(724f) Coarse-Grained Models of Linear Alkanes

The objective of the Materials Genome Initiative (MGI) is a new R&D paradigm enabling accelerated materials design via computation leading to a shortening of the time and cost needed to bring new materials to market. An essential element of meeting this goal for polymers and related soft materials is the development of a coherent and progressive framework for both deriving and utilizing coarse-grained (CG) potentials in molecular dynamics simulations at mesoscopic length scales between the atomistic and continuum. In this work, we conduct a systematic study of a family of CG potentials of polyethylene oligomers (through C150) at various levels of resolution parameterized by the multiscale coarse-graining technique of Izvekov  et al. [J. Chem. Phys. 120, 10896 (2004)] and compare with the behavior of the corresponding atomistic simulations [Jeong and Douglas, in preparation, 2015]. Specifically, we calculate the molecular diffusion and viscosity as a function of chain length in the high temperature regime where the fluid dynamics is straightforward and use this information to determine the Arrhenius parameters a function of the level of coarse-graining, a fundamental procedure that has received very little attention in this area. The study of the atomistic chain dynamics as a function of chain length indicates that the activation enthalpy and entropy closely follow enthalpy-entropy compensation and we anticipate that this phenomenon will also apply under coarse-graining.  Quantification of this compensation effect for the coarse-grained system will provide significant guidance on how to determine the energetic parameters to best preserve dynamics of the original molecular model. We also examine the ability of the CG potentials to capture the structural transition from a rod like molecule to wormlike molecule upon the increase of molecular weight in linear chains.  This is an essential aspect of the dynamics of polymer melts that has not received sufficient attention and which has a large impact on polymer melt properties.