Liquid crystalline behavior

Liquid crystalline behavior occurs in the exocuticle of certain classes of beetles. The bright iridescent colors that are reflected from the surface of Scarabaeid beetles originates from a petrified chiral nematic stmctural arrangement of chitin crystaUites in the exocuticle (38). It is suggested that this chiral nematic texture forms spontaneously in a mobile, Hquid crystal phase that is present during the initial stages of the exocuticle growth cycle. 

Characterization439 Inherent viscosity before and after solid-sate polymerization is 0.46 and 3.20 dL/g, respectively (0.5 g/dL in pentafluorophenol at 25°C). DSC Tg = 135°C, Tm = 317°C. A copolyester of similar composition440 exhibited a liquid crystalline behavior with crystal-nematic and nematic-isotropic transition temperatures at 307 and 410°C, respectively (measured by DSC and hot-stage polarizing microscopy). The high-resolution solid-state 13C NMR study of a copolyester with a composition corresponding to z2/zi = 1-35 has been reported.441... 

Coco, S., Cordovilla, C., Espinet, P., Martm-Alvarez, J. and Munoz, P. (2006) Dinudear gold(I) isocyanide complexes with luminescent properties, and displaying thermotropic liquid crystalline behavior. Inorganic Chemistry, 45, 10180-10187. 

Benouazzane, M., Coco, S., EspineL P., Martm-Alvarez, J. and Barbera J. (2002) Liquid crystalline behavior in gold(I) and silver(I) ionic isocyanide complexes smectic and columnar mesophases. Journal of Materials Chemistry, 12, 691-696. 

Vemula, P.K., Mallia, V.A., Bizati, K. and John, G. (2007) Cholesterol Phenoxy Hexanoate Mesogens Effect of meta Substituents on Their Liquid Crystalline Behavior and in Situ Metal Nanoparticle Smtesis. Chemistry of Materials, 19, 5203-5206. 

N4P4(OR)8 (R = C6H4C6H4OC8H17 and C6H4CH = NC6H4OC7H15) have been prepared and shown to exhibit liquid-crystalline behavior (131). 

In order to determine the necessity and/or the length of the spacer that is required to achieve liquid crystalline behavior from flexible vs. rigid polymers, we have introduced mesogenic units to the backbones of a rigid [poly(2,6-dimethyl-l,4-phenylene oxide) (PPO)] and a flexible [poly(epichlorohydrin) (PECH)] polymer through spacers of from 0 to 10 methylene groups via polymer analogous reactions. 

Alkoxystilbazoles can react with a variety of silver salts, AgX, to yield the two-coordinate linear complexes (19). They show liquid crystalline behavior that strongly depends on the anion, the alkoxy chain length, and other substituents which may be present on the stilbazole.59-65... 

Another approach to get new liquid crystals is the lateral fluorination of the stilbazole ligands,337 which is a common and highly effective tool to exert control over mesomorphism, crystal phase stability, and physical properties. Other modifications include the use of more alkoxy substituents and other alkyl sulfate anions.338-344 Ionic silver amino complexes also display liquid crystalline behavior at rather low temperatures they are of the form [Ag(NH2 -CJl +OJX (X = N03, n = 6,8,10,12,14 X = BF4, = 8,10,12,14).345... 

In this category, the units giving rise to the liquid-crystalline behavior can be in the backbone (as already discussed), in the side chains, or in both [172], Of considerable interest is the orientation of the mesogenic groups and the chain backbones to which they are attached [173,174], Frequently-studied backbones include siloxanes and acrylates [175,176], but a variety of other structures have also been studied, including amphiphilics [177]. 

Block copolymers with PS and a polymethacrylate block carrying a liquid crystalline group, PS-b-poly 6-[4-(cyanophenylazo)phenoxy]hexyl methacrylate, were successfully prepared in quantitative yields and with relatively narrow molecular weight distributions (Scheme 5) [18]. The thermotropic liquid crystalline behavior of the copolymers was studied by differential scanning calorimetry. 

Whereas poly(9,9-dihexylfluorene) (PDHF, 195) is generally considered as amorphous, PF with longer octyl side chains, PFO 196, is crystalline material. Many PFs — dioctyl (PFO 196 [228,230,231]) or 2-ethyl hexyl) (197 [232]) as well as some fluorene copolymers [233] exhibit liquid crystalline behavior, opening a possibility to fabricate polarized LEDs [224,234,235] (Chart 2.45). 

Pang, Li, and Barton indicated, based on viscosity data, that PAEs had a stiff structure in solution [43]. A PAE shows a Mark-Houwink constant as high as a= 1.92 [43], revealing that it has a very stiff structure, similar to rodlike poly-(pyridine-2,5-diyl) [65,66]. Liquid crystalline behavior of PAEs with a meso-gen has been reported [45]. The chemical properties of PAEs have also been investigated with oligomeric model compounds [67-76]. 

A given compoimd may exhibit one t3rpe of liquid crystalline behavior, or several t3rpes, each at characteristic temperature and solvent concentration or solvent type, or it may, of course, only form an isotropic phase. 

Intermediate concentration for many years almost permanently trapped In a strongly enmeshed, Intermediate state with low scattering power. Over a period of weeks or months, other regions may concentrate sufficiently to display the type of liquid crystalline behavior shown In Figure 9b. 

LCP phases are subdivided into thermotropic or lyotropic. Lyotropic liquid crystals are formed by macromolecules that show liquid crystalline behavior in solution. This behavior is strongly concentration dependent. Thermotropic liquid crystals are molecules that show liquid crystalline behavior above the melting point of their crystallites. 

Goodby, J. W., Melting phenomena and liquid-crystalline behavior, Chemlog Highlights, 11, 3-7 (1987). 

Increasing the concentration of surfactants in water to a level above the CMC leads to the formation of rod-like micelles and, subsequently, liquid crystals [251]. Both liquid crystals and liquid-crystalline polymers [252] have been used as media for small particle generation [253, 254] and have also acted as piezoelectric devices [255]. Of particular interest are metallomesogens, the metal complexes of organic ligands which exhibit liquid crystalline behavior [255],... 

Lyotropic liquid crystals they display liquid crystalline behavior when mixed with another material in the right concentration (typically a solvent). They can also be a mixture of more than two components (e.g., cell membranes). 

Many ccllulosic derivatives form anisotropic, i.e.. liquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety ol solvents. The nature of Ihe polymer -solvent interaction determines the concentration at which liquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birclringenl polymer solution. Strong organic acids, eg, trifluoroac etic acid, are most effective and can produce an anisotropic phase with concentrations as low as... 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

In homologous series liquid crystalline behavior is found for molecules with comparable length of RH- and RF-segments, whereas compounds with very different block size are usually crystalline [106, 107]. Substituents at the end of the aliphatic blocks have a significant effect on self assembly. Branching of the... 

For the liquid crystalline behavior, the position of the ester group was crucial. Introduction of an ethyl spacer in 9 led to the complete loss of the mesophase and... 

Lattermann found that the sixfold symmetry is not important for the presence of columnar mesophases in [14]-N4 when appropriate peripheral groups are chosen [105]. As substituents with one terminal alkyl (88a-d) chain and with three terminal alkyl chains (88e) are not able to induce liquid crystalline behavior in 88 (Table 5) [104], a substituent with two alkyl chains, namely 3,4-bis(alkyloxy) benzoyl, was chosen since it has been known to induce liquid crystalline phases in other systems. A mesophase (most likely Colh due to the texture) was observed between 96 and 132 °C for 90 (Scheme 44). 

---