Nematic mesophase thermotropic

When they are heated, mesogenic compounds do not melt directly from the highly ordered crystalline state to an isotropic liquid. They form instead, intermediate phases in which the molecules are orientated in a parallel direction and referred to as smectic (centers of the molecules organized in layers) or nematic (centers of the molecules distributed at random). Smectic and nematic mesophases are in turn divided into a variety of subgroups of thermotropic liquid crystals which will not be dealt with in detail in the present article. 

The synthesis and liquid crystalline properties are presented of two classes of chiral (I-n) and prochiral (Il-m) thermotropic poly(ester 0-sulfide)s. The nematic mesophase behavior of the polymers I-n exhibits distinct even-odd alternations with chemical structure and is compared with that of closely related poly(ester 0-sulfide)s Ill-n and IV-m. 

Gray, D.G. Harkness, B.R. Chiral nematic mesophase of lyotropic and thermotropic cellulose derivatives. In Liquid Crystalline and... 

Aromatic polyesters undoubtedly represent the most important class of thermotropic nematics. Fully aromatic rod-hke homopolymers such as poly(p-oxybenzoate) or poly (p-phenylene terephthalate) melt at temperatures which are too high to form a stable nematic mesophase. However if the regular chemical structure of the homopolymer is disrupted, the melting temperature is reduced and it is possible to obtain thermotropic nematics. ... 

Hydrosilylation reactions have been performed with PFSs containing Si-H functionalities to attach mesogenic azobenzene groups. This has allowed the preparation of calamitic thermotropic side-chain liquid crystalline materials that display a nematic mesophase between ca. 53 and 250... 

These materials represent the first observation of the SmC (zig-zag) and SmO (arrow head) structure in rod-coil diblock copolymers [41] in contrast to the homopolymer of poly( -hexyl isocyanate) which only form a nematic mesophase (both lyotropic [65] and thermotropic [66]). This confirms the idea by Halperin [60, 69] that rod-coil systems are a microscopic model for smectic liquid crystals in general. Although the SHIC rod-coil system has a relatively broad polydispersity, a smectic mesophase over a size scale of as much as 10 xm has been observed (Fig. 4B). This indicates that microphase separation plays a very important role in determining the self-assembly of the liquid crystalline process of these blocks. The existence of only a nematic phase in the rod homopolymer system is probably due to its broad polydispersity in contrast to the fact that a smectic meso-... 

The more obvious choice for a model compound corresponds to exactly one repeat unit of the polymer. The a-ethoxy-m-(4-n-alkoxy-4 -cyanobiphenyl)s exhibit nematic mesophases at n=4-9 and SmA mesophases at n = 8-ll, and therefore match the thermotropic behavior of the polymer better than the vinyl ether monomer. However the SmA mesophase is enantiotropic only at n = 11, and the nematic mesophase is monotropic at all of these spacer lengths. Compounds which take into account only the mesogen and spacer are also good, if not better, models of the polymers. In contrast to the ethyl ethers, all of the SmA and most of the nematic mesophases are enantiotropic, which means that the melting temperature mimics the relative temperature of the glass transition of the polymer backbone better. However, the nematic mesophase still appears at n = 6 -11, and the SmA mesophase doen t appear at n = 5-7, 10, 11. 

In contrast to 9 which exhibits a SmA and a nematic mesophase on heating, the lyotropic 8 is thermotropically not liquid crystalline. After two solid-solid transitions the diol 8 melts at 94 °C directly into the isotropic liquid. The thermotropic data of both compounds are given in Table 5. 

There are two principal categories of mesophases, thermotropic and lyotropic. Thermotropic liquid crystals are formed within a particular range of temperature in a molten material, with no solvent present, whereas lyotropic liquid crystals are formed by some substances when they are dissolved in a solvent. Within each of these categories there are three distinct classes of mesophases, which were first identified by Friedel in 1922. The simplest of these to describe are the nematic and smectic classes, illustrated schematically in fig. 12.16. These phases are formed by long thin rigid molecules which tend to line up parallel to each other. 

Three types of thermotropic liquid crystals have been characterised, the most used for NMR being the nematic mesophase, in which inter-molecular forces cause a long-range order by orienting the long axes of the comparatively linear molecules parallel, although the molecules remain free to move relative to each other, and domains with different orientation occur. 

Polyether dendrimer (thermotropic) Dibromalkanes with 1-(4-hydroxy-4 -biphenylyl-2-(4-hydroxyphenyl) 6-10 methylene units as flexible spacers those with 6, 8, and 10 CH2 units or with benzyl chain end exhibit nematic mesophase. [108]... 

Polyether dendrimers (thermotropic) Monomers 6-bromo-1-(4-hydroxy-4 -biphenylyl)-2-(4-hydroxyphenyl)hexane (TPH) 13-bromo-1 -(4-hydroxyphenyl)-2- [4-(6-hydroxy-2-naphthalenylyl)-phenyl]tridecane (BPNT) 13- bromo-1-(4-hydro-xyphenyl)-2-(4-hydro-4"-p- terphenylyl)tridecane (TPT) Chain ends benzyl or allyl or alkyl. TPH and BPNT show narrow nematic meosphase TPT nematic mesophase extends over 82 °C. Degree of branching for TPT with allyl end is 0.82. [137]... 

The relaxation of liquid-crystal thermotropic polymethyleneterephtha-loyl-bis( -oxybenzoate) was studied in Ref [139] the [3-relaxation occurred at low temperatures while a-relaxation took place at temperatures above 20 °C ((3-relaxation is associated with local motion of mesogenic groups while a-relaxation is caused by the glassing in amorphous state). It was established thin the presence of ordering both in crystals and in nematic mesophase expanded relaxation. 

Chen, Y., Shen, Z., Gehringer, L., Frey, H. and Stiriba, S.-E. (2006) Supramolecular thermotropic liquid crystalline materials with nematic mesophase based on methylated hyperbranched polyethylenimine and mesogenic carboxylic acid , Macromol. Rapid Comm., 21,69-75. 

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