Isotropic-mesophase temperature

Note Mesophase X should be stable at lower temperatures than phase Y. For example, the wewai/c-isotropic transition temperature would be denoted as Tni. 

At higher temperatures and water concentrations, the system may shift into the cubic mesophase structure (see Figure 15). The water is present as spheres totally surrounded by monoglyceride. This phase has a high viscosity and is sometimes called viscous isotropic in the hterature the two terms refer to the same structure. In the presence of more water than can be accommodated in the internal spherical phase, one obtains a mixture of lumps of this cubic structure dispersed in excess water. With a saturated monoglyceride such as GMS, the lamellar structure is the main mesophase found under practical conditions, while with unsaturated monoglycerides this cubic phase is the predominant one at lower temperatures. At lower water concentrations, the spherical water micelles are farther apart, so the viscosity of the mixture becomes lower, approaching that of melted pure surfactant. This is the fluid isotropic mesophase, sometimes referred to as the L2 phase. 

In SGLC-coil systems, the LC mesophase must form within the block microdomain and adapt to the domain boundary conditions [44, 111]. One needs to realize that the ODT in this system is much higher than the isotropic transition temperature. This domain boundary condition can act to stabilize the orientation of the mesogen in one of the domains [111] as indicated in Fig. 14. In Fig. 14, WAXD patterns are shown for sample SIHgFg-41/64 (phase transition SmB... 

Since the ODT of LC block copolymers is in general much higher than the LC isotropic transition temperature (unless the total molecular weight is too small, i. e. a small [43, 62], all the LC mesophase transitions lie below the block microdomain boundary conditions. These microdomain boundary conditions were shown [111] to stabilize the LC mesophase. The orientation of the mesogen within the block microdomain can be recovered when cooling from the isotropic state as already shown in Fig. 14. On the other hand, the texture of the LC block copolymer is very homogeneous and almost defect-free compared to that of... 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

Small molecule liquid crystals suffer a lowering of the mesophase-isotropic transition temperature upon lateral substitution, i,e. replacement of hydrogen on an aromatic ring. As can be seen from the compounds below the same trend is observed for both polymer and the chemically analogous small molecule. It is known that for small molecule mesogens the transition temperature, governed by the relation Tni - A H/AS, decreases upon lateral substitution primarily due to an entropic effect. The... 

Figure 1.11 Transition temperatures for n-ASOB from DSC measurements [111]. Open symbols are for transitions approached from higher temperatures and solid symbols are for transitions approached from lower temperatures. Circles isotropic-mesophase squares mesophase-solid triangles solid-solid.

Data in Tables 1 and 2 show maximum mesophase-isotropic transition temperatures for 1 2 and 1 3 binary combinations respectively. Temperatures in these Tables are given only if the mixed mesophase is more thermally stable than either of the pure components. It is seen that along a column or along a row there is a general trend for a maximum value of smectic-isotropic transition temperature to occur when the alkoxy chain lengths of both components are equal or nearly so. 

There is a wealth of information in Tables 1 and 2 and there are numerous ways in which the data could be presented. We have chosen to use four plots for this purpose (Fig. 1). In each of these plots we have considered one homologous series as the basis set (primary series) and then plotted, as a function of alkoxy chain length in this basis set, maximum mesophase-isotropic transition temperatures of binary mixtures of this primary series with selected members of the other (secondary) series - a short-, a medium- and a long-chained member of this second series. 

Liquid crystallinity is mainly exhibited in the following two temperature regions (1) from (glass transition temperature) to T, (isotropic transition temperature, also known as the clearing point ) (2) from (melting point) to T, where several mesophases can usually be observed. Like ordinary polymers, the transition behavior of liquid polymers is also dependent on their thermal history and relative molecular mass. Therefore, only the samples with sufficient molecular mass and the same thermal history are suitable for the purpose of comparison and study. The transition temperatures for these substances rise with increase in the relative molecular mass, but remain approximately constant when the latter is more than 10 000. The transition heat shows a similar tendency. In addition, the transition entropy from mesophase to isotropic, ASl, is related to the order parameters. 

Figure 19. (a) Pressure-temperature diagram of benzene hexa-n-hexanoate. The mesophase-isotropic transition line extrapolated to atmosphere pressure yields a virtual transition temperature of 89 °C. (From Chandrasekhar et al. [63], reproduced by permission of Academic Press), (b) Miscibility diagram of benzene hexa-n-hexanoate and the heptanoate. The virtual mesophase-isotropic transition temperature for the hexanoate is 89 °C, in agreement with the value obtained from the pressure-temperature diagram (a). From Billard and Sadashiva [64], reproduced by permission of the Indian Academy of Sciences). 

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