Phase behaviour liquid crystals

In addition to TMV the liquid crystal phase behaviour of the semirigid cylindrical bacteriophage feline distemper (fd) has been investigated extensively. The... 

The first of these was by Vieillard-Baron [5] who investigated a system of spherocylinders but failed to detect a liquid crystal phase primarily because the anisometry, L/D, of 2 was too small [37]. He also attempted to study a system of 2392 particles with the larger L/D of 5 but these simulations had to be abandoned because of their large computational cost. However, in view of the ellipsoidal shape of the Gay-Berne particles it is the behaviour of hard ellipsoids of revolution which is of primary relevance to us. 

Examination of the thermal behaviour showed that with three exceptions, all complexes showed a monotropic SmA phase with in almost all cases, melting being observed between 88 and 99 °C, with clearing between 82 and 89 °C. Of the three exceptions, 15-6,8 and 15-8,10 showed no liquid crystal phase at all, while 15-12,6 showed an additional monotropic nematic phase. A curious feature of these complexes is the apparent insensitivity of the melting and clearing points to both n and m. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

Whang and Wu [3] have described the liquid crystalline state of polyimide precursors and shown that certain polyamic acids derived from pyromellitic anhydride exhibit lyotropic behaviour. Liquid crystal phases have also been observed by Wenzel et al. [4] in polyimides derived from pyromellitic anhydride and 2,5-di-n-alkoxy-1,4-phenyl ene diisocyanate. Dezern [5] has disclosed a synthesis for linear polyamide-imides derived from benzophenone dianhydride but the occurrence or otherwise of mesophases is not mentioned. 

Changes in the temperature or composition of the solution may result in a conformational transition of one of the components. For example, intramolecular forces may stabiUze a helical conformation. Such an ordered structure may also be favoured because of the associated decrease in the volume occupied. A helical conformation can affect the phase-separation behaviour in several ways, for example by presenting attractive or repulsive groups on its surface, promoting either polymer-polymer or polymer-solvent interactions, or, if the helical molecule is sufficiently stiff and rod-like, by an isotropic-liquid crystal phase separation. Flory has predicted the general features of the corresponding phase diagram (Fig. 7). 

In low molar mass systems there are maity fascinating trends of mesomorphic behaviour and so this should also be seen in polymeric analogues. The additional ordering on polymerisation causes liquid crystal phases to be more ordered than for the monomeric analogue and transition temperatures and clearing points are higher. Accordingly, if a... 

The response of liquid crystals to electric and magnetic fields is one of their most interesting properties. The presence of orientational order produces a response only possible when the molecules in a macroscopic volume of a substance behave cooperatively. Since the fluid nature of the liquid crystal phase offers little resistance to this cooperative behaviour, only weak electric and magnetic fields are necessary to produce these significant responses. This is what makes liquid crystals nature s delicate phase of matter. 

This chapter includes a discussion of only a small portion of the theoretical work on liquid crystals. Much of what has been done is based on the simple models described here, embellishing them by adding additional interactions or calculating additional quantities. Clearly the task of describing the liquid crystal phase is a difficult one, yet mar of these models present a simple picture for the origin of the most important liquid crystal properties. The use of new analytical techniques and the ability to use more powerful computers continue to refine our understanding of liquid crystals and the reasons behind their complex behaviour. 

In this work, lattice Monte Carlo simulations are used to study the phase behaviour of a ternary system in which surfactants and the inorganic component are depicted by flexible chains of connected sites on a lattice box. Under no inorganic condensation conditions (where the reaction to form a 3-dimensional silica network is avoided due to the high pH of the solution), these systems phase separate into a hybrid liquid crystal phase in equilibrium with a solvent-rich phase [11]. In simulations, the formation of a hybrid-rich phase in equilibrium... 

At high concentrations, amphiphiles tend to self-assemble into ordered structures called lyotropic liquid crystal phases. The prefix lyo- (from the Greek for solvent) indicates that concentration is a controlling variable in the phase behaviour, as well as temperature. Temperature alone controls the self-assembly of thermotropic liquid crystals, which is the subject of the next chapter. Lyotropic liquid crystal phases can be formed in non-aqueous solvents. However, here we shall consider lyotropic liquid crystal phases formed in water, since these are by far the most important and widely studied. 

In contrast to the small effects which temperature change has on the phase behaviour of ionic surfactants [38] there is a very pronounced change in the appearance of phase diagrams of oil-water-non-ionic surfactant systems with increase in temperature. Changes induced by temperature in the relative positions and extent of isotropic and liquid crystal phases present in the ascorbic acid-water-polysorbate 80 system have been recorded by Nixon and Chawla [39] (Fig. 2.21). Temperature increase decreases the width of the liquid crystal band the most pronounced effect occurring between temperatures of 25 and 30° C where the polysorbate concentration at which liquid crystals first appear (Li + LC) is increased from about 35 to 36% to 44% polysorbate in the presence of ascorbic acid. 

Diglycerol esters of samrated fatty acids have recently been extensively studied by Shrestha and co-workers in both aqueous [83] and nonaqueous [84, 85] systems. The phase behaviour of caprate (CIO) and laurate (C12) esters in water were found to be quite different from the solution behaviour of the myristate (C14) and palmitate (Cl6) esters (see Figure 1.5). In the former, a lamellar liquid crystal phase is present in the surfactant-rich region and it absorbs a substantial amount of water. The melting temperature of this phase is practically constant in a wide range of compositions. For the more hydrophobic surfactant the phases with solids and the extent of water solubilization are increased. 

Apart from high strength materials formed from nematic polymer fibres, most applications of nematic liquid crystals depend on their anisotropic optical properties. As a consequence the refractive indices of nematics are of prime importance in the development of materials for applications. The refractive indices are determined by the molecular polarisability coupled to the mientational order of the mesogens in the liquid crystal phase, so refractive indices can provide a direct probe of the order parameter. Furthermore the optical properties of liquid crystal films are frequently used to determine phase behaviour, identify phase types through characteristic optical textures or explore the properties of defects, and such experiments rely on the anisotropy in the refractive index of the material. The first tool of a liquid crystal scientist is the polarising microscope, which emphasises the importance of optical properties in general and refractive indices in particular to the stufy of liquid crystals. 

---