Atomistic simulations, bulk systems

This article reviews progress in the field of atomistic simulation of liquid crystal systems. The first part of the article provides an introduction to molecular force fields and the main simulation methods commonly used for liquid crystal systems molecular mechanics, Monte Carlo and molecular dynamics. The usefulness of these three techniques is highlighted and some of the problems associated with the use of these methods for modelling liquid crystals are discussed. The main section of the article reviews some of the recent science that has arisen out of the use of these modelling techniques. The importance of the nematic mean field and its influence on molecular structure is discussed. The preferred ordering of liquid crystal molecules at surfaces is examined, along with the results from simulation studies of bilayers and bulk liquid crystal phases. The article also discusses some of the limitations of current work and points to likely developments over the next few years. 

In Odegard s study [48], a method has been presented for finking atomistic simulations of nano-structured materials to continuum models of tfie corresponding bulk material. For a polymer composite system reinforced with SWCNTs, the method provides the steps whereby the nanotube, the local polymer near the nanotube, and the nanotube/ polymer interface can be modeled as an effective continuum fiber by using an equivalent-continuum model. The effective fiber retains the local molecular stractuie and bonding information, as defined by MD, and serves as a means for finking tfie eqniv-alent-continuum and micromechanics models. The micromechanics method is then available for the prediction of bulk mechanical properties of SWCNT/polymer com-... 

This section presents a review of atomistic simulations and of a recently introduced mesoscale computational method to evaluate key factors affecting the morphology of CLs. The bulk of molecular dynamics studies in PEFC research has concentrated on proton and water transport in hydrated PEMs (Cui et al., 2007 Devanathan et al., 2007a,b,c Elliott and Paddison, 2007 Jang et al., 2004 Spohr et al., 2002 Vishnyakov and Neimark, 2000, 2001). There has been much less effort in using MD techniques for elucidating structure and transport properties of CLs, particularly in three-phase systems of Pt/carbon, ionomer, and gas phase. 

Table 2. Atomistic simulations of bulk liquid crystal systems.

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

The proposed CG models can also be used for the study of systems more complicated than bulk polymer melts. Possible examples are the study of the diffusion of a penetrant in a polymer matrix, or of block copolymers, blends, etc. [121, 122]. In addition, the method can be directly incorporated into multiscale methodologies, which include multiple levels of simulation, and where both atomistic and mesoscopic descriptions are needed at the same time, but in different regions. An example is the study of the long time scale dynamics of polymers near solid attractive surfaces, where an atomistic description is needed very close to the surface but a mesoscopic description can be used for length scales far from the surface. 

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