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

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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.