Liquid crystal systems

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. 

A5 The Gay-Beme model for liquid crystal systems and some typical arrangements. 

Computer simulations therefore have several inter-related objectives. In the long term one would hope that molecular level simulations of structure and bonding in liquid crystal systems would become sufficiently predictive so as to remove the need for costly and time-consuming synthesis of many compounds in order to optimise certain properties. In this way, predictive simulations would become a routine tool in the design of new materials. Predictive, in this sense, refers to calculations without reference to experimental results. Such calculations are said to be from first principles or ab initio. As a step toward this goal, simulations of properties at the molecular level can be used to parametrise interaction potentials for use in the study of phase behaviour and condensed phase properties such as elastic constants, viscosities, molecular diffusion and reorientational motion with maximum specificity to real systems. Another role of ab initio computer simulation lies in its interaction... 

This article reviews progress in the field of atomistic simulation of liquid crystal systems. The first part of the article provides an introduction to molecular force fields and the main simulation methods commonly used for liquid crystal systems molecular mechanics, Monte Carlo and molecular dynamics. The usefulness of these three techniques is highlighted and some of the problems associated with the use of these methods for modelling liquid crystals are discussed. The main section of the article reviews some of the recent science that has arisen out of the use of these modelling techniques. The importance of the nematic mean field and its influence on molecular structure is discussed. The preferred ordering of liquid crystal molecules at surfaces is examined, along with the results from simulation studies of bilayers and bulk liquid crystal phases. The article also discusses some of the limitations of current work and points to likely developments over the next few years. 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

The use of photoreactive liquid crystal systems in imaging devices is not unprecedented. As early as 1971, Sackmann showed for the first time the photochemical change of pitch in cholesteric liquid crystals (H). Since then several techniques for using liquid... 

In view of sensitivity, the liquid crystal system is much improved than the previously mentioned systems. However, these liquid... 

Toledano J-C, Toledano P (1997) The Landau theory of phase transitions application to structural, incommensurate, magnetic, and liquid crystal systems. World Scientific Pub. Co Inc., Singapore... 

Figure 6 shows the phase diagrams plotting temperature T vs c for PHIC-toluene systems with different Mw or N [64], indicating c( and cA to be insensitive to T, as is generally the case with lyotropic polymer liquid crystal systems. This feature reflects that the phase equilibrium behavior in such systems is mainly governed by the hard-core repulsion of the polymers. The weak temperature dependence in Fig. 6 may be associated with the temperature variation of chain stiffness [64]. We assume in the following theoretical treatment that liquid crystalline polymer chains in solution interact only by hardcore repulsion. The isotropic-liquid crystal phase equilibrium in such a solution is then the balance between S and Sor, as explained in the last part of Sect. 2.2. 

Recently, experimental data of S for various liquid-crystalline polymer solutions have become available. Figure 12 illustrates the concentration dependence of S for three polymer liquid crystal systems (a) PBLG-DMF [92,93], (b) PHIC-toluene [94], and (c) poly(yne)-platinum polymer (PYPt)-trichloro-ethane (TCE) [33]. For systems (a) and (c), with the alignment made by... 

Boy et al. [79] used lyotropic liquid crystals (LC) for the immobilization of HfeHNL. The solid LC phase is not used because of the high viscosity. Therefore, the LC is used in a biphasic system consisting of the LC and an organic solvent. Such biphasic liquid crystal systems consist of organic solvent, water, and surfactant, where poorly soluble substrates and products are dissolved in the organic solvent and the liquid crystal matrix, which contains the enzyme, has a protective effect on it. By optimization and by virtue of the immobilization, it is possible to establish an extractive continuous process [79]. 

Quantum dots used in liquid crystal systems (ranging in size from a few to tens of nanometers) are usually from the II-VI group, and include CdS, CdSe, and CdTe capped with trioctylphosphine/trioctylphosphine oxide (TOP/TOPO) [101-103], CdS as well as CdTe stabilized with thiols [104], and CdSe protected with amines [105]. 

Two-dimensional, disc-shaped nanomaterials are the last class of nanomaterials to be discussed in more detail. Though a great variety of nanodiscs, nanosheets and nanoplatelets based on metals [256], metal oxides [257-260], graphene [261], or semiconductors [262] are frequently described in the literature, the vast majority of studies in liquid crystal systems dealt with some form of nanoclay. 

In the SC lipids form two crystalline lamellar phases.27 The mixture of both phases produces the optimal barrier to water loss from SC. The balance between the liquid crystalline and the solid crystal phases is determined by the degree of fatty acid unsaturation, the amount of water, and probably by other yet undiscovered factors. A pure liquid crystal system, produced by an all-unsaturated fatty acid mixture, allows a rapid water loss through the bilayers with a moderate barrier action. The solid system produced with an all-saturated fatty acid mixture causes an extreme water loss due to breaks in the solid crystal phase.6,23 Studies with mixtures prepared with isolated ceramides revealed that cholesterol and ceramides are very important for the formation of the lamellar phases, and the presence of ceramide 1 is crucial for the formation of the long-periodicity phase.27 The occurrence of dry skin associated with cold, dry weather for example, may result from an extensive, elevated level of skin lipids in the solid state. Therefore, a material that maintains a higher proportion of lipid in the liquid crystalline state may be an effective moisturizer.6... 

The excitation and detection of multiple quantum transitions in systems of coupled spins offers, among other advantages, an increase in resolution over single quantum n.m.r. since the number of lines decreases as the order of the transition increases. This paper reviews the motivation for detecting multiple quantum transitions by a Fourier transform experiment and describes an experimental approach to high resolution multiple quantum spectra in dipolar systems along with results on some protonated liquid crystal systems. A simple operator formalism for the essential features of the time development is presented and some applications in progress are discussed. 

Liquid-crystalline phases are characterized to some degree by the shape of the molecules and by their packing arrangements and ordering in the mesomorphic state. Typically, molecules can have cither disc- or rod-like shapes and can form discotic or calamitic mesophases, respectively. Ferrocene liquid crystal systems that have so far been synthesized tend to have molecular structures that are lath- or rod-like in shape, and consequently the phases observed are calamitic. However, this does not preclude the possibility that a polysubstituted ferrocene could be prepared where the molecular shape is disc-like, thereby holding out the prospect of possibly producing discotic/columnar phases. 

Principal properties of linear rigid-chain liquid crystal systems... 

The introduction of lipophilic substituents is of interest for producing surface-active compounds (surfactants) and liquid-crystal systems. The complexes with allyl substituents at the apical boron atoms are precursors for the synthesis of linear and netlike polymeric clathrochelates. 

A. V. Komolkin, Aatto Laaksonen and A. Maliniak, Molecular Dynamics Simulation of a Nematic Liquid Crystal, J. Chem. Phys. 101 (1994) 4103 M. R. Wilson, Determination of Order Parameters in Realistic Atom-Based Models of Liquid Crystal Systems, J. Mol. Liq. 68 (1996) 23 ... 

K. Ichimura, K. Photoalignment of Liquid-Crystal Systems. Chem. Rev. 100, 1847 (2000). Ichimura, K., Seki, T, Kawanishi, Y., Suzuki, Y., Sakuragi, M., and Tamaki, T. Photocontrol of Liquid Crystal Alignment by Command Surfaces , in Photoreactive Materials for Ultrahigh Density Optical Memory. M. Irie, Ed., Elsevier Amsterdam, (1994), p. 55. 

Ichimura, K. Photoalignment of liquid-crystal systems. Chem. Rev., 100, 1847 (2000). 

In some liquid crystal systems, broad, partially resolved spectra with some structure attributed to proton dipole-dipole interaction have been observed (16, 17, 18). On the other hand, such spectra were not observed in this study, so that the ordering parameters of the samples employed would be expected to be very small. This result is compatible with that of the ESK study reported by Y. Yamada et al. (19). 

The contrast of this work compared to findings for mixtures of small molecule liquid crystals ( ) suggests that entropy plays an important part in determining miscibility in liquid crystal systems. The entropic effect could conceivably enter in two different ways. 

Among the different two-phase liquid/crystal systems, ice/water interfaces are of great interest because of their fundamental presence in nature and importance in chemical, biological, environmental and atmospheric processes [14]. Systematic studies of ice/water interfaces by molecular dynamics simulations began in 1987, when Karim and Haymet [15] simulated for the first time the two-phase coexistence using the SPC model of water molecules. Since 1987, ice/water interfaces were studied with TIP4P [16], CF1 [17], SPC/E [18, 19] and six-site [20] models of water molecules. 

Yamanaka N, Kawano R, Kubo W et al. (2007) Dye-sensitized Ti02 solar cells using imida-zolium-type ionic liquid crystal systems as effective electrolytes. J Phys Chem B 111 4763—4769... 

All the above theories are derived for rigid rod nematic liquid crystal systems. The rheological behavior of chiral nematic liquid crystals is more complex and less understood than that of nematic systems. Rey introduced a model based on rigid rod chiral nematic liquid crystals to describe permeation shear flow and small amplitude oscillatory shear flow. The model can predict some common phenomena of chiral nematic liquid crystals, e.g., the three-region... 

Brostow, W. Arkinay, A. Ertepinar, H. Lopez, B. Phase structure and phase diagrams on polymer liquid crystal systems copolymers of PET and p-hydrobenzoic acid. POLYCHAR-3 Proceedings 1993, 3, 46. 

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