Wu, Zhenghao (2021)
Improved Dynamics in Hybrid Particle-Field Molecular-Dynamics Simulations of Polymers.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00019894
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

Over the past half century, molecular dynamics simulation techniques have advanced considerably, aiming to provide a comprehensive understanding of the structure-property relationships of polymers and to make quantitative predictions of the structural, dynamical and rheological behaviors of polymeric materials. Specifically, molecular dynamics simulations with atomic-level resolution models provide, in principle, the most accurate results among them. Unfortunately, the associated spatial and temporal dimensions involved in these polymer modelings will require computational costs that are unaffordable on a routine basis today. Such hurdles can be mitigated by using coarse-grained models that retain only a few relevant degrees of freedom and treat the others in an averaged manner. This procedure allows for large-scale simulations, but limits the ability to describe chemical variations. Therefore, efficient systematic coarse-graining, which allows direct simulation of the mesoscale behavior of polymeric materials with detailed chemistry, remains an extremely challenging task. In this dissertation, we introduce a novel computational technique for simulations of polymers by combining the hybrid particle-field molecular-dynamics (hPF-MD) simulation method and the multi-chain slip-spring model, namely, slip-spring hybrid particle-field molecular-dynamics approach (SS-hPF). In the original hPF-MD, the non-bonded interactions between particles are computed through the density-functional-field potential. This potential is effectively soft-core, resulting in chain-crossing events in modeling polymer melts or concentrated polymer solutions. Consequently, chain entanglement and associated dynamical mechanisms, e.g., chain reptation and arm retraction, are absent in these systems. The second component, the multi-chain slip-spring model, thus enters to improve the dynamics in original hPF-MD simulations of polymers. The so-called slip springs are transient bonds between polymer chains that artificially mimic the topological constraints between polymer chains. As proof-of-concept examples, we develop and validate SS-hPF simulations of linear polymers using an atomistic model of polyethylene melts and a systematic coarse-grained model of polystyrene melts. Moreover, we generalize the SS-hPF simulation approach to be able to simulate branched polymers, since the presence of a small fraction of chain branching can significantly modify the dynamical and rheological behaviors in polymeric materials. In addition, we study the properties that are likely to be affected by local molecular packing, such as the self-entanglements or knots in polymer melts simulated by soft-core models with and without slip-springs. Finally, we believe our work provides an efficient and practical approach to establish chemistry-specific coarse-grained models for polymers.

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