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Abstract
The characteristic shear rate for nonlinear viscoelastic behavior in a polydisperse polymer system was investigated by the study of the stress overshoot behavior of polybutadiene solutions. The frequency at the crossing point of the dynamic moduli (
) was determined by oscillation frequency sweeps. For the stress overshoot observed in the shear flow at a fixed rate, the dependency of the stress drop, 
, defined as the difference between the stress at the peak and the stress at the steady state, on the shear rate can be well described by a fitting function: 
, and a critical shear rate (
) can be obtained using this fitting function. The correlation between 
and 
for the solutions can be expressed as 
and the proportionality factor is dependent on the degree of chain interpenetration. When the shear rate was normalized by 
, the dependency of the stress drop on this normalized shear rate for the solutions with different concentrations can be superposed well. This observation indicates that 
can be viewed as a characteristic shear rate for nonlinear viscoelastic behavior in a polydisperse polymer solution.
Acknowledgment
The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Grant No. 20734005) and the Program for New Century Excellent Talents in University (Grant No. NCET-08-0382).