Bernini S, Leporini D
Dipartimento di Fisica 'Enrico Fermi', Università di Pisa, Largo B.Pontecorvo 3, I-56127 Pisa, Italy.
J Phys Condens Matter. 2016 May 11;28(18):185103. doi: 10.1088/0953-8984/28/18/185103. Epub 2016 Apr 12.
The random walk of a bonded monomer in a polymer melt is anisotropic due to local order and bond connectivity. We investigate both effects by molecular-dynamics simulations on melts of fully-flexible linear chains ranging from dimers (M = 2) up to entangled polymers (M = 200). The corresponding atomic liquid is also considered a reference system. To disentangle the influence of the local geometry and the bond arrangements, and to reveal their interplay, we define suitable measures of the anisotropy emphasising either the former or the latter aspect. Connectivity anisotropy, as measured by the correlation between the initial bond orientation and the direction of the subsequent monomer displacement, shows a slight enhancement due to the local order at times shorter than the structural relaxation time. At intermediate times-when the monomer displacement is comparable to the bond length-a pronounced peak and then decays slowly as t (-1/2), becoming negligible when the displacement is as large as about five bond lengths, i.e. about four monomer diameters or three Kuhn lengths. Local-geometry anisotropy, as measured by the correlation between the initial orientation of a characteristic axis of the Voronoi cell and the subsequent monomer dynamics, is affected at shorter times than the structural relaxation time by the cage shape with antagonistic disturbance by the connectivity. Differently, at longer times, the connectivity favours the persistence of the local-geometry anisotropy, which vanishes when the monomer displacement exceeds the bond length. Our results strongly suggest that the sole consideration of the local order is not enough to understand the microscopic origin of the rattling amplitude of the trapped monomer in the cage of the neighbours.
由于局部有序性和键的连通性，聚合物熔体中键合单体的随机游走是各向异性的。我们通过分子动力学模拟研究了从二聚体（M = 2）到缠结聚合物（M = 200）的全柔性线性链熔体的这两种效应。相应的原子液体也被视为参考体系。为了区分局部几何形状和键排列的影响，并揭示它们之间的相互作用，我们定义了合适的各向异性度量，重点强调前者或后者。通过初始键取向与后续单体位移方向之间的相关性来测量的连通性各向异性，在短于结构弛豫时间的时间尺度上，由于局部有序性而略有增强。在中间时间——当单体位移与键长可比时——出现一个明显的峰值，然后随着t^(-1/2)缓慢衰减，当位移达到约五个键长，即约四个单体直径或三个库恩长度时变得可以忽略不计。通过Voronoi胞特征轴的初始取向与后续单体动力学之间的相关性来测量的局部几何各向异性，在短于结构弛豫时间的时间尺度上受到笼状形状的影响，同时连通性产生拮抗干扰。不同的是，在较长时间，连通性有利于局部几何各向异性的持续存在，当单体位移超过键长时，这种各向异性消失。我们的结果强烈表明，仅考虑局部有序性不足以理解被困在相邻原子笼中的单体晃动幅度的微观起源。
