Conformational and Dynamic Properties of Poly(ethylene oxide) in BMIM⁺BF₄^–: A Microsecond Computer Simulation Study Using ab Initio Force Fields

The behavior of polymers in complex solvents is interesting from a fundamental perspective and of practical importance from the standpoint of polymer processing. There has been recent interest in the conformational and dynamic properties of polymer in room temperature ionic liquids, with conflicting predictions from computations using models with different resolutions and conflicting results of experiments from different groups. In this work, we develop a first-principles, nonpolarizable united atom (UA) force field for a mixture of poly­(ethylene oxide) (PEO) in the ionic liquid BMIM⁺BF₄^–. The UA force field is benchmarked against ab initio calculations, and the PEO atomic charges are parametrized to implicitly capture the polarization contribution to the solvation energy of a single PEO molecule in BMIM⁺BF₄^–. The UA model allows one to perform multi-microsecond molecular dynamics simulations. This is necessary because the conformational relaxation correlation times are of the order of 100 ns. The simulations predict that the radius of gyration, R_g, scales with molecular weight, R_g ∼ M_w^ν with ν ≈ 0.56 in the temperature range 300–600 K, consistent with experiment, seemingly in between a self-avoiding walk and an ideal chain. An examination of the snapshots of the polymer demonstrates, however, that the polymer conformations are composed of ringlike and linear segments, with ringlike parts of the chain wrapped around cations of the ionic liquid. The slow dynamics arises from the barrier to unwrapping the ringlike segments of the polymer. The mean-square displacement shows three regimes which we interpret as confinement, Zimm, and diffusive. The simulations emphasize the importance of accurate force fields and microsecond simulations in obtaining reliable results for polymers and elucidate important correlation effects for polymers in strongly interacting solvents.