Molecular Dynamics and Relaxation of Entangled Polymers

[721011] larson自1971年de Gennes提出聚合物在纠缠聚合物中的扩散和弛豫的重复理论以来，通过认识到原始路径波动和约束释放等附加机制的重要性，在定量预测简单聚合物弛豫方面取得了逐步的进展。McLeish及其同事介绍的理论概念表明，这些相同的基本机制可以用于预测更复杂分子系统的粘弹性特性，例如具有多个分支点的分子熔体。然而，到目前为止，理论都专门用于特定的系统，如纯线性分子，或单分散或双分散的恒星。此外，约束释放的概念有两种不同的简化描述，一种称为管重组，另一种称为管扩张。在多分散线性聚合物的特殊情况下，称为双重重复的管重组版本似乎成功地描述了松弛数据，而对于星形聚合物，约束释放必须用称为动态稀释的管扩张模型来表现。在目前的资助下，将开发一个理论框架，它可以解释纠缠聚合物中所有已知的慢弛豫机制，并统一管重组和管稀释的概念。为了建立这样一个模型，迄今为止成功的概念将被利用，在许多情况下，浓缩聚合物系统中的纠缠可以被视为两个链之间成对相互作用的集合。当其中一个链移动其链端穿过纠缠位置时，每个纠缠就会消失。因此，假设是，即使对于不同拓扑结构的复杂混合物，也可以通过仅跟踪链端、链分支点和离散纠缠或滑移连接来对不同链的弛豫进行定性和定量预测。将开发一种称为双滑移连接算法的计算算法，该算法原则上可以描述线性或任意分支聚合物的任何分子混合物的弛豫。在初步工作中，对于线性聚合物，该模型简化为双重重复理论，该理论已知对许多多分散线性聚合物是准确的，而对于星型聚合物，它预测的行为类似于Ball-McLeish动态稀释理论。这项初步工作将通过允许分支点的运动来扩展，这将允许模拟任意拓扑和组成的聚合物混合物。该模型的预测将与分析理论和线性分子、星形和绒球及其混合物的实验数据进行比较，并与模拟结果进行比较，以更深入地了解简单和复杂聚合物系统中的弛豫过程。该基金支持聚合物混合物基本弛豫特性的理论研究。使用新开发的计算算法，这些系统的各种其他理论将统一起来。除了具有基本的兴趣外，了解聚合物混合物中的弛豫还具有广泛的工业应用

0072101LarsonFollowing the introduction in 1971 of de Gennes' reptation theory of polymer diffusion and relaxation in entangled polymers, gradual progress has been made in quantitatively predicting the relaxation of simple polymers by recognizing the importance of the additional mechanisms of primative path fluctuations and constraint release. Theoretical concepts introduced by McLeish and co-workers suggest that these same basic mechanisms can allow prediction of the viscoelastic properties of more complex molecular systems, such as melts of molecules with multiple branch points. Heretofore, however, theories have been specialized to particular systems, such as purely linear molecules, or monodisperse or bidisperse stars. Furthermore, the concept of constraint release has taken on two different simplified descriptions, one called tube reorganization and the other tube dilation. In the special case of polydisperse linear polymers, a version of tube reorganization called double reptation appears to successfully describe relaxation data, while for star polymers, constraint release must be manifested in terms of a tube dilation model known as dynamic dilution.In the present grant, a theoretical framework will be developed that is general in scope, which can account for all known mechanisms of slow relaxation in entangled polymers, and unify the concepts of tube reorganization and tube dilution. To develop such a model, the so-far successful notion will be exploited that in many cases entanglements in concentrated polymeric systems can be viewed as a set of pair-wise interactions between two chains. Each entanglement disappears when either chain in the pair moves its chain end through the entanglement position. Thus, the hypothesis is that qualitative and perhaps quantitative predictions of polymer relaxation can be made even for complex mixtures of different topology, by keeping track only of chain ends, chain branch points, and discrete entanglements or slip links that couple pair-wise the relaxation of different chains. A computational algorithm, called the dual slip link algorithm, will be developed that can in principle describe relaxation of any molecular mixture of linear or arbitrarily branched polymers. In preliminary work, for linear polymers, the model reduces to the double reptation theory, which is known to be accurate for many polydisperse linear polymers, while for star polymers, it predicts behavior similar to that of the Ball-McLeish dynamic dilution theory. This preliminary work will be extended by allowing motion of branch points which will then permit simulation of polymer mixtures of arbitrary topolgy and composition. Predictions of this model will be compared with analytic theories and with experimental data for linear molecules, stars and pom-poms, and mixtures thereof, and with simulation results to gain deeper insights into relaxation processes in both simple and complex polymer systems.%%% This grant supports theoretical research on fundamental relaxational properties of mixtures of polymers. Using a newly developed computational algorithm, various other theories of these systems will unified. Besides being of fundamental interest, understanding relaxation in polymer mixtures has wide ranging industrial applications.***