Conformational and Dynamic Properties of Poly(ethylene oxide) in an Ionic Liquid: Development and Implementation of a First-Principles Force Field

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.