Phase Behavior Of Liquid Crystals Research Articles

Synthesis and characterization of side‐chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and benzyl ether based mesogenic groups

AbstractThe synthesis of side‐chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and 4‐methoxyphenyl benzyl ether and 6‐cyano‐2‐naphthyl benzyl ether based mesogenic groups is presented. The phase behavior of both monomeric and polymeric liquid crystals was characterized by differential scanning calorimetry and optical polarized microscopy. All synthesized polysiloxanes present smectic mesomorphism. The insertion of oxygen atoms into the flexible aliphatic spacers leads to decrease in both glass transition and isotropization temperatures of the resulting side‐chain liquid crystalline polymers.

Phase Behavior and Polymerization of Lyotropic Binary Liquid Crystals. A Positron Annihilation Study

Abstract Positron lifetimes and cross-polarized microscopic measurements have established the presence of hexagonal, nematic, and lamellar phases in 43%, 53–63%, and greater than 60% sodium decyl sulfate liquid crystals, respectively, at 65°C. These techniques also revealed hexagonal, nematic, and anisotropic phases in liquid crystals formed from 52% (w/v) sodium 10-undecanoate at 20°C, 30°C, and 44°C. Liquid crystalline sodium 10-undecanoate has been polymerized by ultraviolet irradiation, as well as by heating in the presence of ammonium persulfate as initiator. Irradiation resulted in a sequential change from hexagonal to mixed hexagonal-lamellar, and finally to lamellar phases. Polymerization by heating in the presence of ammonium persulfate resulted in a sequential change from hexagonal to mixed hexagonal-lamellar, and finally to isotropic phases. Cooling of the isotropic phase of polymerized liquid crystalline sodium 10-undecanoate to 20°C reformed the lamellar phase.

New high-pressure computer-assisted differential thermal analysis apparatus

A high-pressure differential thermal analysis apparatus for investigations in the temperature range between 273 and 600 K and at pressures up to 7 kbar is described. The experimental setup and the measuring technique are discussed. Finally, some results on the phase behavior of liquid crystals are presented.

Pressure-induced re-entrant cholesteric phase behaviour of non-polar liquid crystals

Abstract In a previous paper we reported the existence of a pressure-induced re-entrant cholesteric phase in mixtures of non-polar liquid crystals. Now the influence of the mixing ratio on this behaviour has been studied up to 3000 bar and 190°C and the phase boundaries based on light reflection measurements have been confirmed by transmission and texture observations in a diamond anvil cell. Additional thermodynamic investigations show that when the cholesteric/smectic A phase transition line possesses a maximum temperature the pretransition enthalpy and volume disappear.

Synthesis and characterization of liquid crystalline polysiloxanes containing benzyl ether mesogens

AbstractSide chain liquid crystalline polysiloxanes were synthesized by the hydrosilation of poly(methylhydrosiloxane) with p‐(1‐undecenyl‐11‐oxy) benzyl ethers of 4‐cyanophenol (IM), 4‐methoxyphenol (IIM), 4‐cyano‐4′‐hydroxybiphenyl (IIIM), 4‐methoxy‐4′‐hydroxybiphenyl (IVM), and 2‐cyano‐6‐hydroxynaphthalene (VM). The phase behavior of both monomeric and polymeric liquid crystals was characterized by differential scanning calorimetry and optical polarization microscopy. IM is a monotropic liquid crystal, IIM is crystalline, and IIIM and IVM are enantiotropic liquid crystals, whereas VM presents two virtual liquid crystalline transitions and crystalline polymorphism. All the synthesized polysiloxanes present enantiotropic smectic mesomorphism.

The influence of lateral substituents on the phase behaviour of nematogenic liquid crystals

Abstract The influence of lateral substituents or side chains attached to thermotropic nematogens is considered on the basis of the Flory lattice model. The rigid core of the nematogen is characterized by its axial ratio x and the characteristic temperature T∗ which measures the strength of the anisotropic dispersion forces operative between the cores. For the side chain an additional parameter zm which is proportional to its hard core volume enters the partition function. Given these quantities and the free volume as obtained from the thermal expansion coefficient, the nematic-isotropic transition temperature TNI and the temperature dependence of the order parameter may be calculated. The results show that the lowering of T NI by lateral substituents is caused by (1) their interference with molecules packing and (2) the dilution of the anisotropic dispersion forces between the central nematogenic cores. As an additional consequence of this the order parameter is lowered considerably. A survey of recent e...

Mesogenic Transition Metal Complexes Liquid Crystal Phase Behaviour and Crystal and Molecular Structure of Some Nitrile Complexes of the Platinum Metals

Abstract A new range of transition metal-containing liquid crystals is formed by complexing classical organic mesogens directly to a metal. Thus, reaction of [PdCl2(PhCN)2] with 4-n-pentyl-4′-cyanobiphenyl (5CB) leads to [PdCl2(5CB)2] which exhibits a monotropic nematic phase. Complexes of long chain 4-alkyloxy-4′-cyanobiphenyls show enantiotropic mesophases. Analogously, reaction of [PtCl2(PhCN)2] or [Rh2Cl2(CO)4] with 4-alkyl-4′-cyanobiphenyls (L) yields mesogenic [PtCl2L2] or cis-[RhCl(CO)2 L] respectively. The crystal and molecular structure of [PdCl2(5CB)2] and [PtCl2(8CB)2] are also reported and show different packing in the solid state.

Thermal and microscopic phase studies on liquid crystals at high pressures

High pressure studies on the phase behaviour of liquid crystals up to a maximum pressure of 5 kbar were performed using a computer assisted high-pressure differential thermal analysis equipment (DTA) and a diamond anvil cell (DAC) mounted in a polarizing microscope. T− p phase diagrams were determined for some homologues of the series of the trans-4-alkyl-(4-cyanophenyl)cylohexanes (PCHs e.g. alkyl  propyl(3PCH), butyl(4PCH)) and the terephthal-bis-(4-alkylanilines) [TBAAs e.g. alkyl  butyl(TBAA(4)  TBBA), pentyl(TBAA(5)), octyl(TBAA(8)), dodecyl(TBAA(12))]. A pressure-induced nematic phase is found in 4PCH and TBAA(8) above 170 bar, 320 K and 230 bar, 488 K respectively. TBAA(12) exhibits the rare phenomenon of a pressure-induced smectic A phase above 805 bar, 492 K. The smC/smA transition in TBAA(8) is likely to show a tricritical point near about 1 kbar. The influence of pressure on the stability of the different liquid crystalline phases is discussed and compared with the effect of increasing chain length. Finally, recent results for a ternary mixture of cholesterylmyristate, -nonanoate and -formiate are presented giving evidence for a reentrant cholesteric phase behaviour with respect to pressure.

Interaction of a model skin surface lipid with a modified triglyceride

AbstractA study of the interaction between a model skin surface lipid (SSL) and an oil base skin softener (G2) was made by comparing the phase behavior of these substances in combination with the water/triethanolamine: oleic acid (1:1 wt ratio) system. A principle feature of the water/G2/triethanolamine:oleic acid (TEA:OL) system was the formation of a lamellar liquid crystal region that incorporated an average of 40% G2 over the 10–45% water range. The phase behavior of the water/SSL/TEA:OL system demonstrated only over a 4–9% water range. At greater than 5% SSL, there is the complete absence of single phase regions above 10’% water.From comparison of the phase regions and the small angle X‐ray diffraction results, it is found that for G2/SSL mixtures the phase behavior of the lamellar liquid crystal is dominated by the SSL and the interlayer spacing of the lamellar liquid crystal is dominated by G2.

Liquid crystalline materials: physical properties and intermolecular interactions

The physical properties and the phase behaviour of some nematic liquid crystals are discussed, with emphasis on the influence of short-range antiparallel dipole correlation that occurs in mesogenic compounds with a strong terminal dipole moment. Evidence for this effect, which stems especially from dielectric studies, is summarized. Variations of the dipole correlation with molecular structure can explain the sometimes unexpected phase behaviour and physical properties of these substances. A qualitative model is given in terms of a monomer-dimer equilibrium.

Microscopic and thermal phase studies on smectic and nematic liquid crystals at high pressures

High pressure studies on the phase behaviour of liquid crystals were performed using a high pressure differential thermal analysis equipment (DTA) and a diamond anvil cell (DAC). Results are presented and discussed on the polymorphism in the homologous series of the bis-(4,4’-n-alkoxybenzylidene)–1,4-phenylenediamines as well as on the reentrant nematic behaviour of 4-n-octyloxy-4’- cyanobiphenyl (80CB) and 4-cyanobenzylidene-4’-n-octyloxyaniline (CBOOA) and their mixtures for pressures up to 3 kbar.

The physics and display applications of liquid crystals

Abstract After a short introduction, the general characteristics of liquid crystals are reviewed, the three main classes of thermotropic liquid crystals are described and the concept of order parameters introduced. The phase behaviour of thermotropic liquid crystals is considered in some detail and the continuum theory, describing the static behaviour of liquid crystals, is introduced. Electro-optical effects in liquid crystals and display construction methods are then covered and the operating principles and performance of dynamic scattering, twisted nematic and dyed phase-change displays are reviewed and compared. Threshold and realignment mechanisms in liquid crystal displays are then treated in some detail followed by a consideration of the physics of their dynamical behaviour and response times. The physical properties governing the design of liquid crystal materials and pleochroic dyes are discussed in relation to display performance. The applications of simple displays are then reviewed and the nee...