Modelling materials driven far from equilibrium

Materials driven from equilibrium offer a challenging field for both time and space multiscale modelling. In materials under irradiation the elementary damaging process, that is, the nuclear collision leading to the displacement cascade, is modelled by molecular dynamics (≈ 10 ps, 10³ nm³), whereas the later stage recovery process is modelled by kinetic Monte Carlo (≈ years, 10⁴ nm³); at still a coarser scale, grain boundary segregations are modelled by a meanfield approximation of the very same atomistic model as used in kinetic Monte Carlo modelling. Connecting this broad range of time scales helps to elucidate basic issues on the criteria for alloy stability under irradiation. Irradiation effects on plasticity are beginning to be modelled in the same spirit; a first molecular dynamics study of the interaction of cascade debris with gliding dislocations has been reported together with the first results of mesoscale modelling of dislocation dynamics in the course of a nanoindentation test. Such studies can be extended to materials processed by ball milling.     

Deformation mechanisms, electronic conductance and friction of metallic nanocontacts.

Close interactions between experiments and computer simulations of metallic contacts have revealed the central role of mechanical instabilities in the evolution of the electronic conductance of atomic-scale contacts pulled to fracture. At the same time crucial differences in the time scales of relaxation processes that occur in experiments and in simulations have been highlighted and have raised fundamental questions about modelling strategies.     

Multi-scale modeling strategies in materials science—The quasicontinuum method

The problem of prediction of finite temperature properties of materials poses great computational challenges. The computational treatment of the multitude of length and time scales involved in determining macroscopic properties has been attempted by several workers with varying degrees of success. This paper will review the recently developed quasicontinuum method which is an attempt to bridge the length scales in a single seamless model with the aid of the finite element method. Attempts to generalize this method to finite temperatures will be outlined.     

Linear scaling methods for electronic structure calculations and quantum molecular dynamics simulations

In the past five years, notable advances in the field of electronic structure calculations have been made, by the development of linear scaling methods for total energy calculations and quantum molecular dynamics simulations. These are methods implying a computational workload which grows linearly with th system-size,in contrast to standard algorithms where the workload scales as the cube of the system-size. Therefore the use of linear scaling methods can considerably widen the class of systems and type of problems being tackled with quantum simulations. At present, linear scaling methods using semi-empirical Hamiltonians allow one to perform simulations involving up to a thousand atoms on small workstations, and up to ten thousand atoms for tens of picoseconds when using supercomputers. This has made it possible to study problems such as large organic molecules in water, thin film growth on a surface and extended defects in semiconductors. Although the implementation of first-principles linear scaling methods is less advanced than that of semi-empirical methods, promising results regarding organic molecules and metal-alloys have already appeared in the literature.     

Aspects of transition metal mediated polymerizations

Catalysts based on metallocene complexes of early transition metals herald a new era in Ziegler—Natta olefin polymerization. The developments are driven primarily by the flexibility and high activity of metallocene catalysts, which allow excellent control of polymer properties and have led to a remarkably wide spectrum of polymers and copolymers. In-depth mechanistic understanding is proving crucial for catalyst design and has opened up the use of metallocenes in non-Ziegler chemistry, notably as initiators for carbocations polymerizations. Recently developed late transition metal complexes have substantially added to the range of olefin (co)polymers that are now accessible, including copolymerizations with carbon monoxide and polar monomers.     

From catenanes to mechanically-linked polymers

Dramatic recent advances in the synthesis of [2]catenanes, together with an increased understanding of the dynamics and properties of the mechanical bond, have made available for the first time bulk quantities of topologically complex building blocks, which can form the basis of a new generation of mechanically-linked macromolecules. Highlights include the application of ring closing metathesis to catenane synthesis, catenanes formed under thermodynamic control and the preparation of the first linear main chain oligomers and polymers with mechanically-linked backbones.     

Mesoscopic modelling

Mesoscopic modelling covers a wide range of ideas and models. The objective is to study length-scales and time-scales that are not easily accessible to either atomistic or macroscopic continuum methods. ‘Grand-challenge’, large-scale atomistic simulations push at the boundary of the mesoscale at the lower end, while increasing sophistication in micromechanics push at the boundary from the upper end. The increase in computing power is also responsible for the increasing number of 3D as opposed to 2D calculations. Obtaining well-defined, well-validated models remains difficult despite some recent successes.     

Recent developments in anionic polymerization

The past year witnessed very significant advances in the living anionic polymerization of (meth)acrylate monomers, particularly in hydrocarbons at or below 0°C. Block polymerization of alkyl methacrylates with primary alkyl acrylates, although somewhat improved, remains a challenge. Anionic polymerization of styrene, diene and its derivatives was carried out with the aim of synthesizing functional polymers and block copolymers of various architectures. There has been a trend towards combining different living techniques in order to design polymers of unique architectures and properties     

The theory of polymer dynamics

Recent years have brought exciting theoretical advances to understanding the behavior of macromolecular systems under nonequilibrium conditions. Developments in diffusion-controlled reactions of polymers are bringing molecular insights to reactive blending technologies, and improved theories relating to associating polymers should aid in the design of thickening agents and coatings. Recent progress in molecular theories of flow and deformation may facilitate the design of branched polymers with tailored rheological properties and improved adhesives.     

Nanotubes

The field of nanotubes is undergoing an explosive growth, fueled by brakethroughs in synthesis and the promise of unique applications. Highly unusual properties and devices have been predicted and/or observed, including extremely high strength and flexibility, nanoscale electronic devices consisting entirely of carbon, and strong capillary effects leading to the production of exceptionally thin wires, cold cathode field emission and other effects.     

Growth aspects of quantum dots

Recent theoretical developments on growth aspects of quantum dots, which appear during highly mismatched semiconductor epitaxy, include size distribution and lateral and vertical self organization. The stable shape of coherent islands has also recently been investigated, especially regarding the role of surface tension. Strain fields, have proved to play a very important role in these systems, and different ways of modelling the inhomogeneous strain distribution have been used successfully.     

End-tethered chain molecules at liquid interfaces

Polymeric chains attached by one end to an interfce and immersed in a liquid environment provide a powerful tool for the modification of interfacial properties. Recent developments have advanced our understanding and control over interfacial polymer layers at the molecular level, with clear implications to tribological, biosensing and multicomponent systems design considerations.     

Tailoring magnetic and dielectric properties of rubber ferrite composites containing mixed ferrites

Rubber ferrite composites containing various mixed ferrites were prepared for different compositions and various loadings. The magnetic and dielectric properties of the fillers as well as the ferrite filled matrixes were evaluated separately. The results are correlated. Simple equations are proposed to predetermine the magnetic and dielectric properties. The validity of these equations is verified and they are found to be in good agreement. These equations are useful in tailoring the magnetic and dielectric properties of these composites with predetermined properties.     

Molecular modelling of polymers

Significant advances have been made in the application of molecular modelling techniques to understanding the properties of polymers. The highlights include the use of detailed atomistic studies of segmental motions and penetrant motions and amorphous polymers aimed at enhancing the interpretation of experimental data, nd mesoscale modelling of diblock copolymer microphase separation.     

Synthesis of an organic-inorganic hybrid material by solid state intercalation of 2-mercaptopyridine into Na-, Al(III)- and Co(II)-montmorillonite

The preparation of an organic-inorganic hybrid material by solid state intercalation of 2-mercaptopyridine (2Mpy) into Na-, Co(II)- and Al(III)-montmorillonite has been studied using a variety of techniques. The extension ofd ₀₀₁from XRD proves that the intercalation of 2-mercaptopyridine into Na-, Co(II)- and Al(III)-mont occurs at ambient temperature in 5 mn. When the intercalated samples were heated at different temperatures, we found that the d₀₀₁ gave different values. For instance, for intercalated Al(III)- and Co(II)-, d₀₀₁ remained unchanged for a temperature under 500°C. However, for intercalated Na-mont, it shifted to 14 Å for a temperature of 300° C, the washing of different samples with a methanol solution shifted thed ₀₀₁of intercalated Na-mont to 14 Å. However, for intercalated Al(III) and Co(II), it did not change. This proves that in the case of Na-mont, the molecules of 2-mercaptopyridine interact with the clay through hydrogen bindings and physical interactions. However, for Al(III) and Co(II), it forms coordination linking and physical interaction.¹³C NMR and FTIR spectroscopy have been employed for the characterization of the intercalation compounds. Tautomeric equilibrium between thiol and thione species of 2-mercaptopyridine must be taken into account to explain the arrangement of molecular aggregates and their particular orientation in the interlayer space. The isotherm of adsorption-desorption of nitrogen and topographic AFM images prove that intercalation of 2Mpy is accompanied by a total blockage of clay porosity and an increase in roughness.     

Neutron spin echo investigations of polymer dynamics

In 1997, the instrumental basis for neutron spin echo (NSE) investigations of polymer dynamics was considerably enlarged as new and improved spectrometers became available. The scientific highlights include NSE studies on the segmental motion of polymers with various chain architectures in the liquid as well as in the solid state and a direct comparison of NSE findings with the results of molecular dynamics simulations.     

Hydrogen under extreme conditions

Some recent advances in our understanding of the properties of hydrogen and deuterium at megabar pressures are reviewed. The emphasis is on recent spectroscopic experiments to elucidate the nature of the H-A phase (Phase III), the high-pressure phase diagram, and on the reported generation of liquid metallic hydrogen under shock compression conditions. Density Functional and Quantum-Monte Carlo calculations have proved a useful guide to interpreting the experimental results: these will also be reviewed.     

Polymer architecture influence on rheology

The past two years have seen considerable advances in the important challenge of the relationship between polymer chain architecture and melt rheology. The promise of new polymerisation catalysts offering greater control over molecular structure in industrial quantities has given the field greater impetus. Important work in the parallel areas of commercial grade polymers, tailored model polymers and molecular theory has led to a quantitative understanding of linear rheology in some cases, and the promise of at least a qualitative understanding of the puzzling but important behaviour of branched polymers under nonlinear deformations.     

Dislocation modelling at atomistic and mesoscopic scales

Several basic dislocation mechanisms have recently been examined in detail by atomistic simulations. In parallel, mesoscale approaches of dislocation behaviour are being developed for the study of single crystal plasticity. Linking atomistic and mesoscopic simulations is emerging as a new challenge for materials modelling.     

Mesoscopic modelling of high TC superconductors

Advances in experimental techniques, such as magneto-optical imaging, have influenced the way modelling studies are to be approached. In particular, these studies should be coupled with the fundamental understanding of flux lattices, vortex motion, melting and pinning, obtained from phenomenological theories and via various measurement techniques, such as Hall probe arrays, scanning superconducting quantum interference devices and by Lorentz microscopy. There are a range of models and questions to be addressed such as those concerning critical current density in relation to thin films. The ultimate goal is to obtain a cohesive picture of the properties of high T_C superconductors that will be useful from the standpoint of superconducting technology.