Liquid crystalline phases thermotropic

There are now numerous examples of cellulose derivatives that form both lyotropic and thermotropic mesophases. Of course, cellulose itself is unlikely to form a thermotropic liquid crystalline phase because it decomposes prior to melting. 

Other cases, polymers can undergo lyotropic or thermotropic liquid crystalline phase transitions, which can often be observed and recorded in a polarized light microscope. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

More recently, we have found that the terphenyltetracarboxylic acid-based polyimides P-XTP showed thermotropic liquid-crystalline phase [21,22]. As far as we know, this is the first example of the simple polyimides showing thermotropic liquid crystallinity. 

The unusual optical properties of liquid crystals had been remarked upon and described for several centuries before their uniqueness as a state of matter was recognised. Their early reports described the strange melting behaviour and appearance of some naturally occurring materials, either as pure compounds or as gels in water, which have now been shown to be thermotropic or lyotropic liquid crystals. Thermotropic liquid crystalline phases are formed under the action of heat, see Figures 2.1 and 2.2, and the lyotropic liquid crystalline phases are formed by the action of a solvent, such as water, usually with an amphiphilic compound. However, the nature of these materials, or indeed their exact... 

Polymers of the mesogenic vinyl and propenyl ethers mostly assume thermotropic liquid crystalline phases where smectic forms are frequently... 

Both theoretical approaches qualitatively describe the "thermotropic" and "lyotropic" liquid crystalline state of rod-like molecules ( see also D.B. DuPre, R. Parthasarathy, this book). Combination of both theories (Flory, Ronca)(7) slightly improves the predictions compared to the experimental findings. Anisotropic dispersion interactions and/or anisometric molecular shape can thus be the basis for explaining theoretically the appearance of "lyotropic" and "thermotropic" liquid crystalline phases. 

The differentiation between "lyotropic" and "thermotropic" liquid crystalline phases of rod-like molecules is rather artificial, because in bulk as well as in solution the same physical origin causes the anisotropic phase. Originally, the term "lyotropic l.c" was clearly related to micellar liquid crystalline... 

Materials and Processing. Copolyesters of poly(ethylene terephtha-late) (PET) and para-oxybenzoic acid (FOB) were supplied by the Tennessee Eastman Corporation. Past work Indicates the copolyesters form thermotropic liquid crystalline phases at compositions containing more than 30 mole% POB (26,27,28). The composition of the copolyester studied here contains 60 mole% POB. Quiescent liquid crystalline films were made by compression molding the copolyester at 210, 230, 255, and 285 C, and followed by a quench Into Ice water, ambient air, or cooled In the press with the power off. Film thicknesses ranged between 0.05-0.15 mm. Another sa(q>le of the 40/60 PET/POB copolyester was melted at 270 °C In a Mettler hot stage, manually sheared between glass slides, and then ambient air cooled. 

Gray et al. have reported that (acetoxypropyl)cellulose behaves as a thermotropic cholesteric liquid crystal below 164 °C. It has been also observed that some (hydroxy-propyljcellulose forms a thermotropic liquid crystalline phase at temperatures above 160 °C From these results together with our finding, we presume that rigid rod-like... 

Although the compounds that form thermotropic liquid crystalline phases are of a variety of chemical types such as azo compounds, azoxy compounds or esters, the molecular geometries of the molecule have some characteristic features in that they are... 

At 80°C, the polymers with more than 10 carbon atoms are in the liquid crystalline state. Low mobility of the side chain prevents the polymers of n 9 to be in the thermotropic liquid crystal. The existence of the discontinuity is evidence for the solvent like nature of the side chains in the thermotropic liquid crystalline phase. 

As their name implies, liquid crystals are materials whose structures and properties are intermediate between those of isotropic liquids and crystalline solids (2). They can be of two primary types. Thermotropic liquid crystalline phases are formed at temperatures intermediate between those at which the crystalline and isotropic liquid phases of a mesogenic compound exist. Substances which exhibit thermotropic phases are generally rod- or disc-like in shape, and contain flexible substituents attached to a relatively rigid molecular core. Lyotropic liquid crystalline phases are formed by amphiphilic molecules (e.g. surfactants) in the presence of small amounts of water or other polar solvent. In general, the constituent molecules in a liquid crystal possess orientational order reminiscent of that found in the crystalline phase, yet retain some degree of the fluidity associated with the isotropic liquid phase. 

In (3.65) there is more of an aliphatic component than an aromatic one. Phenylene rings are each isolated by aliphatic components. No mesogenic units of rigid-rods are present in the molecule. However, the thermotropic liquid crystalline phase was observed for many of the polymers although... 

A few polyacetylen s with liquid crystalline side groups have been reported. For instance, a polymer (Mw 16 x 10" ) is obtained in high yield with WCl6 from cholesteryl 4-pentynoate. The polymer shows a transition to a thermotropic liquid-crystalline phase at 89 °C and a transition to the isotropic phase at 189 °C. Other examples are poly(l,6-heptadiynes) with mesogenic side groups. Studies on their applications are under way. 

The existence of thermotropic liquid crystalline phases allows the manipulation of morphology and thus their optical properties by external forces," including magnetic fields. Such phases will play a role in future development of aligned layers for polarized emission and other LED-type applications. 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

Fig. 3.18 Some synthetic molecular nanographenes exhibiting thermotropic liquid crystalline phase behavior...

The simplest PPEs are the dialkyl-PPEs 4 made by either alkyne metathesis of dialkyldipropynylbenzenes or by Pd-catalyzed couplings. The materials are thermally and photochemically very stable and form thermotropic liquid crystalline phases of the smectic variety [55], Optical textures under crossed polarizers resemble those of nematic phases due to the homeotropic alignment of the chains on glass... 

Monoglycerides have also been examined by Malkin (1954) and the melting and solidification points of saturated members are given in Table 8.21. It should be pointed out that in the gap between the reported melting point and the solidification point a thermotropic liquid-crystalline phase is formed as described above. 

The melting point of the most stable forms of 1-mono-olein and 1-monoelaidin, which are j8-forms, are 35.0 °C and 58.5 °C respectively. When 1-monoolein solidifies from the melt, a thermotropic liquid-crystalline phase with lamellar structure is formed as a metastable transition state. 

V. Percec, C. Peihwei, G. Ungar, and J. Zhou, calamitic nematic and smectic thermotropic liquid crystalline phases. J. Am. Chem. Soc., 117 11441, 1995. 

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