Liquid crystals isotropic phase

Fig. 7. Schematic representation of a liquid crystal-isotropic phase diagram for rigid rods in solution.

So far, the role of phase separation in gelation has been limited to polymers which are regarded as flexible in solution. Thermoreversible gels can be formed from solutions of many rod-like polymers. " Miller et have reviewed the phase separation and networks formed by some rod-like polymers. The nature of the restoring force in such networks of rods must clearly be different from that of a rubber-like, flexible polymer network. However, that network formation occurs via a phase separation remains a possibility. Flory has developed a detailed theory for the liquid crystal-isotropic phase behaviour of rod-like particles in a diluent. 

Wee EL, Miller WG (1971) Liquid crystal-isotropic phase equilibriums in the system poly (y-benzyl a-L-glutamate)-methylformamide. J Phys Chem 75 1446-1452... 

The second group involves polymers with three-dimensional ordering of side branches (e.g., those forming Mj-phaseXTable 5). On X-ray patterns of these polymers 3-4 narrow reflexes at wide angles are observed. As a rule, the authors define this type of structure as crystalline, or ascribe a smectic type of structure, characteristic for ordered smectics in SE or SH phases. The heats of transition from anisotropic state to isotropic melt are usually small and do not exceed the heats of transition smectic liquid crystal — isotropic melt . The similarity of structural parameters of three-dimensionally ordered smectics and that of crystalline polymers of the type here considered, make their correct identification quite a difficult task. 

The areas bounded by the curves represent the conditions for the stable existence of the four single phases, solid crystals, liquid crystals, isotropic liquid, and vapour. 

There are three principal methods of fabricating PDLC (to disperse liquid crystals in polymer matrix) phase separation, encapsulation and permeation. In the phase separation method, liquid crystal materials, prepoljnner (monomer or oligomer) and photoinitiator (curing agent) are mixed, and then polymerization is brought about by heating [2] or UV irradiation [3]. During the polymerization, the liquid crystal material phase separates from the solution, and liquid crystal droplets and a polymer matrix are formed. By measurement of the nematic-isotropic transition temperature, the purity of the liquid crystal in the droplets can be checked. In the case that the polymerization-induced phase separation is not complete and unreacted prepolymer is dissolved in the liquid crystal droplets, the... 

Arguments for the minimal model system being the traveling cholesteric liquid crystal-isotropic liquid phase boundary are given in this section. The sample is prepared so that the length scale imposed on it by the non-equilibrium driving forces is comparable to the cholesteric liquid crystal s equilibrium length, its pitch. In a pattern formation context, knows time ... 

XI is transformed reversibly at 94°C into a thermotropic (dry) lamellar liquid crystal (D) phase. The identification of this phase was based entirely on X-ray data, as it possessed an unrecognizable optical texture [24]. On further heating in a sealed tube in vacuum [25], the D phase melts at about 126°C to an isotropic liquid. This phase transition is not reversible it is accompanied by slow chemical decomposition. Decomposition was suggested by the fact that the transition temperature decreased on repetition using the same sample. It was confirmed by the analytical detection (using thin-layer chromatography) of decomposition products. Decomposition appears to have occurred by the cyclic elimination of water from two hydroxyl groups, to form a five- or six-membered cyclic ether. 

Smectic phase Liquid crystal Isotropic solution... 

Figure 9. Shear viscosity coefficients r/i, t 2 and t)3, rotational viscosity coefficient y, and isotropic shear viscosity coefficient as a function of temperature for the liquid crystal Nematic Phase V. T, Clearing point temperature.

The bulk of such polymers shows the characteristic phases of liquid crystals (isotropic, nematic and smectic). It raises the following problem how is the polymer backbone inserted inside the mesomorphic molecules ... 

This method is used to locate phase transitions via measurements of the endothennic enthalpy of phase transition. Details of the teclmique are provided elsewhere [25, 58]. Typically, the enthalpy change associated with transitions between liquid crystal phases or from a liquid crystal phase to the isotropic phase is much smaller than the melting enthalpy. Nevertheless, it is possible to locate such transitions with a commercial DSC, since typical enthalpies are... 

Typical shapes of the orientation distribution function are shown in figure C2.2.10. In a liquid crystal phase, the more highly oriented the phase, the moreyp tends to be sharjDly peaked near p=0. However, in the isotropic phase, a molecule has an equal probability of taking on any orientation and then/P is constant. 

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