Soft Matter Invited Talk

Marina Guenza (University of Oregon)

A coarse-graining method that preserves the free energy, structural correlations, and thermodynamic state of polymer melts from the atomistic to the mesoscale

Based on the solution of the Ornstein-Zernike Equation, we present our analytical coarse-grained model that is structural and thermodynamic consistent across multiple length scales [1]. The model in this way is fully predictive, when the potential is used as an input in mesoscale molecular dynamic simulations of polymer melts. The model is analytical and depends on molecular and thermodynamic parameters of the system under study, as well as on the direct correlation function in the k –> 0 limit, c0 [2]. This single non-trivial quantity parameterizes the coarse-grained potential. The value of c0 can be obtained numerically from the PRISM integral equation, or directly from the experimental compressibility of the system. Direct comparison with united atom simulations of both the analytical equations and mesoscale simulations shows quantitative consistency of structural and thermodynamic properties independent of the chosen level of representation [3]. In the mesoscale description, the potential energy of the soft-particle interaction becomes a free energy in the coarse-grained coordinates which preserves the excess free energy from an ideal gas across all levels of description. The total free energy of the coarse-grained system is reduced by only the configurational entropy associated with the removal of degrees of freedom from an ideal chain. The structural consistency between the united-atom and mesoscale descriptions means the relative entropy between descriptions has been minimized without any variational optimization parameters. The approach is general and applicable to any polymeric system in different thermodynamic conditions.

[1] A. J. Clark, J. McCarty, I. Y. Lyubimov, and M. G. Guenza, Phys. Rev. Lett. 109, 168301 (2012).
[2] A. J. Clark, J. McCarty, and M. G. Guenza, J. Chem. Phys. 139, 124906 (2013).
[3] J. McCarty, A. J. Clark, J. Copperman, and M. G. Guenza, J. Chem. Phys. (in press) 2014.