Thermotropic liquid crystalline materials

Thermotropic liquid crystals are composed of moderate-size ( 2-5nm) organic molecules, which are strongly anisometric elongated and shaped like a cigar (so-called calamitic liquid crystals), disc-shape (discotic LCs), or bent-shape (pyramidal or banana-shape). 

Thermotropic liquid crystalline polyesters (TLCP) such as DuPont s Zenite or Ticona s Vectra have unique properties because a very high degree of orientation can develop under the action of shear. This arises because of the presence of rigid groups in the polymer backbone (often based on hydroxybenzoic acid) which form ordered domains in which the chains are aligned in the same direction. In general the advantages of TLCP are as follows  

However, the mouldings tend to be highly anisotropic, have low weld line strength and low toughness with a fracture behaviour resembling that of wood. As polyesters, TLCP are susceptible to hydrolysis by acids and bases. 

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The number of building blocks for supramolecular self-assembly is virtually unlimited. Chapter 6, by Brunsveld, Rowan, Nolte, and Meijer, describes studies on disk-shaped molecules which are programmed to stack in a helical fashion, leading to novel kinds of twisted fibers as well as lyotropic and thermotropic liquid crystalline materials. 

Guan Y, Zakrevskyy Y, Stumpe J, Antonietti M, Paul CFJ. 2003. Perylenediimide surfactant complexes thermotropic liquid crystalline materials via ionic self assembly. Chem Commun 894 895. 

In contrast to the poly(enamine-ketones) prepared by Imai e al. the methyl substituted poly(enamine ketones), J 2, prepared from the condensation of diamines and bis-acetylacetylarenes, were found to be hydrolytically stable and thermally stable from 257 to 340 C, When a linear and rigid aromatic bis(1,3-diketone) was condensed with alkaline dieamines with ten or more carbons or with rigid aromatic diamines, thermotropic liquid crystalline materials were obtained. The monomers and polymers have much higher solubilities and lower transition temperatures as compared with liquid crystals containing only aromatic hydrocarbon rings of similar structures. 

Fig. 4.12 Two-dimensional diffraction patterns of an aligned sample of the thermotropic liquid crystalline material 5-(octyloxy)-2-(4-(octyloxy)phenyl)pyrlmidine a at 100 °C in the SmA phase and b at 50 °C in the SmC phase [23]...

The first thermotropic liquid crystalline material (cholesteryl benzoate, 2), discovered in 1888 by the Austrian botanist Reinitzer, exhibits what is now known as the chiral nematic (N ) phase. Perhaps not sitrprisingly, this natural product, like many others based on cholesterol, has an asymmetric molecirlar stmcture and hence it is chiral and optically active. Cholesteryl benzoate (2) has 8 chiral centres giving a total of 256 stereoisomers however, only one is produced in nature. 

Chen, Y., Shen, Z., Gehringer, L., Frey, H. and Stiriba, S.-E. (2006) Supramolecular thermotropic liquid crystalline materials with nematic mesophase based on methylated hyperbranched polyethylenimine and mesogenic carboxylic acid , Macromol. Rapid Comm., 21,69-75. 

Recent solid state NMR studies of liquid crystalline materials are surveyed. The review deals first with some background information in order to facilitate discussions on various NMR (13C, ll, 21 , I9F etc.) works to be followed. This includes the following spin Hamiltonians, spin relaxation theory, and a survey of recent solid state NMR methods (mainly 13C) for liquid crystals on the one hand, while on the other hand molecular ordering of mesogens and motional models for liquid crystals. NMR studies done since 1997 on both solutes and solvent molecules are discussed. For the latter, thermotropic and lyotropic liquid crystals are included with an emphasis on newly discovered liquid crystalline materials. For the solute studies, both small molecules and weakly ordered biomolecules are briefly surveyed. 

Liquid crystalline materials can be divided into two large groupings thermotropic and lyotropic. Thermotropic LCs are formed when pure molecules such as cholesteryl form ordered structures upon heating. When LCs occur through mixing with solvents they are called lyotropic LCs. 

Thermotropic liquid crystalline PPV derivatives 43 were prepared by the coupling of dihalodialkoxybenzene and divinylbenzene in the presence of a palladium catalyst, as outlined in Scheme 47 [133]. Polarized light microscopy, as a function of temperature, showed evidence of a nematically ordered structure in the material. X-ray diffraction analysis of the pristine polymers showed them to be semi-crystalline in nature, although the crystallinity of the polymer changed dramatically upon heating above 100 °C. 

Several cyclopropanated organofullerene materials have been synthesized for the preparation of fullerene-containing thermotropic liquid crystals. A wide variety of such liquid-crystalline materials were synthesized possessing mono-140-42], hexa-addition [43] pattern, or even dendritic addends [44-46]. 

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... 

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. 

The phase behavior is similar to that of a lower critical solution temperature (LCST), hence it is different from the above systems. The HPC/water system is an interesting model system because of the rich variety of phase structure 01 the material. HPC is a semicrystalline polymer in the solid state (7), but exhibits thermotropic liquid crystalline character at elevated temperatures below the melting point (8). It shows isotropic phase in dilute solutions, but forms an ordered liquid crystalline phase with cholesteric structure in concentrated solutions (4). 

PDES) is a thermotropic liquid-crystal material. It exists in the mesomorphic state for a certain temperature range after melting of the crystalline phase (i5). This property is due primarily to the side chains. This material has two crystalline forms, a and P, and these crystalline forms go through isomorphic transitions from al to a2 and from pi to P2 when the temperature increases. These transitions occur because of ethyl group reorientation (i6), which has an activation energy of 9.3 kcal/mol. The previous experimental results (16) did not identify the initial and final orientations. In this study, we identified these orientations by using conformational analysis. 

The mesophase exhibiting a homeotropic texture can still shew stir opalescence, which is a method of identification particularly suited for thermotropic polymers. This somewhat crude method of characterizing a liquid crystalline material is performed by shearing a thin film of the mesophase and looking for momentary appearances of turbidity in the otherwise transparent melt. No microscope is need to observe stir opalescence, but simple shearing of the homeotropic melt between crossed-polars can also reveal the mesophase. 

There has been a great deal of interest in thermotropic, liquid crystalline polymers in the past twenty years or so since the discovery of useful materials based on them. Many critical factors such as structure of mesogenic units, presence and structure of flexible spacers or rigid kinks, molecular weight and its distribution, and thermal history influence thermal, physical and thermotropic properties of liquid crystalline polymers(1-13). 

Jackson and Kuhfuss reported a thermotropic polymers system comprised of polyethylene terephthalate) modified with HBA (4). They demonstrated not only the thermotropic liquid crystalline behavior but also the excellent melt processability of these materials to yield specimens with high modulus values. However these copolymers had very low use temperatures as reflected by low heat deflection temperatures. 

Isomerism, optical activity and liquid crystalline materials exhibit intersting photophysical features where luminescence is often a valuable probe. Fluorescence analysis of thermotropic biphenyl liquid crystalline polyesters has shown a... 

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