New mesogens with cubic phases: hydrogen-bonded bipyridines and siloxane-containing benzoic acids II. Structural studies

The supramolecular structure of new hydrogen-bonded mesogens composed of dipyridyl and 4-(oligodimethylsiloxyl)alkoxybenzoic acids was investigated by X-ray diffraction and deuterium NMR. These H-bonded mesogens exhibit a cubic thermotropic liquid crystalline phase at a temperature below a smectic A phase. Moreover, above the smectic A phase two optically isotropic, low viscosity phases (I₁ and I₂) exist with the lower temperature I₁ phase comprising aggregated mesogens. Two broad, temperature-independent, wide angle X-ray reflections suggest that the aggregation is driven by nanophase-separation; the aggregate adopts a structure with one domain siloxane rich and the other hydrocarbon rich. In the isotropic melt near the I₁-to-I₂ phase transition, a very weak small angle reflection, indicates incipient lamellar-like clustering of the aggregated mesogens. With decreasing temperature the intensity of the small angle reflection increases and in the smectic A phase, the reflection becomes very sharp. In the cubic phase the small angle reflection splits into two peaks which can be indexed with a face-centred cubic (FCC) structure. The deuterium NMR measurements in the isotropic and smectic A phases are indicative of aggregation and an oriented, lamellarlike structure, respectively. In the cubic phase, the collapse of quadrupolar splittings is consistent with an isotropic average of the quadrupole interaction caused by mesogen translational diffusion through a cubic supramolecular structure.     

Structure and shear orientation of a side chain liquid crystal polymer studied by small angle X-ray scattering

We have studied the structure and shear flow behaviour of a side chain liquid crystal polymer, namely P₁₂, in the smectic (SmA_d) phase. Simultaneous oscillatory shear and small angle X-ray scattering experiments showed that, as expected, the shearing of P₁₂ at high frequency and large strain amplitudes leads to the alignment of the smectic layer normals parallel to the neutral direction. It was also found that, on erasing the memory of the prior shear alignment through melting, P₁₂ can unexpectedly orient with the smectic layers perpendicular to the shear flow direction (b-configuration). We speculate that the b-configuration is adopted due to a counter-balance between the rather long spacer in P₁₂ which decouples mesogens from the backbone, and the oscillatory character of the shear applied to P₁₂.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis(4-n-decyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis(4-n-decyloxybenzoyl)hydrazine, BABH(10), was investigated under pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope (POM) equipped with a high pressure optical cell. The reversible change in structure and optical texture between the cubic (Cub) and smectic C (SmC) phases was associated with a change from a spot-like X-ray pattern and dark field for the Cub phase to the Debye-Sherrer ring pattern and sand-like texture for the SmC phase under both isobaric and isothermal conditions. The Cub phase was found to disappear at pressures above about 11 MPa. The phase transition sequence, low temperature crystal (Cr₃)-intermediate temperature crystal (Cr₂)-high temperature crystal (Cr₁)-Cub-SmC-isotropic liquid (I) observed at atmospheric pressure, is maintained in the low pressure region below 10 MPa. The transition sequence changes to Cr₃-Cr₂-(Cr₁)-SmC-I in the high pressure region. Since the Cub-SmC transition line determined by POM has a negative slope (dT/dP) in the T-P phase diagram, a triple point is estimated approximately at 10-11 MPa, and 143-145°C for the SmC, Cub and Cr₁ phases, giving the upper limit of pressure for the observation of the cubic phase.     

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.     

New mesogens with cubic phases: hydrogen-bonded bipyridines and siloxane-containing benzoic acids I. Preparation and phase behaviour

Several 4-(oligodimethylsiloxyl)alkoxybenzoic acids and their hydrogen-bonded complexes with 4,4-dipyridyl or 1,2-bis(4-pyridyl)ethane were prepared and their phase behaviour studied by DSC and polarized optical microscopy. The neat acids showed no liquid crystalline phases. The 4,4-dipyridyl complex of 4-(n-heptamethyltrisiloxyl)hexyloxybenzoic acid (Si3C6BA) exhibits an optically isotropic, highly viscous liquid crystalline phase below a smectic A phase. The 1,2-bis(4-pyridyl)ethane complex of Si₃C₆BA also shows an optically isotropic liquid crystalline phase above its smectic C phase. Its behaviour is similar to that of the well known cubic D phase found in 4-n-alkoxy-3-nitrobiphenyl-4-carboxylic acids. In the hydrogenbonded mesogens studied herein, the cubic phase appears to assemble spontaneously in order to take account of the chemical incompatibility between the siloxane moiety terminating the H-bonded complex and the stiff aromatic cores. The transition temperatures of the cubic phases in these materials is around 100^oC, hence they are amenable to a variety of physical measurements.     

P NMR investigation of a ringing gel phase and adjacent phases

The temperature-surfactant concentration phase diagram was examined for the dodecyltrimethylammonium dimethylphosphate/3-methyl-3-methoxybutanol/water ternary system. The phase diagram contained a highly elastic gel phase which is known as a “ringing gel phase”. The ringing gel phase and adjacent phases in the ternary system were investigated by polarized optical microscopy, freeze-fracture transmission electron microscopy, and ³¹P NMR. Globular textures were observed in an optically isotropic gel phase. Since the globules were larger than those found in an isotropic solution, the texture consists of domains of aggregated units in the cubic (I₁) phase. Structure units of domains are equivalent to microemulsions which are constructed by surfactant molecules and swollen by alcohol in the isotropic (L₁) phase. Characteristic polarized microscopic textures were visualized in two phases with higher surfactant concentrations. These phases were identified as being hexagonal (H₁) and lamellar (L) liquid crystals which was confirmed by transmission electron microscopy. The ³¹P NMR signal of the ringing gel showed a sharp singlet the same as that of the L₁ phase, indicating the fully averaged anisotropic interaction of the aggregates. The characteristic NMR signals of the anisotropic hexagonal and lamellar liquid crystal phases displayed chemical shielding with an asymmetric lineshape.     

Thermal behaviour of the thermotropic cubic mesogen 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) under hydrostatic pressure

The phase behaviour of 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) was investigated under hydrostatic pressures up to 200 MPa using high pressure differential thermal analysis. The phase transition sequence crystal 4 (Cr4)-crystal 3 (Cr3)-crystal 2 (Cr₂)-crystal 1 (Cr₁)-smectic C (SmC)-Cubic (Cub)-smectic A (SmA)-'structured liquid' (I₁)-isotropic liquid (I₂) was observed for a virgin sample on heating at atmospheric pressure. The stable temperature region of the optically isotropic cubic phase becomes narrower on increasing pressure and disappears at pressures above 65 MPa. The T vs. P phase diagram exhibits the existence of a triple point (65 MPa, 207.6°C) for the cubic phase, a new mesophase (X), and the SmA phase, indicating the upper limit for the cubic phase. The new mesophase, denoted here as X, appears in place of the cubic phase at pressures above 65 MPa. The phase diagram also indicates that the Cr₄-Cr₃, Cr₃-Cr₂, and Cr₂-Cr₁ transition lines merge at about 40-50 MPa and then only the Cr₄-Cr₁ transition is observed in the solid state at higher pressures. Thus the phase transition process on heating changes from the sequence Cr₄-Cr₃-Cr₂-Cr₁-SmC-Cub-SmA-I₁-I₂ at atmospheric pressure to Cr₄-Cr₁-SmC-X-SmA-I₁-I₂ in the high pressure region above 65 MPa, via Cr₄-Cr₃-Cr₂-Cr₁-SmC-(X)-Cub-SmA-I₁-I₂ in the low pressure region.     

Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure-property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

X-ray diffraction study on liquid crystals from 4-hexylphenyl 4-cyanobenzoyloxybenzoate and 4-heptyloxyphenyl 4-cyanobenzoyloxybenzoate

X-ray investigations were performed on 4-hexylphenyl 4-cyanobenzoyloxybenzoate (DB6CN) and on 4-heptyloxyphenyl 4-cyanobenzoyloxybenzoate (DB7OCN). For both compounds, the smectic A₂ structure is observed. From the small angle meridional reflections, the angular distribution of the smectic units around the normal to the smectic layers and the longitudinal correlation length were studied. The orientational order parameter is calculated from the wide angle equatorial reflections. Based on the assumption of a layer structure with undulation in the liquid crystalline smectic A phase, the relationship between the dimensionless parameters χch/dL (correlation length/thickness of the smectic layer) and FWHM/2θ₀ (full width at the half maximum/maximum point of the scattering peak) is simulated.     

Short range order in the isotropic phase of antiferroelectric liquid crystals

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C^*_A, and smectic Q. In the other system the phase sequence is smectic C^*, smectic C^*_A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results.     

Effect of terminal polar substituents on the nature of smectic A liquid crystals of a series of 4-(4'-octyloxybenzoyloxy)benzylidene-4'-substituted anilines

The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4'-octyloxy-benzoyloxy)benzylidene-4'-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO₂ and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.     

Phase behaviour of the thermotropic cubic mesogens 1,2-bis-(4-n-undecyl- and 4-n-dodecyl-oxybenzoyl)hydrazine under pressure

The phase transition behaviour of two optically isotropic, thermotropic cubic mesogens 1,2-bis-(4-n-undecyloxy- and 4-n-dodecyloxy-benzoyl)hydrazine, BABH(11) and BABH(12), was investigated under hydrostatic pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope equipped with a high pressure optical cell. It is found that for BABH(11) and BABH(12), a smectic C (SmC) phase is induced between the isotropic liquid (I) and the cubic (Cub) phases by applying pressures above 10-12 and 16-17 MPa, respectively. A sea-island texture consisting of bright sand-like sea regions (SmC phase) and areas of dark islands (Cub phase) appears in the mesophase under pressures up to 140 MPa, while the sand-like texture of the SmC phase is formed predominantly on cooling under pressure. These observations indicate the destabilization of the cubic phase with increasing pressure. The phase transition sequence of BABH(11) and BABH(12), Cr-Cub-I at atmospheric pressure, changes to Cr-Cub-SmC-I under intermediate pressures and would change to Cr-SmC-I under elevated pressure.     

Reinvestigation of liquid crystalline property of dimesogenic compound consisting of cholesterol and cyanobiphenyl

The dimesogenic compound consisting of cholesterol and cyanobiphenyl mesogens interlinked byω-oxyalkanoyl spacer was synthesized.The mesomorphic properties were investigated by differential scanning calorimetry(DSC),polarizing optical microscopy(POM),and X-ray diffraction(XRD).The experimental results indicated that this compound exhibited mesophase over a much wider temperature range and a new mesophase blue phase(BP).Focal conic domains(FCDs) and droplets texture to the smectic A phase(SA),oily streaks...     

Coexistence of two different side chain packing structures in a smectic phase of liquid-crystalline side chain polymers

Ten varieties of liquid-crystalline side chain polymers, poly(cholesteryl-ω-(methacryloyloxy)alkanoates) (pChMO-n, n = 1, 2, 3, 4, 5, 7, 9, 10, 11 and 15; the carbon number of the alkyl chain), were studied by differential scanning calorimetry and small angle X-ray scattering. On and after the first cooling run from the isotropic state, these polymethacrylates gave the same smectic phase. X-ray investigations showed that pChMO-n with short spacers (n = 1-7) has a two layer (bilayer) S_A packing structure, and pChMO-n with a longer spacer (n= 15) has a single layer (monolayer) S_A packing structure. However, these two types of packing structure appear simultaneously in pChMO-n (n = 9s-11) below their phase transition temperature. To clarify the manner of the coexistence of the two different structures the smectic layer spacing and X-ray diffraction patterns were examined by small angle X-ray scattering at various temperatures.     

Monocyclic rod-type liquid crystals; 2,5-dialkanoyloxytropones and 5-alkanoyloxy-2-alkoxytropones

Two kinds of monocyclic troponoid mesogens, 2,5-dialkanoyloxytropones (4) and 5-alkanoyloxy2-alkoxytropones (5), were prepared. The former showed monotropic smectic A phases and the virtual isotropic liquid-smectic A transitions of the latter were determined by extrapolation of results in a binary phase diagram. Comparing the mesogenic properties between the tropones 4 and the 2-alkanoyloxy-5-alkoxytropones (1), the alkanoyloxy group at C-5 enhances both the melting points and the transition temperatures of the smectic A phases. From the comparison between 5 and 1 , the alkanoyloxy group at C-2 lowers the melting points.     

Detailed study of a two component smectic 4TPB-8OCB system with a nematic gap. Phase diagram and viscosity study

The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4'-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η₁, η₂, η₃ and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.     

Miscibility studies of the smectic phases of trans,trans-4'-alkylbicyclohexyl-4-carbonitrile (CCH) liquid crystals

Complete temperature-composition phase diagrams for binary mixtures of 4'-ethyl-, 4'-propyl-, and 4'-butylbicyclohexyl-4-carbonitrile (CCH-2, CCH-3, and CCH-4, respectively) in each of the three possible combinations have been constructed from differential scanning calorimetry and thermal microscopy data. The highest temperature smectic phases of each of the pure mesogens are immiscible with one another, even though CCH-3 and CCH-4 have both been previously assigned the bilayer crystal-B structure on the basis of X-ray diffraction studies. The present studies indicate that the enantiotropic smectic phase of CCH-4 is slightly higher-ordered than is the monotropic smectic phase of CCH-3. The smectic phase of CCH-2, which previously has been found to be characterized by rhombohedral packing on the basis of X-ray diffraction data, is miscible with the second, previously uncharacterized (monotropic) smectic phase (S₂) of CCH-3. Photographs illustrating the subtle differences in the microscopic textures of these smectic phases are also presented.     

A 2H NMR study of order and dynamics in the cubic I1 phase of the dodecyltrimethylammonium chloride/water system

The micellar cubic phase I₁ of the dodecyltrimethylammonium chloride (C₁₂TACl)/water system has been studied by ²H NMR spectroscopy. Relaxation rates of selectively deuteriated surfactant were measured at several frequencies down to 0.5 MHz in the temperature range 25-61°C. The results are interpreted within models for the reorientational dynamics in I1 phases and it is concluded that the remaining order after surface diffusion and aggregate rotation is small and does not vary substantially with temperature. The results are discussed in relation to the two prevailing models for the structure of these phases as suggested (separately) by Fontell and Vargas. At 0°C the ²H NMR spectrum from perdeuteriated n-dodecane solubilized in this phase shows static quadrupole splittings with a magnitude that is in good agreement with the results obtained from the analysis of the relaxation data.