Liquid crystals original references

Highly aromatic thermoplastic polyesters first beeame available in the 1960s but the original materials were somewhat difficult to process. These were followed in the 1970s by somewhat more processable materials, commonly referred to as polyarylates. More recently there has been considerable activity in liquid crystal polyesters, which are in interest as self-reinforeing heat-resisting engineering thermoplastics. 

The lyotropic mesophases of cellulose and cellulose derivatives were first observed only relatively recendy (1-3). It is of interest to note that Flory in his now classical papers (44) i icted in 1956 that cellulose or cellulose derivatives should exhitnt liquid crystal behavior. Since Werbowyj and Gray (I) first reported mesophases of hydroxylpropyl cellulose in water, the field has expanded rapidly (for reviews see References 6 and 7). Undoubtedly, the activity in this area originates from a desire to prepare fibers or films of cellulose or cellulose derivatives with supoior properties as well as to understand the purely scientific aspects of the systems. 

The chapter starts with a discussion of the basics of liquid crystal phase behavior, the types of mesophase that are formed and methods for their characterization those requiring more detail and/or breadth are directed to the volumes of reference. This chapter does not cover all liquid crystals which might be considered as being somehow inorganic in origin, and so, for example, systems containing boron clusters, and the lyotropic mesophases of metal oxides, etc., are omitted. Similarly, polymeric systems are not covered in any detail as since some earlier reviews, there has been relatively little development in the field. 

The extrapolated line of log S-log C crossed each other at a critical concentration Cq at which S stays constant and independent of temperature. These results suggest that the temperature dependence of the cholesteric pitch would inflect at the concentration higher than Cq This is analogous to the behavior of thermotropic liquid crystals composed of cholesteric solute and nematic solvent, where the sign of dS/dT reverses at a critical concentration. It is understood that the behavior of both thermotropic and lyotropic liquid crystals is comparable provided that the nematic substances of the former are substituted with the solvents of the latter. The critical concentration Cq is about 0.41 vol/vol and this value is very close to the concentration at which the side chains on neighboring molecules of the polymer come to contact each other ( refer to fig.5 ). From these results, it is expected that the origin or mechanism of twist would change at this concentration Cq. The... 

Liquid crystals (LCs) represent an intermediate state of matter between the solid and liquid phases, often referred to as the fourth state of matter, and exhibit the regularity of crystalline solid and the fluidity of isotropic liquid [1-4], The unique thermal, mechanical, optical, and electrical properties of LCs originate from the molecular self-organization facilitated by weak intermolecular interactions, which is sensitive to external stimuli. Stimuli-responsive LCs are at the forefront in the development of electro-optic devices such as LC displays (LCDs) and continue to attract great interest in view of both fundamental research and practical applications. 

Part 5 covers special structures such as liquid crystals, solid surfaces and mesoscopic and nanostructured materials. The chapter on liquid crystals covers physical properties of the most common liquid crystalline substances as well as some liquid crystalline mixtures. Data compiled in the chapter on solid surfaces refer to atomically clean and well characterized surfaces. The values reported are mainly averages from different authors where reference to the original papers is made. In the chapter on nanostructured materials emphasis is placed on size and confinement effects. The properties associated with electronic confinement are addressed and particular attention is drawn to semiconductor-doped matrices. The two main applications of nanostructured magnetic materials, spintronics and ultrahigh-density data storage media, are also treated. 

FIGURE 4.12. Variation with voltage of the optical characteristics of twist cells between parallel polaroids d = 30 /xm, a mixtiure of nematic liquid crystal with Ae = +0.22, T = 23 C) [66]. (1) Transmission with a cell conventionally arranged to the polarization of the incident ray (2) transmission when the cell is rotated around the normal through 45 from the original position (3) phase lag calculated from curve 2. Axes to the right and bottom refer to (1) and (2) and axes to the left and top refer to (3). 

Chiral nematic liquid crystals, as the name suggests, are optically active variants of nematic liquid-crystalline compounds the incorporation of a chiral centre imparts properties which are unique to the chiral nematic phase and are responsible for their utilisation in a variety of differing display technologies and other related applications. The term cholesteric liquid crystal was originally used to describe this phase, and originates from the structural nature of the earliest chiral nematic liquid crystals which were derivatives of cholesterol [1,2], Nowadays, the term chiral nematic is used primarily because the materials are clearly derived from nematic type liquid crystals [3, 4], Despite these differences in definition, the terms cholesteric and chiral nematic phase are interchangeable and it is common to find references to either term in the literature. 

The majority of current commercial liquid crystal displays are based on the twisted nematic electrooptic effect using active matrices to give, for example, complex computer lap-top screens [13]. It is often overlooked that these displays are based on long pitch chiral nematic materials to remove so-called reverse twist, and the original papers [254, 255] referred to the effect as a positive planar cholesteric to nematic phase... 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as liquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many liquids in real time (14). 

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