Xu Xiaolei, Chen Jizhong
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Jilin, Changchun 130022, People's Republic of China.
J Chem Phys. 2016 Jun 28;144(24):244905. doi: 10.1063/1.4955098.
The properties of unentangled star polymers with arm length Nf = 20 beads and functionality f (3 ≤ f ≤ 60) are investigated at equilibrium and under shear flow by coarse-grained molecular dynamics simulations. At equilibrium, the star polymer shows a crossover from a linear, freely penetrable, extremely soft object to a spherical, slightly hard object with an impenetrable center with increasing f. The results confirm that the arm relaxation is essentially independent of f and stars of large f form a liquid-like structure. In shear flow, the polymer deformation and alignment are calculated as well as the shear-induced rotational dynamics as function of shear rate. These properties are found to exhibit qualitative changes at an f-independent shear rate, γṗ, which is a consequence of competition between chain relaxation and imposed flow. Shear thinning is characterized by shear viscosity and normal stress differences. With increasing f, the critical shear rate for the onset of shear thinning decreases from γṗ for f = 3 to a smaller value. Our results also show that shear thinning of stars of large f arise from the collapse of liquid-like structures at low shear rates (γ̇≪γṗ), where chains have no deformation; at high shear rates (γ̇≫γṗ), shear thinning is mainly attributed to the chain stretching and orientation as linear polymers.
通过粗粒化分子动力学模拟，研究了臂长为Nf = 20个珠子且官能度f（3≤f≤60）的非缠结星形聚合物在平衡态和剪切流作用下的性质。在平衡态下，随着f的增加，星形聚合物呈现出从线性、可自由穿透、极其柔软的物体到具有不可穿透中心的球形、稍硬物体的转变。结果证实，臂的松弛基本上与f无关，且f较大的星形聚合物形成类似液体的结构。在剪切流中，计算了聚合物的变形和取向以及作为剪切速率函数的剪切诱导旋转动力学。发现这些性质在与f无关的剪切速率γṗ处呈现出定性变化，这是链松弛和外加流动之间竞争的结果。剪切变稀由剪切粘度和法向应力差来表征。随着f的增加，剪切变稀开始时的临界剪切速率从f = 3时的γṗ降低到一个较小的值。我们的结果还表明，f较大的星形聚合物的剪切变稀源于低剪切速率（γ̇≪γṗ）下类似液体结构的坍塌，此时链没有变形；在高剪切速率（γ̇≫γṗ）下，剪切变稀主要归因于链的拉伸和取向，如同线性聚合物一样。
