Liquid crystals made of highly polar compounds

Abstract

Liquid crystals are states of matter with symmetries intermediate between those of isotropic liquids and crystalline solids. The role of permanent dipoles in determining the stability o f the nematic phase was discussed soon after the discovery of liquid crystals. Though it is now w ell known that polar interactions are not essential for the stability of the mesophases, strong longitudinal dipoles give rise to many unusual phenomena like reentrant phases, polymorphism of smectic A and nematic liquid crystals, etc. The article gives a summary of these phenomena and phenomenological as well as molecular theories which have been developed to describe them. I IntroductionThermotropic liquid crystals made of low molecular weight 200-400 organic compounds were discovered more than 100 years ago. The simplest liquid crystal is the nematic with a long range orientational order of rod like Fig. 1 or disc-like molecules 1,2 . The average direction of alignment of the molecules is denoted by a dimensionless unit vector called the director, n. The molecular interactions responsible for the stability of the nematic have been the subject of much research activity for a long time. Indeed the rst theory was proposed by Max Born in 1916 3 in which h e proposed that the interactions between the permanent dipoles stabilise the orientational order in the medium which w ould hence have a permanent polarization. The nematic-isotropic NI transition would be analogous to a ferroelectric-paraelectric transition and would be second order in nature. E orts made to detect the polarization did not yield any positive results and further, many compounds like p-quinquephenyl which do not have molecules with net dipole moments were found to exhibit the nematic phase. The N-I transition has also been found to have a rst order characteristic in all known cases. Further, all the physical properties of the nematic indicate that the directorn is apolar in nature even when the molecules have strong dipole moments. For example, the topological defects of the nematic are half-strength disclinations around which a thin nematic sample exhibits two dark brushes between a pair of crossed polarizers. This implies a continuous rotation of the director by rads around the core of the defect which is possible only ifn and-n are physically equivalent. As such the orientational order parameter in the nematic is a second rank tensor which for cylindrically symmetric molecules takes the simple form: S = 3 cos 2 ,1 2 = P 2 cos 1 in which is the angle made by the molecular long axis with the director and the angular brackets signify a statistical average. This means that S O and S O will imply very di erent distribution functions peaking around = O and =2 rads respectively. As such, in the Landau-deGennes theory of the nematic, the 3rd order term has to be taken into account. The free energy density reads as:

Rheology of nematic liquid crystals with highly polar molecules

“…There are also some reports on new electrooptic effects in nematic liquid crystals with highly polar molecules [6]. Apart from applications, these compounds also exhibit several unusual features such as reentrant phase transitions, smectic-A polymorphism, nematic to nematic transition, and strong suppression of director fluctuation by fields [7][8][9][10][11][12][13]. When the nematic liquid crystals are sheared, they exhibit three different viscosities depending on the orientation of the director (the average molecular orientation direction) with respect to the flow and velocity gradient directions, and they are called Miesowicz viscosities [14].…”

Anchoring Energy Measurements at the Aqueous Phase/Liquid Crystal Interface with Cationic Surfactants Using Magnetic Fréedericksz Transition

“…This difference may be due to the extent of the nesting of LC and surfactant . LC nematic layer is composed of the 5CB molecules oriented face-to-face along their aromatic moieties, which have conjugated electrons . Consequently, the alkyl chain of the surfactant is adsorbed at the interface between the 5CB molecules.…”

“…13 LC nematic layer is composed of the 5CB molecules oriented face-to-face along their aromatic moieties, which have conjugated electrons. 28 Consequently, the alkyl chain of the surfactant is adsorbed at the interface between the 5CB molecules. We saw from Figure S2 that C 14 TAB adsorbed more than C 12 TAB at the hydrophobic 5CB phase and the Γ required to trigger 5CB orientation for C 14 TAB is lower than C 12 TAB.…”

Studies on binary mixtures of nematic liquid crystals made of strongly polar molecules with identical cores and antagonistic orientation of permanent dipoles

“…However, physical studies on the binary mixtures of nematic liquid crystals with the identical core structures and highly antagonistic dipole orientation are meagre. High dipole moments of the molecules are known to make an important contribution to the intermolecular interactions and give rise to distinct physical properties of liquid crystals [14][15][16][17][18].…”