4.7 Article

Extended Ensemble Approach to Transferable Potentials for Low Resolution Coarse-Grained Models of Ionomers

Journal

JOURNAL OF CHEMICAL THEORY AND COMPUTATION
Volume 13, Issue 5, Pages 2185-2201

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.jctc.6b01160

Keywords

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Funding

  1. Penn State Academic Computing Fellowship
  2. DOE Basic Energy Sciences [0000219054]
  3. Neutron Scattering Program
  4. National Science Foundation [MCB 1053970, CHE 1565631]
  5. Cyberinfrastructure, a unit of Information Technology Services at Penn State
  6. Theoretical and Computational Biophysics group at the Beckman Institute, University of Illinois at Urbana Champaign
  7. Direct For Biological Sciences
  8. Div Of Molecular and Cellular Bioscience [1053970] Funding Source: National Science Foundation
  9. Division Of Chemistry
  10. Direct For Mathematical & Physical Scien [1565631] Funding Source: National Science Foundation

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We develop an extended ensemble method for constructing transferable, low-resolution coarse-grained (CG) models of polyethylene-oxide (PEO)-based ionomer chains with varying composition at multiple temperatures. In particular, we consider ionomer chains consisting of 4 isophthalate groups, which may be neutral or sulfonated, that are linked by 13 PEO repeat units. The CG models represent each isophthalate group with a single CG site and also explicitly represent the diffusing sodium counterions but do not explicitly represent the PEO backbone. We define the extended ensemble as a collection of equilibrium ensembles that are obtained from united atom (UA) simulations at 2 different temperatures for 7 chemically distinct ionomers with varying degrees of sulfonation. We employ a global force-matching method to determine the set of interaction potentials that, when appropriately combined, provide an optimal approximation to the many-body potential of mean force for each system in the extended ensemble. This optimized xn force field employs 'long-ranged Coulomb potentials with system-specific dielectric constants that systematically decrease with increasing sulfonation and temperature. An empirical exponential model reasonably describes the sensitivity of the dielectric to sulfonation, but we find it more challenging to model the temperature-dependence of the dielectrics. Nevertheless, given appropriate dielectric constants,the transferable xn force field reasonably describes the ion pairing that is observed in the UA simulations as a function of sulfonation and temperature. Remarkably, despite eliminating any explicit description of the PEO backbone, the CG model predicts string-like ion aggregates that appear qualitatively consistent with the ionomer peak observed in X-ray scattering experiments and, moreover, with the temperature dependence of this peak.

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