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

Molecular dynamics simulations have been performed on solutions of low mole cular weight poly(ethylene oxide) (PEO) and LiI at 363 and 450 K for compos itions ether oxygen:Li (EO:Li) = 48:1, 15:1, and 5:1. An explicit atom quan tum chemistry based force field has allowed us to make quantitative predict ions of polymer dynamics, ion mobilities, and complex lifetimes in these so lutions. In the more dilute PEO/LiI solutions we found dynamical behavior c onsistent with a separation of the solutions into salt-rich and polymer-ric h (PEG-like) domains. Dihedrals with oxygen atoms bound to Li+ cations (com plexed dihedrals) were found to have significantly slower conformational dy namics than those dihedrals not bound to Li+ cations (uncomplexed dihedrals ). In the dilute solutions, the dynamics of the complexed dihedrals were fo und to be only weakly dependent on composition, and the dynamics of the unc omplexed dihedrals were found to resemble closely those of pure PEG. For th e EO:Li = 5:1 system, the conformational dynamics of both complexed and unc omplexed dihedrals were dramatically slower than in the more dilute solutio ns, 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 sign ificant increase in the Rouse time and a significant decrease in the polyme r self-diffusion coefficient, was also observed. Chain dynamics exhibited b ehavior consistent with salt-rich and PEG-rich domains for EO:Li greater th an or equal to 15:1. The slowing down of chain dynamics was found to be cor related with an increase in the torsional correlation time due to restricti on of the conformational space available for complexed dihedrals, resemblin g behavior seen in simulations of polymer melts approaching the glass-trans ition temperature. The ether oxygen-cation bond was found to be quite labil e, with an average lifetime of approximately 100-200 ps, while cations tran slate 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 c ross-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 we re predicted to within 1 order of magnitude of experimental values for simi lar systems.