Rheological Images of Poly(Vinyl Chloride) Gels. 4. Nonlinear Behavior in a Critical Gel State

Nonlinear rheology was examined for a critical gel of poly(vinyl chloride) in dioctyl phthalate (PVC/DOP). PVC crystal domains worked as the cross-linking domains in this physical gel. In the linear regime, the gel exhibited power-law dependence of storage and loss moduli on frequency ω, G‘(ω) ∝ G‘‘(ω) ∝ ωn with n ≅ 0.7. In stress relaxation experiments the gel exhibited nonlinear damping of the relaxation modulus G(t,γ) with increasing step-strain γ, but the magnitude of damping was much smaller than that for homogeneous homopolymer liquids. In addition, for the gel, the G(t,γ) data for the largest strain examined (γ = 5) were in close agreement with the linear G(t) measured after imposition of this strain. On start-up of shear flow at a rate γ, the viscosity growth function η+(t,γ) of the gel followed the linear η+,L(t) at a short time scale, deviated downward from η+,L(t) but still increased gradually at an intermediate time scale, exhibited an apparent plateau (pseudo-steady-flow behavior) over a long...