Thermotropic liquid crystals isotropic-nematic phase transition

The isotropic-to-nematic transition is determined by the condition [1 — (2/3)TBBWBB/k T] = 0 whereas the spinodal line is obtained when the denominator of XAA is equal to zero. These conditions are evaluated in the thermodynamic limit (Q = 0) in Fig. 7 for a Maier-Saupe interaction parameter Web/I bT = 0.4xAb and for NA = 200, N = 800, vA = vB = 1. When the volume fraction of component A(a) is low, the isotropic-to-nematic phase transition is reached first whereas at high < >A the spinodal line is reached first. In the second case, the macromolecules do not have a chance to orient themselves before the spinodal line is reached. This RPA approach is a generalization of the Doi et al. [36-38] results (that were developed for lyotropic polymer liquid crystals) to describe thermotropic polymer mixtures. Both approaches cannot, however,... 

The thermotropic liquid crystal, 4,4 -diheptylazoxybenzene (HAS), exhibiting isotropic, nematic and smectic phases, has been studied through e NMR. The temperature dependence of e chemical shifts and spin-lattice relaxation times of the Xe gas dissolved in HAS showed clear signatures of the phase transitions. Theoretical models have been used to understand the influence of the different phases on the isotropic and anisotropic parts of the chemical shielding. From the studies it is also inferred that in the smectic phase, Xe atoms preferentially occupy interlayer spacings rather than the interiors. Bent-core or banana-shaped molecules display an array of novel chiral liquid crystalline phases. NMR studies on two of the banana core moieties have been analyzed using ab initio structure calculations and the steric inertial frame model. ... 

In a recent work, isotropic-nematic-smectic A phase transitions in thermotropic liquid crystals were also induced by applying an electric field [140]. The liquid crystal investigated (a mixture of 8CB and lOCB) showed a first order isotropic to smectic A transition. When in the isotropic phase and near the spontaneous transition temperature, a field-induced first order transition was observed from a paranematic to a nonspontaneous nematic phase. For higher values of the applied electric field, another first order transition occurred from the nonspontaneous nematic to a phase exhibiting the same order as a smectic A phase. A phenomenological Landau-de Gennes model has been developed to describe these transitions [141],... 

FIGURE 2.29 An example of differential scanning calorimetric (DSC) data for a thermotropic liquid crystal (8CB) measured on a heating run of 207min. Two peaks can be observed. The first peak with an onset at 32.16° is the smectic-A-to-nematic phase transition, and the second peak at 39.56° is the nematic-to-isotropic liquid phase transition. Delta H Is the enthalpy of the transitions. 

Li), the lamellar Ld), and the nematic one (No), which in this case consists of disk-like micelles. One outstanding feature of the system is the extremely wide range of stability of the lamellar as well as of the nematic phase. Moreover, the capability to perform phase transitions lamellar/nematic/ isotropic by temperature variation makes the handling of the samples easy. The short, perfluorinated chains of the surfactant are responsible for the unusual properties of this lyotropic liquid crystal which in some aspects behaves very similar to a thermotropic phase. Both of the briefly introduced achiral lyotropics have been used extensively as host phases for the induction of phase chirality by means of chiral dopants. 

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