Molecular dynamics simulations of poly(ethylene oxide)/LiI melts. 2. Dynamic properties

Molecular dynamics simulations have been performed on solutions of low molecular weight poly(ethylene oxide) (PEO) and LiI at 363 and 450 K for compositions ether oxygen:Li (EO:Li) = 48:1, 15:1, and 5:1. An explicit atom quantum chemistry based force field has allowed us to make quantitative predictions of polymer dynamics, ion mobilities, and complex lifetimes in these solutions. In the more dilute PEO/LiI solutions we found dynamical behavior consistent with a separation of the solutions into salt-rich and polymer-rich (PEG-like) domains. Dihedrals with oxygen atoms bound to Li+ cations (complexed dihedrals) were found to have significantly slower conformational dynamics than those dihedrals not bound to Li+ cations (uncomplexed dihedrals). In the dilute solutions, the dynamics of the complexed dihedrals were found to be only weakly dependent on composition, and the dynamics of the uncomplexed dihedrals were found to resemble closely those of pure PEG. For the EO:Li = 5:1 system, the conformational dynamics of both complexed and uncomplexed dihedrals were dramatically slower than in the more dilute solutions, and it was no longer possible to observe dynamical behavior consistent with separate salt-rich and polymer-rich domains. A slowing down of polymer chain dynamics with increasing salt concentration, characterized by a significant increase in the Rouse time and a significant decrease in the polymer self-diffusion coefficient, was also observed. Chain dynamics exhibited behavior consistent with salt-rich and PEG-rich domains for EO:Li greater than or equal to 15:1. The slowing down of chain dynamics was found to be correlated with an increase in the torsional correlation time due to restriction of the conformational space available for complexed dihedrals, resembling behavior seen in simulations of polymer melts approaching the glass-transition temperature. The ether oxygen-cation bond was found to be quite labile, with an average lifetime of approximately 100-200 ps, while cations translate the length of a polymer chain on a nanosecond time scale. Despite the high lability of the ether oxygen-cation bonds, interchain hopping events were rare, with an estimated frequency of 1 interchain hop/cation/10-100 ns. For systems with Rouse times less than the hopping time, we found the ion mobilities to he highly correlated with the polymer center-of-mass motion. For the EO:Li = 5:1 solutions with much longer Rouse times and a lightly cross-linked system, some decoupling of the ion motion from polymer motion, indicative of a change in mechanism, was observed. Finally, in contrast to previous simulations, conductivities and ion self-diffusion coefficients were predicted to within 1 order of magnitude of experimental values for similar systems.