Conformational and Structural Relaxations of Poly(ethylene oxide) and Poly(propylene oxide) Melts: Molecular Dynamics Study of Spatial Heterogeneity, Cooperativity, and Correlated Forward-Backward Motion

Performing molecular dynamics simulations for all-atom models, we characterize the conformational and structural relaxations of poly(ethylene oxide) and poly(propylene oxide) melts. The temperature dependence of these relaxation processes deviates from an Arrhenius law for both polymers. We demonstrate that mode-coupling theory captures some aspects of the glassy slowdown, but it does not enable a complete explanation of the dynamical behavior. When the temperature is decreased, spatially heterogeneous and cooperative translational dynamics are found to become more important for the structural relaxation. Moreover, the transitions between the conformational states cease to obey Poisson statistics. In particular, we show that, at sufficiently low temperatures, correlated forward-backward motion is an important aspect of the conformational relaxation, leading to strongly nonexponential distributions for the waiting times of the dihedrals in the various conformational statesComment: 13 pages, 13 figure

Systematic Computational and Experimental Investigation of Lithium-Ion Transport Mechanisms in Polyester-Based Polymer Electrolytes

Understanding the mechanisms of lithium-ion transport in polymers is crucial for the design of polymer electrolytes. We combine modular synthesis, electrochemical characterization, and molecular simulation to investigate lithium-ion transport in a new family of polyester-based polymers and in poly(ethylene oxide) (PEO). Theoretical predictions of glass-transition temperatures and ionic conductivities in the polymers agree well with experimental measurements. Interestingly, both the experiments and simulations indicate that the ionic conductivity of PEO, relative to the polyesters, is far higher than would be expected from its relative glass-transition temperature. The simulations reveal that diffusion of the lithium cations in the polyesters proceeds via a different mechanism than in PEO, and analysis of the distribution of available cation solvation sites in the various polymers provides a novel and intuitive way to explain the experimentally observed ionic conductivities. This work provides a platform for the evaluation and prediction of ionic conductivities in polymer electrolyte materials

Hydrogen adsorption kinetics on Pd/Ce0.8Zr0.2O2

Hydrogen adsorption on Pd/Ce0.8Zr0.2O2 was studied by temperature-programmed reduction, volumetric measurements and IR spectroscopy. Hydrogen uptake and reduction rate at 353 K are strongly dependent on the hydrogen pressure. At relatively high hydrogen partial pressure, reduction involves PdO, the surface and a significant fraction of the bulk of the ceria based oxide. Formation of oxygen vacancies even at low temperature ( F-2(7/2) electronic transition of Ce3+ with hydrogen pressure and surface dehydroxylation. This "severe'' reduction has a negative effect on the subsequent hydrogen adsorption capability. The decrease of hydrogen uptake capacity and rate during adsorption can be associated with the partial loss of superficial OH and the presence of Ce3+, which deactivates Pd electronically

Kinetics of hydrogen chemisorption on high surface area Pd/Ce0.8Zr0.2O2

The effect of pre-reduction conditions on the H2 chemisorption kinetics of high surface area Pd(0.53 wt.%)/Ce0.8Zr 0.2O2 was investigated by means of temperature-programmed reduction, oxygen-storage capacity, IR spectroscopy and volumetric hydrogen measurements. In the 20-80 \ub0 C range, the pre-reduction rate was very fast and dependent on temperature, with an apparent activation energy of 58 kJ mol-1. Distinction between PdO reduction and H spillover processes is only possible at low H2 pressures. The total H2 uptake and the chemisorption rate strongly decrease by the use of high H2 pressures during the pre-reduction process. Irreversible reduction with the concomitant oxygen vacancies formation occurs at low temperature, as demonstrated by in situ IR experiments. However, H2 uptake on the Pd(0.53 wt.%)/Ce0.8Zr0.2O2 during pre-reduction and chemisorption is recovered by reoxidation at 400 \ub0 C