Sylvie Neyertz

Molecular dynamics simulation of crystalline poly(ethylene oxide)

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...

A general pressure tensor calculation for molecular dynamics simulations

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...

Local structure and mobility of ions in polymer electrolytes: A molecular dynamics simulation study of the amorphous PEOxNaI system

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 PEOxNaI (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 i...

A computer simulation study of the chain configurations in poly(ethylene oxide)‐homolog melts

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.

Molecular dynamics simulation of the crystalline phase of poly(ethylene oxide)-sodium iodide, PEO3NaI.

Abstract The polymer-polymer interaction potential used in an earlier reported MD simulation of crystalline PEO [Neyertz, Brown and Thomas, J. Chem. Phys. , 101 , 10064 (1994)] is here transferred to the crystalline phase of poly(ethylene oxide)-sodium iodide, PEO 3 NaI. An appropriate set of ion-ion and ion-polymer interactions are also introduced. Force-field and other computational parameters are optimized to reproduce satisfactorily the behaviour of crystalline PEO 3 NaI. From the MD simulations performed at 300 K, results are presented for the pressure tensor, mean-square displacements, distributions of torsion angles and radial distribution functions. These are compared with the X-ray determined PEO 3 NaI and pure PEO structures.

Tutorial: Molecular Dynamics Simulations of Microstructure and Transport Phenomena in Glassy Polymers

This synopsis illustrates the current possibilities and limitations of classical molecular dynamics (MD) simulations with respect to fully‐atomistic models of high‐performance glassy polymers such as polyimides. Realistic molecular models are potentially able to characterize the microstructures and transport mechanisms at the atomistic level, and thus complement experimental evidence. A hybrid pivot Monte Carlo – MD generation procedure, which allows for chain configurations characteristics of the equilibrium bulk melt to be created at the required temperature from single‐chain sampling under a local energy approximation, has been developed and validated for several different polymers. More recently, an original procedure, loosely based on the experimental solvent‐casting process, has been designed for creating membrane models. Direct applications include realistic bulk and membrane models of polyimide matrices, in an attempt to link macromolecular structure and dynamics to the transport mechanisms for a series of small penetrants. Polymers have first been studied in the pure bulk state in order to characterize the influence of the chemical structure on configurations, preferential interactions, void‐space morphologies or chain mobilities. In a second stage, gas or water molecules have been inserted into the pre‐prepared polymer matrices. Their diffusion and solubility properties have led to further studies on specific aspects of these models, such as the influence of simulation box size, or the presence of confined and freestanding interfaces on penetrant transport.

Preparation of bulk melt chain configurations of polycyclic polymers

The configurations of oligomers of polyimide and polyetherketone polycyclic polymers in the melt are predicted by a new hybrid pivot Monte Carlo (PMC)/molecular dynamics (MD) single-chain sampling technique restricted to a limited number of near-neighbor interactions. These are then compared to configurations obtained for the same models by running MD simulations on the corresponding multichain systems in the bulk melt. A new phantom-atom technique is introduced which avoids interlocking rings during construction of the bulk melt samples. Similar to earlier work carried out on polyethylene, polyvinylchloride and uncharged polyethylene oxide, both theoretical and bulk melt sampled conformational and configurational properties are found to be in very good agreement. This confirms that the new hybrid PMC/MD sampling is a promising and cost-effective technique for preparing polymer samples prior to subsequent MD simulations of the bulk amorphous phase.

A new all‐atom force field for crystalline cellulose I

The details of a new all-atom force field designed to reproduce the phases of the native I-α and I-β forms found in crystalline cellulose I are reported in this article. The energy differences, densities, unit cell parameters, and moduli are in close agreement with experimental evidence. Analyses of the modular dynamics simulations also included thermodynamic data and angle distributions as well as characterization of the intrachain, intrasheet, and intersheet hydrogen-bond networks for both phases under study.

The local energy approximation and the predictability of chain configurations in polymer melts

The configurations of a series of short polar model molecules for poly(vinyl chloride), PVC, and poly(ethylene oxide), PEO, have been determined from extensive molecular dynamics simulations of the corresponding bulk melts. Their conformational and configurational properties are compared to those sampled for the same models by a pivot Monte Carlo procedure based on the assumption that chain configurations in the melt depend only on highly localized near‐neighbor intramolecular interactions. The comparison proves favorable for all neutral and polar chains, with the exception of the realistic model leading to the gauche effect in PEO. Discrepancies in the latter case are related to the failure of the single‐chain local energy approximation to account for the specific competition between intramolecular ‘‘1...5’’ C–H...O and intermolecular C–H...O electrostatic interactions in the PEO bulk melts.

Copolyimides with trifluoromethyl or methoxy substituents. NMR characterization

Abstract In the first stage, a series of aromatic diamine compounds such as 2-methoxy-5,4′-diaminodiphenyl ether (ODAOMe) and 2-trifluomethyl-4,4′-diaminodiphenyl ether (ODACF 3 ) were synthesized. These aromatic diamines and 4,4′-diaminodiphenyl ether (ODA) were then used to prepare copolyimides with 4,4′-oxydiphthalic anhydride (ODPA) and bicyclo[2.2.2]-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA). Both chemical composition and the arrangements of repetitive units were characterized by 1 H and 19 F NMR. It was shown that solubility and thermal stability are related to the BCDA fraction in the copolymers and to the chemical structure of the diamine.