Comparison of ring and linear polyethylene from molecular dynamics simulations
Comparison of ring and linear polyethylene from molecular dynamics simulations
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DOI：
10.1021/ma060274s
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发表时间：
2006-05
期刊：
影响因子：
5.5
通讯作者：
Kahyun Hur;R. Winkler;D. Y. Yoon
中科院分区：
文献类型：
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作者：
Kahyun Hur;R. Winkler;D. Y. Yoon
高分子熔体的构象和动力学性质受到拓扑约束的强烈影响。约束成为主导的程度取决于各种聚合物参数，如分子量和结构。在这方面，线性聚合物的构象特征和动力学已被广泛的实验和理论研究。1已证实，它们在熔体中的构象性质与未扰动链的构象性质相同。2，3关于它们的动力学，当分子量超过一定值时，从由Rouse模型描述的各向同性运动1转变为由相邻链的不可交叉性引起的局部各向异性运动。后一种动力学可以用爬行理论模型来近似，4这一点已被计算机模拟5-7和实验所证实。8-10在传统的蠕动模型中，链端的存在对线性聚合物在熔体中的松弛过程起着主导作用。对于其他聚合物结构，例如聚合物环，其显然没有末端，必须预期不同的弛豫过程。提出了一种新的凝胶中的环状聚合物模型，即所谓的“晶格动物”，11并进行了线性基质中的环状聚合物12和环状基质中的线性聚合物13的实验。在各种流变学测量中，14-16发现超过一定分子量的环的粘度表现出与线性分子类似的分子量依赖性。预测的缠结分子量高于可比的线性分子。然而，迄今为止还没有得到结论性的实验结果，因为未打结和非连接的“纯”环聚合物的合成仍然是一个悬而未决的问题。[14]最近，利用键涨落模型对熔体中的环状聚合物进行了蒙特卡罗模拟研究[17-20]。这些模拟表现出严重的影响，拓扑约束的聚合物构象以及它们的动力学。然而，预测环状聚合物与其线性对应物相比显示出更高的扩散常数，17与实验结果明显不一致。21
The conformational and dynamical properties of macromolecular melts are strongly affected by topological constraints. The extent to which the constraints become dominant depends on various polymer parameters, such as molecular weight and architecture. In this regard, the conformational characteristics and dynamics of linear polymers have been studied extensively experimentally and theoretically. 1 It has been confirmed that their conformational properties in a melt are identical to those of unperturbed chains. 2,3 With respect to their dynamics, a transition occurs from an isotropic motion, described by the Rouse model, 1 to a locally anisotropic motion induced by the noncrossability of neighboring chains when the molecular weight exceeds a certain value. The latter dynamics may be approximated by the reptation theoretical model, 4 as confirmed by computer simulations 5-7 and experiments. 8-10 In the conventional reptation model, the presence of the chain ends plays a dominant role in relaxation process of a linear polymer in a melt. For other polymer architectures such as polymer rings, which obviously have no ends, different relaxation processes have to be expected. A new model of a ring polymer in a gel, a so-called “lattice animal”, was proposed, 11 and experiments of ring polymers in linear matrices 12 and linear polymers in ring matrices13 were performed. In various rheological measurements, 14-16 it was found that the viscosity of rings beyond a certain molecular weight exhibit a molecular weight dependence similar to that of linear molecules. The predicted entanglement molecular weight is higher than that of comparable linear molecules. However, no conclusive experimental results have been obtained so far, since the synthesis of unknotted and nonconcatenated “pure” ring polymers remains an unresolved issue. 14 Recently, Monte Carlo simulation studies 17-20 on ring polymers in a melt have been performed exploiting the bondfluctuation model. These simulations exhibit a severe influence of topological constraints on polymer conformations as well as their dynamics. However, ring polymers are predicted to show a higher diffusion constant compared to their linear counterparts, 17 in apparent disagreement with experimental results. 21