School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Eonyang-eup, Ulju-gun, Ulsan 689-798, South Korea.
Soft Matter. 2017 Nov 22;13(45):8644-8650. doi: 10.1039/c7sm01644a.
We present a detailed analysis on the effect of short-chain branches on the structure and dynamics of interfacial chains using atomistic nonequilibrium molecular dynamics simulations of confined polyethylene melts in a wide range of shear rates. The intrinsically fast random motions of the short branches constantly disturb the overall chain conformation, leading to a more compact and less deformed chain structure of the short-chain branched (SCB) polymer against the imposed flow field in comparison with the corresponding linear polymer. Moreover, such highly mobile short branches along the backbone of the SCB polymer lead to relatively weaker out-of-plane wagging dynamics of interfacial chains, with highly curvy backbone structures in the intermediate flow regime. In conjunction with the contribution of short branches (as opposed to that of the backbone) to the total interfacial friction between the chains and the wall, the SCB polymer shows a nearly constant behavior in the degree of slip (d) with respect to shear rate in the weak-to-intermediate flow regimes. On the contrary, in the strong flow regime where irregular chain rotation and tumbling dynamics occur via intensive dynamical collisions between interfacial chains and the wall, an enhancement effect on the chain detachment from the wall, caused by short branches, leads to a steeper increase in d for the SCB polymer than for the linear polymer. Remarkably, the SCB chains at the interface exhibit two distinct types of rolling mechanisms along the backbone, with a half-dumbbell mesoscopic structure at strong flow fields, in addition to the typical hairpin-like tumbling behavior displayed by the linear chains.
我们使用原子非平衡分子动力学模拟，在较宽的剪切速率范围内研究了短支链对界面链结构和动力学的影响。短支链的固有快速随机运动不断干扰整个链构象，导致短支链（SCB）聚合物的链结构比相应的线性聚合物更紧凑、变形更小。此外，SCB 聚合物主链上的这些高迁移短支链导致界面链的面外摇摆动力学相对较弱，在中间流动状态下具有高度弯曲的主链结构。结合短支链（相对于主链）对链与壁之间总界面摩擦力的贡献，SCB 聚合物在弱至中等流动范围内，其滑动度（d）与剪切速率呈近乎恒定的关系。相反，在强流动范围内，由于界面链与壁之间的剧烈动态碰撞，导致链从壁上脱离的增强效应，SCB 聚合物的 d 值比线性聚合物的斜率更大。值得注意的是，界面处的 SCB 链在强流场中沿主链表现出两种不同类型的滚动机制，具有半哑铃状的介观结构，除了线性链显示的典型发夹状翻滚行为外。
