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 chlor ide) 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 freq uency omega, G'(omega) proportional to G ''(omega) proportional to ome ga(n) with n congruent to 0.7. In stress relaxation experiments the ge l exhibited nonlinear damping of the relaxation modulus G(t,gamma) wit h increasing step-strain gamma, but the magnitude of damping was much smaller than that for homogeneous homopolymer liquids. In addition, fo r the gel, the G(t,gamma) data for the largest strain examined (gamma = 5) were in close agreement with the linear G(t) measured after impos ition of this strain. On start-up of shear now at a rate gamma, the vi scosity growth function eta(+)(t,gamma) of the gel followed the linear eta(+,L)(t) at a short time scale, deviated downward from eta(+,L)(t) but still increased gradually at an intermediate time scale, exhibite d an apparent plateau (pseudo-steady-flow behavior) over a long time s cale and finally decreased with further increases of time. These featu res of eta(+)(t,gamma) and G(t,gamma) were discussed in relation to fl ow/strain-induced changes of the fractal structure in the gel.