Liquid-crystalline state

Liquid crystals represent an intermediate state of order (mesophase) between crystals and liquids. Crystals have a three-dimensional long-range order of both position and orientation (Fig. 5.1-la). Liquids, in contrast, do not show any long-range order (Fig. 5.1-lb). In plastic crystals (disordered crystals, Fig.5.1-lc), positional order is maintained, but orientational order is lost. In mesophases, imperfect long-range order is observed, and thus they are situated between crystals and liquids. The reasons for the formation of a mesophase can be the molecular shape or a microphase separation of amphiphilic compounds. 

More than 100 000 individual liquid crystals have been prepared until now [1.1-4]. About 2000 of them have been tested for physical properties and technical applications [1.5-14]. These materials can be classified 

Generally, molecules of liquid crystalline substances have the following shapes  

The simplest and most widespread liquid crystalline phase is the nematic phase. The molecules are statistically distributed within the medium, but the long axes are orientated in one direction, the director (Fig. 5. l-2a). A special class of nematic phases is the cholesteric phase 

Disk-like molecules Discotic liquid crystals Lyotropic liquid crystals Monophilic liquid crystals 

H Phase structure Calamitic liquid crystals Discotic liquid crystals 

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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. 

A block copolymer composed of liquid crystalline polymer (LCP) segments or that composed of segments having an LCP unit in their main chain or side chain was synthesized [67,68]. The latter showed partial compatibility and second-phase separation even when in a melt liquid crystalline state. 

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. 

Papkov, S. P., Kulichikhin, V. G. Liquid-crystalline state of polymers (in Russian), Moscow Khimiya 1977... 

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. 

Major determinants of membrane fluidity may be grouped within two categories [53] (1) intrinsic determinants, i.e., those quantifying the membrane composition and phase behavior, and (2) extrinsic determinants, i.e., environmental factors (Table 1). In general, any manipulation that induces an increase in the molal volume of the lipids, e.g., increase in temperature or increase in the fraction of unsaturated acyl chains, will lead to an increase in membrane fluidity. In addition, several intrinsic and extrinsic factors presented in Table 1 determine the temperature at which the lipid molecules undergo a transition from the gel state to liquid crystalline state, a transition associated with a large increase in bilayer fluidity. 

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... 

Ten layers of dimyristoylphosphatidylethanolamine (DMPE) LB films were deposited on the QCM. The QCM was immersed into temperature-controlled water phase, and the frequency was followed with time. Typical time-courses of frequency changes are shown in Figure 8. The phase transition temperature from solid to liquid crystalline states of... 

From the frequency measurements of the LB-film-deposited QCM plate in water, the behavior of phospholipid LB films can be classified into three types (i) phospholipids having relatively hydrophilic head groups such as DPPC and DPPG are hydrated and then easily flaked from the substrate in the fluid liquid crystalline state above Tc (ii) DPPE and DPPS having the less hydrophilic head groups are hydrated only near their Tc (iii) cholesterol LB films show relatively large hydration behavior even at low temperatures due to the water penetration into the structure defects in the membrane. 

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. 

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