Rheological properties of chiral liquid crystals possessing a cholesteric–smectic A transition

We have measured the rheological properties of two cholesterol derivatives (cholesteryl myristate and cholesteryl nonanoate) in the vicinity of their cholesteric–smecticA transitions. The results for the two compounds differ qualitatively, and are in agreement with results based on optical observations of new defects in cholesteryl nonanoate showing that this material, traditionally considered as a typical cholesteric, in fact exhibits a TGBA phase between the cholesteric and smectic A phases.     

Investigation of the blue phase in a compensable cholesteric mixed system

Pitch dependence of cholesteric blue phase stability has been investigated in a compensable mixed system of cholesteryl nonanoate/chloride. Two critical pitches above which no blue phase exists were found depending on the structure of the liquid crystal. The clearing enthalpy does not exhibit discontinuities over the whole mixing range.     

Determination of the anchoring constant for a compensated binary cholesteric mixture

The optical transmission and the electric power was simultaneously recorded for the binary compensated cholesteric mixture cholesteryl chloride + cholesteryl nonanoate (70:30 molar). The values of the critical voltages U₁ and U₂ (at which the planar texture changes into a conic focal one, and the cholesteric-nematic transition take place) at different temperatures were obtained. The temperature dependence of the anchoring constant is determined.     

An apparatus for thermoanalytical microscopy and its applications

A high-sensitivity apparatus for thermoanalytical microscopy under reflected light has been designed and has been applied to studies of (1) the mechanism of the polymorphic transition of stearic acid, (2) the fine structure of the smectic—cholesteric transition of cholesteryl nonanoate and (3) the process of the multiplication of yeast bacteria.     

Optical activity of a side group cholesteric liquid crystalline silicone

We measured, using a phase modulation technique, the optical rotation in the isotropic and blue phases of a side group cholesteric liquid crystalline silicone polymer. For comparison, similar measurements were performed on cholesteryl nonanoate (CN) and CE6. The polymer has a selective reflection close to that of CE6 and a chiral mesogenic side group chemically similar to CN. The optical activity data for the polymer, unlike those for low molecular mass liquid crystals, does not follow a simple Landau-de Gennes temperature dependence.     

Adsorption of Binary Liquid Crystals onto Cellulose Membrane for Thermo-Responsive Drug Delivery

A thermo-responsive membrane was developed by adsorbing the binary liquid crystals (LC) onto the cellulose nitrate membrane. The binary LC mixture is consisted of 36% cholesteric oleyl carbonate (COC) and 64% cholesteryl nonanoate (CN). The physico-chemical and the thermo-responsive properties of this binary LC adsorbed-membrane were examined. The results indicate that the adsorption behavior of binary LC mixture on the cellulose membrane was quick and reached to the equilibrium about 10 min, and linearly correlated with the increase of LC concentration used. The hydrophilic membrane was hydrophobicized after adsorption of binary LC mixture. The focal conic fan textures of binary LC mixture were observed from the surface of cellulose membrane. Due to the obedience and reproducibility of the binary LC used, the binary LC mixture-adsorbed membrane continuously exhibited an on-off thermo-responsive sensitivity to control the drug penetration.     

Latent heat measurements of the smectic A-cholesteric transition on binary mixtures

New differential scanning calorimetry measurements of the smectic A-cholesteric latent heat of transition for binary mixtures of cholesteryl nonanoate (C₉) and cholesteryl heptanoate (C₇), cholesteryl nonanoate and cholesteryl caproate (C₆) are reported for cooling cycles. These measurements give evidence of a tricritical point at approximately the reduced temperature T_SACh/T_Chl ≅ 0·92 for 63·1 mol per cent C₉ in the mixture of C₆ and C₉. This tricritical point occurs at reduced temperatures and concentrations different from those already obtained for heating cycles for the same mixture.     

Reflectance properties of polymer-stabilised cholesteric liquid crystals cells with cholesteryl compounds of different functionality

In this paper, we describe the synthesis of cholesteryl compounds of different functionality. A series of polymer-stabilised cholesteric liquid crystal (PSCLC) cells are prepared by photo-polymerisation of a cholesteric liquid crystal (Ch-LC) mixture containing a non-reactive liquid crystal, a nematic diacrylate and the above cholesteryl compounds. The effect of cholesteryl compound functionality on the morphology and the reflectance properties of PSCLC cells is evaluated, as are parameters such as the polymerisation temperature. The results indicate that the higher functional cholesteryl compound is more effective for broadening the reflection band of the Ch-LC composites, which is speculated to be a result of the pitch differences in the local network environment. Scanning electron microscopy is used to examine the morphologies of the polymer network of the PSCLC cells. We have found that the morphologies of the polymer network are determined by the functionality of the cholesteryl compounds as well as polymerisation temperature, which further influence the reflectance properties of the composites.     

Behaviour of the pitch of the cholesteric and chiral smectic C helix near and at the cholesteric-smectic A-chiral smectic C multicritical point

A cholesteric-smectic A-chiral smectic C multicritical point was established in binary mixtures of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate with cholesteryl myristate or cholesteryl benzoate. At this point the cholesteric phase, smectic A phase, and chiral smectic C phase become indistinguishable. Whereas the pitch of the cholesteric helix at the S_A-Ch phase transition is infinite already in the vicinity of the multicritical point, the pitch of the cholesteric helix at the S*_C-Ch transition becomes infinite only at this point. In accord with the theory of Beresnev the pitch of the chiral smectic C helix remains finite at the multicritical point. Additional high pressure experiments support the results obtained at atmospheric pressure.     

Ultrasonic absorption studies in the cholesteric and isotropic phases of cholesteryl valerate

Ultrasonic absorption has been measured in the cholesteric and isotropic phases of cholesteryl valerate as a function of both temperature and frequency. The anomalous absorption observed in the cholesteric phase has been interpreted by considering the coupling of the sound wave to the director fluctuations. In the isotropic phase coupling of the sound wave to the tensor order parameter is considered to analyse the absorption results.     

Cholesteric liquid crystal displays as optical sensors of barbiturate binding

The influence on the optical properties of cholesteric liquid crystal displays (LCDs) was examined for neutral molecule binding by mesogen/receptors in the mesomorphic phase. The motivation was to prepare neutral molecule sensors that use a colour change to signal analyte binding. A receptor that binds barbiturate analytes was modified with two or one cholesteryl groups to yield compounds 2 and 3, respectively. LCDs were prepared by incorporating one of the receptor/mesogen compounds into a cholesteric LC blend along with a potential H‐bonding guest. The optical properties of the LCDs were then determined by measuring the absorbance of the displays. For various LCDs, the colour of the display depended upon several factors: the amount of guest molecule used, the number of cholesteryl side chains on the receptor and the mole concentration of receptor/mesogen in the blend. In particular, complementary host/guest binding of H‐bonding analytes by the bis(cholesteryl) receptor 2 in a cholesteric LCD caused a change of up to +70 nm, which was observed by the naked eye as a blue‐to‐orange colour change. Control experiments confirm that the colour of an LCD is a consequence of molecular recognition in the mesomorphic phase.     

Effect of composition on the pressure-induced re-entrant cholesteric phase behaviour of mixtures of non-polar cholesteryl n-alkanoates

Recently we reported a pressure-induced re-entrant cholesteric phase for ternary systems of non-polar cholesteryl n-alkanoates. In these systems one component contains a shorter n-alkyl chain than the other two. We now show that the former produces a positive excess volume in the smectic phase, which is probably responsible for the pressure-induced re-entrant phase behaviour. The maximum temperature of the cholesteric/smectic A phase boundary is found to decrease drastically with decrease of the n-alkyl chain length of this particular component.     

Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Optical activity in the isotropic and blue phases of two cholesterics

Optical activity measurements were performed in the isotropic phase and blue phases of two short-pitch cholesteric liquid crystals (cholesteryl oleate and cholesteryl oleyl carbonate). The data in the isotropic phase are interpreted in terms of five structural modes of the order parameter. Coupling between these modes gives rise to an extremum and a change of sign in the pretransitional optical rotation. The intensities of the polarized and depolarized component of the transmitted beam were measured as an extral tool to locate the phase transitions.     

Optical activity in the isotropic and blue phases of two cholesterics

Optical activity measurements were performed in the isotropic phase and blue phases of two short-pitch cholesteric liquid crystals (cholesteryl oleate and cholesteryl oleyl carbonate). The data in the isotropic phase are interpreted in terms of five structural modes of the order parameter. Coupling between these modes gives rise to an extremum and a change of sign in the pretransitional optical rotation. The intensities of the polarized and depolarized component of the transmitted beam were measured as an extral tool to locate the phase transitions.     

Activity coefficients of butanol isomers in liquid crystalline cholesteryl tridecylate according to data of reverse phase gas chromatography and tensiometry

The reverse phase gas chromatography technique was used to determine the activity coefficients of butanol isomers at infinite dilution in the smectic, cholesteric and isotropic phases of cholesteryl tridecylate. Experimental parameters with which the chromatographic process may be viewed as balanced and proceeding within the bulk of the liquid crystal were defined. Alcohol activity coefficients in binary systems with cholesteryl tridecylate determined by the gas chromatographic and tensiometric techniques were found to be in good agreement. A conclusion is reached on the suitability of the reversed-phase gas chromatographic technique for studying smectic and cholesteric liquid crystalline phases.     

Thermische analyse binärer systeme von mesogenen cholesterylestern und azoxybenzolen

Zusammenfassung Die binären Systeme von Cholesterylnonanoat und 4,4-Bis(hexyloxy)azoxybenzol (1) und mit 4,4-Bis(heptyloxy)azoxybenzol (2) besitzen induzierte smektischeA-Phasen mit Umwandlungspunktmaximum. Während bei (1) die smektischeA-Phase beim Erhitzen in ihrem ganzen Existenzbereich zur cholesterischen Phase umgewandelt wird, ist bei (2) das Maximum gegenüber der cholesterischen Phase stabilisert und geht direkt in die isotrope Schmelze über. Beide Maxima weisen das Molverhältnis 11 auf. Im System (2) tritt außerdem eine smektischeC-Phase auf. Cholesterylhexadecanoat bildet mit Cholesterylnonanoat (3) und mit Cholesteryltetradecanoat (4) smektische und cholesterische Mischkristallreihen nach Typus I bzw. III Roozeboom. Durch die relative Stabilisierung von instabilen Festkristallmodifikationen der Komponenten in bestimmten Mischung sbereichen kommt es zur Ausbildung zusätzlicher, metastabiler Eutektika, die sich z.T. nicht in die thermodynamisch stabilen Formen umwandeln lassen. Dadurch ist es bei früheren Untersuchungen zu Irrtümern gekommen. In den Systemen (1) bis (3) bilden die Festkristalle einfache Eutektika, im System (4) Mischkristalle nach Typus V Roozeboom.

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The binary systems of cholesteryl nonanoate with 4,4-bis(hexyloxy)azoxybenzene (1) and with 4,4-bis(heptyloxy)azoxybenzene (2) show induced smecticA phases with transition point maximum. Whereas in (1) the smecticA phase is transformed into the cholesteric phase on heating within the whole range of its existence, in (2) the maximum of the smectic phase is stabilized compared with the cholesteric phase and clears into the isotropic melt directly. The molecular ratio of the maximum is 11 in both cases. Moreover in system (2) a smecticC phase was observed.Cholesterylhexadecanoate forms series of smectic and cholesteric mixed crystals of type I resp. III Roozeboom with cholesteryl nonanoate (3) and with cholesteryl tetradecanoate (4). As the unstable solid modifications of the components become relatively stabilized in some mixtures, additional metastable eutectics are formed, which hardly allow to be transformed into the thermodynamic stable solid crystals. Thus errors happened at previous investigations. In the systems (1) to (3) the solid crystals show simple eutectics, in system (4) mixed crystals type V Roozeboom are formed.

     

Some novel cholesteric liquid crystals

A novel series of cholesteric liquid crystals (CNTⁿ). made by quaternization of the new mesogenic unit cholesteryl isonicotinate (CN) with n-alkyl salts, C_nH_2n + ₁ (T) (n = 2 to 8, T = 4-MeC₆H₄SO₃) has been prepared. Differential scanning calorime-try and polarizing optical microscopy studies reveal that all of these salts exhibit an enantiotropic cholesteric phase but for the cholesteryl isonicotinate unit the cholesteric phase is monotropic. For all members of the series the cholesteric-isotropic transitions are not reversible as these salts begin to decompose before their cholesteric-isotropic transition point is reached.     

Influence of spacer variation on phase behaviour of liquid crystalline copolymethacrylates containing cholesteryl and S-(-)-4-chloropropionyloxybiphenyl-4-yl groups

Liquid crystalline side group copolymers containing cholesteryl and S-(-)-chloropropionyloxybiphenyl groups were synthesized and investigated by size exclusion chromatography, 1H NMR spectroscopy, polarization microscopy, X-ray diffraction and differential scanning calorimetry. The spacer of the cholesteryl-containing side groups was changed by the substitution of two methylene groups by ether groups. The existence of a cholesteric phase on copolymerization of two monomers containing chiral tail groups, from which the homopolymers exhibit only smectic phases, could be observed. This cholesteric phase only exists over a narrow range of copolymer composition. The temperature dependence of the reflection wavelength for the cholesteric phase was determined. For the cholesteryl-containing homopolymer, an SA bilayer phase was observed, whereas the S-(-)-chloropropionyloxybi2 phenyl homopolymer showed a higher ordered SB phase below an SA monolayer phase. The layer periodicities of the SA phases of the copolymers depend on the composition. The substitution of two methylene groups of the spacer by ether groups led to disappearance of the cholesteric phase. Simultaneously, over a small range of copolymer composition a biphasic region was obtained.