Liquid crystalline phases classes

A number of polyphosphazenes of repeat unit [-PRR N-] also exhibit liquid-crystalline phases [166-168]. It is certainly intriguing that apparently the only classes of flexible chains that extensively exhibit liquid-crystalline phases are the polysiloxane and polyphosphazene semi-inorganic polymers. 

Carbenes form stable complexes with many metals in the periodic table of the elements, and gold is no exception. In fact, the chemistry of this class of organogold compounds for some time has been one of the fastest growing subdisciplines. While the corresponding chapters were still short in previous accounts,1 2 the inventory for this review is now particularly rich and diverse.230 As for other classes of carbene complexes, this upsurge is based on expectations for potential applications in various fields such as NLO materials, liquid crystalline phases, and catalysis. Where applicable, this is indicated for each of the entries in this chapter. 

It is intriguing that even some flexible siloxane polymers form mesomorphic (liquid-crystalline) phases.34 139-166 Some illustrative data are given in Table 4.2. Both poly(diethylsiloxane) and pol y(di-n-propylsiloxane) show two crystalline modifications as well as a mesomorphic phase. (The other major class of semi-inorganic polymers, the polyphosphazenes, are also relatively flexible, and show similarly interesting behavior.)10167... 

This chapter focuses on the fixation of lyotropic liquid crystalline phases by the polymerization of one (or more) component(s) following equilibration of the phase. The primary emphasis will be on the polymerization of bicontinuous cubic phases, a particular class of liquid crystals which exhibit simultaneous continuity of hydrophilic — usually aqueous — and hydrophobic — typically hydrocarbon — components, a property known as bicontinuity (1), together with cubic crystallographic symmetry (2). The potential technological impact of such a process lies in the fact that after polymerization of one component to form a continuous polymeric matrix, removal of the other component creates a microporous material with a highly-branched, monodisperse, triply-periodic porespace (3). 

The cholesterol activity on lipid bilayers has been subjected to extensive studies. As described above, cholesterol belongs to the class of neutral (non-polar) lipids and can be characterized as an amphiphilic molecule having a hydroxyl group at the C-3 position and a non-polar region. Frequently, cholesterol is incorporated into lipid membranes and studies concerning the influence on the phase transition between gel and liquid crystalline phases of the lipid bilayers hav been published [26, 27]. 

An earlier paper by Tschierske and co-workers32 dealt with bolaphilic polyols 21 as a novel class of amphotropic liquid crystals. They also noted that n-alkane-l,2-diols 20, with n > 5, exhibit liquid crystalline phases and can be greatly stabilized in their bolaamphiphilic form (Figure 7). A key element for increased stability of these assemblies was... 

The saturated aqueous solution of surfactant at 25°C is in equilibrium with a liquid crystalline phase which contains about 25 wt% water. This phase dispersed in solution is the same as the phase formed by water vapor sorption into initially dry surfactant, according to nmr spectroscopy (which virtually eliminates the possibility, mentioned in the Introduction, of a complicating triple point in the two-component system). This hydrated liquid crystal is probably lamellar, to judge by the similarities in texture with lamellar liquid crystals of phospholipids and water (36). It is not uncommon for surfactants for form liquid crystalline phases by absorption of water, or hydrocarbon, or both (37). Moreover the true solubility of many other surfactants (particularly alkyl aryl sulfonates) in water, in salt water, and in hydrocarbon is small, sometimes as small as 0.003 wt% in water, below the Krafft point (38,39). Hence the present finding of liquid crystalline phase in equilibrium with isotropic aqueous solution at surfactant levels above 0.1 wt% may be representative of broad classes of surfactants, including some of interest in connection with... 

Over the past two decades, liquid crystal polymers (LCP s) have received a considerable amount of attention in both academic and industrial laboratories. Often termed mesomorphic (meaning having "middle form"), liquid crystalline phases have a degree of order between that of the zero ordered liquid and that of the three dimensional crystal lattice. Recent reviews of liquid crystal polymers have provided a fundamental understanding of the synthesis, classification, morphology, and rheology of this unique class of materials (52-541. 

Liquid crystals can be divided into two main classes those similar to the cho-lestrayl derivatives, whose liquid crystalline phases are formed when the pure compound is heated, arc called thermotropicy and those where the hquid crystalline phase forms when the molecules are mixed with a solvent are referred to as lyotropic. The thermotropic class also includes enantiotropic types, whae the liquid crystalline phases can be seen on both the heating and the cooling cycles, and monotropic types, where the mesophase is stable only on supercooling from the isotropic melt. 

In the preceding section, it was seen that the monoglycerol fatty acid esters could not form any liquid-crystalline phases in liquid paraffin, squalane, and squalene. Phase-behavior studies have shown that the surfactants of this class do not form any liquid-crystalline phases in n-alkanes also. At normal room temperature, there is a solid phase in equilibrium with an excess oil phase in glycerol a-monolaurate... 

A new class of hexabenzocoronene derivatives, e.g. compounds (Scheme 7.13) can self-organize into liquid crystalline phases composed of molecular stacks that orient spontaneously parallel to the surface [177, 178]. Field effect transistors based on these materials show high charge-carrier mobilities, high on/off ratios, and low tum-on voltages. So far compound 44 exhibits the best field effect transistor properties achieved for a columnar discotic material. Polarized fight microscopy revealed that these materials tend to orient their columns parallel to the surface upon thermal annealing. 

---