Heterogeneous and homogeneous dynamics in a simulated polymer melt: Analysis of multi-time correlation functions
Heterogeneous and homogeneous dynamics in a simulated polymer melt: Analysis of multi-time correlation functions
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模拟聚合物熔体中的非均相和均相动力学：多时间相关函数分析
DOI：
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
K. Okun
中科院分区：
文献类型：
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作者：
A. Heuer;K. Okun
一般来说，非指数弛豫可以由异质（不同指数过程的叠加）或均匀情况（本质上相同的非指数过程）产生。本文首次对这两种情况给出了严格的定义。提出了一种方法，通过比较两次和三次相关函数，可以估计非指数性的异质和同质贡献，即松弛的类型。在此基础上，对劳斯模型进行了松弛类型的解析计算。劳斯模型的松弛主要是同质的。此外，还计算了模拟聚合物熔体在玻璃化转变以上的动力学弛豫类型。在这项工作中使用的聚合物模型是众所周知的键波动模型。在大长度尺度和高温下，聚合物熔体的弛豫基本上是均匀的，与劳斯模型的定量一致。然而，对于短长度尺度和/或低温，聚合物熔体的动力学包含显著的非均相贡献。这直接表明了局部分子间相互作用与聚合物动力学的相关性。进一步分析了非均匀速率分布内的波动。波动的时间尺度与松弛过程本身具有相同的顺序。
In general, nonexponential relaxation can result either from a heterogeneous (superposition of different exponential processes) or a homogeneous scenario (identical intrinsically nonexponential processes). In the present paper for the first time a strict definition of both scenarios is formulated. A procedure is presented which allows to estimate the heterogeneous and homogeneous contributions to the nonexponentiality, i.e., the type of relaxation, from comparing a two-time and a three-time correlation function. On this basis the type of relaxation is calculated analytically for the Rouse model. The relaxation of the Rouse model turns out to be mainly homogeneous. Furthermore, the type of relaxation is calculated for the dynamics of a simulated polymer melt above the glass transition. The polymer model used in this work is the well-known bond fluctuation model. For large lengthscales and high temperatures the relaxation of the polymer melt is mainly homogeneous in quantitative agreement with the Rouse model. However, for short length scales and/or low temperatures the dynamics of the polymer melt contains significant heterogeneous contributions. This is a direct indication of the relevance of local intermolecular interactions for the polymer dynamics. Furthermore the fluctuations within the heterogeneous rate distribution are analyzed. The time scale of fluctuations turns out to be of the same order as the relaxation process itself.