Smectic transitions

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]. 

The smectic phases Ai, A2 and A have the same macroscopic symmetry, differing from each other in the wavelength of spacing. Hence it is possible to go from Ai to Aa or from Aa to A2 by varying only the layer periodicity in a continuous or discontinuous way(with the jump in the layer spacing d). Smectic-smectic transition lines of first order may terminate at a critical point, where the differences between the periodicities of the smectic A phases vanish, providing a continuous evolution from an Aa to bilayer A2 phase [12]. 

Liquid Crystalline Polymers at their Nematic-Smectic Transition 206... 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

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...

Figure 31. Coupling between the nematic order parameter S and the smectic order parameter 4/ for the calamitic system GB(3, 5, 2, 1) (TV = 256) at three state points along the isochor at density p = 0.32. At the nematic phase (T = 1.194 bottom), at the smectic phase (T = 0.502 top), and at the nematic-smectic transition region (T = 0.785 middle). The order parameters are for instantaneous configurations. (Reproduced from Ref. 161.)...

Looking back, the earliest written report concerning the possibility of having a smectic phase with twisted layers comes from the work of Wolfgang Ullrich Muller at the Technischen Universitat Berlin in 1974 [40], In his thesis entitled Verhalten cholesterischer Mesophasen unter dem Einfluss von Phasenum-wandlungen , Muller examined the mesophase behavior at the chiral nematic to smectic transition for various mixtures of cholesteryl oleoyl carbonate (COG), 5, and cholesteryl chloride (CC), 6. Muller came to the conclusion from his work that the smectic phase must have a helical structure and that the heli-axis must lie in the plane of the layers, and the structure must also have... 

Real nematics are, of course, elastically anisotropic. In certain situations, as for example at temperatures close to the nematic-smectic transition, the anisotropy becomes very large and certainly cannot be ignored. We shall now investigate some of the consequences of elastic anisotropy on the properties of disclinations. 

Thermal properties of selected examples of polymers 1 (T smectic transition, ., nematic transition, H heat of fusion) ... 

A eutectic mixture of two smectic materials has a crystal to smectic transition which is lower than either of the components. 

I. Janossy and L. Bata, Study of Elastic Properties near a Nematic-Smectic Transition, Preprint KFKI, Budapest, 1977. 

The nonlinear P 0) dependence close to the smectic-C-smectic- transition may also be responsible for the strong variation of the helical smectic-C pitch near the transition [17]. The helical structure is described by gq ... 

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