Thermotropic smectics

Note 3 A focal-conic domain built around an ellipse and an hyperbola is the most common type of defect in thermotropic smectic A phases. The hyperbola passes through a focus of the ellipse and the ellipse passes through the focus of the hyperbola (see Fig. 24). Note 4 In a particular limiting case of an ellipse-hyperbola focal-conic domain, the ellipse becomes a straight line passing through the center of a circle. 

Decher et al. [420] have developed the idea of using freely suspended smectic liquid crystals and have used it to form thin hlms on solid substrates. A thermotropic smectic liquid crystal is drawn across an aperture in a solid support and is capable of bridging the aperture (which can be up to 15 mm in diameter) with a him which can be between two and several hundred layers thick. The him consists of a smectic structure with the layers lying in the plane of the him. The him is formed a short distance above a solid substrate and the apparatus is constructed so that a difference of pressure between the two sides of the him can be used to force the him down in contact with the substrate. These authors have thus formed good quality hlms up to an area of about 1 cm2. In the work described the material used was ethyl-4 -n-octyloxybiphenyl-4-carboxylate. 

This study has concentrated on the defects observed in lyotropic lamellar phases, and it has put into evidence the specific character of the textures compared to classical thermotropic smectic phases. In leci-... 

Little work seems to have been done on thin oriented layers of lyotropic liquid crystals although there is one recent report of preparation of such a layer of the lecithin-water lamellar phase (JO). As indicated by Brochard and de Gennes (II), theories of the hydrodynamics of thermotropic smectic materials can be adapted to describe oriented layers of lamellar liquid crystal in lyotropic systems. 

In comparison to nematic liquid crystals, examples of smectic liquid crystals doped with quasi-spherical nanoparticles became more elusive over the last few years. This is surprising especially considering recent work by Smalyukh et al., who found that nanoscale dispersion (based on /V-vinyl-2-pyrrolidone-capped gold nanoparticles with 14 nm diameter) in a thermotropic smectic liquid crystal (8CB) are potentially much more stable than dispersions of nanoparticles in nematics [367]. 

For small-molecule thermotropic smectic-A phases, typical values of two elastic constants are K 10 dyn and B 10 dyn/cm (Ostwald and Allain 1985). For lyotropic smectics, such as those made from surfactants in oil or water solvents, the layer compression modulus B can be much lower (see Chapter 12). From B and K, a length scale A. = ( 1 /B) 1 nm is defined it is called the permeation depth and its magnitude... 

The rheology of lyotropic smectic phases under shear has some similarities, as well as some differences, with respect to that of thermotropic smectics discussed in Section 10.4. In particular, the shear viscosity rj shows a power-law shear-thinning region as a... 

As discussed m Section It).4.8, small-molecule thermotropic smectics, such as 8CB, show two flow alignment regimes under steady shearing (see the left half of Fig. 10-38 labeled SmA ). At high shear rates and high temperatures, 8CB orients preferentially in the a, or perpendicular, direction under steady shear, the same as is seen in block copolymers in region II. Recent theory indicates that layer fluctuations destabilize the c, ... 

Contact preparations of complexes 106b and 111 with alkanes revealed the induction of a nematic phase with the complete destabilization of the thermotropic smectic phases. For both complexes, mixing with solvents resulted in the substantial depression of both melting and clearing temperatures. In the case of 106b, the nematic range... 

The general phase behavior of mono-tailed amphiphiles is shown in Fig. 6. Starting at a certain chain length of the lipophilic part of the molecule, a thermotropic smectic phase... 

According to Fig. 3, classical thermotropic smectic phases of amphotropic liquid crystals are (SmA ), colunmar hexagonal (Col ), bicontinuous cubic (Cub, i), or discontinuous cubic (Cubjis) [169]. All these meso-phases include a disclination surface between the hydrophilic and the lipophilic parts of the unordered molecules. This surface can be uncurved (SmA), curved in one direction (columnar), curved in two directions with the same sign (discontinuous cubic), or curved in two directions with opposite sign (bicontinuous cubic). 

In the classical view only membranes and their components seem tolerable, with their close bilayer similarities to thermotropic smectics and head/tail abstracted lyotropics [7a, 33p, q, 54-57]. While nucleic acids and proteins represent both structure and system individualities, membranes, as huge collectives of highly differentiated mem-... 

The increase of T with further lengthening of the spacer (m>8) in the present series has to be most likely attributed to the occurrence of a thermotropic smectic mesophase... 

Aromatic polyesters forming thermotropic smectic mesophases, J.Polym.Sci., Polym.Lett.Ed. 20 109 (1982). 

Structural forces due to long-range positional order are quite easily observed in the smectic A liquid crystals. SFA measurements have been performed on lamellar lyotropic smectics [42,43] and in thermotropic smectics [44-46]. These works extend to a nanometer scale the early studies on elasticity, viscoelastic response and layers instability of smectic A, observed in macroscopic wedge-shaped piezoelectric cells [47,48]. 

Triblock partially fluorinated hydrocarbons F/nHiiFin(m = 12, n = 8 m = 10, n = 10 m = 12, n = 12) have also been examined and all are found to exhibit very narrow (< ) range of thermotropic smectic B phases between their crystalline and isotropic liquid phases [88]. When the polymethylene portions of such molecules are replaced by a much stiffer pam-substituted aromatic ring, the mesogenic character is lost [89]. 

The term "lipid nanoparticles" includes all colloidal systems where the nanoparticles consist of a kind of lipid matrix whereas the matrix lipid can occur in different physicochemical states (Figure 9.1) isotropic liquid (e.g. conventional fat emulsions), liquid crystalline (e.g. lyotropic cubic and thermotropic smectic ) or solid crystalline (SLN). A further distinction can be made if the lipid matrix is continuous (emulsions, SLN) or presents a discontinuous network of e.g. lipid bilayers (e.g. cubic nanoparticles). However, it should be kept in mind that lipid nanoparticles in several physicochemical states may coexist in one formulation. Generally the mean size of the nanoparticles is in the mid to lower nm-range normally between 100 and 500 nm. 

In contrast to lipid nanoparticles based on lyotropic mesophases discussed above, nanoparticles with a lipid matrix in the thermotropic smectic state possess a continuous, compact,... 

Cholesterol itself, not being a mesogen, induces high spontaneous polarization in thermotropic smectic C matrices [52]. Moreover, it can induce ferroelectric properties even in lyotropic mesophases [53, 54]. Other biologically active molecules also induce Pg values [55]. 

Fig. 1 A typical structure for the thermotropic smectic A phase of a glycolipid. ...

Locally oblate lyotropic elastomers with lamellar phase structure (L -phase) can be oriented by uniaxial compression, as outlined above for thermotropic smectic-A elastomers. Fischer et al. synthesized crosslinked polysiloxane elastomers carrying non-ionic amphiphilic side-groups attached with their hydrophobic end to the polymer backbone. They were able to compress elastomer samples between Teflon half-cylinders to about half of their original thickness. The orientation of the phase structure - except for some unoriented domains - was demonstrated by means of H-NMR spectroscopy on the directly deuterated samples as well as by X-ray scattering. The preferred orientation of the director, and hence the amphiphilic side chains, was found to be parallel to the axis of compression with the amphiphilic bilayers aligned perpendicularly [98, 99]. 

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