Li Ting, Yang Xiaozhen, Nies Erik
Polymer Research Division, Department of Chemistry, The Leuven Mathematical Modeling and Computational Science Centre (LMCC) and the Leuven Materials Research Centre (LMRC), Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium, State Key Laboratory of Polymer Physics & Chemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100080, Peoples' Republic of China, Laboratory of Polymer Technology, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands.
J Chem Theory Comput. 2011 Jan 11;7(1):188-202. doi: 10.1021/ct100513y. Epub 2010 Dec 9.
The conformational properties of a finite length polyethylene chain were explored over a wide range of temperatures using a replica exchange molecular dynamics simulation providing high quality simulation data representative for the equilibrium behavior of the chain molecule. The radial distribution function (RDF) and the structure factor S(q) of the chain as a function of temperature are analyzed in detail. The different characteristic peaks in the RDF and S(q) were assigned to specific distances in the chain and structural changes occurring with the temperature. In S(q), a peak characteristic for the order in the solid state was found and used to determine the equilibrium melting temperature. A detailed scaling analysis of the structure factor covering the full q range was performed according to the work of Hammouda. In the Θ region, a quantitative analysis of the full structure factor was done using the equivalent Kuhn chain, which enabled us to assign the Θ region of our chain and to demonstrate, in our particular case, the failure of the Gaussian chain approach. The chain conformational properties at the equilibrium melting temperature are discussed using conformational distribution functions, using the largest principal component of the radius of gyration and shape parameters as order parameters. We demonstrate that for the system studied here, the Landau free energy expression based on this conformational distribution information leads to erroneous conclusions concerning the thermodynamic transition behavior. Finally, we focus on the instantaneous conformational properties at the equilibrium melting temperature and give a detailed analysis of the conformational shapes using different shape parameters and a simulation snapshot. We show that the chain does not only take the lamellar rod-like and globular conformational shapes, typical of the solid and liquid states, but can also explore many other conformational states, including the toroidal conformational state. It is the first demonstration that a flexible molecule like PE can also take a toroidal conformational state, which is normally linked to stiffer chains.
使用副本交换分子动力学模拟，在很宽的温度范围内探索了有限长度聚乙烯链的构象性质，该模拟提供了代表链分子平衡行为的高质量模拟数据。详细分析了链的径向分布函数（RDF）和结构因子S(q)随温度的变化。RDF和S(q)中不同的特征峰被指定为链中的特定距离以及随温度发生的结构变化。在S(q)中，发现了固态有序的特征峰，并用于确定平衡熔点。根据Hammouda的工作，对覆盖整个q范围的结构因子进行了详细的标度分析。在θ区域，使用等效的库恩链对整个结构因子进行了定量分析，这使我们能够确定链的θ区域，并在我们的特定情况下证明高斯链方法的失效。使用构象分布函数，以回转半径的最大主成分和形状参数作为序参量，讨论了平衡熔点下的链构象性质。我们证明，对于本文研究的体系，基于这种构象分布信息的朗道自由能表达式会导致关于热力学转变行为的错误结论。最后，我们关注平衡熔点下的瞬时构象性质，并使用不同的形状参数和模拟快照对构象形状进行了详细分析。我们表明，链不仅呈现出固态和液态典型的层状棒状和球状构象形状，还可以探索许多其他构象状态，包括环形构象状态。这首次证明了像PE这样的柔性分子也可以呈现环形构象状态，而这种状态通常与更刚性的链相关联。
