A hybrid method for the calculation of the pressure tensor in molecular dynamics (MD) simulations, which combines both the thermodynamic and mechanical definitions, is presented here. The need for a new approach was motivated initially by MD simulations of crystalline poly(ethylene oxide) (PEO) (S. Neyertz, D. Brown, and J. O. Thomas, 1994, J. chem. Phys., 101, 10064), as the combination in this model of effectively ‘infinite’ chains, holonomic constraints and the long-range Coulomb potential calculated via the Ewald summation method, presented difficulties with regard to the traditional routes for calculation of the pressure tensor. However, this technique is quite general and thus applicable also to most MD simulations performed using periodic boundary conditions. In this paper, we address in detail the contribution of forces arising from different parts of the potential and the constraints to the pressure tensor. The method is then illustrated in a model of crystalline PEO, where the sensitivity of the pressure tensor to details of the potential provides an effective criterion for assessing possible parameter sets.

Molecular dynamics (MD) simulation holds great promise as a source of otherwise elusive information concerning ionic conduction mechanisms occurring in the amorphous phases of polymer electrolytes. However, most polymer/salt complexes have a multiphase character at temperatures of interest. Insights into crystalline phases may thus prove meaningful in the subsequent design of strategies to decrease the degree of crystallinity in these systems. We report here the full details of a molecular dynamics model (‘‘md’’ model) for the crystalline phase of the widely used host‐polymer poly(ethylene oxide) (PEO). Force‐field and computational parameters are optimized to give realistic behavior for crystalline PEO. Analyses of the structure and dynamics obtained from the MD simulations performed at 300 K include mean‐square displacements, x‐ray powder diffractograms, distributions of bond and torsion angles, and radial distribution functions. These are compared with experimental data, the static x‐ray determined PEO structure and that obtained using a ‘‘tethered’’ model, in which the atoms are attached by springs to their initial crystalline positions, and allowed to move according to their experimental mean‐square displacements. Agreement is good, but the reliability of the x‐ray refined positions is questioned.

The configurations of a series of diethyl ether chain molecules with the general formula C2H5–O–(CH2–CH2–O)m–C2H5, i.e., homologs of poly(ethylene oxide), PEO, have been determined from NpT molecular dynamics simulations of the pure melts at 400 K. The fraction of trans conformers, the mean square radii of gyration, and the mean square end-to-end distances have been compared to those predicted by a pivot Monte Carlo sampling method based on the assumption that chain configurations in the melt are largely determined by highly localized intramolecular near-neighbor interactions. Discrepancies are significant and point to inadequacies in the latter picture when Coulomb potentials are explicitly taken into account.

Solid polymer electrolytes are ionically conducting phases formed by dissolving salts in an amorphous polymer matrix. In this study, the local structure and dynamics of Na⁺ and I⁻ ions in molecular dynamics (MD) simulations of the amorphous poly(ethylene oxide)‐based electrolyte PEO_xNaI (x=48,20,3) are analyzed at both 400 and 500 K. The fully atomistic model reproduces many phenomena seen experimentally and provides a picture of the complex correlations between cation, anion, and polymer host in these systems. The composition of the first coordination shell around the cations illustrates the concentration‐dependent competition between iodines and PEO backbone oxygen atoms to coordinate the positively charged cations. Contiguous polymer segments tend to form near‐planar polydentate loops around the sodiums while the anions are usually placed above and/or below the PEO...Na⁺ quasiplane. This geometry results in optimal coordination of both types of ligands to the cation in a similar pattern to that found in crystalline PEO‐based systems and crown ethers. Although the observation of large‐scale diffusion is still effectively precluded by the currently accessible MD time scale, discrete ligand exchanges do occur in the coordination shell of the cations. Examples of these dynamical events, including various jumps between local potential coordination sites, are shown and their net result on ionic motion is discussed.