The dynamics in polyethyleneoxide–alkali iodide complexes investigated by neutron spin-echo spectroscopy and molecular dynamics simulations

Abstract

We determined the self part of the intermediate scattering function in liquid polyethyleneoxide ͑PEO͒ and PEO-alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics ͑MD͒ computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm Ϫ1to approximately 20 nm Ϫ1 . We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt ͑1 metal ion per 15 monomers͒ leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data. © 2000 American Institute of Physics. ͓S0021-9606͑00͒51525-6͔ Amorphous polymer electrolytes provide an environment-friendly alternative for liquid electrolytes used in batteries, fuel cells, electrochemical displays, and chemical sensors.1 A polymer electrolyte is a complex of a polar polymer with a metal salt. In order to optimize performance of applications, it is of fundamental importance to understand the mechanism of ion transport, which is closely coupled to the segmental motions of the polymer chain. The systems most studied are poly͑ethyleneoxide͒ ͑PEO͒ and poly͑propyleneoxide͒ ͑PPO͒ salt complexes.From Brillouin light scattering of PPO-salt systems 2 and MD simulations of PEO-NaI systems 3 it appears that the Na ϩ ions form crosslinks between different oxygen atoms within a polymer chain, which causes slowing down of movement of polymer segments. Quasielastic neutron scattering measurements on the PPO-LiClO 4 complex have confirmed this effect, but because of the limited energy resolution it was impossible to obtain quantitative results for the effect of solvated salt on the structural relaxation.4 Londono et al. 5 have performed neutron diffraction with isotopic Li substitution in combination with MD simulations in order to determine the partial pair distribution function g Li,O (r). They obtained a Li-O coordination number of about 3.5 for PEOLiI ͑O:Mϭ5, which is the number of ether oxygens of the polymer chain per metal ion͒, confirming crosslinking between cations and ether oxygens. It has been shown that the conductivity characteristics for PPO-Li salt and PPO-Na salt are very similar.6 Therefore, we expect that the influence of Li and Na on the polymer dynamics in PEO is similar.Until today, no quantitative results were available on the local dynamics of the backbone segments of the polymer nor on the influence of various parameters such as salt concentration, polymer chain length, and different ions. Neutron spin echo ͑NSE͒ is the technique of choice regarding energy resolution and wave-vector rang...

Molecular dynamics simulation of the polymer electrolyte poly(ethylene oxide)/LiClO4. II. Dynamical properties

“…10, where one sees the good agreement with experimental data obtained by the neutron scattering spectroscopy of a PEO sample with 23 EO repeat units, i.e., ͑-CH 2 CH 2 O-͒ n where n = 23. 52 Confident that the united atom model used in the MD simulations is a reasonable one concerning the structural relaxation of PEO, Fig. 11 compares structural relaxation in pure PEO and PEO/ LiClO 4 .…”

Quasi-Elastic Neutron Scattering Investigation of Dynamics in Polymer Electrolytes

“…For this polymer a complex dynamics has been characterized with a relaxation shape parameter ) 0.6 and a temperature dependence for the relaxation times resembling the rheological one. [12][13][14] In the case of neat PPO, the partial superposition between the methyl group dynamics and the segmental dynamics hinders a detailed distinction and characterization of the latter process. In fact, a number of reports exist on QENS characterization of the segmental dynamics in PPO, and these indicate different values for the parameter in the above T g regime: these values fall in the range ∼0.45, 15 0.7, 40 0.5, 16 and ∼0.4.…”

“…The effect of salt addition on the amorphous PEO dynamics has been studied above the melting point, thus limiting the accessible temperature range. [11][12][13][14] In the case of PPO, as it does not crystallize, a larger temperature range could be studied in the amorphous phase (both liquid and glassy states). [15][16][17][18] The general results arising from these studies indicate that, as many other polymers do, both PPO and PEO show a distinct non-Arrhenius and non-Debye segmental dynamics above their glass transition.…”

TiO2 nano-particles in polymer electrolytes: surface interactions

“…Computational chemistry methods were early employed to reveal the dynamic properties of binary salt/poly (ethylene oxide), PEO, polymer electrolytes [19 -21] and have gained renewed interest [22,23]. These molecular dynamics (MD) studies have also recently been extended to nanoparticle/MX/PEO systems [24,25].…”