Computer Simulations of nonlinear dynamics of entangled polymer liquids

利用分子动力学模拟方法，研究了珠数N=10的短链模型聚合物熔体在过冷态下的动力学。在静态条件下，计算了应力松弛函数，该函数在n松弛时间的时间尺度上呈现伸展的指数松弛，并最终遵循Rouse动力学。在剪切稳定状态下，在较强的剪切条件下，链发生剪切变薄并伸长成椭球状。在如此强的剪切下，我们发现链经历了随机翻滚运动，呈拉伸和紧凑交替的形状。接下来，我们可视化了聚合物熔体中的纠缠。在静止状态下，处于纠缠状态的粒子与属于不同链的粒子持续长时间地相互作用。如果每个粒子支持的链间相互作用能在比单体弛豫时间长得多的时间间隔内平均，则可以识别每条链上的纠缠区域。然后，我们施加一个比纠缠运动快得多的剪切流。随着应变的增加，链条呈之字形，在缠绕区域发生弯曲。链首先被拉伸成网络，但随后发生解缠事件，导致实验观察到的应力超调。

Using molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N=10 in supercooled states. In quiescent conditions, the stress relaxation function is calculated, which exhibits a stretched exponential relaxation on the time scale of the n relaxation time and ultimately follows the Rouse dynamics. In sheared steady states, shear-thinning and elongation of chains into ellipsoidal shapes take place for a strong shear regime. In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively.We next visualized entanglements in polymer melts. In quiescent states, a particle at an entanglement interacts with those belonging to a different chain persistently for long times. Entangled regime on each chain can be identified if the inter-chain interaction energy supported by each particle is averaged over a time interval much longer than monomeric relaxation time. We then apply a shear flow with rate much faster than the entanglement motion. With increasing strain the chains take zigzag shapes with bending taking place at the entangled regions. The chains arc first stretched as a network but disentanglement events subsequently occur, resulting in stress overshoot observed experimentally.