Article pubs.acs.org/JPCB
Conformational and Dynamic Properties of Poly(ethylene oxide) in an Ionic Liquid: Development and Implementation of a First-Principles Force Field Jesse G. McDaniel,† Eunsong Choi,‡ Chang-Yun Son,† J. R. Schmidt,† and Arun Yethiraj*,† †
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, United States
‡
S Supporting Information *
ABSTRACT: The conformational properties of polymers in ionic liquids are of fundamental interest but not well understood. Atomistic and coarse-grained molecular models predict qualitatively different results for the scaling of chain size with molecular weight, and experiments on dilute solutions are not available. In this work, we develop a first-principles force field for poly(ethylene oxide) (PEO) in the ionic liquid 1-butyl 3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) using symmetry adapted perturbation theory (SAPT). At temperatures above 400 K, simulations employing both the SAPT and OPLS-AA force fields predict that PEO displays ideal chain behavior, in contrast to previous simulations at lower temperature. We therefore argue that the system shows a transition from extended to more compact configurations as the temperature is increased from room temperature to the experimental lower critical solution temperature. Although polarization is shown to be important, its implicit inclusion in the OPLS-AA force is sufficient to describe the structure and energetics of the mixture. The simulations emphasize the difference between ionic liquids from typical solvents for polymers.
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INTRODUCTION The properties of polymer/solvent mixtures determine many interesting processes in biology and materials science, dictated by thermodynamic and kinetic phenomena whose explanations often test the limits of current theory. In particular, polymer/ ionic liquid (IL) mixtures have recently received interest due to their potential wide-ranging application, including use in fuel cells, batteries, and gas separation.1 For these applications, the specific choice of IL/polymer combination is critical, as various systems exhibit dramatically different properties, with miscibility dependent on both the polymer molecular weight and temperature.2,3 The development of “design rules” for realization of optimal properties of these composite materials is thus an important goal.1 Poly(ethylene oxide) (PEO) is a commonly used polymer due to its low cost and low toxicity, and PEO/IL mixtures employing the prototypical imidazolium-based, 1-ethyl-3methylimidazolium (EMIM), or 1-butyl-3-methylimidazolium (BMIM) ILs have recently been the subject of both experimental3−11 and theoretical studies.11−15 Experiments and simulations suggest PEO displays extended conformations in dilute solution, at room temperature. Neutron-scattering experiments on PEO in [BMIM][BF4] performed by Triolo et al.4 show that in semidilute solutions the radius of gyration, Rg, scales with concentration, c, as Rg ∼ c−0.25, which implies that Rg ∼ N in dilute solution, where N is the degree of polymerization. Analogous behavior of dilute PEO in [BMIM][BF4] was subsequently observed at room temperature by Mondal et al.14 in replica exchange molecular © 2015 American Chemical Society
dynamics (MD) simulations employing the OPLS-AA force field. This behavior is surprising because PEO is not charged, and the driving force behind chain stretching is not clear. Mondal et al.14 speculated this could be due to strong association of the oxygen atoms of the PEO with the BMIM cations as had also previously observed by Costa and Ribeiro.12 The phase behavior of PEO in ILs is also different from typical polymer solutions. Lee and Lodge8,9 have experimentally demonstrated the existence of a lower critical solution temperature (LCST) for the mixing of PEO/[BMIM][BF4]. Interestingly, the LCST occurs at a high polymer wt % and is largely independent of polymer molecular weight; in addition, it is also significantly affected by the length of the imidazolium alkyl chain, the nature (hydroxy/methoxy) of the PEO terminal groups, and methylation at the most acidic imidazolium hydrogen position.9 On the basis of temperature-dependent IR spectra, Li and Wu16 attributed the LCST phase behavior of PEO/[EMIM][BF4] to two different hydrogen-bond-like interactions, namely, PEO-oxygen/imidazolium-hydrogen and PEO-hydrogen/BF4-fluorine. Additionally, Choi and Yethiraj,15 utilizing MD simulations, showed that the LCST for PEO/ [BMIM][BF4] is driven by the temperature dependence of the PEO-oxygen/imidazolium-hydrogen interaction, which has a strong entropic component. Notably, the prediction of an Received: October 14, 2015 Revised: December 18, 2015 Published: December 21, 2015 231
DOI: 10.1021/acs.jpcb.5b10065 J. Phys. Chem. B 2016, 120, 231−243
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The Journal of Physical Chemistry B LCST was dependent on the specific details of the force field, with scaled-charge force fields showing no phase separation. A strict comparison of experiment and theory for these systems is difficult due to the disparity in both polymer molecular weight and concentration. Simulations are typically restricted to much lower molecular weight polymers (
