Conformational dynamics of polymers in ethylammonium nitrate from advanced sampling methods

Abstract

The atomistic simulations of isolated poly (ethylene oxide) (PEO) in ionic liquid Ethylammonium nitrate (EAN) at 300 K, for degrees of polymerization, n = 9, 18, 27 and, 40 were carried out to understand the polymer dynamics using advanced sampling methods. We used various simulation techniques for this study: free energy simulations using a coupling parameter, replica-exchange molecular dynamics, and umbrella sampling simulations. The contributions from entropy and energy to the solvation free energy were also calculated from the finite-difference temperature derivative of the free energy at each reaction coordinate interval. We found that the polymer shows self-avoiding walk behavior evident from the scaling exponent is 0.55, and the end-to-end distance distribution has a single peak. EAN acts as a poor solvent compared to water in correlation with a previous experimental study. The solvation is entropically unfavorable and energetically favorable in IL, whereas it is entropically favorable and energetically unfavorable in water. However, PEO is more expanded in water for PEO with n = 9, whereas it is more expanded in EAN for other degrees of polymerization values. The mean force of the potential is found to be profoundly repulsive between two PEO chains with n = 9 in EAN compared to that in water, and the energetic solvent-polymer interactions attribute to repulsion. The cation alkyl chains of the IL interact with the polymer’s surface, and ammonium groups of the cations stay away from the polymer surface due to the strong hydrogen bonding between the IL’s negative and positive ions.