Chemistry and development of mesophase in pitch
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The chemical and physical processes involved in formation of mesophase pitch are described. Carbonaceous mesophase exhibits properties of both nematic liquid crystals and glass-like pitches. Mesophase development requires polymerization of aromatic pitch components to a sufficient molecular size, and a proper concentration balance between molecules with different mesophase forming tendencies.The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'NX ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the NX phase to exhibit an electrooptic response consistent with a nematic-like phase.
Pentaerythritol
Biaxial nematic
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The chemical and physical processes involved in formation of mesophase pitch are described. Carbonaceous mesophase exhibits properties of both nematic liquid crystals and glass-like pitches. Mesophase development requires polymerization of aromatic pitch components to a sufficient molecular size, and a proper concentration balance between molecules with different mesophase forming tendencies.
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Abstract An interesting binary system consisting of a non-mesogen and a smectogen viz. p-chlorophenyl-p′-methoxycinnamate (A) and p-chlorophenyl-p′-n-octyloxycinnamate (B), respectively, which belong to the same homologous series1 viz. p-chlorophenyl-p′-n-alkoxycinnamates is now reported in which mixed monotropic nematic and mixed enantiotropic nematic mesophases emerge over a wide range of temperature and concentration. It is observed that the mixed smectic mesophase gets depressed as the concentration of the non-mesogen increases, yielding simultaneously enantiotropic mixed nematic phase; the mixed phase diagram exhibits nematic as well as smectic phases. As the concentration of the non-mesogen increases further, the mixed smectic phase enters into monotropic state while the mixed nematic phase continues to be enantiotropic. However, with more than about 60 mole% (A), the mixed nematic enantiotropic mesophase becomes monotropic nematic which ends by about 92 mole% of the non-mesogen (A). The characteristic modifications in the mixed mesomorphism are correlated with the structural characteristics of the components of the molecular forces displayed by them.
Mesogen
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Abstract The degree of order, θ33 of a paramagnetic probe, vanadyl acetylacetonate, has been measured in the nematic mesophase of eight liquid crystals. Significant deviations from a common curve are observed when θ33 is plotted as a function of the reduced temperature. Calculations using the specific form suggested by Maier and Saupe for the intermolecular potential function, describing both solvent-solvent and solute-solvent interactions, fail to provide a quantitative account of the deviations. The calculations do, however, account qualitatively for the behaviour of different solutes aligned in the same nematic mesophase. The deviations appear to depend, in a systematic way, on the nematic-isotropic transition temperature which suggests that a modification to the intermolecular potential could describe the experimental results.
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Texture (cosmology)
Component (thermodynamics)
Biaxial nematic
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Abstract The peculiar liquid crystalline behavior of two poly(urethane‐ester)s TDI‐C6C4 and TDI‐C6C8 is reported. Evidence is provided of the formation of two nematic mesophases, a cybotactic nematic and a conventional nematic mesophase, in TDI‐C6C8 and of one cybotactic nematic mesophase in TDI‐C6C4. IR spectroscopy indicates that different hydrogen bonded and non‐bonded structures occur with varying temperature. The transition from the nematic cybotactic mesophase to the nematic mesophase, or to the isotropic phase, is accompanied by a substantial decrease of the strength of the hydrogen bonds. This is also reflected in the dynamic‐mechanical behavior of these poly(urethane‐ester)s which is similar to the one of slightly crosslinked thermoreversible networks.
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Biaxial nematic
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Abstract An x‐ray investigation on powder specimens and stretched oriented fibers of poly(urethaneester)s TDI‐CmCn, derived from various mesogenic alkylene di[4‐(ω‐hydroxyalkyloxy‐4‐oxybenzoyl)oxybenzoate]s (CmCn; m = 2, 4, or 6, and n = 4, 6, 8, or 10) and 2, 4‐toluenediisocyanate (TDI), is reported. Evidence is provided for the formation of two different nematic mesophases in the polymers, namely a cybotactic nematic and a conventional nematic mesophase. Whereas samples TDI‐C2C6, TDI‐C6C4, and TDI‐C6C10 formed one cybotactic nematic mesophase, samples TDI‐C4C6, TDI‐C6C6, and TDI‐C6C8 exhibited both cybotactic nematic and conventional nematic mesophases in a sequence with increasing temperature, which were connected by a first‐order transition. The analysis of the various features of the small‐angle x‐ray diffraction patterns indicates that two structural arrangements, namely smectic C‐like and conventional nematic structures, coexist inside the cybotactic nematic mesophase of these poly(urethane‐ester)s. © 1995 John Wiley & Sons, Inc.
Mesogen
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Abstract The peculiar mesophase structure of a series of thermotropic poly(urethane-ester)s is discussed. The polymers form different nematic phases, namely a nematic mesophase with cybotactic groups and a conventional nematic mesophase. The X-ray diffraction analysis suggests that two types of structural arrangements, namely smectic C-like and conventional nematic structures, coexist inside the cybotactic nematic mesophases, to which different internal degrees of correlation correspond. The correlation length parallel to the director of the nematic mesophase ranges from 38 to 45 Å. In the cybotactic groups the correlation lengths parallel and perpendicular to the director range from 260 to 560 Å and 130 to 270 Å, respectively.
Thermotropic crystal
Biaxial nematic
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