Moore J D, Cui S T, Cochran H D, Cummings P T
Department of Chemical Engineering, University of Tennesee, Knoxville, Tennessee 37996-2200, USA.
Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics. 1999 Dec;60(6 Pt B):6956-9. doi: 10.1103/physreve.60.6956.
Using nonequilibrium molecular dynamics simulation, we have studied the response of a C100 model polymer melt to a step change from equilibrium to a constant, high shear rate flow. The transient shear stress of the model polymer melt exhibits pronounced overshoot at the strain value predicted by the reptation model, in striking similarity to melts of longer, entangled polymer governed by reptation motion. At the maximum of shear stress overshoot, the molecular orientational order and the alignment angle are found to be midway between those characteristic of Newtonian flow and full alignment with the flow. The Doi-Edwards theory is found to be applicable but only by taking into account the shear-rate-dependence of the terminal relaxation time. We further analyze the molecular origins of such behavior in short polymer chains by decomposing the total stress into the contributions from various molecular interactions.
通过非平衡分子动力学模拟，我们研究了C100模型聚合物熔体对从平衡态到恒定高剪切速率流动的阶跃变化的响应。该模型聚合物熔体的瞬态剪切应力在由蛇行模型预测的应变值处呈现出明显的过冲现象，这与受蛇行运动支配的更长、缠结聚合物熔体极为相似。在剪切应力过冲的最大值处，发现分子取向序和排列角处于牛顿流动特征值与完全与流动方向对齐之间的中间状态。结果表明，只有考虑到末端弛豫时间对剪切速率的依赖性，Doi-Edwards理论才适用。我们通过将总应力分解为各种分子相互作用的贡献，进一步分析了短聚合物链中这种行为的分子起源。
