Elastic constants and molecular association of cyano-substituted nematic liquid crystals

Abstract

The dielectric and elastic properties of cyano-substituted nematic liquid crystals are investigated. The results are discussed in terms of molecular properties and molecular association.

A new model for dipole-dipole association in mesogenic systems

“…The contribution of short-range dipole correlation to the parallel and perpendicular components of the permittivity tensor can also be determined in terms of quantities gt II and gx• defined in [1]. For anti-parallel association gl values are less than one, and results have been reported [2] as low as 0.45 for strongly polar nematogens such as Py.7 (5-n-heptyl-2-(4'cyanophenyl)-pyrimidine). Weakly polar materials like MBBA (4-methoxy-~" Permanent address: Hitachi Ltd., Mobara Works, 3300 Hayano, Mobara, 297 Japan.…”

“…There is however plenty of evidence to show that molecular orientations are correlated over long distances in mesophases, and in isotropic phases ctose to mesophase transition temperatures. This correlation is independent of dipole direction, and may be conveniently described in terms of the correlation factor g2 g2 = 1 + ~ (P2(cos Oij)), (2) j*i or via the correlation function :…”

From the discovery of the partially bilayer smectic A phase to blue phases in polar liquid crystals

“…They exhibit Δε values in the range of 10-25 at temperature 20 K below clearing points (T N-I ). [106] Other compounds with the same terminal CN group exhibit much lower Δε values, for example compounds 5 (CCH, Δε≈4.0 [106]), 11 (EPCH, Δε 20 ≈8.0 [45]) and 14 (CHCN, Δε≈8.0 [106]) (see also Figure 4). Such distribution of Δε values may be explained by the different tendencies of forming dimeric aggregates in different compounds.…”

“…6 R. Dąbrowski ray studies but also by dielectric spectroscopy. [105][106][107][108][109][110][111] Dimerisation process should decrease the parallel electric permittivity (ε || ) in the nematic and smectic phases as well as in the isotropic phase (ε i ). Moreover, it was found [107][108][109][110] that there exists substantial difference between the association behaviour of different mesogens which depends on the molecular shape and polarisability.…”

Pyrimidine as a Structural Fragment in Calamitic Liquid Crystals

“…[192,[224][225][226][227] present the results of XRD of some pyrimidine derivatives in their crystalline phases. It can be proposed that the electronic and geometric structures of the pyrimidine ring and reference fragments [5,6,65,66,190] play a very important role in the intra-and intermolecular interactions [255][256][257], which affect the packing of the molecules and which predominantly influences mesophase stability [44,[255][256][257][258]. Anisotropic dispersion interactions, and consequently the anisotropy of polarizability, depending on the electron density distribution in the molecular fragments under consideration, also influence the packing and hence the stability of the mesophases but play a secondary role compared to the steric factors [258].…”