The non-linear stress relaxation behavior of scarcely entangled polymer chains was investigated with primitive chain network simulations in the transitional regime of molecular weight for the type-A and type-B damping. After checking the consistency of simulation results with experimental data for linear viscoelasticity, stress relaxation was examined under large step shear deformations. As reported in experiments, the time-strain separability was observed in the examined molecular weight regime so that damping function was obtained. The simulated damping function increased (the nonlinearity became less significant) with decreasing chain length, which was in accord to experimental data for polystyrene solutions. Then, the mechanism of molecular weight dependence of the damping function was analyzed on the basis of kinetics of the sliplink network. For the short chains showing the type-B behavior, the equilibration of sliplink network is faster than that for longer chain due to high concentration of chain end that enhances recreation of the sliplinks. This fast equilibration of sliplink network smears the inhomogeneity of tension along the backbone and reduces the nonlinearity of damping function. The transition from the type-A to type-B behavior mainly reflects the difference of equilibration rate of entanglement network structure.
2013 The Society of Rheology, Japan