Nematic -> smectic transition

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]. 

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... 

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.)...

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. 

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

Many of the compounds of type I exhibit one or several smectic phases besides a. nematic phase. It has been shown recently [6], that smectic liquid crystals can in principle be aligned homogeneously by cooling an ordered nematic phase below the nematic-smectic transition temperature. Accordingly, smectic phases are also suited as anisotropic solvents for spectroscopic applications. 

Some nematic liquid crystals show positive 3 values in the neighbourhood of a nematic/smectic transition. For positive Oj the sign of the torque component does not depend on 0. This leads to a continuous rotation if the director is orientated in the shear plane (0=90°). The sign of the torque component Fq depends on 0, that is, the director is stabilized in the shear plane for two quarters of a revolution and destabilized for the other two quarters. Because of the additional influence of surface alignment and elastic torques the real movement of the director is difficult to predict [31]. 

A number of publications deal explicitly with the theory of elastic constants at phase transitions. The behaviour of nematic elastic constants in the vicinity of the nematic-smectic transition has been analysed by means of the Ginzburg-Landau Hamiltonian [295]. At the transition to the smectic A phase, a critical divergence of K22 and A 33 is predicted. 

Polymers which form smectic and nematic phases are easily oriented in the nematic phase, the mesogenic groups are positioned along the axis of orientation, and the nematic-smectic transition is then possible in the oriented sample (fiber). If the mesogenic groups in the smectic phase formed are tilted toward the layer, then a chevron-like structure is formed, where the smectic layers form a certain angle with the axis of the fiber [55]. 

The nematic-isotropic transition temperature [13] for 7CB is 315.7 K, and for 8CB Tm is 314.0 K. 8CB also has a smectic A phase, with a nematic-smectic transition of 294.3 K crystallisation takes place at 302.7 K for 7CB and at 302.5 K for 8CB. Dielectric experiments have been performed by different authors for both compounds using frequency domain [14-16] and time domain [17,18] techniques. 

At the nematic-smectic transition, a jump in the order parameter is observed which is consistent with results for conventional liquid crystals. 

One can find a well detailed analysis of the elastic properties of the smectics in the review article of Miyano and Ketterson [l]. The essential point is that there are two elastic quantities needed to describe the state of the sample - the density and the strain perpendicular to the layers. We note p the relative change of the density from a reference state chosen to be the nematic-smectic transition point, and Xz the second elastic variable, xz can be also seen as the relative variation of the layer thickness. 

Of special interest is the fact that, in the smectic state, the anisotropy depends on the degree of polymerization. For short chains, is smaller than R in both the nematic and the smectic A phases. By contrast, for long chains, the anisotropy reverses itself at the nematic-smectic transition so that the polymer backbone seems to adopt an oblate conformation in the smectic state. It should be noted, however, that the conclusions drawn from these SANS experiments must be considered with great caution, the polysiloxanes under interest having been labelled in the terminal methoxy group of the side chains. 

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