There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

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Cellulose nanomaterials review: structure, properties and nanocomposites

Advanced polyimide materials: Syntheses, physical properties and applications

In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

https://iopscience.iop.org/article/10.1088/0953-8984/17/15/R01/pdf

Effects of confinement on material behaviour at the nanometre size scale

Polymeric CO2/N2 gas separation membranes for the capture of carbon dioxide from power plant flue gases

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.

Molecular dynamics simulation of crystalline poly(ethylene oxide)

Fully atomistic molecular dynamics (MD) simulations have been carried out on a series of bulk melt pure PEO oligomers and PEO oligomer−silica systems, which differed by their macromolecular end groups. A realistic hybrid model of a silica nanoparticle combining an ionic core as well as the fine-tuning of the surface thickness and number of OH groups per unit surface area was used. The PEO oligomers were decorrelated in all systems under study in order to prevent any artifacts related to the preparation procedure. Significant changes were found to occur in the immediate vicinity of the interface with flattened PEO backbones arranged in densily packed shells and stabilized by the added PEO−silica interactions. Their conformations were also more coiled in order to better adapt to the surface structure. While methyl end groups did not show special characteristics other than their steric effect, hydroxyl end groups had a much higher affinity for the surface and tended to position themselves perpendicular to th...

Interface between End-Functionalized PEO Oligomers and a Silica Nanoparticle Studied by 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...

A general pressure tensor calculation for molecular dynamics simulations

Extensive molecular dynamics (MD) simulations of 6FDA−6FpDA, 6FDA−6FmDA, and 6FDA−DAM polyimides with CO2 weight percentages up to ∼30%, were carried out to characterize the atomic level features associated with CO2 diffusivity in these glassy matrices. The fluorinated polyimide models were first loaded with CO2 in increments of 2% in order to mimic the experimental procedure of progressive loading and to avoid the necessity of artificially preswelling the simulation boxes. The sorption phase was then followed by a progressive desorption phase in decrements of 2%. This work covered nominal CO2 concentrations up to ∼200 cm3(STP) cm−3 and amounted to a total of more than 80 simulations of 5000 ps each at 308 K, as well as an additional 20 simulations at higher temperatures. In all cases, CO2 trajectories display the basic hopping-type mechanism, i.e. a combination of oscillations within available free volumes in the polymer matrix associated with occasional jumps from one site to another. There are no long-...

Carbon Dioxide Diffusion and Plasticization in Fluorinated Polyimides

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.

Sylvie Neyertz

Papers

Molecular dynamics simulation of crystalline poly(ethylene oxide)

Interface between End-Functionalized PEO Oligomers and a Silica Nanoparticle Studied by Molecular Dynamics Simulations

A general pressure tensor calculation for molecular dynamics simulations

Carbon Dioxide Diffusion and Plasticization in Fluorinated Polyimides

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