Liquid crystal isotropization temperature

Note The inherent viscosity is in dl/g Tm melting point Ti isotropization temperature Tic isotropic liquid to liquid crystal transformation temperature in cooling Tc crystallization temperature. All the Ts are the peak temperatures in the DSC curves and are all in °C. 

Table I. Liquid Crystal/Isotropic Liquid Transition Temperatures...

Some chiral liquid crystals, as temperature is decreased, exhibit the mesophases isotropic cholesteric —> smectic-A. Because of the propoly shown by Equation (1.70), there is no... 

For lyotropic liquid crystals, the temperature plays a secondary role in the formation of the individual mesophases. The primary influence on the phase sequence is exerted by the solvent concentration. The solvent concentration is directly connected to the packing parameter and thus to the micellar shape (cf. Sect. 3.1), which largely determines the mesophase type. At low solvent concentrations lamellar phases are usually formed. By increasing the solvent concentration, columnar and nematic phases appear. At very high solvent concentrations an isotropic micellar solution dominates. An illustration of this phase behavior is shown in the theoretical phase diagram depicted in Fig. 3.9 (c [14]). The individual phases in Fig. 3.9 are separated by biphase regions. 

Fig. 2 Sketch of liquid crystal confined in the gap between a modified microsphere and a glass substrate (not to scale). Dependent on the anchoring strength, liquid crystal and temperature the substrate induces prenematic or/and presmectic alignment. If both surfaces approach each other, isotropic liquid crystal condenses into a nematic phase in the gap between both surfaces, causing attraction...

It is well known that certain aromatic esters give nematic or smectic liquid crystals. The temperatures of the solid-liquid crystalline and liquid crystalline-isotropic fluid transitions have been shown to be sensitive to the symmetry of the molecular structure, as influenced by side groups, and to the type of end groups on the molecule. 

Li JL, Crandall KA, Chu P, Percec V, Petschek RG, Rosenblatt C (1996) Dendrimeric liquid crystals isotropic—nematic pretransitional behavior. Macromolecules 29 7813-7819 Lin YG, Winter HH (1991) High-temperature recrystallization and rheology of a thermotropic liquid crystalline polymer. Macromolecules 24 2877-2882 Lin Q, Yee AF (1994) Elastic modulus of in-situ composites of a liquid crystalline polymer and polycarbonate. Polym Compos 15 156-162... 

From Equation (8.16) one can see that the molecular orientational nonlinearity in the isotropic phase of a liquid ciystal is directly proportional to the laser-induced order parameter Q. In typical anisotropic liquids (e g., CS2 or liquid crystals at temperatures far above Tq), the value of Q may be obtained by a statistical mechanics approach. In the completely random system, the average orientation is described by a distribution function G) ... 

The liquid-crystal transition between smectic-A and nematic for some systems is an AT transition. Depending on the value of the MacMillan ratio, the ratio of the temperature of the smectic-A-nematic transition to that of the nematic-isotropic transition (which is Ising), the behaviour of such systems varies continuously from a k-type transition to a tricritical one (see section A2.5.91. Garland and Nounesis [34] reviewed these systems in 1994. 

Liquid crystals represent a state of matter with physical properties normally associated with both soHds and Hquids. Liquid crystals are fluid in that the molecules are free to diffuse about, endowing the substance with the flow properties of a fluid. As the molecules diffuse, however, a small degree of long-range orientational and sometimes positional order is maintained, causing the substance to be anisotropic as is typical of soflds. Therefore, Hquid crystals are anisotropic fluids and thus a fourth phase of matter. There are many Hquid crystal phases, each exhibiting different forms of orientational and positional order, but in most cases these phases are thermodynamically stable for temperature ranges between the soHd and isotropic Hquid phases. Liquid crystallinity is also referred to as mesomorphism. 

The phase behavior of a-ester sulfonates has been studied in detail with methyl laurate and methyl palmitate [58]. In both cases, at higher temperatures, as the surfactant concentration increases, there is a transition from an isotropic solution to a hexagonal liquid crystalline phase and finally, at high surfactant concentrations, to a lamellar liquid crystal (Fig. 4). The crystal/liquid-crys-tal phase transition occurs at even higher temperatures as the chain length increases. On the other hand, chain length has practically no influence on the... 

Unlike low molar mass liquid crystals, these materials do not undergo a nematic-isotropic transition. Instead, they adopt liquid crystal behaviour throughout the region of the phase diagram for which they are in the melt. Above a particular temperature, rather than adopting an isotropic liquid structure, they decompose. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

The question arises as to how useful atomistic models may be in predicting the phase behaviour of real liquid crystal molecules. There is some evidence that atomistic models may be quite promising in this respect. For instance, in constant pressure simulations of CCH5 [25, 26] stable nematic and isotropic phases are seen at the right temperatures, even though the simulations of up to 700 ps are too short to observe spontaneous formation of the nematic phase from the isotropic liquid. However, at the present time one must conclude that atomistic models can only be expected to provide qualitative data about individual systems rather than quantitative predictions of phase transition temperatures. Such predictions must await simulations on larger systems, where the system size dependency has been eliminated, and where constant... 

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]. 

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

Here we have used the zero-field nematic distribution function PQ( ) for convenience of notation. The degree of net polar alignment can be seen to be enhanced in the liquid crystal over the isotropic case. The limiting cases are isotropic distributions and the Ising model (in which only 6=0 and 6=n are allowed orientations). By retaining only the leading terms in the last equation one sees that in the high temperature limit... 

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