Analytical coarse-grained description for polymer melts.
ABSTRACT Starting from the Ornstein-Zernike equation the authors derive an analytical theory, at the level of pair correlation functions, which coarse grains polymer melts into liquids of interacting soft colloidal particles. Since it is analytical, the presented coarse-graining approach will be useful in developing multiscale modeling procedures to simulate complex fluids of macromolecules. The accuracy of the theory is tested by its capacity to reproduce the liquid structure, as given by the center-of-mass intermolecular total pair correlation function. The theory is found to agree well with the structure predicted by molecular dynamics simulations of the liquid described at the united atom level as well as by molecular dynamics simulations of the liquid of interacting colloidal particles. The authors perform simulations of the liquid of interacting colloidal particles having as input the potential obtained from their analytical total pair correlation function by enforcing the hypernetted-chain closure approximation. Tests systems are polyethylene melts of chains with increasing degrees of polymerization and polymer melts of chains with different chemical architectures. They also discuss the effect of adopting different conventional approximations for intra- and intermolecular monomer structure factors on the accuracy of the coarse-graining procedure, as well as the relevance of higher-order corrections to their expression.
- SourceAvailable from: Marina Guenza[Show abstract] [Hide abstract]
ABSTRACT: Structural and thermodynamic consistency of coarse-graining models across multiple length scales is essential for the predictive role of multi-scale modeling and molecular dynamic simulations that use mesoscale descriptions. Our approach is a coarse-grained model based on integral equation theory, which can represent polymer chains at variable levels of chemical details. 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. A numerical solution to the PRISM integral equations is used to determine c0, by adjusting the value of the effective hard sphere diameter, dHS, to agree with the predicted equation of state. This single quantity parameterizes the coarse-grained potential, which is used to perform mesoscale simulations that are directly compared with atomistic-level simulations of the same system. We test our coarse-graining formalism by comparing structural correlations, isothermal compressibility, equation of state, Helmholtz and Gibbs free energies, and potential energy and entropy using both united atom and coarse-grained descriptions. We find quantitative agreement between the analytical formalism for the thermodynamic properties, and the results of Molecular Dynamics simulations, independent of the chosen level of representation. 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 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.The Journal of chemical physics. 05/2014; 140(20):204913.
- [Show abstract] [Hide abstract]
ABSTRACT: We studied unentangled poly(ethylene-alt-propylene) (PEP) in a composite with hydrophobic silica particles as a function of the filler concentration. Our neutron spin echo (NSE) experiments cover both the internal dynamics as well as the center of mass diffusion beyond the Rouse time. The key experimental results are (i) all of the chains are equally mobile, (ii) the basic segmental (Rouse) relaxation rate is unaffected even at highest filler concentrations, and (iii) apparently the obstacles reduce significantly the translational center of mass motion. This happens, even in the case when the particles do not significantly confine the polymer. (iv) A transition from regular to anomalous diffusion in the Rouse regime at the highest particle fractions is clearly evidenced. In order to understand the microscopic mechanisms underlying the experimental observations, we performed coarse grained simulations. We demonstrate that the geometrical confinement only affects the dynamics at a long time scale outside the experimental window and therefore it is not able to explain the results found in the NSE experiments. The consideration of inter-chain interactions, however, results in a significant influence even at shorter times and a quantitative agreement between the experiments and simulations was found. The simulations clearly demonstrate that the interfaces cause a deceleration of the chains in their close vicinity. Then the inter-chain interactions carry this slowing down to the other chains at a time-scale of the Rouse relaxation time. Hence, in the experimental datasets an overall slowing down is observed.Soft Matter 03/2013; 9(16):4336-4348. · 4.15 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: We present the equation of state for a coarse-grained model of polymer melts where each chain is represented as a soft colloidal particle centered on its center-of-mass. The formalism is based on the solution of the Ornstein−Zernike equation and is analytical, allowing for the formal investigation of the elements that ensure thermodynamic consistency in coarse-grained models of polymer melts. By comparing predictions from our expressions with those from computer simulations of the coarse-grained system and with atomistic polymer integral equation theory, we demonstrate that both structural and thermodynamic consistency with the atomistic level description is maintained during our coarse-graining procedures.