The Smectic Phases

A smectic liquid crystalline phase is characterized primarily by its layered structure. In addition to the overall alignment of the molecules with a particular c director, as in the nematic phase, smectics have an additional degree of order, the layer structure. Smectic phases are typically much more viscous than the nematic phase and occur at lower temperatures. A large number of different smectic-like phases have been observed and studied, each with their own structural features. In this section, we describe some of these. 

FIGURE 2.7 Focal conic texture of the smectic A phase imaged using polarized optical microscopy at lOOX magnification. 

Molecules within the layers move freely, with no defined packing arrangement. There is no correlation between molecules from layer to layer in this phase. The layer-to-layer ordering of the smectic phase can be approximated to a density wave, and this can be incorporated in a modified form of the order parameter. 

The simplest form of the smectic C (SmC) phase is a variant on the SmA phase in that the basic layered structure is the same, but the molecules are 

This transition can be described as a second-order phase transition as there is no discontinuity in order parameter at the transition point (i.e., the tilt varies smoothly, tending to zero at the transition). 

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Using this order parameter, the free energy in the nematic phase close to a transition to the smectic phase can be shown to be given by [20, 88, 89, 91]... 

An exception to the mle that lowering the temperature causes transitions to phases with iacreased order sometimes occurs for polar compounds which form the smectic phase. Decreasiag the temperature causes a transition from nematic to smectic but a further lowering of the temperature produces a transition back to the nematic phase (called the reentrant nematic phase) (22). The reason for this is the unfavorable packing of the molecules ia the smectic phase due to overlap of the molecules ia the center of the layers. As the temperature is lowered, the steric iateractions overpower the attractive forces, causiag the molecules to pack much more favorably ia the nematic phase. The reentrant nematic phase can also be produced from the smectic phase by iacreasiag the pressure (23). 

FIGURE 5.50 The smectic phase of a liquid crystal. Not only do the molecules lie parallel to one another, they also line up next to one another to form sheets. 

The three classes of liquid crystals differ in the arrangement of their molecules. In the nematic phase, the molecules lie together, all in the same direction but staggered, like cars on a busy multilane highway (Fig. 5.49). In the smectic phase, the molecules line up like soldiers on parade and form layers (Fig. 5.50). Cell membranes are composed mainly of smectic liquid crystals. In the cholesteric phase, the molecules form ordered layers, but neighboring layers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules (Fig. 5.51). 

In 8CB, continued cooling into the smectic phase reveals the appearance of a broad ultra-low-frequency feature centred at around 10 cm where no other modes are seen. This is shown in Fig. 15. This feature appears to be unique to the smectic phase and has been tentatively attributed to intermolecular dipolar coupling across smectic layers [79]. In principle this should be a generic feature of smectics but it will be necessary to explore this issue through extensive computer simulations using realistic, shape-dependent potentials for... 

Complementary information about the structure of the smectic phases is contained in the X-ray scattering patterns, as in studies of real mesogens. The intermolecular scattering patterns calculated (see Sect. 3) for GB(4,4, 20,0, 1,1)... 

Fig. 14. A snapshot of a configuration showing the stripe-like structure of the smectic phase formed hy the polar mesogen GB(3.0, 5.0, 1, 3) and the antiferroelectric compensation in adjacent layers. The different orientations of the dipoles are indicated hy the different shading of the ellipsoids...

Fig. 17a,b. Snapshots viewed along orthogonal directions showing the molecular organisation within the smectic phase formed hy chiral GB(3.0, 5.0, 1, 2) with c equal to 0.8 at p of 0.30 and T of 0.75 the neighbouring periodic images are also included... 

The compounds crystallise in noncentrosymmetric space groups namely PI, P2i, C2, and P2i2i2i (but with priority of P2i) due to the chirality of the molecules. Most of the compounds have a tilted layer structure in the crystalline state. The tilt angle of the long molecular axes with respect to the layer normal in the crystal phase of the compounds is also presented in Table 18. Some compounds show larger tilt angles in the crystalline state than in the smectic phase. In the following only the crystal structures of some selected chiral liquid crystals will be discussed. 

A lattice model of uniaxial smectics, formed by molecules with flexible tails, was recently suggested by Dowell [29]. It was shown that differences in the steric (hard-repulsive) packing of rigid cores and flexible tails - as a function of tail chain flexibility - can stabilize different types of smectic A phases. These results explain the fact that virtually all molecules that form smectic phases (with only a few exceptions [la, 4]) have one or more flexible tail chains. Furthermore, as the chain tails are shortened, the smectic phase disappears, replaced by the nematic phase (Fig. 1). 

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]. 

These compounds present a new class of LC molecules having a perfluoroalkyl tail on one end and an ordinary alkyl tail on the other. Earlier studies of several groups indicated that terminal fluorination considerably extends the temperature range of the smectic phase [81]. Moreover, mesogens incorporating... 

To determine whether the 8CB droplets condensed above 41°C (trapped in the isotropic phase) sit on a trilayer or on bare silicon, we used the ATM tip to mechanically spread the droplets and thus accelerate their conversion to a stable configuration. The SPFM images shown in Fignre 15 were obtained after such tip-induced spreading. A layered structure with 32-A-high steps typical of the smectic phase is obtained. The first, or bottom, layer is 41 A thick, while the layers above it are all 32 A thick. This indicates that the bottom layer of the film is a trilayer and that the remaining snbstrate is dry silicon, i.e.. 

More precise description of the structure of the smectic phases can be found in ... 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

The reasons behind this accelerated rate behavior have been attributed to a decrease in chain transfer processes (28,29) and a decreased termination rate (24,25) indicated by molecular weight measurements (26). Recently, direct evidence of decreases in the termination rate have been shown (27) and in these studies both the termination and propagation kinetic constants were determined for polymerizations exhibiting enhanced rates in a smectic phase. The propagation constant, kp, decreases slightly in the ordered phase from the isotropic polymerization. Such a decrease would be expected because of the lower temperature in the smectic phase. The termination kinetic constant, kt, however, decreases almost two orders of magnitude for the ordered polymerization, indicating a dramatically suppressed termination rate. 

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