Cholesteric liquid crystal polymer phase

Under certain conditions, stiff rod-like helical polymers can spontaneously form lyotropic or thermotropic cholesteric liquid crystal (TChLC) phases. 

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. 

Huang et al., studied a series of aliphatic esters of HPC-CnPC, where n = 2,3,5, 6, 7, 10. The authors observed that as one increases the number of methylene units in the side-chain of the cholesteric liquid crystal polymers, the window of the thermotropic phase transition narrows (Huang et al. 2007). Although the authors presented a similar study to the one published by Kosho et al. in 1999 (Kosho et al. 1999), wide-angle X-ray diffraction (WAXD) studies permitted to detect that the layer spacing of the cholesteric liquid crystals in this series increases linearly with an increase in the methylene units in the side chains. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

To form cholesteric liquid crystalline polymers, one either polymerizes cholesteric monomers or mixes low molecular mass cholesteric liquid crystals with polymers. In the latter case, two components may be mixed homogeneously or in such a way that the polymers act as a matrix while the small molecular mass cholesteric liquid crystals are in droplets, known as the polymer-dispersed liquid crystals (PDLC) (Doane et al., 1988) or the nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition, there are many polymers in nature exhibiting the cholesteric phase such as PBLG, cellulose, DNA, etc. 

To our knowledge, this is the first example of the coexistence of both twisted smectic and cholesteric phases in thermotropic liquid crystal polymers. Previous preparations of thermotropic polymers by the use of chiral derivatives both incorporated in the macromolecular backbone and pendant to it as side chain substituents (comb-like polymers) resulted in either cholesteric or smectic " polymeric products. 

There are two classic approaches for encapsulation emulsification [17] and phase separation [18]. The main difference between the two methods depends on how to make and process encapsulated droplets. In the emulsification method, water is used as a solvent to dissolve a polymer and to form a viscous solution, and cholesteric liquid crystals are mixed with the aqueous solution. By a shearing device like a propeller blade, small droplets of micrometer scale are formed and finally emulsified. The resulting emulsion is then printed on a plastic film and dried by evaporation of water. The disadvantage of this method is the broad size... 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

Absorption measurements in the visible/UV region have been employed widely in the study of liquid crystal polymers displaying cholesteric phases. In that case absorption occurs through the selective scattering of light due to the regularly twisted structure. 

Cholesteric droplets have been extensively studied during the last decade, especially after Crooker and Yang suggested to use polymer-dispersed cholesteric liquid crystals for reflective color displays [63], Lavrentovich and Nastishin [64], [65] reported on an intriguing phenomenon liquid crystal droplets dispersed in an isotropic matrix (glycerin with lecithin) divided into smaller ones when one decreases the temperature of the sample, and passes from the cholesteric to the smectic A phase through the TGB phase. The reader is referred to the recent reviews [66]-[68], and to the contribution of Crawford, Svensek, and Zumer in this book for more details about dispersed liquid crystals. 

Polymer dispersed liquid crystal (PDLC) devices usually contain N phases as the liquid crystal material [71] and are used in vision products [72], e.g., privacy windows, projection displays [73], and direct view displays [74, 75]. Cholesteric liquid crystals have also been used [76]. All these devices relax back to the original ground state when the field is removed. Ideally such films consist of dfoplets of liquid crystal in a polymer matrix the reverse situation (reverse phase) consists of a liquid crystal continuum with polymer balls dispersed within it. The latter films are not desirable, because they do not provide reversible electrooptic effects. 

The -dependence of the decay rate F of tracer polystyrene in polymethylmethacrylate benzene was measured by Numasawa, et a/.(60). The value of V/q increases at large q, as also seen for dilute polymers in simple solvents as discussed in Chapter 11. The effect of a phase transition on Ds was observed by Russo, et al, who examined poly(y-benzyl-a,L-glutamate) pyridine(61). An isotropic-cholesteric liquid crystal phase transition occurs for this rodlike polymer at elevated c. The value of Ds c) increases dramatically at the transition, but on both sides of the transition Ds c) decreases as c is increased. 

Emoto A, Uchida E, Fukuda T (2012) Optical and physical applications of photocontrollable materials azobenzene-containing and liquid crystalline polymers. Polymers 4 150-186 Ericson LM, Fan H, Peng HQ, Davis VA, Zhou W, Sulpizio J, Wang Y, Booker R, Vavro J, Guthy C et al (2004) Macroscopic, neat, single-walled carbon nanotube fibers. Science 305 1447-1450 Etchegoin P (2000) Blue phases of cholesteric liquid crystals as thermotropic photonic crystals. Phys Rev E 62 1435-1437... 

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