Observation of a re-entrant isotropic phase in a pure disc-like liquid crystal

Abstract

We report the observation of an optically isotropic phase in the new disc-like mesogen 2,3,7,8,12,13-hexa(octadecanoyloxy)truxene. On cooling, this optically isotropic phase goes over into a nematic phase, while on heating a columnar phase grows in. To our knowledge this is the first example of a pure discotic mesogen exhibiting an isotropic phase below the temperature existence of a columnar phase. Our findings are supported by refractive index measurements of this compound as a function of temperature.     

A polycatenar mesogen with a perfluorinated moiety showing continuous phase transformation between a smectic A phase and a structured,fluid, optically isotropic phase

A polycatenar mesogen consisting of a four aromatic ring system with three long alkoxy chains and a bulky perfluorinated substituent has been synthesized and found to show the polymorphism of a cubic phase, a columnar phase and a smectic A phase, including a structured, fluid, optically isotropic phase with the phase transformation between the smectic A phase and the isotropic phase occurring continuously.     

Structural ordering and molecular-optical properties of a discotic liquid crystal (Colho)

The infrared absorbance and the dispersion of the refractive indices in the region 0.49-0.63 µm of the discotic mesogen 2,3,6,7,10,11-hexaheptyloxytriphenylene were studied for the columnar and isotropic phases. The temperature dependence of the orientational order parameter S and the local field parameters of a light wave acting on the sample have been evaluated. It is shown that the mixing of molecular excitations modifies the behaviour of S as a function of temperature. Molecular-optical properties of the triphenylene derivative, such as the anisotropy of the polarizability, Δγ and the mean polarizability, &gammatilde; were investigated on the basis of the generalized Lorenz-Lorentz relation for uniaxial liquid crystals. The experimentally observed changes of these properties in the columnar phase seem to be affected by the induction mechanism of the change in the molecular polarizability with increasing S.     

Phase behavior of a thermotropic cubic mesogen 4'-n-hexadecyloxy-3'-nitro- biphenyl-4-carboxylic acid under pressure

The phase behavior of an optically isotropic cubic mesogen 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-16) was investigated under hydrostatic pressures up to 200 MPa using a high-pressure DTA, a polarizing optical microscope equipped with a high-pressure hot-stage and a wide-angle X-ray diffractometer equipped with a high-pressure vessel. In the T vs. P phase diagram constructed in the heating mode, a triple point exists at 54±1 MPa and 205±1°C for the SmC, cubic, and SmA phases. A new mesophase, denoted here as X, appears in place of the cubic phase under pressures above about 60 MPa, while the X phase appears on cooling in the whole pressure region studied. Thus the X phase is a monotropic (metastable) phase between the SmA and Cub phases in the low pressure region, while being an enantiotropic phase between the SmA and SmC phases in the high pressure range. The X phase exhibits broken-fan or sand-like textures under pressure and a spot-like diffraction pattern, indicating the birefringent feature and no layered structure. It is suggested that the X phase is tetragonal or hexagonal columnar phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aggregation-driven,re-entrant isotropic phase in a smectic liquid crystal material

An azobenzene-core chiral mesogen designed for a photoactive ferroelectric liquid crystal system with switchable polarisation displays a highly unusual phase sequence, with a re-entrant, optically isotropic, fluid phase found below smectic phases in mixtures with high enantiomeric purity. The re-entrant isotropic phase is found on the basis of X-ray scattering and freeze-fracture transmission electron microscopy experiments not to be a cubic or other highly ordered phase but instead a translationally disordered liquid. The material also forms a gel under a wide range of concentrations in 50:50 ethanol/chloroform solutions. Ultraviolet/visible and infrared spectroscopy and quantum chemistry calculations suggest that the primary unit in the re-entrant isotropic and gel phases is a dimer composed of molecules crossed by about 90°, which hinders the formation of crystal phases and forms tubules of helical aggregates in the gel phase.     

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.     

Liquid crystalline hexa-amides and side chain polysiloxanes of azacrown [18]-N6

The hexa-4-dodecyloxybenzoyl derivative 1 of azacrown [18]-N₆ was originally reported to have a 'tubular' mesophase on the basis of its large central ring and 6-fold symmetry. Starting in the mesophase, annealing of 1 under a cover slip results in formation of a new crystalline phase that melts directly to an isotropic liquid at the temperature previously observed for the mesophase to isotropic transition. Thus the phase behaviour of 1 is kinetically controlled. The analogous hexa-3,4-bisdodecyloxybenzoyl derivative 2 of azacrown [18]-N₆ has no kinetic limitations to its phase changes and has an enantiotropic columnar liquid crystalline phase. We have synthesized side chain copolysiloxanes with a (CH₂)₁₁ spacer and 75-84 per cent by weight of the same 4-dodecyloxybenzoyl-[18]-N₆ mesogen. The polysiloxanes also display a liquid crystalline phase.     

Discotic liquid crystals Physical parameters of some 2,3,7,8,12,13-hexa(alkanoyloxy)truxenes. Observation of a re-entrant isotropic phase in a pure disc-like mesogen

The synthesis of a new discotic mesogen, 2,3,7,8,12,13-hexa(octadecanoyloxy)-truxene is reported. This new compound has been studied using polarization microscopy, differential scanning calorimetry, small angle X-ray scattering and adiabatic calorimetry. The results are compared with those of two other truxene derivatives from the same series: 2,3,7,8,12,13-hexa(decanoyloxy)truxene and 2,3,7,8,12,13-hexa(tetradecanoyloxy)truxene. Refractive index measurements have been performed on these three truxene derivatives as a function of temperature. A re-entrant isotropic phase has been observed in 2,3,7,8,12,13-hexa(octadecanoyloxy)truxene. This is the first report of such a phase in a pure mesogen. We also report measurements of the ratio of the Frank constants for splay and bend in the nematic regime of this new mesogen as a function of temperature using the magnetic Frederiks transition technique.     

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.     

Ordered nanostructures at two different length scales mediated by temperature: A triphenylene‐containing mesogen‐jacketed liquid crystalline polymer with a long spacer

A mesogen‐jacketed liquid crystalline polymer (MJLCP) containing triphenylene (Tp) moieties in the side chains with 12 methylene units as spacers (denoted as PP12V) was synthesized. Its liquid crystalline (LC) phase behavior was studied with a combination of solution ¹H NMR, solid‐state NMR, gel permeation chromatography, thermogravimetric analysis, polarized light microscopy, differential scanning calorimetry, and one‐ and two‐dimensional wide‐angle X‐ray diffraction. By simply varying the temperature, two ordered nanostructures at sub‐10‐nm length scales originating from two LC building blocks were obtained in one polymer. The low‐temperature phase of the polymer is a hexagonal columnar phase (Φ_H, a = 2.06 nm) self‐organized by Tp discotic mesogens. The high‐temperature phase is a nematic columnar phase with a larger dimension (a′ = 4.07 nm) developed by the rod‐like supramolecular mesogen—the MJLCP chain as a whole. A re‐entrant isotropic phase is found in the medium temperature range. Partially homeotropic alignment of the polymer can be achieved when treated with an electric field, with the polymer in the Φ_H phase developed by the Tp moieties. The incorporation of Tp moieties through relatively long spacers (12 methylene units) disrupts the ordered packing of the MJLCP at low temperatures, which is the first case for main‐chain/side‐chain combined LC polymers with MJLCPs as the main‐chain LC building block to the best of our knowledge. The relationship of the molecular structure and the novel phase behavior of PP12V has implications in the design of LC polymers containing nanobuilding blocks toward constructing ordered nanostructures at different length scales. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 295–304     

Supermolecular self organization via molecular aggregation. Non-discotic three chain diols displaying a hexagonal columnar phase

A new class of mesogen formed by three chain diols is described. These compounds consist of a central part to which three aliphatic chains of various length (n = 4-14) are attached at one end and two hydroxy groups at the other. X-ray scattering, dilatometric measurements, dielectric and Kerr relaxation studies were carried out. Both X-ray investigations, dielectric and electro-optic studies revealed that the molecules are aggregated in the isotropic fluid state and that hydrogen bonding and dipole correlation contribute to this aggregation. Columnar assemblies of up to several hundred molecules are orientationally correlated in the isotropic fluid phase. Correlated reorientations occur in the presence of external electric fields. The X-ray data show that the aggregates pack in a hexagonal way at lower temperatures giving a structure which strongly resembles that of the hexagonal disordered columnar phase. This anisotropic phase can be quenched to a glassy liquid-crystalline state. It is evident that the mesogenic properties of this new class of compounds are a function of the disc-like shape of the aggregates rather than the shapes of the individual molecules.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic ‘banana phase’. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic ‘banana phase’ occur.     

Thermodynamic parameters for the dissolution of n-alkanes and n-alkanols in a liquid-crystalline polypropylenimine dendrimer: A gas-chromatographic study

Thermodynamic parameters for the dissolution of n-alkanes (C₇-C₁₁) and n-alcohols (C₅-C₉) under infinite dilution in the columnar and isotropic phases of a polypropylenimine dendrimer have been determined by inverse gas chromatography. Thermodynamic functions have been investigated as a function of the phase state of a mesogen, the length of alkyl chains of sorbates, and temperature. For the dissolution of low-molecular-mass organic compounds, a higher endothermic effect corresponds to a larger entropy factor with its positive deviation from ideality. The dominant effect of the entropy constituent of the Gibbs energy on the dissolution of sorbates in a high-molecular-mass LC solvent has been revealed. The Rohrschneider constants have been estimated for the stationary phase based on the dendrimer. The experimental data indicate the low polarity of the sorbent that is close to those of phenyl-containing siloxane phases.     

Phase behaviour and structure of a non-ionic discoidal amphiphile in water

The phase behaviour and structure of mixtures of a novel non-ionic discoidal amphiphile, 2,3.6,7,10,11-hexa-(1,4,7-trioxaoctyl)-triphenylene, and water have been investigated using deuterium NMR spectroscopy and X-ray diffraction. The feature of special interest is the occurrence of a nematic phase, intermediate to an isotropic micellar solution at higher temperatures and a columnar hexagonal phase at lower temperatures. This nematic phase exists over wide concentration (0.1 to 0.5 volume fraction of amphiphile) and temperature intervals. The molecules are stacked in columnar (rod-shaped) aggregates. In the isotropic micellar solution and nematic phase the length of the micelle is of the order of the centre-to-centre separation.     

A liquid-crystalline silsesquioxane dendrimer exhibiting chiral nematic and columnar mesophases

A hexadecamer, first-generation, octasilsesquioxane liquid-crystalline dendrimer was synthesized by a platinum-catalyzed hydrosilylation reaction of the parent first-generation vinyl octasilsesquioxane dendrimer with a modified, laterally substituted mesogen. The structure and purity of the octasilsesquioxane substrate was confirmed by 1H, 13C, and 29 Si NMR spectroscopy, microanalysis, and size exclusion chromatography (SEC). The mesogenic substrate was found to exhibit only a chiral nematic phase, whereas the resulting hexadecamer dendrimer displays enantiotropic chiral nematic, disordered hexagonal columnar, and disordered rectangular columnar phases, with a glass transition below room temperature. The lateral or side-on attachment of the mesogen to the dendritic core was found to be a key design feature in the formation of the mesophases.     

New mesogen with thermotropic cubic phase: 3,4,5‐tris‐(11,11,12,12,13,13,14,14,15,15,16,16,16‐tridecafluorohexadecyloxy)benzoic acid

A new mesogen, 3,4,5‐tris(11,11,12,12,13,13,14,14,15,15,16,16,16‐tridecafluoro‐hexadecyloxy)benzoic acid, with a thermotropic cubic phase was synthesized. The phase behaviour of the fluorinated compound and its corresponding non‐fluorinated analogue was investigated by differential scanning calorimetry, polarizing optical microscopy, and synchrotron small‐angle X‐ray diffraction. The fluorinated compound, which contains three partially perfluorinated alkoxy moieties and one carboxylic acid group capable of forming hydrogen bonding, exhibits a very interesting sequence of isotropic, cubic and hexagonal columnar phases, whereas its non‐fluorinated compound does not have a liquid crystalline phase. The cubic phase exists over a wide temperature range (from ～200°C to ～30°C on cooling) and is hence amenable to various physical measurements and potential applications. The ability to self‐assemble molecules into ordered superstructure via both partially perfluorinated chains and hydrogen bonding provides new insight for the development of novel liquid crystalline materials.     

Long- and short-range order in the mesophases of laterally substituted calamitic mesogens and their radial octapodes

The mesomorphic behavior of a calamitic mesogen (4'-undecyloxybiphenyl-4-yl-4-octyloxy-2-(pent-4-en-1-yloxy)benzoate) and of a supermesogenic octapode formed by the side-on attachment of the mesogen to a octasilsesquioxane central core is studied by X-ray diffraction and polarizing optical microscopy. The calamitic compound is found to have a nematic phase that has biaxial domains (cybotactic clusters) of tilted layers throughout its entire temperature range. Domains of analogous structure are also found in both the nematic and the hexagonal columnar mesophases exhibited by the obctapode compound. The spacing of the layers forming the domains is found to have the same, essentially temperature independent value for the calamitic monomer and for the octapode, in both its mesophases. Comparison with compounds of analogous structure shows that this value is determined by the length of the rigid part of the mesogenic unit. Variation of the latter length is shown to have no effect on the size of the hexagonal lattice of the octapode columnar phase or on the stacking distance within the columns. The presence of the biaxial domains in the nematic phase is discussed in connection with the phase biaxiality that has been observed in structurally related tetrapode compounds and the possibility of field induced macroscopic biaxial nematic order.     

Properties of a liquid crystalline polyester with a mesogen containing the bicyclooctylene ring

A liquid crystalline polyester with a decamethyene flexible spacer and a mesogenic unit containing a central 1,4-bicyclo(2,2,2)octylene ring was prepared for comparison with the equivalent polymer containing a central p-phenylene ring in the mesogen. Both polymers formed a nematic phase on melting, and as expected, the former had a much lower isotropization temperature than the latter. The polymer containing the bicyclooctylene ring, however, showed a batonnet texture, indicative of a smectic phase, at a temperature above that of the nematic state, especially on cooling from the isotropic melt.     

Thermotropic liquid crystals of 1H-imidazole amphiphiles showing hexagonal columnar and micellar cubic phases

The linear and polycatenar type 1H-imidazole amphiphiles showing a strong self-assembly tendency to build various supramolecular structures in bulk were synthesized by the esterification reaction of 4′-alkyloxy phenols (for 1-4) and hydroxyphenyl trialkyloxybenzoates (for 5-7) with 4-chlorocarbonyl imidazole. The linear 1H-imidazole amphiphiles formed thermotropic smectic phases, but compound 4 with an ethyl group instead of hydrogen on 1N did not show a mesophase. The polycatenar type 1H-imidazole amphiphiles (5-7) formed thermotropic hexagonal columnar and cubic phases. A phase transition was observed from a columnar phase to a cubic phase as the temperature increased on heating, and vice versa on cooling. In the POM study for compounds 5-7, optically isotropic phases first appeared on cooling from the isotropic melts, and then birefringent mesophases with a nonspecific texture appeared on further cooling. The X-ray analysis shows that the optically isotropic phases were very likely micellar cubic phases with Pm3n symmetry. The birefringent phases were confirmed as hexagonal columnar phases.