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°C. The PVC/DINP system exhibited power law type relaxation modulus,
G(
t,
γ) ∝
t-n’, over a wide range of time
t. The slope
n’ increased with elevating temperature
T irrespective of step-strain
γ. At 190°C, the
n’ value of the system was 0.75 showing a critical gel temperature (
Tgel), because 0.75 was in agreement with the values obtained from the linear dynamic viscoelasticity reported previously. Time-strain separability
G(
t,
γ) =
G(
t)
h(
γ) is applicable over a wide range of
t. Here,
G(
t) is linear relaxation modulus. The magnitude of damping depended strongly on
T. Below
Tgel, in other words, in a gel state,
h(
γ) 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(
γ) showed still more stronger strain dependence similar to branch polymers. It was found that
h(
γ) of PVC/plasticizer system strongly depended on temperature near
Tgel.
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