Isotropic-nematic phase transition dynamics

The SD is a phase separation process usually occurring in systems consisting of more than two components such as in solutions or blends. However, in the present case the system employed is composed of one component of pure PET. In this case, what triggers such an SD type phase separation Doi et al. [24, 25] proposed a dynamic theory for the isotropic-nematic phase transition for liquid crystalline polymers in which they showed that the orientation process... 

A. Dynamics across the Isotropic-Nematic Phase Transition... 

The capillary condensation phenomenon is of course not exclusive to water. It can be found in any confined system, where the surfaces prefer one phase over another and there is a first order phase transition between the phases of the material between the surfaces. A nematic liquid crystal is an example of such a system exhibiting a first order phase transition between the isotropic and the nematic phase. For this system, the nematic capillary condensation has been predicted by P. Sheng in 1976 [17]. Since the isotropic-nematic phase transition is only weakly first order, the phenomenon is not easy to observe. One has to be able to control the distance between the surfaces with a nanometer precision and the temperature within 10 K, which is unachievable to methods like NMR, SEA, DSC, etc., and very difficnlt to achieve in dynamic light scattering experiments [18,19]... 

Here, 1 will show that the measurement of the dynamic heterogeneity revealed the characteristics of the random structure in the B4 phase [64]. Figure 10.30a shows the time autocorrelation function of the BX phase (at 27 °C) and the B4 phase (at 34 °C) of a mixed system of 80 wt% 5CB. Figure 10.30b shows the nematic and isotropic phases of pure 5CB for comparison. Large fluctuations are present around 0.1 ms in the Bx phase, but the corresponding fluctuation in the B4 phase has disappeared. In addition, the Bx-B4 transition temperature of the mixed system is almost the same as the phase transition temperature of pure 5CB, 34.10 °C. In the phase diagram, Bx-B4 transition temperature corresponds to the extrapolated isotropic-nematic phase transition temperature phase of 5CB. From these two experiments we reason that the fluctuations that had been observed in the Bx phase come from the orientation fluctuations of the 5CB nematic... 

Torre, R., Tempestini, F., Bartolini, P. and Righini, R. (1995). Collective and single particle dynamics near the isotropic-nematic phase transition. Philos. Mag. B 77 645-653. 

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... 

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]. 

It is observed that in the nematic phase of a liquid crystal, the solvation dynamics of coumarin 503 are biexponential [184a]. The slowest time constant decreases from 1670 ps at 311.5 K to 230 ps at 373 K. The solvation time is not affected by the nematic-isotropic phase transition. Thus, it appears that the local environment and not the long-range order controls the time-dependent Stokes shift. A theoretical model has been developed to explain the experimental findings. This model takes into account the reorientation of the probe as well as the fiuctuation of the local solvent polarization. Similar results are also obtained for rhodamine 700 in the isotropic phase of octylcyanobiphenyl [184b]. 

Figure 13. Orientational dynamics of the Lebwohl-Lasher lattice model (N = 1000) at temperatures near the isotropic-nematic transition, (a) Time evolution of the single-particle second-rank orientational time correlation function in a log-log plot at temperatures T = 1.213,1.176, 1.160,1.149,1.134. Temperature decreases from left to right, (b) Decay of the OKE signal in a log-log plot at short-to-intermediate time window at temperatures T = 1.176, and 1.149. Temperature decreases from top to bottom on the left side of the plot. The dashed lines are the simulation data and the continuous lines are the linear fits to the data. The system undergoes a transition from the isotropic to the nematic phase at T 1.14. (Reproduced from Ref. 115.)...

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