Cholesteric liquid crystals applications

Liquid crystals are mainly used for decorative purposes in cosmetics. Cholesteric liquid crystals are particularly suitable because of their iridescent color effects, and find applications in nail varnish, eye shadow, and lipsticks. The structure of these thermotropic liquid crystals changes as a result of body temperature, resulting in the desired color effect. In recent times, such thermotropic cholesteric liquid crystals have been included in body care cosmetics, where they are dispersed in a hydrogel. Depending whether this dispersion requires stirring or a special spraying process, the iridescent liquid crystalline particles are distributed statistically in the gel (Estee Lau-... 

Optically active polymerizable liquid-crystal monomers, (I), providing polymeric cholesteric liquid-crystal films were prepared by Seki et al. (2) and used in optical applications. 

Another saturated tetrahydrofuryl core has found application as a component of liquid crystals. Cholesteric liquid crystal polymers are useful as photostable UV filters in cosmetic and pharmaceutical preparations for the protection of human epidermis and hair against UV radiation, especially in the range 280-450nm <2000DEP19848130>. Fused bifuran 81 is a suitable monomer for the preparation of these desired polymers as it contains the requisite characteristics of having more than one chiral, bifunctional subunit type which is capable of forming a cholesteric liquid crystal phase with a pitch of <450 nm. It also contains an achiral aromatic or cycloaliphatic hydroxyl or amino carboxylic acid subunit, achiral aromatic or cycloaliphatic dicarboxylic acids, and/or achiral aromatic or cycloaliphatic diols or diamines. Polymers prepared from suitable monomers, such as diol 81, can also be used as UV reflectors, UV stabilizers, and multilayer pigments. 

Problem 10.6 Name two applications of cholesteric liquid crystals. 

Thus, side-chain systems can exhibit many properties in between, well-oriented and solid materials. Many applications for cholesteric, nematic, and smectic cyclic siloxanes have been proposed. Most of them use cholesterics. Cholesteric liquid crystals (n ) or tilted smectic phases reflect the incident light in a specific wavelength range and with circular polarization. The... 

The reflective wavelength of cholesteric liquid crystals varies according to temperature. Such an effect has been made useful in thermography. It has been applied in the diagnosis of cancers by displaying the skin temperature distribution. It has also been applied to test faults in integrated circuits. The applications also include thermometers and temperature warning indicators and non-destructive detection. 

In the above applications, cholesteric liquid crystals need to be sealed between two glass plates or in the form of micro encapsulates. However, cholesteric liquid crystalline polymers can easily form thin films or be coated on substrates. 

Their thermal stability is low and there is little chance that they will find applications in lubrication, except maybe, in biological systems. We think, however, that they are implicated in the orientation of nematic (or cholesteric) liquid crystals by their interactions with the superficial atmospheric layer, which explains the role of the salts of fatty acids as lubricant additives. 

Reports on the applications of h.p.l.c. to specific problems include an efficient separation of the reduction products of progesterone, of the conjugates of natural bile acids, and of 2-hydroxy- and 2-methoxy-oestrogens ( catechol oestrogens). Fluorescence detection is reported to be some 500 times more sensitive than u.v. absorption for h.p.l.c. of oestriol. The h.p.l.c. behaviour of compounds in the vitamin D series appears to be correlated with the degree of molecular planarity. " A first report on the use of cholesteric liquid crystals as stationary phases for h.p.l.c. shows promise. Various cholesteryl esters coated on or bonded to Corasil II showed increased capacity factors (k ) when steroids were chromatographed, and permitted some useful separations. 

R.A. van Delden, T. Mecca, C. Rosini, B.L. Feringa, A chiroptical molecular switch with distinct chiral and photochromic entities and its application in optical switching of a cholesteric liquid crystal. Chem. Eur. J. 10, 61-70 (2004)... 

The second application uses the converse flexoelectric effect, i.e. a field-induced splay-bend distortion, to generate a fast, symmetric and thresholdless linear electro-optic effect in a cholesteric liquid crystal. 

A multifunctional cholesteric liquid crystal polymer functionalized with amine has been synthesized and tested for biomedical applications [104]. 

Mercedes PM, Boualem H. SAXS and SANS investigation of synthetic cholesteric liquid-crystal polymers for biomedical applications. J Mater Sci Eng 2013 B 3(2) 104-15. 

In bistable Ch reflective display applications, it is desirable that the threshold of the transition from the planar state to the focal conic state be high, so that the cholesteric liquid crystal can remain in the planar state and the display does not exhibit flicker under colimin voltage in addressing. 

Optical ProportiGS. All of the optical applications of LCTs have considered the cholesteric phase. The cholesteric phase is interesting because thin films of a cholesteric liquid crystal oriented such that the helical axis is perpendicular to the plane of the film will selectively reflect circularly polarized light with a wavelength determined by the pitch of the helix. For low molar mass liquid crystals. 

The characteristic functionalities of naturally occurring polymers are, in most cases, related to their specific chiral structure. In nature, proteins, nucleic acids, and polysaccharides are constructed of readily available chiral monomers such as sugars and amino acids. Both natural and synthetic chiral polymers are finding application as chromatographic supports, polymeric reagents and catalysts, chiral membranes, and materials for preparation of cholesteric liquid crystal polymers (471,472). 

In fact, our theoretical analysis in Secs. II and III, using the Bloch picture should be applicable to all such materials. Bloembergen and Sievers have considered possibilities for phase matching of a number of nonlinear interactions in periodic layers of GaP and GaAs. This layer medium lacks inversion symmetry, thus permitting the observation of second-order nonlinear effects. However, it is difficult to fabricate and not so directly tunable. The great advantage of cholesteric liquid crystals is their inherent and variable periodicity. 

This effect has been observed experimentally in comparatively thick cells (d 50 /xm) [113]. In cells with d 20 /xm, the final twisted state (in the field) proves to be insufficiently stable and the nematic liquid crystal layer is gradually transformed into a planar structure. The addition of small quantities of cholesteric liquid crystals to the initial nematic mixture enables a stable twisted structure to be achieved with the application of a field and improves the electrooptical characteristics of the device. The electrooptical response of electrically induced twist nematic cells includes intensity oscillations observed both in the switching on and switching off regimes [114]. These oscillations take place due to the variation of birefringence, which are not important in the usual twist effect. 

If the cell walls are treated to produce a homeotropic orientation in a cholesteric liquid crystal with As < 0, for a particular relation between the thickness and the helical pitch (1 < Po/d < 2) a new texture appears under the influence of the electric fleld. These are the bubble domains [77] mentioned in Section 6.1.1. When there is no field present the liquid crystal has a homeotropic nematic structure, since the helix is untwisted by the walls. The application of a low-frequency field induces an electrohydrodynamic instability. After the removal of the field a new stable texture appears in the form of cholesteric bubbles dispersed through the homeotropic nematic phase. Thus a memory is created, while erasure of the information can be performed either by a high-frequency field or by the mechanical... 

Khandelwal H, Loonen RCGM, Hensen JLM, Schenning APHJ, Debije MG (2014) Application of broadband infrared reflector based on cholesteric liquid crystal polymer bilayer film to windows and its impact on reducing the energy consumption in buildings. J Mater Chem A 2 14622-14627... 

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