Molecular dynamics isotropic-nematic transition

Figure 11.5 Complex dynamic viscosity as a function of temperature for a main-chain polyether consisting of a methyl stilbene mesogen and a mixture of seven-and nine-carbon aliphatic spacers. The polymer has a molecular weight of 36,000. The diamonds and squares are for temperature ramp rates of 0.1 °C and 2.0°C/min, respectively the open and closed symbols are for heating and cooling, respectively. The dashed line marks the isotropic-nematic transition. (From Gillmor et al. 1994, with permission from the Journal of Rheology.)...

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 concentrated regime is where n > 1/dL ox cp >ap. In this range the dynamic properties of the fibers can be severely affected by fiber-fiber interactions and can lead to solid-like behavior. It is interesting to note that most fiber composites of industrial interest typically have fiber concentrations of ( > 0.1 and fall within the concentrated regime. In addition to the three regimes defined above, molecular theories define a critical concentration in which molecules will preferentially align to form a nematic liquid crystalline phase, a phase intermediate to a purely crystalline phase and an isotropic liquid phase. However, it has yet to be proved that fiber suspensions will also go through this transition (Larson, 1999). 

MD-EPR approach has also been sueeessfully applied to study the dy-namies and ordering of the molecules in the bulk phases of soft matter systems sueh as nematic liquid crystals nCB doped with nitroxide spin probes. MD simulations have been reported at both coarsegrained and fully atomistic levels. Predicted ehanges in molecular order, dynamics and variable temperature EPR line shapes across the nematic (N) to isotropic (I) phase transitions showed excellent agreement with experiment. A combined MD-EPR approach provides a new level of detail to descriptions of molecular motions and order. Figure 7 shows snapshots of isotropie (top) and nematic (bottom) states of 8CB with doped CLS spin probe. It also presents comparison between predicted and measured EPR spectra of 8CB along the N-I phase transition curve... 

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