Liquid crystals molecular behavior

Supermolecular level liquid crystals are not of major concern in this review. In fact, they form a class of materials which should best be separated from the normal liquid crystals . Although there are structural similarities to the molecular liquid crystals, molecular motion and transition behavior of these liquid crystals based on supermolecular structure is completely different. 

The two most common molecular motifs that lead to liquid crystal phase behavior are the rod aud the disk. Clearly rodlike molecules have one unique axis that is longer than the other two, while diskUke molecules have one unique short axis and two longer axes (Figure 1). RodUke molecules organize into nematic or smectic phases, while disklike systems form nematic or colunmar phases. Figure 2 shows schematic diagrams of molecules arranged in a nematic, smectic A (SmA), and smectic C (SmC) liquid crystal phase (= mesophase). There are very many smectic phases of which SmA and SmC are only two. ... 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

In the case of simple, pure liquids and smooth substrates, mesoscopic precursor Films, that is, films whose thickness is much larger than molecular sizes, are also controlled by general laws, while specific behavior shows up at molecular scale. In the case of liquid crystals, specific behavior is present whatever the scale. How and why such behavior is observed is the subject of the present contribution, with specific focus on the nanoscopic thickness scale. At least in the case of liquid crystals, many questions are still waiting for answers. 

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

Marcos, M., Ros, M.B., Serrano, J.L., Sola, M.A., Oro, L.A. and Barbera, J. (1990) Liquid-crystal behavior in ionic complexes of silver(I) molecular structure-mesogenic activity relationship. Chemistry of Materials, 2, 748-758. 

On a molecular level the director is not rigorously defined, but the molecular director is typically considered to be the average long axis of the molecules, oriented along the macroscopic director with some order parameter less than one. This type of anisotropic order is often called long-range orientational order and, combined with the nonresonant optical properties of the molecules, provides the combination of crystal-like optical properties with liquidlike flow behavior characteristic of liquid crystals. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

This article reviews the following solution properties of liquid-crystalline stiff-chain polymers (1) osmotic pressure and osmotic compressibility, (2) phase behavior involving liquid crystal phasefs), (3) orientational order parameter, (4) translational and rotational diffusion coefficients, (5) zero-shear viscosity, and (6) rheological behavior in the liquid crystal state. Among the related theories, the scaled particle theory is chosen to compare with experimental results for properties (1H3), the fuzzy cylinder model theory for properties (4) and (5), and Doi s theory for property (6). In most cases the agreement between experiment and theory is satisfactory, enabling one to predict solution properties from basic molecular parameters. Procedures for data analysis are described in detail. 

The possible transitions of plastic and condis crystal-forming materials are shown in Fig. 4. For plastic crystals, this diagram is fully based on information on low molecular weight materials. No flexible, linear macromolecules which resemble plastic crystalline behavior have been reported (see Sect. 5.2.3). Similarly, little attention has been paid in the past to conformationally disordered mesophases in small molecules. In fact, some of the plastic crystals of larger organic molecules may actually be condis crystals (see Sects. 5.2,2 and 5.3.3). Since the positional order is preserved in both plastic and condis crystals, the possible phase relations are similar. The major difference from the liquid crystals is the possibility of partial mesophase formation. 

The transition behavior of a number of liquid crystals with side-chain mesogens is summarized in Table 5. The most obvious feature of macromolecular liquid crystals is the frequent absence of fully ordered crystals at low temperatures. If fully ordered crystals are observed, crystallization is incomplete, i.e. the observed phase states are to be described by an area on the right side of Fig. 3. Glass transitions, which were hard to find in low molecular weight liquid crystals (see Table 3), are now prominent. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

K. Hongladarom, W. R. Burghardt, S. G. Baek, S. Cementwala, and J. J. Magda, Molecular alignment of polymer liquid crystals in shear flows. I. Spectroscopic birefringence technique, steady-state orientation, and normal stress behavior in poly(benzyl glutamate) solutions, Macromolecules, 26, 772 (1993). 

In the following sections, we shah demonstrate that the observed behavior of electro-optic activity with chromophore number density can be quantitatively explained in terms of intermolecular electrostatic interactions treated within a self-consistent framework. We shall consider such interactions at various levels to provide detailed insight into the role of both electronic and nuclear (molecular shape) interactions. Treatments at several levels of mathematical sophistication will be discussed and both analytical and numerical results will be presented. The theoretical approaches presented here also provide a bridge to the fast-developing area of ferro- and antiferroelectric liquid crystals [219-222]. Let us start with the simplest description of our system possible, namely, that of the Ising model [223,224]. This model is a simple two-state representation of the to-... 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

Chandrasekhar, 1977). This cooperative behavior results in weak elastic properties. Then, the application of an electric field can easily change the molecular orientation, which is initially fixed by the mechanical boundary conditions. The concomitant changes in the optical properties form the basis of liquid crystal displays (LCD). 

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