Nonlinear Stress Relaxation of Plasticized Poly(vinyl chloride)

Large deformation, nonlinear stress relaxation behavior was examined for poly(vinyl chloride) in di-isononyl phthalate (PVC/DINP) system having PVC content of 45.5 wt% at 180, 190, and 200 degrees C. The PVC/DINP system exhibited power law type relaxation modulus, G(t, gamma) proportional to t(-n'), over a wide range of time t. The slope n' increased with elevating temperature T irrespective of step-strain gamma. At 190 degrees C, the n' value of the system was 0.75 showing a critical gel temperature (T-gel), because 0.75 was in agreement with the values obtained from the linear dynamic viscoelasticity reported previously. Time-strain separability G(t, gamma) = G(t)h(gamma) is applicable over a wide range of t. Here, G(t) is linear relaxation modulus. The magnitude of damping depended strongly on T. Below T-gel, in other words, in a gel state, h(gamma) exhibited very weak strain dependence like cross-linking rubbers. In a critical gel state, the magnitude of damping was a little larger than that for the gel state. Above Tgel, in a sol state, h(gamma) showed still more stronger strain dependence similar to branch polymers. It was found that h(gamma) of PVC/plasticizer system strongly depended on temperature near T-gel.