Small-molecule smectics

Other, nonfocal defects also occur in smectics. These include walls, such as the tilt wall depicted in Fig. 10-34a, as well as dislocations such as the edge and screw dislocations depicted in Fig. 10-34b and 10-34c. The most common defects in small-molecule smectics are those that maintain a constant lamellar spacing, such as focal domains, screw dislocations, and walls. Edge dislocations seem to be more common in lamellar block copolymers, which also have smectic symmetry (see Chapter 13). 

The bend and especially the compressive moduli of lamellar block copolymers are therefore typically lower, or at least no higher, than those of small-molecule smectic-A liquid crystals, while the viscosities of the former are usually much larger than the latter. By analogy with nematics, for layered materials one can define characteristic Ericksen numbers as Erj = r]yh /K and Er = where is a characteristic viscosity and... 

Such a mobility does not exist in layered LC-main chain polymers (Fig. 1 IB). The mobility required for the definition and existence of a molten state results in LC-main-chain polymers exclusively from a gliding of chains along each other. Such a motion is only possible when the intermolecular forces between the mesogens are relatively weak. Therefore only smectic -A and smectic -C phases are true LC-phases. In contrast to small molecules smectic -B (and higher ordered smectic phases) are solid mesophases. The difference between a solid smectic mesophase and a smectic crystalline phase lies in the extent of the three dimensional order and is usually difficult to determine experimentally (see Sect. 7). 

Let us halt for a moment and consider the relationship between block copolymer materials and liquid crystals. A block copolymer exhibiting a lamellar microstructure can be classified as a smectic A liquid crystal because it has positional disorder in two dimensions and a periodic pattern in the third. Here, the periodic pattern can be identified with the variation in the local monomer compo,sition. In small-molecule smectics, the analogous periodicity arises from the molecular center-of-mass distribution. A block copolymer exhibiting a cylindrical microstructure is analogous to a columnar liquid crystal. The OBDD and spherical microstructures have three-dimensional order and have no liquid cry.stal analogs. 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

The flow properties of cholesterics have scarcely been studied at all. Figure 10-26 shows one of the few sets of measurements of the viscosity of a cholesteric-forming small-molecule material, cholesteryl myristate, as a function of shear rate in flow through a eapillaiy at various temperatures (Sakamoto et al. 1969). As the temperature is lowered, cholesteryl myristate passes through isotropic, chiral nematir smectic, and nrystalline phases Figure... 

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... 

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, ... 

Figure 2.108 illustrates the chemical stmcture and a thermal analysis curve of a typical small molecule with hquid-crystal and condis-crystal phases, OOBPD. The mesogen is the rigid bisoxybenzalphenylenediamine. Two flexible octyl groups enable conformational disorder by rotation about the C- C and O - C bonds. The letter N represents the nematic phase, letters C, I, G , and H the increasingly better ordered smectic phases, and K designates condis phases. Note that phase Kj has still not... 

Soap-like molecules are examples of condis crystals of small molecules. This is illusU ated on the case of the homologous series of thallium carboxylates, TK, in Fig. 5.147. These soaps are amphiphilic molecules and form lamellar, smectic liquid crystals without the help of a mesogen by ordering on both sides of the interface between the polar and nonpolar parts of the nanophase-separated molecules. The only regular increases of entropy with chain-length n occur, when adding all transition... 

How should these observations be interpreted Let us compare the overall shapes of a small molecule such as methane and a molecule of 8GB (see Fig. 9.1). Whereas the tetrahedral form of methane is quite close to spherical, 8CB is more like a long rod. Such a shape opens the way to arrangements intermediate between crystal and liquid. It is not difficult to imagine a liquid made up of parallel rods with randomly distributed centres of mass (see Fig. 9.0b). This is the nematic phase. We could also imagine these rods organised in plane layers, within which their centres of mass are still distributed randomly as in a liquid. On average, the rods are perpendicular to the planes. The layers are themselves stacked up n a periodic way. This is the smectic phase (see Fig. 9.0d). 

Since the first discovery of the liquid crystalline phase over one hundred years ago, the classification of the distinct liquid crystalline phases in small-molecule liquid crystals has been well established (7,2). As shown in Figure 1, the least ordered liquid crystalline phase is the nematic phase that only possesses molecular orientational order due to the anisotropy of the molecular geometric shape. The next ordering level introduced is the layer structure in addition to the molecular orientation to lorm a smectic A (S/J or a smectic C (Sc) phase. Following the phase the hexatic B (Ho), smectic crystm B (So) and smectic crystal E (S ) phases are observed. In this series the long axis of the molecules is oriented perpendicular to the layer surface while order is increasingly developed from positional order normal to the layer in bond... 

In summary, TPPs show complicated phase transition behaviors. Their phase diagrms are established and various phases are identified via the thermodynamic transition properties obtained from DSC, the structural order and symmetry determined by WiOCD, and morphology and defects observed under PLM and TEM. In particular, the WAXD fiber patterns in different phases play the most important role in determining the phase structures and symmetry. It is evident that the concepts of highly order smectic phases developed in small-molecule liquid crystals can also be utilized in the main-chain liquid crystalline polymers. 

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