range, 1-5%. Under all conditions, the n values fall between 0.64 and 0.72, a range consistent with theoretical treatments of Martin et al. and Muthukumar et al. The critical gel strength, S, increases with concentration approximately as S is-proportional-to C1.33 in the concentration range 4-14%. Within experimental error, S is approximately consistent with an empirical relation suggested in the recent literature, S = G(e)(1-n)eta0n, where G(e) and eta0 are, respectively, the equilibrium modulus of the fully developed gel and the shear viscosity of the initial sol. The implications of these observations with respect to the structure of the critical gel are discussed.