Liquid crystals Metastability

Keywords liquid crystals, metastability, symmetry breaking, weak disorder, Irmy-Ma scaling... 

Solvent viscosity vs, concentration plots for cellulose dissolved in TFA-CH2CI2 (70/30, v/v) do not exhibit a maximum (1I,S1) in contrast to the typicid behavior of polymer liquid crystal solutions. This same behavior is exhibited by other cellulose-solvent systems (52,fiQ). Conio et al. (59) si gest that due to the close proximity of the cholesteric mesophase to its solubility limit, it is only observed in a metastable condition. 

Fig. 3. Schematic diagram of the possible states of a liquid crystal forming material. Only the left side of the diagram corresponds to equilibrium. The states towards the right are increasingly metastable or unstable, but are often reached for flexible, linear macromolecules...

As a result of the strong drive to equilibrium, it is usually difficult to quench the isotropic melt to an amorphous glass when liquid crystal formation is possible. Extraordinary quenching techniques may be needed. Once produced, the amorphous state loses its metastability on heating above the glass transition Tga. The melt is quite unstable, so that it may not be possible to keep the melt from changing to the mesophase 21. ... 

Lipid-water gel phases were previously regarded as metastable structures that are formed before separation of water and lipid crystals when the corresponding lamellar liquid crystal is cooled. New information on gel phases (see below) reveals that they can form thermodynamically stable phases with very special structural properties. This characteristic makes them as interesting as the lamellar liquid crystals from a biological point of view. 

Liquid crystals with a large molecular mass are able to form a glassy state with mesomorphic behaviour. Yitzchaik and coworkers38 report about a new class of copolymers with non-linear optical properties, and a monomeric representative with a glassy metastable... 

Spiropyrans have been incorporated as photochromic moieties into many polymers [4,67]. Krongauz and his co-workers [68-72] have extended this work to include several types of SCLC polymers. Small molecule LC materials containing a spiropyran moiety have not yet been reported, presumably because typical spiropyrans are not sufficiently elongated. The covalent attachment of spiropyrans to rodlike mesogenic groups does afford quasi liquid crystals compounds with unusual metastable mesophases [4]. However, these compounds do not show LC... 

Most (but not all) LCDs require that the liquid crystal be uniformly oriented i.e., that the LC exist in the form of a single domain in either the OFF or ON condition. The required alignment can be brought about by imparting to the confining surfaces a unidirectional sense, either by rubbing or by the oblique evaporation of SiO or other compounds (Kahn, 1977). A variety of boundary conditions can therefore be set up, including some that result in a metastable behavior of the enclosed liquid crystal phase. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

Fig. 15. Intermediate stage of the transition from the metastable equilibrium between the liquid crystal and the isotropic solution to the stable equilibrium between the polymeric crystallosolvate and the isotropic solution in a PBA-sulphuric acid system. The photograph is obtained in crossed polaroids (according to...

Figure 3. Schematic free energy diagram for an enantiotropic liquid crystal, where the mesophase is metastable (see text).

Generally, the metastability is a phenomenon associated with the persistence of the given phase well below the stability domain, bordered by the first order transition, for instance (/) the glass transition phenomenon, (//) metastable systems studies linked to spinodals - absolute stability limits, with particular attention towards the inherently metastable negative pressure domain (///) metastability near a critical point, (/v) the quest for the liquid - liquid near-critical transition in one component liquid, (v) the issue of liquid crystals where... 

The second drawback is that the method is usually used in out-of-equilibrium conditions. In some systems, such as those involving lyotropic liquid crystals, the time required to reach equilibrium can be very long metastable phases can also be encountered. 

A. Water. Other systems of the substance water. B, Sulphur. Metastable systems. Bivariant systems. C. Tin. Transition point. D. Phosphorus, E. Liquid Crystals or Anisotropic Liquids. Nature of liquid cryst s. Equilibrium relations in the case of liquid crystals. 

In the 1930 s it was shown by E. Vorlander [Trans. Faraday Soc., 29, 907 (1933)] that some liquid crystals could be quick frozen to a metastable brittle glassy state. By working with a polymeric molecule, a synthetic polypeptide called poly-y-benzyl-L-glutamate, we have been able to obtain stable solid films with a liquid crystalline local structure. These films can be obtained in conditions describable as rubbery, leathery, or glassy, as is common for polymer films. The unusual local structure of the molecules in the liquid crystalline phase gives rise to magnetic and optical properties heretofore not obtainable with polymeric systems. 

Kauzmann did not accept this implication, however. He argued instead that in the temperature interval between 7 and Tk, the probability of crystallization was increasing so that before the isoentropic point could be reached, the time scale for crystallization would become the same as that for configurational relaxation in the amorphous phase, as discussed in Section II above. This would precipitate a first-order phase transition by the spontaneous growth of fluctuations in the appropriate direction. Such an event would necessarily terminate the liquid-state metastable free-energy surface and make the apparent entropy-crossing problem metaphysical and, Kauzmann therefore reasoned, of no consequence. 

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