Influence of spacer and terminal group lengths on the smectic ordering of cholesterol-containing dimer liquid crystals

The smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 times the length of the molecule. Three series of alkoxybiphenylylcontaining dimers exhibit two other smectic modifications. For short spacers, a smectic layer spacing is observed that is about the same length as the molecule and for long spacers the smectic layer spacing is about half the length of the molecule. In the latter cases the entropy change at the SmA-N* transition is clearly larger than for the compounds with the other smectic modifications. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd number of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.     

Influence of spacer and terminal group lengths on the smectic ordering of cholesterol-containing dimer liquid crystals

The smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 times the length of the molecule. Three series of alkoxybiphenylylcontaining dimers exhibit two other smectic modifications. For short spacers, a smectic layer spacing is observed that is about the same length as the molecule and for long spacers the smectic layer spacing is about half the length of the molecule. In the latter cases the entropy change at the SmA-N* transition is clearly larger than for the compounds with the other smectic modifications. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd number of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.     

Dimorphism SmA-B2 in bent-core mesogens with perfluorinated terminal chains

Several banana-shaped liquid crystal materials with perfluorinated terminal chains have been synthesized. A dimorphism SmA-B₂ was proved by X-ray studies and electro-optical measurements. The unusual ratio d/L (d = layer spacing, L = molecular length) and the unexpected behaviour of the layer spacing at the phase transition SmA > B2 can be interpreted by a change of the packing of the bent molecules.     

Nature of smectic A*-C* phase transitions in a series of ferroelectric liquid crystals with little smectic layer shrinkage

The smectic layer spacing of two homologous series of ferroelectric liquid crystal compounds was characterized by small-angle x-ray diffraction and different degrees of smectic layer shrinkage on cooling from the SmA* into the SmC* phase were observed. The smectic A*-smectic C* phase transition was further studied by measuring the thermal and electric field effects on the optical tilt angle and the electric polarization. With decreasing length of the alkyl terminal chain the phase transition changes from tricritical exhibiting high layer shrinkage to a pure second-order transition with almost no layer shrinkage. This is explained by the increased one-dimensional translational order of the smectic layers, which seems to promote the "de Vries"-type [Mol. Cryst. Liq. Cryst. 41, 27 (1977)] smectic A*-C* phase transition with no or little layer shrinkage.     

Organic lyotropic lamellar liquid crystals

X-ray diffraction experiments on smectic A and C forming thermotropic liquid crystals reveal that the smectic layer spacing increases with the addition of organic solvents to the host material. The rate of this increase indicates the formation of an organic lyotropic lamellar liquid crystal phase in which the solvent is intercalated between the smectic layers of the host liquid crystal.     

Liquid crystalline behaviour of hydrogen-bonded complexes of alkoxycinnamic acids with octyloxystilbazole

Hydrogen-bonded liquid crystalline complexes have been obtained through 1:1 (molar ratio) complexation of 4- n -alkoxycinnamic acids ( n CNA: n = 4, 8, 10, 12, where n is the number of carbons in the alkyloxy chain) and trans -4-octyloxystilbazole (8Sz). These hydrogen-bonded complexes ( n CNA8Sz) form stable mesophases. The mesomorphic range was extended by the mixing of complexes. Hexatic modification of smectic B (SmB h ), smectic C (SmC), smectic A (SmA), and nematic mesophases of these complexes were determined by a combination of X-ray diffraction and polarizing optical microscopy. Transitions between the various smectic phases were deduced from the temperature-dependent layer spacing of n CNA8Sz. The layer spacing of these complexes in the SmB h and SmA phases gradually increased with increasing alkoxy chain length. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding.     

Liquid crystallinity of newly synthesized glucose derivatives with mesogenic side chains

Synthesis of glucose derivatives by direct esterification of the five available sites on D-(+)-glucose with side chains containing a biphenyl mesogenic moiety, a pentyl spacer and an alkyl tail is reported for the first time. Liquid crystalline phase behaviour of these glucose derivatives was studied by optical microscopy, thermal and X-ray diffraction methods. A layered arrangement of the smectic A (SmA) type was commonly observed in the above materials. An increase in the length of the alkyl tail results in a change of the phase structure from smectic Ai(SmAi), intercalated layer phase, to smectic As(SmAs), segregated bilayer phase. From the transition temperature and enthalpy observations, the SmAs phase has higher order than the SmAi phase. Cholesteric and chiral smectic C phases were also observed in addition to the SmA for some of the glucose derivatives, demonstrating a potential for preparing chiral liquid crystals.     

X-ray diffraction study of smectic A layering in terminally fluorinated liquid crystal materials

We report an X-ray study of smectic A layering for mesogenic compounds with fluorinated substituents in terminal positions. The measurements were carried out using diffractometers with one- and two-coordinate proportional chambers. It was found that in contrast to -CN or -NO₂ terminated smectics, the polar -OCF₃ compounds form only a monolayer smectic A₁ phase. The ratio of the intensity of the second harmonic to the first in smectic A phases formed by molecules with lengthy perfluorinated chains was found to be two orders of magnitude higher than is commonly reported for low molar mass thermotropic mesogens, indicating deviations of the density distribution function p(z) from a pure sinusoidal form. The layer periodicity d for these mesogens exceeds the molecular length L: d/L ≃ 1·1, which corresponds to a smectic A_d phase consisting of parallel or antiparallel dimers. We have observed that lateral fluorine substitution in the benzene ring adjacent to the perfluorinated chain leads to the disturbance of uniform smectic A layering and to the formation of a defect-modulated smectic A structure of a chequer-board type. For polyphilic compounds containing both hydrogenous and perfluorinated units in the terminal chain, the in-plane structure factor displays double-peaked liquid-like profiles indicating the existence of nearest-neighbour molecular stacking at different distances. The peculiarities of smectic A layering in fluorinated mesogens are discussed in terms of steric coupling and enhanced conformational rigidity of fluorine containing moieties.     

Thermal and ionic‐conductive behaviors of liquid crystalline polymers with alkali metal salt

In order to investigate the relationship between ionic conductivity and liquid crystallinity, we prepared the main‐chain type polyester having 1,4‐bisstyrylbenzene units and ethyleneoxide chain in the repeating unit. The main‐chain type polyester with lithium salt at the ratio of 0.04 per polymer repeating unit exhibited a smectic phase. However, the polyester with lithium salt (0.11) showed a nematic phase. The ionic conductivity of the polyester with lithium salt increased with increasing lithium salt concentration. The trans‐type polyester exhibited a liquid crystalline phase, while the cis‐type polyester did not show any mesophase. We found that the ionic conductivity of the trans‐type polyester with lithium salt (0.11) was larger than that of the cis‐type polyester with lithium salt (0.11). However, a liquid crystalline phase was found in the side‐chain type polyether with alkoxy chain length of below 12. A smectic phase was induced for the non‐mesomorphic polyethers with lithium salt. The layer spacing of the smectic A phase for the non‐mesomorphic polyether with lithium salt decreased from 55 to 41 Å with increasing temperature. The ionic conductivity of the polyether with lithium salt increased with decreasing the layer spacing. Copyright © 2001 John Wiley & Sons, Ltd.     

Liquid crystalline behaviour of hydrogen-bonded complexes of alkoxycinnamic acids with octyloxystilbazole

Hydrogen-bonded liquid crystalline complexes have been obtained through 1:1 (molar ratio) complexation of 4-n-alkoxycinnamic acids (nCNA: n = 4, 8, 10, 12, where n is the number of carbons in the alkyloxy chain) and trans-4-octyloxystilbazole (8Sz). These hydrogen-bonded complexes (nCNA8Sz) form stable mesophases. The mesomorphic range was extended by the mixing of complexes. Hexatic modification of smectic B (SmB_h), smectic C (SmC), smectic A (SmA), and nematic mesophases of these complexes were determined by a combination of X-ray diffraction and polarizing optical microscopy. Transitions between the various smectic phases were deduced from the temperature-dependent layer spacing of nCNA8Sz. The layer spacing of these complexes in the SmB_h and SmA phases gradually increased with increasing alkoxy chain length. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding.     

First Examples of de Vries‐like Smectic A to Smectic C Phase Transitions in Ionic Liquid Crystals

In ionic liquid crystals, the orthogonal smectic A phase is the most common phase whereas the tilted smectic C phase is rather rare. We present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core whereas the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature‐dependent layer spacing as well as their translational and orientational order parameters were studied by X‐ray diffraction. The X‐ray tilt angles derived from X‐ray studies of the layer contraction and the optically measured tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries‐like behavior with a layer shrinkage that is far less than that expected for conventional materials. These materials can thus be considered as the first de Vries‐type materials among ionic liquid crystals.     

Synthesis, characterization and crystal structures of two polyphilic mesogenic compounds

Three homologous series of semi-perfluorinated liquid crystals: 4-(2,2,3,3,4,4,4-heptafluoro-butyloxycarbonyl)phenyl, 4-(2,2,3,3,4,4,5,5-octafluoropentyloxycarbonyl)phenyl and 4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxycarbonyl)phenyl 4-[(n-perfluoroalkyl)alkyloxy]benzoates have been synthesized. Their mesomorphic properties were studied by DSC, microscopic observation and X-ray diffraction. Their layer spacings are smaller than the molecular length (L). The ratio d_SA/L is about 0.7. The crystal structure of two derivatives of the first family have also been investigated. In both structures the molecules pack in smectic C-like sheets where neighbouring molecules are antiparallel, interacting through dipolar and van der Waals forces. The interactions between contiguous sheets, through the ends of perfluorinated chains are very weak. The X-ray diffraction results on the mesophases and on the crystalline structures of two compounds of the first family are compared. From this, we propose a model of the smectic phases with a zig-zag structure.     

Preliminary in situ X-ray diffraction measurements of UV-induced photomechanical effects in a mesogenic material

The aim of this work was to determine by X-ray diffraction the nature of the UV-induced changes in the smectic C* organization of a calamitic liquid crystalline material containing the UV-sensitive azobenzene group in the rigid core moiety. As a result of the reversible trans-cis-isomerization, a decrease of all transition temperatures (1.1°C for the SmA-SmC* transition) was observed. In the SmC* phase, this corresponds to an increase in layer spacing (up to 0.5 Å); in the SmA phase, the detected increase in layer spacing was less than a tenth of this. The increase in layer spacing is interpreted in terms of molecules in the cis- configuration expelled from the smectic layers.     

Effect of the chemical constitution of the side-chain on the formation and the structure of mesophases in some polysiloxanes

The synthesis and properties are described for a series of new liquid crystal side-chain polymethylsiloxanes with the same mesogenic rigid core but different substitutents or flexible spacers. The effect of the chemical structure of these polymers upon the polymorphism is studied by means of optical, DSC and X-ray experiments. By comparison with conventional mesogens, the partially bilayered structure of the smectic A phases (i.e. when the layer spacing is between once and twice the length of the side-chain) could be explained, taking into account the asymmetric character of the side-chains.     

Correlation of structure and phase behaviour for a series of modular, chiral liquid crystal diacrylates based on lactic acid

The mesogenic properties of a family of chiral liquid crystal (LC) diacrylates based on a 4-[4-(1R-methyl-2-hydroxyethoxy)phenyl]phenyl 4-hydroxybenzoate core were studied as a function of different tail lengths. In general, this family of LCs was found to exhibit a strong preference for adopting the chiral smectic A phase. Systematic variation of the alkyl spacer lengths on either side of the chiral core revealed that the onset of smectic A behaviour is highly sensitive to the length of the tail adjacent to the chiral unit. However, no correlation between phase transition temperatures and the length of the spacer on the other side of the core was observed. With a fixed spacer length on the chiral side of the core, systematic changes in the length of the other tail resulted in the formation of a monotropic smectic B phase and an increased tendency to supercool.     

Correlation of structure and phase behaviour for a series of modular,chiral liquid crystal diacrylates based on lactic acid

The mesogenic properties of a family of chiral liquid crystal (LC) diacrylates based on a 4-[4-(1R-methyl-2-hydroxyethoxy)phenyl]phenyl 4-hydroxybenzoate core were studied as a function of different tail lengths. In general, this family of LCs was found to exhibit a strong preference for adopting the chiral smectic A phase. Systematic variation of the alkyl spacer lengths on either side of the chiral core revealed that the onset of smectic A behaviour is highly sensitive to the length of the tail adjacent to the chiral unit. However, no correlation between phase transition temperatures and the length of the spacer on the other side of the core was observed. With a fixed spacer length on the chiral side of the core, systematic changes in the length of the other tail resulted in the formation of a monotropic smectic B phase and an increased tendency to supercool.     

The effect of CoPt-coated reduced-graphene oxide nanosheets upon the Smectic-A to Smectic-C* phase transition of a chiral liquid crystal

ABSTRACT

We report on high-resolution calorimetry and small-angle X-ray scattering measurements along the Smectic-A to chiral Smectic-C* phase transition of the liquid crystal 4-(2-methyl butyl) phenyl 4-n-octylbiphenyl-4-carboxylate with dispersed, CoPt nanoparticle-coated reduced-graphene oxide nanosheets. The temperature dependence of heat capacity and smectic layer spacing are obtained in the vicinity of Smectic-A to chiral Smectic-C* phase transition. Though no remarkable pretransitional effects are present, the critical fits show a crossover from mean-field near a tricritical point to classical mean-field compared to pure liquid crystal. The X-ray data yield a dilation of smectic layer thickness, indicating the assembly of graphene oxide nanosheets between the smectic layers.     

A novel class of materials for ferroelectric liquid crystals containing siloxy chain end groups

Novel liquid crystals containing a siloxy chain as an end tail group instead of an alkyl chain were synthesized. The substitution effects were studied for ferroelectric liquid crystal materials. It was found that the temperature range for the chiral smectic C phase was reduced and shifted to lower temperature in comparison with the analogous alkyl chain derivatives. The crystallinity of the siloxy chain derivatives decreased and cholesteric phases were not observed. The influence of siloxy chains on ferroelectric liquid crystal properties, especially spontaneous polarizations and tilt angles, also greatly depended upon the mesogenic group structure. The X-ray diffraction results showed that the end tail group occupied a larger thickness in the chiral smectic C layer for the siloxy chain derivative than that for the alkyl chain derivative.     

Mesogenic properties of 1,2,3-tri-[3'-(4'-alkoxyphenyl)-3'-oxo-1'-propenylamino]propane

The properties of compounds incorporating three mesogenic cores linked to a central 1,2,3-trisubstituted propane unit are reported. They form S_A and S_c calamitic liquid crystalline phases with a layer spacing in the orthogonal smectic phase significantly smaller than a single molecular length. The mesogenic properties of related molecules with two mesogenic cores depend on the position of the enaminoketone ring with respect to the joint. Intramolecular dipolar interactions of the enaminoketone rings are suggested as being responsible for the hindered frmation of the trans-conformational structure of the cores in the dimeric molecules.     

Smectic A and C materials with novel director tilt and layer thickness behaviour

We report a smectic liquid crystal in which a tilt of the molecular orientation away from the layer normal in the Smectic C phase of up to 24 occurs with minimal layer contraction. This characteristic, accompanied by an exceptionally large underlying negative thermal layer expansion coefficient, enables the formation of chiral smectic electro-optical cells having nearly the ideal 'bookshelf' layer geometry, free from the complexities of the chevron layer structure produced by contraction.