The liquid crystalline state

Liquid crystals can broadly be classified into lyotropic and thermotropic. Lyotropic liquid crystals are formed by amphiphilic substances in the presence of a solvent. The 

The local direction of alignment of molecules in liquid crystals is described by a unit vector 7j, called the director, which gives the direction of the preferred axis at each point 

In the last few years disc-like molecules have been shown to form liquid crystals (Chandrasekhar, 1994). Typical of them are hexasubstituted esters of benzene (I) and certain porphyrin esters (II) (see below). In the liquid crystalline state, the disc-like molecules are stacked aperiodically in columns (liquid-like), the different columns packing in a two-dimensional array (crystal-like). The phases have translational periodicity in two dimensions but liquid-like disorder in the third. In addition to the columnar phase(D), the disc-like molecules also exhibit a nematic phase (Nj,). A transition between D and phases has been reported. 

Recently, it has been shown that certain spiropyrans exhibit some of the features of liquid crystals, but have structures that are different from conventional liquid crystals these solids have been called quasiliquid crystals (Shvartsman Krongauz, 1984). 

ARRANGEMENT ADJ. MOLECULES 1 SMECTIC 1 1 NEMATIC 1 1 SMECTIC NEMATIC 1 

MAJOR MOL. AXIS TYPE A 1 TYPE C 1 TYPE A J 1 TYPEC 

A material in the liquid crystalline state can be in a pure form such as hydronium, the liquid crystalline form of water. In this case, hydronium is described as a macromolecule consisting of a large number of individual water molecules 

25Brown, G. Wolken, J. (1979) Liquid crystals and biological structures. NY Academic Press. 

26Brown, G. (1977) Structures and properties of the liquid crystalline state of matter. J. Colloid Interface Sci. Vol. 58, pg. 534 

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J. Cognard. Lubrication with liquid crystals. In G. Biresaw, ed. Tribology and the Liquid-Crystalline State. Washington, DC American Chemical Society, 1990, pp. 1 7. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

The nematic phase of all the compounds CBn is characterized by a coherence length of about 1.4 times the elongated structure of the molecule. Based on this behaviour local associations in form of dimers with cyano-phenyl interactions were postulated. For the smectic A phase a partial bilayer arrangement of the molecules (SAd) is most likely. But there are also example for the smectic A phase with a monolayer (Sai) or a bilayer (Sa2) arrangement of the molecules as well as a commensurate structure A large number of X-ray measurements were carried out in the liquid crystalline state to clear up the structural richness and variability (see Chap. 2, this Vol. [52]). 

It is interesting that all three compounds show small angle reflections in the liquid crystalline state which indicates the formation of associates with a length of about twice the molecular length (for CCH5 17.5 A in the crystal phase I, 31.2 A in the Sb phase, and 27.2 A in the nematic phase) [73]. 

Stacked together along the a-axis. The long molecular axes are tilted by 13° within the layers. By Raman spectral measurements Tashiro et al. found that the biphenyl group of the compound has a twisted structure in the liquid crystalline state as well as in the a-form. 

An orthogonal layered structure in the solid state of rod-like molecules is the exception rather than the rule. Therefore, there is no conclusive evidence that a tilted layer structure in the solid state melts to a tilted smectic phase. In other words, if we consider the solid state as precursor for the type of the liquid crystalline state, no real precursor for an orthogonal fluid smectic phase would exist. As demonstrated in Fig. 19, the compound B-A for example exhibiting a smectic A phase has a tilted layer structure in the solid state. 

From X-ray measurements in the liquid crystalline phase it is impossible to determine the conformation of the molecules in the condensed state. Computer simulations give us information about the molecules internal freedom in vacuum, but the conformations of the molecules in the condensed state can be different because of intermolecular repulsion or attraction. But it may be assumed that the molecular conformations in the solid state are among the most stable conformations of the molecules in the condensed matter and therefore also among the most probable conformations in the liquid crystalline state. Thus, as more crystallo-graphically independent molecules in the unit cell exist, the more we can learn about the internal molecular freedom of the molecules in the condensed state. 

The liquid crystalline state may be identified as a distinct and unique state of matter which is characterised by properties which resemble those of both solids and liquids. It was first recognised in the middle of the last century through the study of nerve myelin and derivatives of cholesterol. The research in the area really gathered momentum, however, when as a result of the pioneering work of Gray in the early 1970 s organic compounds exhibiting liquid crystalline properties were shown to be suitable to form the basis of display devices in the electronic products. 

Demus, D., Goodby, J., and Gray, G. W., Handbook of Liquid Crystals, New York Wiley-VCH, 1998 Biresaw, G., Tribology the Liquid-Crystalline State," ACS Symposium Series 441, Lavoisier, 1990. 

As indicated by Puig et al. (35). surfactant retention and attendant pressure buildup in the rock can be greatly reduced if the surfactant dispersion is converted into the liquid crystalline state. Unilameller vesicles are preferred in the field work rather than the multilamellar... 

It has been shown frequently that without the presence of strong intermolecular interactions, discotic molecules are highly mobile in the liquid crystalline state.1 They undergo both lateral as well as rotational translations, resulting in the absence of positional order. Similarly, such discotics also freely rotate in the columnar aggregates they form in solution. This lack of positional order in the columns accounts for the absence of chiral or helical supramolecular order. We will demonstrate this characteristic using results obtained for triphenylenes. 

Photoisomerization of the azobenzene amphiphile was found to be strongly affected by molecular packing and orientation in the aqueous bilayer solutions. A rate constant of trans to cis isomerization was extremely faster in the liquid crystalline state than in the crystalline bilayer membrane [33]. Photoreaction of the aqueous bilayer membrane of CgAzoCioN+ Br was... 

Polyarylates prepared from cyclohexyl-HQ (Ch-HQ) and PEC (Ch-HQ/PEC) did not show liquid crystallinity due to the more bulky substituent on the HQ unit compared to those on f Bu-HQ and Ph-HQ. As-spun fibers of Ch-HQ/PEC exhibited lower moduli than those of fBu-HQ/PEC and Ph-HQ/PEC. Therefore, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state (7j — 7j,) is an influential factor, as shown in Table 19.1. 

Therefore, we expected that the polyarylates synthesized from substituted HQs and BB would show higher stabilities of the liquid crystalline state and higher moduli than those produced from substituted HQs and substituted PEC. 

Because as-spun fibers of Me-HQ/Cl-PEC showed a higher modulus than those of Ph-HQ/Cl-PEC, the flexural modulus of injection molded specimens of Me-HQ/Cl-PEC exhibited a higher flexural modulus than those of Ph-HQ/Cl-PEC due to the liquid crystalline state. Although flexural-fractured injection molded specimens of Me-HQ/Cl-PEC exhibited highly oriented fibrils, Ph-HQ/ Cl-PEC no longer displayed fibrils because of the lack of liquid crystallinity. 

Thus, the influential factors in obtaining high moduli for injection molded specimens are as follows (1) the stability of the liquid crystalline state, (2) the rigidity of the polymer chain, and (3) the packing density of the polymer chain. 

However, the combination of HQs with bulky substituents such as /Bu-HQ and Cl-PEC was not suitable for formation of the liquid crystalline state and did not exhibit any improvement in the Tg. 

Some drug substances can form mesophases with or without a solvent [19-26]. In the absence of a solvent, an increase in temperature causes the transition from the solid state to the liquid crystalline state, called thermotropic mesomorphism. Lyotropic mesomorphism occurs in the presence of a solvent, usually water. A further change in temperature may cause additional transitions. Thermotropic and/or lyotropic liquid crystalline mesophases of drug substances may interact with meso-morphous vehicles as well as with liquid crystalline structures in the human organism. Table 1 presents drug substances for which thermotropic or lyotropic mesomorphism has been proved. 

More recently, Ingram et al. and Wright et al. independently tried to develop new polymer hosts with secondary structures similar to that of a liquid crystalline state, so that ion transport could occur with a higher degree of freedom in the highly oriented environments and become at least partially decoupled from the polymer segmental relaxations. Ion conductivities approaching liquidlike values have been obtained on the condition that the liquid crystalline state could be maintained. However, the incorporation of these novel polymer materials in electrochemical devices remains to be tested. 

To this purpose, Fossheim et al. reported temperature sensitive liposomal Gd(III)-based probes (137-138). The composition of the liposomes was chosen in order to tune the temperature of transition between the gel-crystalline, where the liposome is water-impermeable, to the liquid-crystalline state, where water has free access to the interior of the liposome. This means that at temperatures below the transition, the relaxivity of the system is very low (paramagnetic contribution close to zero), whereas at higher temperature the Gd(III) complex ([GdDTPABMA(H20)] is the reference) is no longer silent... 

Unusual properties of fully aromatic polyesters are observed if they have at least partially a rigid planar chain structure. In particular, they can form thermotropic liquid crystalline states (see Example 4-5). As already discussed in Sect. 1.2.4 an important structural prerequisit for LCPs of Type A in order to attain the liquid crystalline state of aromatic polyesters (and aromatic polyamides, see Example 4-14), is a rigid main chain according to the following construction principle ... 

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