Roberto - Improved dynamics in self-consistent-field molecular-dynamics simulations of polymers

主要目标是开发一种新的分子模拟算法，允许研究聚合物中的现象，特别是纳米复合材料中的现象，这些现象具有科学意义和技术相关性，但到目前为止还无法模拟。这种情况很常见，因为可用的计算能力不足以同时使用真实模型、建模足够大的样本和模拟足够长的时间来观察所有相关的物理过程。我们提出的方法结合了最近开发的两种方法。第一种成分是米兰(Salerno)和川崎(仙台)的所谓自洽场MD方法(MD-SCF)。它通过辅助场表示来加速计算密集型非键合相互作用的计算。它既可以用于原子化的聚合物模型，也可以用于粗粒度的聚合物模型，并且已经被证明以很低的计算成本非常准确地再现了聚合物材料的分子结构和热力学性质。然而，在设计上，它无法正确地捕捉聚合物的分子动力学和由此产生的所有性质，如扩散、熔体粘度和流变参数。这一失败是由于MD-SCF模型中不同链上的珠子之间缺乏排除体积相互作用造成的。没有防止链交叉，没有链缠绕和旋转，导致了定性错误的聚合物动力学。在这一点上，第二种成分进入。最近，我们与名古屋的Masubuhi合作，提出了聚合物链之间临时移动交链的概念，作为纠缠的模型。我们已经将这些所谓的滑移连接成功地应用到聚合物的耗散粒子动力学(DPD)模型中。DPD也没有珠之间的核心斥力，与MD-SCF在聚合物动力学方面存在完全相同的问题。然而，在引入滑移环之后，DPD再现了聚合物熔体动力学的所有基本特征。因此，我们将在MD-SCF方法中实施滑移连接，这是对DPD方法的改进，因为它允许更多的化学特性。项目阶段为：(1)MD-SCF/滑移连接组合的开发和实施及其技术验证。(2)解决在某些情况下可能导致该方法失败的一些科学风险。例如，在哈密顿量中引入滑移弹簧可能会使计算的性质发生变化，以至于必须以某种方式对其进行补偿。(3)找到了适用于聚合物动力学性质计算的新方法的操作参数范围。(4)最后，我们希望将新方法首次应用于最近涉及聚合物复合材料的流变性和其他动力学性质的问题，这些问题可以用其他方法来解决，但很困难，或者根本不能解决。

The main objective is to develop a new molecular-simulation algorithm, which allows the investigation of phenomena in polymer and especially nanocomposites that are scientifically interesting and technologically relevant, but which hitherto cannot be simulated. This is commonly the case, because available compute power is not sufficient to simultaneously use realistic models, model large enough samples, and simulate them long enough to observe all relevant physical processes.Our proposed approach combines two recently developed methods. The first ingredient is the so-called self-consistent-field MD method (MD-SCF) of Milano (Salerno) and Kawakatsu (Sendai). It accelerates the calculation of the compute-intensive nonbonded interactions by an auxiliary field representation. It can be used with both atomistic and coarse-grained polymer models and has been shown to reproduce molecular structure and thermodynamic properties of polymer materials very accurately at low computational cost. By design, however, it is unable to capture correctly the molecular dynamics of polymers and all properties resulting from it, such as diffusion, melt viscosity and rheological parameters. This failure results from the lack of excluded-volume interactions between beads of different chains in the MD-SCF model. Chain crossing is not prevented, chain entanglement and reptation are absent, and a qualitatively wrong polymer dynamics results.At this point, the second ingredient enters. Working with Masubuchi (Nagoya), we have recently developed the concept of temporary mobile cross-links between polymer chains as a mock-up for entanglements. We have implemented these so-called slip-links successfully into the dissipative-particle-dynamics (DPD) model for polymers. DPD also lacks a hard-core repulsion between beads and has exactly the same problems with the polymer dynamics as has MD-SCF. After the intro-duction of slip-links, however, DPD reproduced all essential features of polymer melt dynamics. We will therefore implement slip-links into the MD-SCF method as well, which is an improvement over DPD, because it allows for more chemical specificity.The project phases are: (1) Development and implementation of the MD-SCF/slip-link combination and its technical validation. (2) Addressing a number of scientific risks, which might cause the method to fail in certain cases. For example, introducing slip-springs into the Hamiltonian could shift the calculated properties to such an extent that they would have to be compensated in some way. (3) Finding a range of the operational parameters of the new method which is suitable for the calculation of dynamical properties of polymers. (4) Finally, we would like to do a first application of the new method to very recent problems involving rheological and other dynamical properties of polymer composites, which can be addressed by other methods only with difficulties or not at all.