Smectic —> isotropic transition

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Analytical approaches to understanding the effect of molecular flexibility on orientational order have concentrated on both the isotropic-nematic and the nematic-smectic transition [61, 62] and mean field theory has shown that cholesteric pitch appears not to depend on the flexibility of the molecule [63]. 

Note 4 T occurs below the clearing temperature, usually by about 1 K in isotropic-to-nematic transitions and increases to at least 10 K for isotropic-to-smectic transitions. 

Fig. 11. DSC-trace of bis-(4 -n-octyloxybenzal)-l,4-phenylenediamine at 5 K/min. The lowest temperature transition is the fully ordered crystal-to-smectic transition, followed by four transitions between the five smectic polymorphs. Next is the smectic-nematic transition, followed by the nematic-to-isotropic liquid transition. Drawn after data by Petrie581...

Fig. 9-6. Phase diagram of ferrocenes 11. Melting point nematic/isotropic liquid transition smectic A/isotropic liquid transition A isotropic liquid/smectic A transition O isotropic liquid/nematic transition.

Kozlovsky, M. V., Meier, J. G., and Stumpe, J. Chiral side-chain copolymers. 4. Kinetics of the phase transition from conventional isotropic liquid to the TGB-like isotropic smectic state. Macromol. Chem. Phys. 201, 2377 (20()0). 

The Effect of Polymer Heterogeneity on the Enthalpy. The kinetics of the isotropic-smectic phase transition were studied for two of the polymers HPX-C9 and HPX-C11, and in Figure 8 a summary of the calorimetric data for the former is presented. The behaviour of the HPX-C11 polymer was similar. Two processes are in fact revealed by these data (a) at high temperatures (7 471.9 K) the two processes have approximately the same induction time and rate and are therefore not resolved, (b) at intermediate temperatures (468.9heat evolved in the slower process is 30-50% of that evolved in the more rapid process. There is a tendency for an increase in Ah0 for the slow process with increasing temperature for HPX-C9, Ah0 was equal to 13 kJ/kg at 468.9 K and 20kJ/kg at 470.9 K. The same trend was observed for HPX-C11. (c) at low temperatures (TS467.9 K), the slower process was much retarded and not observed within the experimental window. 

All physical parameters mentioned above are material specific and temperature dependent (for a detailed discussion of the material properties of nematics, see for instance [4]). Nevertheless, some general trends are characteristic for most nematics. With the increase of temperature the absolute values of the anisotropies usually decrease, until they drop to zero at the nematic-isotropic phase transition. The viscosity coefficients decrease with increasing temperature as well, while the electrical conductivities increase. If the substance has a smectic phase at lower temperatures, some pre-transitional effects may be expected already in the nematic phase. One example has already been mentioned when discussing the sign of Ua- Another example is the divergence of the elastic modulus K2 close to the nematic-smecticA transition since the incipient smectic structure with an orientation of the layers perpendicular to n impedes twist deformations. 

Fig. 2.7.4. The measured X-ray reflectivity from the free surface of the isotropic phase of 12CB at temperatures very close to the isotropic-smectic A transition point. The step-like form of the intensity curve reveals the quantized nature of the layer growth at the surface. (After Ocko et...

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