Mesophases nematic

The mesophases of calamitic mesogens are classified in two groups nematic and smectic. The nematic mesophase (N) is characterized by an orientational order of the molecules that are aligned along a preferred direction (defined by a director n) (Figure 8.2). The molecules can slide and move in the nematic mesophase (while roughly keeping their molecular orientation) and rotate around their main axis. This is the less ordered mesophase and it is usually very fluid. 

In the smectic mesophases the molecules are oriented, as in a nematic mesophase, with their principal axis roughly parallel to the director, but they are also defining layers. These layers can be perpendicular to the director, as in the smectic A mesophase (SmA), or tilted, as in the smectic C (SmC). The SmA and SmC mesophases are the less ordered and more common smectic mesophases. Other less common types of smectic mesophases are known, which differ in the degree or kind of molecular ordering within and between the layers [2]. 

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

The nematic nanoparticies have been prepared by a two step synthetic process. First, gold nanopartides are covered with an alkylthiol monolayer (hexyl- and dodecylthiol) in a second step, the alkylthiol-nanoparticles are reacted with the functionalized thiol mesogen in dichloromethane at room temperature to obtain the monolayer-protected liquid crystal gold nanopartides. These materials are chemically stable and display a nematic mesophase at room temperature [67, 68]. Other examples include liquid crystal gold nanopartides functionalized by hexaalkoxy-substituted triphenylene [69]. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

Lyotropic liquid crystalline nanoparticles have also been described. Concentrated solutions of gold nanorods in water in the presence of a surfactant (cetyltrimethyl-ammonium bromide) display a nematic mesophase stable up to 200 °C [74[. The N mesophase was identified by optical microscopy by their typical nematic droplets texture. 

Liquid-crystalline 3,4-disubstituted furoxans such as 464 (R = 4-alkoxy-benzoyl, Ph) have been prepared by cyclodimerization of 4-ACOC6H4CNO, followed by hydrolysis to 464 (R = H) and acylation. The products form a nematic mesophase (537). 

PFO 196 is clearly crystalline with a melting point temperature around 150°C, above which a nematic mesophase exists up to ca. 300°C. Nanoscale crystallinity of PFO 196 was demonstrated by x-ray diffraction (XRD) experiments (Figure 2.8) [236,237], For the crys-... 

Note 2 The adjective diseotic is also employed to describe the nematic mesophases formed by diseotic mesogens. The mesophases formed by a columnar stacking of disc-like molecules are described as columnar mesophases. 

Note 1 See Fig. 1 for an illustration of the molecular organization in a uniaxial nematic mesophase. 

Assembly of molecules in a nematic mesophase having a short-range smectic-like array of the constituent molecules. 

See Fig. 3 for an illustration of the helicoidal molecular distribution in a chiral nematic mesophase. 

Note 1 Locally, a chiral nematic mesophase is similar to a uniaxial nematic, except for the precession of the director n about the axis, Z. 

Note 3 Chiral nematic mesophases exhibit Bragg scattering of circularly polarised light at a wavelength proportional to the pitch P (Xr = P, where is the mean refractive index). 

Note 4 The director precession in a chiral nematic mesophase is spontaneous and should be distinguished from an induced twisted structure produced by a mechanical twist of a nematic mesophase between confining surfaces. 

Note 5 The term chiral nematic mesophase or chiral nematic is preferred to cholesteric mesophase or cholesteric. 

Note 3 With chiral nematic substances forming chiral nematic mesophases of short pitch (<700 nm), up to three blue phases occur in a narrow temperature range between the chiral nematic phase and the isotropic phase. 

Note 4 The local structure in the nematic mesophase of certain dimers exhibit an intercalated smectic mesophase. 

Nematic mesophase in which disc-shaped molecules, or the disc-shaped portions of macromolecules, tend to align with their symmetry axes parallel to each other and have a random spatial distribution of their centers of mass. 

Note 2 The symmetry and structure of a nematic mesophase formed from disc-like molecules is identical to that formed from rod-like molecules. It is recommended therefore, that the subscript D is removed from the symbol No , often used to denote a nematic formed from disc-like molecules. 

Note 3 In some cases the discotic nematic mesophase is formed by compounds that do not have molecules of discotic shape (for example, phasmidic compounds, salt-like materials and oligosaccharides). 

Note 4 Chiral discotic nematic mesophases, N, also exist. 

Fig. 13. Illustrating the organization of molecules in a discotic nematic mesophase.

Note 3 In lyotropic systems, biaxial nematic mesophases have been identified from the biaxial symmetry of their tensorial properties. 

Note 2 Short board-like shaped molecules usually form biaxial nematic mesophases. It is recommended that the use of the term disordered sanidic mesophases for such mesophases be discontinued (see Definition 3.3.1, Note 5). 

Note 3 Two TGBA structures are possible in one, the number of blocks corresponding to a rotation of the layer normal by 2ti is an integer, while in the other, it is a non-integer. Note 4 A TGBA is found in a phase diagram between smectic A and chiral nematic mesophases or between a smectic A mesophase and an isotropic phase. 

Note 1 See 3.1.1 for the definition of a uniaxial nematic mesophase, 5.8.1 for the definition of uniaxial mesophase anisotropy, and Definitions 3.3 and 5.8.2 relating to biaxial mesophases. 

Spherical droplet that forms during a transition from an isotropic phase to a nematic mesophase. It has characteristic textures that depend on the droplet size and the director orientation at the nematic-isotropic interface. 

Note Nematic droplets display a texture characteristic of a nematic mesophase since they occur nowhere else. 

---