Molecular scale methods

Next to metals, probably the synthetic polymer-based composites have been modeled most by hierarchical multiscale methods. Different multiscale formulations have been approached top-down internal state variable approaches, self-consistent (or homogenization) theories, and nanoscale quantum-molecular scale methods. 

X-ray absorption fine structure spectroscopy has become one of the key molecular-scale methods in studies of reactions mechanisms of aqueous cations and anions at environmental interfaces. 

MODELING AND SIMULATION TECHNIQUES 9.3.1 MOLECULAR SCALE METHODS... 

The modeling and simulation methods at molecular level usually employ atoms, molecules or their clusters as the basic units considered. The most popular methods include molecular mechanics (MM), MD, and MC simulation. Modeling of polymer nanocomposites at this scale is predominantly directed toward the thermodynamics and kinetics of the formation, molecular stracture, and interactions. The diagram in Figure 1 describes the equation of motion for each method and the typical properties predicted from each of them [17-22]. We introduce here the two widely used molecular scale methods— MD and MC. 

A fiirther theme is the development of teclmiques to bridge the length and time scales between truly molecular-scale simulations and more coarse-grained descriptions. Typical examples are dissipative particle dynamics [226] and the lattice-Boltzmaim method [227]. Part of the motivation for this is the recognition that... 

It is possible to use computational techniques to gain insight into the vibrational motion of molecules. There are a number of computational methods available that have varying degrees of accuracy. These methods can be powerful tools if the user is aware of their strengths and weaknesses. The user is advised to use ah initio or DFT calculations with an appropriate scale factor if at all possible. Anharmonic corrections should be considered only if very-high-accuracy results are necessary. Semiempirical and molecular mechanics methods should be tried cautiously when the molecular system prevents using the other methods mentioned. 

These apparent restrictions in size and length of simulation time of the fully quantum-mechanical methods or molecular-dynamics methods with continuous degrees of freedom in real space are the basic reason why the direct simulation of lattice models of the Ising type or of solid-on-solid type is still the most popular technique to simulate crystal growth processes. Consequently, a substantial part of this article will deal with scientific problems on those time and length scales which are simultaneously accessible by the experimental STM methods on one hand and by Monte Carlo lattice simulations on the other hand. Even these methods, however, are too microscopic to incorporate the boundary conditions from the laboratory set-up into the models in a reahstic way. Therefore one uses phenomenological models of the phase-field or sharp-interface type, and finally even finite-element methods, to treat the diffusion transport and hydrodynamic convections which control a reahstic crystal growth process from the melt on an industrial scale. 

A full-scale treatment of crystal growth, however, requires methods adapted for larger scales on top of these quantum-mechanical methods, such as effective potential methods like the embedded atom method (EAM) [11] or Stillinger-Weber potentials [10] with three-body forces necessary. The potentials are obtained from quantum mechanical calculations and then used in Monte Carlo or molecular dynamics methods, to be discussed below. 

Generally, wet chemical methods require the availability of compounds which are soluble in water or in another solvent. The dissolution in a solvent allows intimate mixture on an atomic or molecular scale. However, it has to be ensured that no compound precipitates before the others when the solvent is evaporated, or else insoluble products are formed. The importance of wet chemical methods lies in the possibility of simple upscaling to industrial needs. 

An interpenetrating polymer network (IPN) is defined as a material comprising two or more networks which are at least partly interlaced on a molecular scale, hut not covalently bonded to each other. These networks caimot he separated unless chemical bonds are broken. Two possible methods exist for preparing them, as follows ... 

In literature, some researchers regarded that the continuum mechanic ceases to be valid to describe the lubrication behavior when clearance decreases down to such a limit. Reasons cited for the inadequacy of continuum methods applied to the lubrication confined between two solid walls in relative motion are that the problem is so complex that any theoretical approach is doomed to failure, and that the film is so thin, being inherently of molecular scale, that modeling the material as a continuum ceases to be valid. Due to the molecular orientation, the lubricant has an underlying microstructure. They turned to molecular dynamic simulation for help, from which macroscopic flow equations are drawn. This is also validated through molecular dynamic simulation by Hu et al. [6,7] and Mark et al. [8]. To date, experimental research had "got a little too far forward on its skis however, theoretical approaches have not had such rosy prospects as the experimental ones have. Theoretical modeling of the lubrication features associated with TFL is then urgently necessary. 

H. Goodman and G. Banker. Molecular-scale drug entrapment as a precise method of controlled drug release I Entrapment of cationic drugs by Polymeric flocculation, J. Pharm. Sci. 59 1131-1137, 1970. 

Mechanical and chemical methods for qualitative and quantitative measurement of polymer structure, properties, and their respective processes during interrelation with their environment on a microscopic scale exist. Bosch et al. [83] briefly discuss these techniques and point out that most conventional techniques are destructive because they require sampling, may lack accuracy, and are generally not suited for in situ testing. However, the process of polymerization, that is, the creation of a rigid structure from the initial viscous fluid, is associated with changes in the microenvironment on a molecular scale and can be observed with free-volume probes [83, 84]. 

Both preparation procedures (Ev and SG) aimed at fine distribution of the elements and at obtaining solid solutions. However, the sol-gel methods are known to be particularly powerful to achieve molecular scale dispersion in mixed oxides. The studied samples are shown in Table 1. Taking into account the fact that SrO was easily carbonated, that the surface of SrNd-Ev was covered by carbonate and SrNd-SG was partly composed of bulk carbonate (as it was shown by CO2 TPD, FTIR, and XRD), SrC03 was used instead of SrO. 

Nanotechnology involves the manipulation of matter on atomic and molecular scales. This technology combines nanosized materials in order to create entirely new products ranging from computers to micromachines and includes even the quantum level operation of materials. The structural control of materials on the nanometer scale can lead to the realization of new material characteristics that are totally different from those realized by conventional methods, and it is expected to result in technological innovations in a variety of materials including metals, semiconductors, ceramics, and organic materials. 

The already critical need for molecular-scale compositional mapping will increase as more complex structures are assembled. Currently, electron microscopy, scanning probe microscopy (SPM) and fluorescence resonance energy transfer (FRET) are the only methods that routinely provide nanometer resolution. 

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