STUDIES OF SHEARWAVE DISPERSION IN CRITICAL-GEL SYSTEMS

The viscoelastic dispersion of shearwaves in a gelling system about th e gel-point is examined from the viewpoint of rheometrically exploitab le features. Marked changes in the wave dispersion occur due to the ch anging pattern of frequency dependence of the complex shear modulus G in the viscoelastic-liquid to viscoelastic-solid transition. These ch anges are compared with theoretical predictions of the shearwave dispe rsion characteristics of critical-gel systems at the gel-point, where G has a frequency dependence described by the Chambon-Winter Gel Equa tion, in which G'G '' similar to (omega (0 < n < 1). An Oldroyd theolo gical model is used to simulate the propagation of a transitional visc oelastic liquid-like to viscoelastic solid-like behaviour from high- t o low-frequencies, a notion consistent with the macroscopic gel-like c haracteristics presented by growing molecular clusters to high-frequen cy shearwaves in cross-linking systems. The results of this treatment are in good agreement with critical-gel behaviour for n in the range p redicted by theoretical treatments of gel-point. The latter are based on self-similar connectivity of branched macromolecules and a scaling theory of fractal correlations, in which n is in the range 0.67 < n < 1. The measured wave dispersion between 400 Hz and 1.2 kHz in a system for which n lies within this range at gel-point, is in good agreement with the theoretical prediction.