Smectic Phase Transitions

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

We have recently reported that compound 7a shows a smectic-smectic phase transition associated with the change of the photoluminescent color (Fig. 7) [33]. Compound 7a has a 2,6-diethynylanthracene group as a photoluminescent core. This molecule has fork-like mesogens which consist of tetra(ethylene oxide) and p-(4-/ra .v-pentylcyclohexyl)phenyl moieties. A similar molecular design was previously applied to induce smectic liquid crystallinity for rotaxanes and catenanes [39—41 ]. 

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. 

Figure 19.3. The time evolution of the morphology of an A3B7 surfactant melt, as predicted by dissipative particle dynamics [84], Mean field theory predicts the correct equilibrium hexagonal phase but does not provide any insights into how this equilibrium morphology is reached. The simulations showed that the dynamics of ordering is determined by the percolation of tubes, subsequently destabilized by a nematic or smectic phase transition and also that hydrodynamic effects are important in reaching equilibrium for this system. See the insert showing the colored figures for a better view.

Now that we have discussed all of the structures of the phases that play a part in the formation of twist grain boundary phases, let us now consider the events that can occur at a nematic to smectic phase transition. At a normal chiral... 

When the terminal ester functionality in the phenyl propiolates is replaced with a carbonyl group, i.e., a keto function, TGB phases disappear altogether indicating that the increased size of the lateral dipole at terminus to the core has detrimental effects on the stability of TGB phases [35]. These small variations in structure show how delicate the stability of the TGB phase is, nevertheless, most chiral nematic to smectic phase transitions are accompanied by a TGB phase no matter how transient it is. 

The smectic layering transition [45] is a phenomenon, where smectic layers grow one by one, parallel to the surface, when the temperature approaches the isotropic-smectic phase transition from above. Ellipsometric observation of these steps is possible due to the coupling between the smectic order and the nematic (orientational) ordering. Consequently, the orientational order in... 

Interaction of long-chain n-alkylammonium halides with a halide of a divalent metal leads to the formation of salts of the general formula (RNH3)2MX4 with peculiar phase changes in the solid state [65]. Bis(n-alkylammonium)bromo zinc-ates with n = 10-16 display two reversible high entropy solid crystalline-solid crystalline phase transitions and a solid crystalline-liquid crystalline (smectic phase) transition up to at least 450 K. 

Experimentally there are changes in the components of the permittivity at nemat-ic/smectic and smectic/smectic phase transitions, as illustrated in Figs. 6 and 7. 

Figure 6. Dielectric permittivities for 95S showing effect of smectic phase transitions [21].

An internal/external field competition [33] may be at work in the SmCfg phase observed in an applied electric field [35] when the helix structure is suppressed by an applied electric field. However, while our understanding of the re-entrant mechanism behind the behavior illustrated by Fig. 10 is relatively complete [34], nonlinear theories of smectic phase transitions [36] have not yet accounted for the observed helielec-... 

If there is a phase transition from a nematic to a smectic phase, pretransitional effects are observed in the neighbourhood of the transition [9, 44, 49-51]. Figure 10 [9] shows this behaviour for 4-/7-octyloxy-4 -cyanobiphenyl (80CBP) with a nematic-smectic phase transition at 340.3 K. Pretransitional effects cause a divergence of the shear viscosity coefficient 772 and the rotational viscosity /j. 

In comparison to the above discussed dialkyl- and dialkylazoxybenzenes, it has to be taken into account that only one chain length is changed, which should reduce the odd -even effect. The divergence at the nematic-smectic phase transition causes an addi-... 

Close to the phase transitions point (such as the weak first-order isotropic to nematic transition or the second-order nematic to smectic transition), the order of the low-temperature phase occurs transiently in the finite small size of fluctuations, which is called cybotactic clusters. When the correlation length of the fluctuation increases towards the transition point, the relaxation time diverges. For example, near the nematic-smectic phase transition point, it can be seen that the correlation length becomes longer towards the transition point by X-ray diffraction experiments (Fig. 10.21). In addition, the increase of the relaxation time of the fluctuations can be measured using dynamic light scattering method, which wiU be introduced in detail in Sect. 10.4.3.2. 

Figure 10.4 Normalized force versus distance curve measured in 8CB between a glass sphere (radius 8.5-10 pm) and a glass plate with an AFM [1134], The force curve was recorded in the nematic phase at 34.2 °C that is, 0.7 K above the nematic to smectic phase transition. It was...

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