Molecular mechanics liquid crystals

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

Liquid crystals are materials which exhibit characteristics of both liquids and crystalline solids. From a continuum mechanical point of view, they possess a local unit vector n, the director, corresponding to the molecular direction. Liquid crystals generally exist in nature in three forms (a) nematics, in which the microstructure is oriented by direction, but not by position (b) cholesterics, in which the orientation is helical and (c) smectics, in which the microstructure is oriented by position, i.e. in layers. The smectic case is further divided into cases A,C, and others. We are concerned here with the smectic A case, in which the director is oriented normal to the layers. 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

The dipolar mechanism is sensitive to the molecular shape. By dimensional considerations one can estimate the flexocoefficients due to dipolar mechanism as ei, 63 < /Xe/a, where /Xg 1-5 debye (1 D = 3.3x 10 Cm) is the molecular dipole moment and a 2-4 nm is the typical molecular dimension for a low molecular weight liquid crystal. This means that e and 63 are expected to be of the order of pCm. Assuming a random three-dimensional distribution of the centre of masses of the constituent bent-core (banana-shaped) molecules, Helfrich and Derzhanski and Petrov derived a more precise expression for the macroscopically testable bend fiexo-electric coefficient ... 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

Development of new liquid crystalline (LC) polymeric materials has been a subject of intense interest because of the combination of unusual optical, electrical, and magnetic properties of low-molecular-weight liquid crystals and the mechanical performance and processibility of polymers. Application areas of LC polymers are very diverse, from engineering plastics to LC displays and erasable compact disks. However, the development of conducting and liquid crystalline polymers went on separately in the past in spite of the similarity of the molecular structures of typical main-chain liquid crystals and some conductive polymers. 

Among homogeneous ER fluids, there are systems that show an anomalous ER efiect. Strong dielectric, low molecular weight liquid crystals [48] and polar polymer solutions [49, 50] exhibit a negative ER effect, in which the viscosity and dynamic mechanical properties decrease. 

An interesting feature of polymer nematics was discovered in studying the orientation of some polymethacrylate polymers and cross-linked LC elastomers based on polysiloxanes [57]. Most nematic polymers form an optically positive, uniaxial, homeotropic structure under the effect of a mechanical Held such polymers have a positive birefringence (An > 0), like most low-molecular-weight liquid crystals. 

Another early work in this area is that of DeMeuse and Jaffe from Celanese who used several techniques such as rheology. X-ray diffraction, and calorimetry to demonstrate that a model system consisting of blends of two LCP polyesters of differing HBA/HNA ratios were in fact not miscible. This is in contrast to low molecular weight liquid crystals in which two liquid crystals which form the same type are expected to be miscible. This paper also demonstrated that transesterification did not occur at an appreciable rate under the conditions used for melt rheology measurements. They also discuss the concept that for copolymers in which the distribution of copolymer ratio is extremely wide, there could be phase separation within a nominally homogeneous copolymer. This is precisely the mechanism invoked to explain the anomalous temperature dependence of viscosity of HIQ LCP. ... 

Parameters (ii)-(vii) depend on the dielectric, mechanical and optical properties of the mesogens. To optimize a dis compromise between different molecular characteristics is often required and mixtures of liquid crystals are usually commercial displays. 

Kresge C T, Leonowicz M E, Roth W J, Vartuli J C and Beck J S 1992 Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism Nature 359 710-12... 

Chapter 9, on entropy and molecular rotation in crystals and liquids, is concerned mostly with statistical mechanics rather than quantum mechanics, but the two appear together in SP 74. Chapter 9 contains one of Pauling s most celebrated papers, SP 73, in which he explains the experimentally measured zero-point entropy of ice as due to water-molecule orientation disorder in the tetrahedrally H-bonded ice structure with asymmetric hydrogen bonds (in which the bonding proton is not at the center of the bond). This concept has proven fully valid, and the disorder phenomenon is now known to affect greatly the physical properties of ice via the... 

An essential requirement for device applications is that the orientation of the molecules at the cell boundaries be controllable. At present there are many techniques used to control liquid crystal alignment which involve either chemical or mechanical means. However the relative importance of these two is uncertain and the molecular origin of liquid crystal anchoring remains unclear. Phenomenological models invoke a surface anchoring energy which depends on the so-called surface director , fij. In the case where there exists cylindrical symmetry about a preferred direction, hp the potential is usually expressed in the form of Rapini and Popoular [48]... 

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. 

Molecular mechanics force fields have largely been parameterised using the best available data from the gas phase and (in some cases) from liquid phase or solution data. The question therefore arises as to how applicable molecular mechanics force fields are to predicting structures of molecules in the liquid crystal phase. There is now good evidence from NMR measurements that the structure of liquid crystal molecules change depending on the nature of their... 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

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