Comparison of the dielectric behavior of several Schiff-base and phenyl benzoate liquid crystals

Abstract

The dielectric anisotropies of a number of terminally substituted Schiff-base and phenyl benzoate liquid crystals are reported. These permittivities are interpreted by use of the Onsager equation for the dielectric behavior of polar liquids. It is demonstrated how the effective dipole moment observed for oriented liquid crystals in fact represents an isotropic contribution as well as the anisotropic contribution, dependent upon the order parameter as previously described by Maier and Meier. Literature values for the group dipole moments are shown to permit quick estimation of the sign and magnitude of the dielectric anisotropy. Group moments, therefore, indicate the relative direction they would have with respect to the remaining structure if one is to optimize the sign and magnitude of the dielectric anisotropy.Liquid crystalline materials are fluids that are anisotropic in their physical properties.1 Their anisotropy occurs spontaneously, usually between the crystalline melting point, Tx, and a higher temperature, Tc, at which the fluid becomes isotropic. Several intermediate phases, or mesophases, may occur beween Tx and Tc; these various phases are classified according to the local symmetry of the anisotropy that the fluid possesses. The mesophase possessing the

The effect of inversion of the ester group on the mesophase behaviour of some azo/ester compounds

“…Since the mesophase stability of a liquid crystalline compound depends mainly upon intermolecular attractions in which molecular polarity plays a significant role, it has been shown [7] that within one series of compounds, e.g. I8 a-e , the dipole moments of its individuals are determined by the nature of the substituent.…”

“…However, non-symmetric dimers exhibited liquid crystalline behaviour, not commonly found on the symmetric dimers, such as intercalated and interdigitated phases. The great majority of liquid crystalline materials are aromatic in nature and of a general formula X-Ar-A-B-Ar-Y, where X and Y represent a range of terminal substituents such as alkyl, alkoxy, carboxy, nitro and cyano groups; Ar represents aromatic rings that may be two or more rings; A-B represents a linkage unit in the core structure, for example, CH=N, N=CH, N=N, N=NO, COO, OOC, [6][7][8][9] etc. Aromatic esters are known [10] to be thermally stable and relatively resistant to hydrolysis; in addition, conjugation interactions between the terminal substituent and the ester group, through the intervening benzene rings, do lead to double bond character.…”

Nematic Liquid Crystals: Physical Properties, Section 2.2

“…Below the figures, the chemical formulas and the group dipole moments are also given [34]. Also the slopes of EO and &is,O are different indicating an interaction of the molecular dipole moments.…”

Nachweis der re-entrant nematischen Phase an verschiedenen homologen biaromatischen Flussigkristallen durch Untersuchung binarer Mischsysteme / Re-entrant Nematic Phase Established for Several Homologous Biaromatic Liquid Crystals by Investigation of Binary Mixed Systems

“…Eine wesentlich stärkere Änderung der Molekül struktur stellt die Einführung der Estergruppe -COO-dar, die unter anderem gegenüber der -C H = N -Gruppe ein erheblich höheres Dipol moment besitzt [10]. In der Reihe der 4-w-Alkyloxybenzoesäure-4'-cyanophenylester (w-O-CN; n = 8-12) weist im Gegensatz zu den bisher beschrie benen Verbindungen (n CBP, CN-n, n-CN) erst das Cg-Homologe eine smektische Phase auf (vgl.…”