Properties of polymer liquid crystals: choosing molecular structures and blending

This chapter describes the most important physical parameters which mainly determine the electrooptical behavior of liquid crystal,cells. According to existing phenomenological theories we first: 

(i)

introduce these parameters, then;




(ii)

illustrate their dependence on the concrete molecular structure;




(iii)

then give evidence as to how to measure them and how to develop new liquid crystalline mixtures, having optimal values for them.

Properties of the Materials

Two homologous achiral five-ring mesogens with lateral fluorine substituents on the core and on the outer rings are presented. Both compounds exhibit two B5 phases and the B2 phase. The high-temperature B5 phases possess an antiferroelectric ground state. The low-temperature B5 phase was found to be ferroelectric, which is indicated by the bistable switching. The assignment and characterization of the mesophases is based on XRD measurements, 1H-, 19F-, 13C-NMR and electro-optical measurements.

Ferroelectric and antiferroelectric “banana phases” of new fluorinated five-ring bent-core mesogens

Data on the synthesis, structures and properties of ionic, coordination, organometallic and organoelement compounds with liquid-crystal properties are surveyed. The substances examined are new in the chemistry of the liquid-crystal compounds. Their potential applications are described. The bibliography includes 211 references.

http://iopscience.iop.org/article/10.1070/RC1993v062n04ABEH000019/pdf

Metal-containing liquid-crystal phases

Thermotropic columnar mesophases were observed with soap molecules a long time ago. Since then, they have been widely investigated thanks to the synthesis of disk-shaped molecules. In such mesophases, the molecules are packed one upon another to form columns. These then organize according to a two-dimensional hexagonal lattice. In recent years, a variety of compounds with quite different molecular architectures which also exhibit columnar mesophases have been reported. A review of the most recent significant systems is presented. It concerns discotic molecules, polycatenar molecules, side-chain polymers without mesogenic moieties, dendrimers, pseudo-polymeric chains of metal soaps, self-assembled systems driven by specific interactions such as hydrogen bonding or dipolar interactions, and polar columnar phases. Typical examples are presented, with emphasis given to the relationships between the columnar structure and the molecular architecture, along with the specific interactions responsible for the establishment of this type of supramolecular organization.

Columnar Order in Thermotropic Mesophases

A new type of liquid crystals made up of bowl-like molecules was predicted by Lin in 1982 and was recently synthesized. In is paper, the experimental findings about these bowlic mesophases are briefly summarized. Possible mesophases (polar nematic, non-polar nematic, cholesteric, stringbean, donut, columnar, etc.) are proposed and discussed. The possible breakdown of the macroscopic symmetry n → – n (n is the director) in these mesophases is emphasized. Novel properties including phase transitions and other physical aspects (optical, elastic, flexoelectric and hydrodynamic, as well as defects) are presented. Molecular engineering to produce new compounds (e.g. anisotropic ferromagnetic fluid and bowlic polymers) is suggested. Possible instabilities and applications are discussed. The possibility of second harmonic generation and simple sub-microsecond electro-optical switching (without the complications encountered in ferroelectric smectic C*) in these bowlic mesophases are pointed out. These bow...

Bowlic Liquid Crystalsl

Equations of motion for nematic liquid crystals under time-dependent shear are derived. Soliton solutions are investigated. When the soliton velocity is large and the shear varies slowly in time, approximate analytic solutions for single solitons of the A and B types are found with use of multiple-scale analysis. These perturbed solitons move with time-dependent velocities but are constant in shape and carry no tails. The velocity is proportional to the shear rate. Numerical calculations of the director equation of motion are performed and are in agreement with the analytic results. A recent experiment in which dark lines (under white light) excited by a periodically moving plate at one end of a nematic cell are observed is analyzed and interpreted according to our theory. Good agreement between theory and experiment is obtained.

Perturbed solitons in nematic liquid crystals under time-dependent shear.

The isotropic to nematic transition in bowlic liquid cyrstals are theoretically examined with the help of the work by Krieger and James on a molecular crystal model. In addition to the conventional non-polar nematic phase, a polar nematic phase can also exist. The polar nematic–isotropic and the polar nematic–nonpolar nematic transitions can he either first or second order, and two tricritical points as well as one triple point exist (while the nonpolar nematic–isotropic transition is always first order).

Phase Transitions of Bowlic Liquid Crystals

Equations of motion of director and velocity of nematic liquid crystals under pressure gradients are derived from the Ericksen-Leslie theory. Under suitable approximations, these coupled equations are reduced to a single (2 + 1) nonlinear partial differential equation for the director angle. Boundary effects are taken into account. Similarity analysis of this equation is presented. Properties of steady-state solutions are discussed. Numerical solutions are obtained. Traveling solutions of the (2 + 1) equation are given numerically showing the existence of solitons propagating along the direction of the pressure gradient (in both directions). Corresponding optical interference patterns of these solitons under both monochromatic and white lights are calculated. Good agreement between our theoretical results and recent experiments are obtained.

Solitons Generated by Pressure Gradients in Nematic Liquid Crystals

Experiments on ring-shaped propagating single solitons in nematic liquid crystals are reported. These solutions with constant velocities and widths are generated by pressure gradients in the radial direction in homeotropic nematic disc cells. The pressure at the rim of the cell is maintained at atmospheric pressure while that at the center is varied. In suitable pressure gradient regimes these solitons appear as dark rings under transmitted parallel white light passing vertically through two crossed polarizers sandwiching the liquid crystal cell. These results are contrasted with those observed when the pressure at the rim is varied (in which both dark and white rings propagating towards the center are generated).

Lin Lei

Papers

Bowlic Liquid Crystalsl

Perturbed solitons in nematic liquid crystals under time-dependent shear.

Phase Transitions of Bowlic Liquid Crystals

Solitons Generated by Pressure Gradients in Nematic Liquid Crystals

Experiments On Ring-Shaped Solitons In Nematic Liquid Crystals