Cholesteric polyesters

Kricheldorf has studied extensively cholesteric polyesters 60 prepared from the mentioned dianhydroalditols even cyclic polyesters from 1 were isolated by a selective extraction of the linear polymers [39]. 

Strzelecki and coworkersfound that the use of (+) 3-methyladipic acid in the synthesis of liquid crystal polyesters led to a cholesteric polymer. They prepared a homopolymer and a series of copolyesters of the following structures ... 

Blumstein and coworkers studied the cholesteric behavior of polyesters with azoxybenzene mesogenic units and the same chiral spacer, (+) 3-methyladipic acid. They could clearly observe oily streak textures, which are typical of low molecular weight cholesterics, for the following homopolymer and copolymers ... 

In contrast, a racemic mixture of the 3-methyladipic acid residues in the polyester resulted in the formation of a nematic phase. Similar cholesteric polyesters based on azobenzene mesogenic units and (-f) 3-methyladipic acid were reported by Japanese workers... 

Recently Krigbaum and coworkers prepared cholesteric polyesters from the reaction of 4,4 -dihydroxy-a-methylstilbene with mixtures of (-f) 3-methyladipic acid and adipic acid. They could change the morphology of the cholesteric state formed by the (4-) 3-methylapidic acid homopolymer by mixing it either with a low molecular... 

The finding that the PEIs of 27b and monosubstituted hydro quinones form broad nematic phases, but show little propensity to crystallize, has prompted various modifications of their structures and properties. In this connection it should be stated that non-crystalline LC-polymers have found little interest in the past decades, but they may be attractive for various applications provided that the Tg can be varied between 90 and 250 °C. For instance, the absence of crystallinity has the advantage that the mechanical properties do not depend on the thermal history, and thus on the processing conditions. The temperature allowing a convenient processing may be reduced below 200 °C, which is of interest for the processing of LC-polymer reinforced blends and composites. Furthermore, non-crystalline nematic FC-polyesters are a useful basis for the synthesis of cholesteric lacquers, films or pigments (Sect. 5). 

On the other hand, liquid crystalline polymers applied to optical information storage has attracted great attention. The liquid crystalline polymer is applied mainly in terms of the thermo-optical effect. The backbone of liquid crystalline polymer can be polysiloxane, polyacrylate, or polyesters. In order to enhance the absorption coefficient for the writing laser beam, the dyes may be either dissolved into the liquid crystalline polymer in the guest-host model or attached to the backbone of the liquid crystalline polymer to form a copolymer. The nematic, cholesteric and smectic liquid crystalline polymers are all be able to be utilized in optical information storage. 

A.Blumstein, S.Vilasagar, S.Ponrathnam, S.B.Clough, and R.B. Blumstein, Nematic and cholesteric thermotropic polyesters with azoxybenzene mesogenic units and flexible spacers in the main chain, J.Polym.Sci., Polym.Phys.Ed. 20 877 (1982). 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

Phase transitions in thermotropic nematic and cholesteric main chain polyesters were investigated by polarizing microscopy and differential scanning calorimetry ... 

R B Blumstein. Nematic and Cholesteric Thermotropic Polyesters with Azoxybenzene Mesogenic Units and Flexible Spacers in the Main Chain. J.Poly.Sci Poly.Phys. 20 877 (1982). 

Cholesteric polyesters were prepared from silylated derivatives of 2,3-di-(9-isopropylidene-D-threitol, DAS, or DAM with dicarboxylic acid dichlorides by polycondensation in solution [34]. Trifluoroacetic acid-water allowed an easy cleavage of the isopropylidene group without hydrolysis of the polyester. All these polyesters formed a broad cholesteric phase, and the polymers containing 5 or 10 mol per cent sugar diol displayed a blue Grandjean texture. 

Photosetting cholesteric polyesters derived from DAS and 4-hydroxycinnamic acid [35], and from 2,5-bis(dode cyloxy)terephthalic acid and4,4 -dihydroxybiphenyl, were also described [36]. 

The first apparently cholesteric polyester derived from an alditol such as l,2 5,6-di-(9-isopropylidene-D-mannitol was reported by Chiellini etal. [54]. Surprisingly, an acidolyrtic deprotection yielded a seemingly thermotropic polyester. 

Schwarz G., Kricheldorf H.R., New polymer synthesis. LXXXlll. Synthesis of chiral and cholesteric polyesters from sUylated sugar diols , J. Polym. Sci. Part A Polym. Chem., 34, 1996,603-611. 

Sapich B., Stumpe J., Krawinkel T., Kricheldorf H.R., New polymer syntheses. 95. Photosetting cholesteric polyesters derived from 4-hydroxycinnamic acid and isosorbide, Macromolecides, 31, 1998, 1016-1023. 

Kricheldorf H.R., Wulff D.F., Layer structures. 11. Cholesteric polyesters derived from isosorbide, 2,5-bis(dodecyloxy)tere phthalic acid and 4,4 -dihydroxybiphenyl, Polymer, 39, 1998,6145-6151. 

With the help of cholesteric liquid crystals, intensity distributions of microwave fields can be rendered visible [12]. A suitable detector (Fig. 2) consists of a thin resistive metal film with a surface resistance of 200 12/a, to 400 2/d coated with a 10 jLtm to 20 jum layer of a cholesteric liquid crystal. A polyester film deposited on both sides protects the device. When this foil is introduced into a microwave field of sufficient power density, absorption in the metal film produces a temperature profile proportional to the field intensity distribution. The liquid crystal temperature indicator then makes this profile visible. 

A thin layer of cholesteric liquid crystal is sandwiched between a bottom layer of polyester blackened on the opposite side and a top layer of transparent polyester. The black layer serves both as a conducting lower electrode (approx. 1 kl2 per square) and as an absorber for the transmitted light. The bottom polyester layer presents an orienting surface for the liquid crystal alignment. The top polyester... 

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