Liquid smectic

All solid forms lacking order to a substantial degree and ordered liquids (smectic, nematic) are called at times mesomorphic forms. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Fig. 9-6. Phase diagram of ferrocenes 11. Melting point nematic/isotropic liquid transition smectic A/isotropic liquid transition A isotropic liquid/smectic A transition O isotropic liquid/nematic transition.

Such an assignment is, however in conflict with that made by Van Luyen and Strzelecki who reported the poly(, i -diphenylsebacate) to be nematic. X-ray patterns of oriented specimens will be necessary to substantiate the nature of this mesophase. Bosio et al reported from texture observations that the lower homologues in series XVI may exhibit a 3-dimensional ordered smectic E phase in addition to 2 D-liquid smectic A and nematic phases. At low temperature, the inner sharp diffraction ring and the four sharp outer rings are consistent with the formation of such a highly ordered mesophase. 

Monodomains of the 2 D-liquid smectic phase of the polyester VI were obtained by careful melting of oriented fibers Schematic diffraction pattern is shown in figure 27 b the two sharp small angle reflections on meridian... 

Crystal 67 smectic A 106 liquid smectic A liquid crystal... 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

L. The liquid-expanded, L phase is a two-dimensionally isotropic arrangement of amphiphiles. This is in the smectic A class of liquidlike in-plane structure. There is a continuing debate on how best to formulate an equation of state of the liquid-expanded monolayer. Such monolayers are fluid and coherent, yet the average intermolecular distance is much greater than for bulk liquids. A typical bulk liquid is perhaps 10% less dense than its corresponding solid state. 

The liquid-crystal transition between smectic-A and nematic for some systems is an AT transition. Depending on the value of the MacMillan ratio, the ratio of the temperature of the smectic-A-nematic transition to that of the nematic-isotropic transition (which is Ising), the behaviour of such systems varies continuously from a k-type transition to a tricritical one (see section A2.5.91. Garland and Nounesis [34] reviewed these systems in 1994. 

Smith D P E, Hdrber H, Gerber Ch and Binnig G 1989 Smectic liquid crystal monolayers on graphite observed by scanning tunnelling microscopy Science 245 43... 

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

The smectic A phase is a liquid in two dimensions, i.e. in tire layer planes, but behaves elastically as a solid in the remaining direction. However, tme long-range order in tliis one-dimensional solid is suppressed by logaritlimic growth of tliennal layer fluctuations, an effect known as tire Landau-Peierls instability [H, 12 and 13]... 

Detailed x-ray diffraction studies on polar liquid crystals have demonstrated tire existence of multiple smectic A and smectic C phases [M, 15 and 16]. The first evidence for a smectic A-smectic A phase transition was provided by tire optical microscopy observations of Sigaud etal [17] on binary mixtures of two smectogens. Different stmctures exist due to tire competing effects of dipolar interactions (which can lead to alternating head-tail or interdigitated stmctures) and steric effects (which lead to a layer period equal to tire molecular lengtli). These... 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

It has not proved possible to develop general analytical hard-core models for liquid crystals, just as for nonnal liquids. Instead, computer simulations have played an important role in extending our understanding of the phase behaviour of hard particles. Frenkel and Mulder found that a system of hard ellipsoids can fonn a nematic phase for ratios L/D >2.5 (rods) or L/D <0.4 (discs) [73] however, such a system cannot fonn a smectic phase, as can be shown by a scaling... 

The nematic to smectic A phase transition has attracted a great deal of theoretical and experimental interest because it is tire simplest example of a phase transition characterized by tire development of translational order [88]. Experiments indicate tliat tire transition can be first order or, more usually, continuous, depending on tire range of stability of tire nematic phase. In addition, tire critical behaviour tliat results from a continuous transition is fascinating and allows a test of predictions of tire renonnalization group tlieory in an accessible experimental system. In fact, this transition is analogous to tire transition from a nonnal conductor to a superconductor [89], but is more readily studied in tire liquid crystal system. 

Gray G W and Goodby J G W 1984 Smectic Liquid Crystais Textures and Structures (Glasgow Hill)... 

Leadbetter A J 1979 Structural studies of nematic, smectic A and smectic C phases The Moiecuiar Physics of Liquid Crystais ed G R Luckhurst and G W Gray (London Academic)... 

McMillan W L 1971 Simple molecular model for the smectic A phase of liquid crystals Phys.Rev A 4 1238-46... 

Flard spherocylinders (cylinders witli hemispherical end caps) were studied using computer simulations [118]. In addition to a nematic phase, such particles also display a smectic-A phase, in which tire particles are arranged in liquid-like layers. To observe tliis transition, ratlier monodisperse particles are needed. The smectic-A phase was indeed observed in suspensions of TMV particles [17]. 

Liquid Crystallinity. The Hquid crystalline state is characterized by orientationaHy ordered molecules. The molecules are characteristically rod-or lathe-shaped and can exist in three principal stmctural arrangements nematic, cholesteric, and smectic (see Liquid crystalline materials). 

Fig. 9. Smectic fan-shaped texture observed between crossed polarizers. Courtesy of G. H. Brown, Liquid Crystal Institute, Kent State University.

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Di-n-heptyloxyazoxybenzene [2635-26-9] M 426.6, m 75 , 95 (smectic nematic) and 127 (nematic - liquid), pK j —5. Purified by chromatography on AI2O3 (" benzene), recrystd from hexane or 95% EtOH and dried by heating under vacuum. The liquid crystals can be sublimed in vacuo. [Mellifiori et al. Spectrochim Acta Part A 37(A) 605 1981 Dewar and Schroeder J Am Chem Soc 86 5235 1964-, Weygand and Glaber J Prakt Chem 155 332 1940]. 

S. H. J. Idziak, I. Koltover, J. N. Israelachvili, C. R. Safinya. Structure in a confined smectic liquid crystal with competing surface and sample elasticities. Phys Rev Lett 76 1477-1480, 1996. 

Let us enter the world of liquid crystals built by the purely entropic forces present in hard body systems. The phase diagram of hard spherocylinders (HSC) shows a rich variety of liquid crystalline phases [71,72]. It includes the isotropic, nematic, smectic A, plastic, and solid phases [73]. In a plastic crystal the particle centers lie on lattice sites, but the orientations of the... 

Similar lamellar structures are formed for l-alkyl-3-methylimida2olium cations with [PdCl4] when n > 12. As with the pyridinium systems, mesomorphic liquid crystal structures based on the smectic A structure are formed [24]. 

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