Cholesteric liquid crystal polymer

Cholesteric liquid crystal pol5miers are commercially available, such as the HELICONE series. This type of polymer has been described in detail (42). Cholesteric siloxanes are liquid-crystaUine side chain polymers (41). Also, compoimds based on cyclic organo-siloxanes with side chains containing cholesterol and methacryloyl groups have been described (43). 

---

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. 

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. 

D.-K. Yang and J.W. Doane, Cholesteric liquid crystal/polymer gel dispersions reflective displays, SID Inti. Symp. Digest Tech. Papers, 23, 759 (1992). 

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). 

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... 

Yang DK, Doane JW (1992) Cholesteric liquid crystal/polymer gel dispersion Reflective display application. SID Symp Dig Technic Pap 23 759-761... 

Yang, D.-K., Chien, L.-C., Doane, J.W. Cholesteric liquid crystal/polymer dispersion for haze free light shutters. Appl. Phys. Lett. 60, 3102-3104 (1992)... 

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. 

Hou H, Reuning A, Wendorff JH, Greiner A (2000) Tuning of the pitch height of thermotropic cellulose esters. Macromol Chem Phys 201(15) 2050-2054 Huang B, Ge JJ, Li Y, Hou H (2007) Aliphatic acid esters of (2-hydroxypropyl) cellulose—effect of side chain length on properties of cholesteric liquid crystals. Polymer 48(l) 264-269 Isogai A, Saito T, Fukuzumi H (2011) TEMPO-oxidized cellulose nanofibers. Nanoscale 3(l) 71-85... 

The complex of poly(Fglutamic acid) with PEG [48,49] and complexes of PNVP with polyphenols [48] are examples of hydrogen bonded complexes other than those containing PMAA or PAA. The former complexes form in organic solvents, typically DMF or dioxane/water, with PEG filling interstitial spaces between parallel alpha helices of PGA to become integral constituents of lyotrophic cholesteric liquid crystals. Polymers based on the copolymer of phenol with formaldehyde can form complexes [48] with crosslinked PNVP gels in water/ethanol environments. 

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

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. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Coatings derived from cholesteric liquid crystalline polymers are used commercially as reflective sheets and polarisers. The liquid crystal is cooled below the vitrification temperature resulting in a solid polymer that is amorphous but contains large regions of frozen liquid crystalline order. Such structures are also found in nature in the iridescent, almost metallic colours of beetles and other insects, which result from helical cholesteric structures in the outer layer of the carapace. 

Organic materials with large optical rotations include cholesteric liquid crystals, molecules and polymers with chiral jt-conjugated systems, especially [n]helicenes [21, 31, 139]. The most important factor contributing to their large optical rotations is anomalous optical rotatory dispersion (ORD), which is associated with the presence of absorption (or reflection) with large rotational strength (Fig. 15.30). 

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... 

In Fig. 23 the relationship between B/kg and XA/D are shown for the polypeptide liquid crystals in various solvents. Following Eq. (25), the compensation temperature is determined by the ratio (fi/kg)/(lA/D) at a constant polymer concentration. The solid and broken lines shown in Fig. 23 correspond to the theoretical values calculated for Tjj = 25 °C and 80 °C, respectively. The solvents located above and below the solid line support the right-handed and left-handed cholesteric liquid crystals, at 25 °C, respectively. The situation is the same for the broken line at 80 °C. The solvents located between the two straight lines invert the cholesteric sense from right-handed to left-handed in the range of measurements. 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

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. 

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. 

A lot of biological polymers exhibit cholesteric liquid crystal characteristics as pointed out first by Robinson et al. (1958). The notable examples are RNA and DNA. The well-known double helical structure, shown in Figure 6.25, makes them rod-like in conformation. 

In main chain cholesteric liquid crystalline polymers, the mesogenic groups and flexible spacers are linked alternatively. The flexible units contain asymmetrical carbon atoms which enable the polymers to possess chirality and thus form cholesteric liquid crystals. By varying the ratio of chiral to non-chiral parts, the cholesteric temperature range and pitch can be changed. The cholesteric range depends on the mol fraction of the polymers. A typical main chain cholesteric liquid crystalline polymer is shown in Figure 6.27. 

---