Thermotropic liquid crystal behaviour of alkaline-earth-metal dihexadecyl phosphate salts

Alkaline-earth-metal dihexadecyl phosphate salts were synthesized, and their thermal stability was evaluated by thermogravimetry. Their thermotropic liquid crystal behaviour was investigated by differential scanning calorimetry, polarizing optical microscopy, dilatometry, and X-ray diffraction. On heating, the calcium, strontium, and barium salts exhibit columnar liquid crystal phases, whereas the less ionic beryllium and magnesium salts melt directly into isotropic liquids.     

Thermotropic liquid crystal behaviour of gemini imidazolium-based ionic amphiphiles

Thermotropic ionic liquid crystals (LCs) are useful for a number of applications such as anisotropic ion transport and as organised reaction media/solvents because of their ordered fluid properties and intrinsic charge units. A large number of different ionic LC architectures are known, but only a handful of examples of gemini (i.e. paired or dimeric) ionic LCs have been prepared and studied. In this work, a series of 20 new symmetric, imidazolium-based, gemini cationic LCs containing two bridged imidazolium cations and two pendant alkyl chains was synthesised, and the thermotropic LC behaviours were characterised. The imidazolium unit provides a highly tunable and modular platform for the design and synthesis of gemini cationic LCs which offers excellent structure control. As expected, the thermotropic LC properties of these new amphilphilic, gemini ionic LCs were found to be strongly influenced by the length of the spacer between the imidazolium units, the length of the pendant alkyl tails, and the nature of the anion. Smectic A (SmA) thermotropic LC phases were observed in more than half of the gemini imidazolium LC systems studied.     

Thermotropic liquid crystalline behaviour of piperazinium and homopiperazinium alkylsulphates

A series of 1,4-piperazinium di-n-alkyl sulphates was synthesized and compared with an analogous series of 1,5-homopiperazinium di-n-alkyl sulphates. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. For the piperazinium salts, two ordered smectic phases were established, in which the lateral packing of the molecules within the layers is rectangular or tetragonal. Both phases are characterized by an alternate periodic packing of the positive and negative ionic groups in the polar sublayer, while the lipophilic sublayers of the alkyl chains are in a disordered conformation. Substitution of the piperazinium by the less symmetrical homopiperazinium group disfavours organization within the smectic layer and as a result smectic A phases were obtained.     

Thermotropic liquid crystalline behaviour of piperazinium and homopiperazinium alkylsulphates

A series of 1,4-piperazinium di- n -alkyl sulphates was synthesized and compared with an analogous series of 1,5-homopiperazinium di- n -alkyl sulphates. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. For the piperazinium salts, two ordered smectic phases were established, in which the lateral packing of the molecules within the layers is rectangular or tetragonal. Both phases are characterized by an alternate periodic packing of the positive and negative ionic groups in the polar sublayer, while the lipophilic sublayers of the alkyl chains are in a disordered conformation. Substitution of the piperazinium by the less symmetrical homopiperazinium group disfavours organization within the smectic layer and as a result smectic A phases were obtained.     

Preliminary Communications Thermotropic liquid crystal behaviour of 5,5'-[(4-hexadecyloxyphenyl)ethynyl]-2,2'-bipyridine

5,5'-[(4-Hexadecyloxyphenyl)ethynyl]-2,2'-bipyridine was synthesized by a palladium(O) cross-coupling reaction. Its thermotropic liquid crystalline behaviour was analysed using differential scanning calorimetry, polarizing microscopy, and X-ray diffraction. Three enantiotropic tilted smectic phases were identified upon heating: S_F, S_I, and S_C.     

Thermotropic and lyotropic liquid crystalline behaviour of 4-alkoxyphenyl beta-D-glucopyranosides

The liquid crystalline properties of a series of 4-alkoxyphenyl beta -D-glucopyranosides (methoxy to decyloxy and dodecyloxy) were studied using polarized light microscopy and differential scanning calorimetry. The compounds with the shortest alkoxy substituents are not liquid crystalline. The butoxy derivative displays a monotropic smectic A phase and the higher homologues display enantiotropic smectic A phases. The lyotropic behaviour was studied as a function of concentration and temperature. Hexagonal, cubic and lamellar phases were observed for compounds with alkoxy chains longer than butoxy. The nonyloxy derivative forms long ribbons in dilute solution as revealed by electron microscopy.     

Thermotropic and optical behaviour of new PDLC systems based on a polysulfone matrix and a cyanoazomethine liquid crystal

The thermotropic and optical characterisations of a novel polymer dispersed liquid crystal (PDLC) system based on polysulfone UDEL P-1700 as polymer matrix and a low molecular weight liquid crystal compound containing a mesogenic azomethine core and a cyano-substituent were investigated. The PDLC samples were prepared by solvent-induced phase separation (SIPS) and thermally induced phase separation (TIPS) methods using various compositions in the two components. Information on the morphology and phase structure was obtained by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. PDLC systems with well-defined droplets were obtained for the composite with medium content of liquid crystal compound. The optical characterisation of these materials was performed by analysing their UV–visible absorption and photoluminescence emission as a function of the liquid crystal aggregation degree.     

Interaction of poly-L-arginine with dihexadecyl phosphate/phosphatidylcholine liposomes

In the present study, mixed liposomes of dihexadecyl phosphate sodium salt:phosphatidylcholine:cholesterol at a 1:19:9.5 molar ratio were allowed to interact with poly-L-arginine at temperatures below and above the main phase transition of the liposomal membrane. The interaction led to the formation of aggregates, which gradually increased in size and eventually precipitated. It was, however, possible, during the initial stage of the experiments, when the ratio of guanidinium group relative to phosphate was smaller than ca. 40%, to determine their size and charge and observe their morphology in aqueous dispersion. Fluorescence experiments established that the liposomes are not ruptured during their interaction with poly-L-arginine. Instead, they are attached at the polypeptide chain through the guanidinium-phosphate complementary pair. Fluorescence quenching experiments indicated that the poly-L-arginine chain is accessible for interaction with iodides dissolved in the aqueous phase when the temperature of the liposomal dispersion is below the main lipid phase transition. It is, however, partitioned in the interior of the membrane at temperatures exceeding this main lipid phase transition.     

Fluorescence behaviour of cyanobiphenyl liquid crystal molecules in liquid crystal/polymer composite films

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer ( 100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacrylate structures and polymerization conditions. The different fluorescence behaviour has been discussed in terms of differences in mobility and/or aggregation degrees of 5CB molecules arising from dominant molecular interactions with the substrate and polymer surfaces.     

Thermotropic diacetylenic liquid crystals

Further studies are reported on polar diacetylenes with conjugated side chains which show liquid-crystalline behaviour. Results obtained using differential scanning calorimetry, thermogravimetry, and hot stage microscopy are reported. One group of materials forms a smectic liquid crystal phase and all polymerize from a nematic liquid crystal phase.     

Thermotropic liquid crystalline glycolipids

Are the liquid crystalline properties of the materials of living systems important in biological structures, functions, diseases and treatments? There is a growing consciousness that the observed lyotropic, and often thermotropic liquid crystallinity, of many biological materials that possess key biological functionality might be more than curious coincidence. Rather, as the survival of living systems depends on the flexibility and reformability of structures, it seems more likely that it is the combination of softness and structure of the liquid-crystalline state that determines the functionality of biological materials. The richest sources of liquid crystals derived from living systems are found in cell membranes, of these glycolipids are a particularly important class of components. In this critical review, we will examine the relationship between chemical structure and the self-assembling and self-organising properties of glycolipids that ultimately lead to mesophase formation.     

Thermotropic phase behaviour of long-chain alkylmaltosides

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.     

Liquid crystal tetramers: influence of molecular shape on liquid crystal behaviour

The synthesis and characterization of two homologous series of tetramers in which four mesogenic units are linked via three alkyl spacers are reported. Both series contain a hexamethylene central spacer while the length of the two outer spacers, n, is varied from three to 12 methylene units. The two series differ only in the substitution pattern around the inner two mesogenic units. The series in which one mesogenic unit is attached to the central spacer in the 4-position while the other is connected at the 3-position is referred to as the n-p6m-n series, while in the n-m6m-n series both inner units are attached in the 3-position. All the members of the n-p6m-n series exhibited a nematic phase while no liquid crystallinity was observed for the n-m6m-n series. The thermal behaviour of this series is compared with that of the n-p6p-n series and also with that of the corresponding series containing a pentamethylene central spacer. The trends observed are interpreted in terms of the average molecular shapes of these tetramers.     

Thermotropic liquid crystal behavior on PBLG‐PDMS‐PBLG triblock copolymers

In this contribution, the preparation of rod‐coil‐rod triblock copolymers based on polydimethylsiloxane and polypeptide [poly(γ‐benzyl‐L ‐glutamate)] is reported. Firstly, self‐assembly in rod‐like structures was demonstrated via polarized optical microscopy and small‐angle light scattering. Further structuration details were obtained using X‐ray scattering and AFM imaging to establish the formation of a double‐hexagonal structure and to accurately define the morphological dimensions of the rodlike structures. The thermal behavior of these structures was investigated using dynamic mechanical analysis and differential scanning calorimetry. We conclude by addressing an unexpected reversible thermal transition within the 130–150 °C temperature range and the ensuing associated organizational modifications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4668–4679, 2006     

Thermophysical behaviour of monofunctional acrylate and liquid crystal systems

The phase behavior of monofunctional acrylate and low molecular weight nematic liquid crystals (LC) is considered. Systems involving the monomeric 2-ethylhexylacrylate (2-EHA), the eutectic LC mixture known as E7 and the 4-cyano-4^′-n-pentyl-biphenyl (5CB) are investigated. A similar investigation is performed on mixtures involving a polymer poly-2-EHA with molecular weight M_w=48,000 g/mol and both LCs. The experimental phase diagrams are established using polarized optical microscopy and analyzed using a theoretical formalism which combines the Flory-Huggins theory of isotropic mixing and the Maier-Saupe theory of nematic order. The results lead to characterization of the miscibility of E7 and 5CB with monomeric and analogous polymeric 2-EHA systems.     

Structure and photoconductive behaviour of a sanidic liquid crystal

A series of [1]benzothieno[3,2-b][1]benzothiophene-2,7-dicarboxylate derivatives consisting of a flat, conjugated aromatic moiety containing sulphur hetero-atoms and substituted by terminal chains - ethyl, decyl, Z-4-decenyl or 2-(2-butoxyethoxy)ethyl - has been synthesized. These materials exhibit a smectic liquid crystalline phase, the stability of which was found to be strongly dependent on the length and the nature of the terminal chains. In the mesophase, the molecular arrangement within the smectic layer is characterized by a local stacking of the rigid parts similar to that found in discotic columnar systems. Finally, clear photoconductive behaviour was found in mechanically aligned samples of these liquid crystals.     

Thermotropic main chain liquid crystalline polyesters containing ferrocene and phosphate units: synthesis and characterization

A new series of liquid crystalline main chain copolyesters were prepared, having ferrocene in the mesogenic segment and a methyl phosphate group along with a methylene spacer. The even numbered methylene groups were varied from two to ten. Liquid crystalline behaviour was investigated on a hot stage optical polarized microscope. Thermal properties of the polymers were analysed by TGA and DSC, revealing that the polymers yield high char products, probably caused by the formation of phosphorus and iron oxides. The glass transition (T_g) temperatures of the polymers were found to be fairly low, the result of the incorporation of bulky phosphorus and ferrocene moieties in the chain. The phase behaviour was analysed and correlated with the structure of the polymers. The liquid crystalline textures of the polymers became more transparent with increasing spacer length. Energy minimized structures for the polymer repeating units reveal that both the ferrocene and phosphorus moieties produce more molecular entanglement, thus reducing the T_g and T_m of the polymers.     

Structure and photoconductive behaviour of a sanidic liquid crystal

A series of [1]benzothieno[3,2-b][1]benzothiophene-2,7-dicarboxylate derivatives consisting of a flat, conjugated aromatic moiety containing sulphur hetero-atoms and substituted by terminal chains - ethyl, decyl, Z-4-decenyl or 2-(2-butoxyethoxy)ethyl - has been synthesized. These materials exhibit a smectic liquid crystalline phase, the stability of which was found to be strongly dependent on the length and the nature of the terminal chains. In the mesophase, the molecular arrangement within the smectic layer is characterized by a local stacking of the rigid parts similar to that found in discotic columnar systems. Finally, clear photoconductive behaviour was found in mechanically aligned samples of these liquid crystals.     

The synthesis and liquid crystal behaviour of monosubstituted ferrocenomesogens

The synthesis and characterization of several new series of monosubstituted ferrocenyl-containing liquid crystals has been achieved. The results indicate that a terminal ferrocenyl group can promote stabilization of a nematic liquid crystal state. At least three phenyl rings are required in the molecular core in order to provide nematic properties and addition of a fourth ring substantially enhances it. Bulky lateral substituents or the introduction of linker groups that introduce kinks inhibit liquid crystal phase formation. Short highly polarising terminal groups or terminal groups that support hydrogen bonding support liquid crystal behaviour.