Shear thinning behavior of linear polymer melts under shear flow via nonequilibrium molecular dynamics

摘要

The properties of both untangled and entangled linear polymer melts under shear flow are studied by nonequilibrium molecular dynamics simulations. The results reveal that the dependence of shear viscosity η on shear rate γ, expressed by η ～ γ?n, exhibits three distinct regimes. The first is the well-known Newtonian regime, namely, η independent of shear rate at small shear rates γ < τ_0~(?1) (where τ 0 is the longest polymer relaxation time at equilibrium). In the non-Newtonian regime (γ > τ_0~(?1)), the shear dependence of viscosity exhibits a crossover at a critical shear rate γ_c dividing this regime into two different regimes, shear thinning regime I (ST-I) and II (ST-II), respectively. In the ST-I regime (τ_0~(?1) < γ < γ_c), the exponent n increases with increasing chain length N, while in the ST-II regime (γ > γ_c) a universal power law η ～ γ~(?0.37) is found for considered chain lengths. Furthermore, the longer the polymer chain is, the smaller the shear viscosity for a given shear rate in the ST-II regime. The simulation also shows that a characteristic chain length, below which γ_c will be equal to τ_0~(?1), lies in the interval 30 < N < 50. For all considered chain lengths in the STII regime, we also find that the first and second normal stress differences N_1 and N_2 follow power laws of N_1 ～ γ 2/3 and N_2 ～ γ 0.82, respectively; the orientation resistance parameter m_G follows the relation mG ～ γ 0.75 and the tumbling frequency f_(tb) follows f_(tb) ～ γ ~(0.75). These results imply that the effects of entanglement on the shear dependences of these properties may be negligible in the ST-II regime. These findings may shed some light on the nature of shear thinning in flexible linear polymer melts.