An Ia3¯d thermotropic cubic phase from N-alkylpyridinium tetrahalocuprates

The thermotropic liquid crystalline behaviour of two N-alkylpyridinium tetrahalocuprates was investigated. The n-pentadecyl tetrachloro and tetrabromo derivatives (CU15-CL and CU15-BR, respectively) exhibit very similar behaviour on cooling, and specifically, a high temperature smectic phase followed by a cubic phase. We fully elucidated the Ia3¯d space group for the cubic phase from X-ray diffraction on single crystals. The type of cubic phase is still under discussion, but we suggest that it is a direct type of structure based on the known phase sequence of lower and higher homologues. A model for the local organization of CU15-CL and CU15-BR in the cubic and smectic mesophases is also proposed.     

Electron and light microscopical investigation of defect structures in mesophases of pharmaceutical substances

 Transmission electron microscopy of freeze fractured and replicated samples (TEM) and polarizing light microscopy (PLM) are used to investigate the defect structures of the thermotropic and lyotropic mesophases of the non-steroidal antiinflammatory drug fenoprofen sodium and of the thermotropic mesophase of the nonionic surfactant sucrose oleate (O1570). All mesophases have a layered, smectic structure. The thermotropic liquid crystal of feno-profen sodium is an interdigitated smectic A phase (smectic Ad) having the highest viscosity of the investigated samples. The thermotropic mesophase of the sugar ester is also of the type smectic A, likely to be of subtype smectic A2 (bilayered smectic structure). The lyotropic mesophase is of lamellar liquid crystalline nature and has a much lower viscosity than the thermotropic mesophases. In the PLM the lyotropic fenoprofen mesophase has a strong tendency to form a pseudoisotropic texture, indicating a strong tendency to form undisturbed layered structures. Other textures exhibited in the PLM are fan-shaped texture and maltese-cross texture. Confocal domains, cylinders, pits and peaks as well as screw dislocations are found in great number in the TEM. However, no greater regions of undisturbed lamellar arrangement in the lyotropic mesophase could be detected. The only texture of the thermotropic fenoprofen mesophase visible in the PLM is the fan-shaped texture, indicating confocal domains as predominant structural elements. However, no confocal domains (tori or Dupin cyclides) are found in the TEM. In the PLM the sugar–ester mesophase exhibited a fan-shaped texture, maltese crosses and oily streaks as dominant textures. In the TEM only a few +π and −π disclinations and imperfect confocal domains could be detected. The discrepancies in the appearance of defect structures and textures between the mesophases as well as the discrepancies in the findings in the PLM and in the TEM investigations are caused by the different sample preparation and the different viscosities of the mesophases. Received: 28 May 1997 Accepted: 2 September 1997     

Self-assembled liquid crystals by hydrogen bonding between bipyridyl and alkylbenzoic acids: solvent-free synthesis by mechanochemistry

A series of thermotropic hydrogen-bonded liquid crystalline structures based on 4,4′-bipyridyl and aliphatic carboxylic acids was prepared by a mechanosynthesis technique. This series was characterised by polarising optical microscope, differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray powder diffraction and ¹H NMR relaxometry experimental techniques. In these complexes, the bipyridyl component, a non-mesogenic substance by itself, acts as a double H-bond acceptor, whereas the alkylbenzoic acid acts as a H-bond donor, in a 1:2 proportion. The so-formed complexes exhibit mesophases that are not observed by the single components. A characteristic phase (smectic A) is identified and shown to be affected by the alkyl chain length. The isotropisation temperature is increased by the supramolecular aggregation through the H-bonds.     

Rigid‐chain metallomesogenic polymers containing vanadyl or copper(II) ions coordinated in the main chain

Rigid‐chain metallomesogenic polymers containing Cu(II) or VO(II) were prepared and characterized. All the polymers were soluble and melted without decomposition. They showed a thermotropic liquid–crystal (LC) behavior, and the mesophases were invariably preserved for a long time at room temperature in a metastable condition, with respect to the semicrystalline state. The nature of the mesophases of the Cu(II)‐containing polymers was similar to that observed in analogous organic rigid‐chain polymers having long lateral alkyl chains packed as extended ribbons. The VO(II)‐containing polymers showed an LC polymorphism involving a smectic A and a nematic phase. For all the polymers in a smectic or smecticlike state at room temperature, X‐ray diffraction data suggested short mean distances among the metal ions arranged in layers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2342–2349, 2001     

Liquid crystalline 3-, 4- and 6-armed star molecules based on nitromethane-trispropanol or pentaerythritol without mesogen spacers

Some molecules of unusual shape and apparently non-linear geometry, with mesogens tied directly to a central unit (for example, tetrahedral pentaerythritol), have been investigated for liquid crystalline behaviour. It was found that these three-, four- and six-armed 'star' molecules generated liquid crystalline mesophases, which were characterized by DSC, optical microscopy and X-ray diffraction. The temperature ranges of the fluid mesophases for the thermotropic compounds were above 158°C for the three-armed molecules and above 219°C for the four-and six-armed 'stars'. All the liquid crystalline compounds exhibited a smectic phase, which appears to be smectic A, with the molecules in their fully extended conformations within the layers. Some of the compounds also had a smectic phase of higher order or a nematic phase.     

Mesophase formation in lead(II) decanoate

Abstract

Structures of the thermotropic mesophases of lead(II) decanoate are reassigned following optical and X-ray diffraction studies. These results, and those of D.S.C., Raman and ²⁰⁷Pb N.M.R. spectroscopy, indicate formation of a lower temperature mesophase involving mainly increased lateral disorder, and a higher temperature L_α (smectic A) phase resulting from chain disordering and decreased lead-carboxylate interaction. Comparison of experimental thermodynamic data for the phase transitions with theoretical data in the literature indicates that the entropy change for the lower to higher mesophase transition is dominated by the increase in chain disorder.     

Small-Molecule Modulation of Soft-Matter Frank–Kasper Phases: A Method for Adding Function to Form

Incorporation of small amounts of α-tocopherol (vitamin E) in blends with the cellobiose–triazole-linked atactic poly(4-methyl-1-pentene) (CB-aPMP) sugar–polyolefin conjugate can be used to exert external control over thermotropic phase behavior and provide access to non-canonical soft matter Frank–Kasper A15 and σ phases. These results establish a paradigm that can be used for the further design and development of scalable quantities of soft matter FK phases of increased structural complexity and functional capability.     

STUDIES ON THE KINETICS OF PHASE TRANSITIONS OF A MAIN CHAIN THERMOTROPIC POLYESTER

The kinetics of phase transitions including a transition between mesophases were studied for a main-chain thermotropic polyester by means of DSC and depolarizing transmittance techniques. The isothermal process of these transitions was found to be described by the Avrami equation to high conversions. The Avrami exponents n are about 2,4, 5.3 and 2.2 for liquid crystallization, transition between mesophases and crystallization from mesophase respectively. The liquid crystallization from isotropic liquid phase occurs at very low undercoolings with high transformation rate. This behavior is explained as the results of the smaller value of the surface free energy for mesophase than that for crystallites which is evidenced by the very weak temperature dependence of liquid crystallization rate.     

The effects of substituent size and molecular weight on the liquid crystalline properties of poly(2-alkyl-1,4-phenylene terephthalate)s

A series of low molecular weight, thermotropic poly(2-alkyl-1,4-phenylene terephthalate)s was prepared by the solution polycondensation reaction of terephthaloyl chloride and alkylhydroquinones containing n-alkyl substituents of increasing size from methyl to dodecyl. Samples of the low molecular weight polymers so obtained were also further polycondensed in the solid state to obtain high molecular weight polymers. The liquid crystalline phase behaviors and textures were determined, and the effects of polymer structure and molecular weight on these properties are discussed. All of the polymers obtained formed thermotropic, nematic mesophases, which were less stable for the lower molecular weight polymers, as expected, than were the mesophases formed by the higher molecular weight polymers. © 1994 John Wiley & Sons, Inc.     

Thermotropic mesomorphic changes in racemic and meso lead(II) 9,10-dihydroxyoctadecanoates

Data are presented for the heats and entropies of phase changes for the racemic and meso lead(II) 9,10-dihydroxyoctadecanoates. The total enthalpy change accompanying the transformation crystal → liquid phase is lower for the lead(II) 9,10-dihydroxyoctadecanoates than for the lead(II) octadecanoate. This suggests the solid phase of the former to be more disordered than that of the latter.Optical examination under a polarising microscope suggests the phase sequence: solid I → mesophase → liquid for the racemic isomer, and solid I → solid II → mesophase → liquid for the meso isomer. The mesophase and solid II phase are absent on reheating the racemic and meso soaps respectively. The behaviour is interpreted in terms of subtle structural rearrangement of the liquid phase arising from extensive hydrogen bonding. The texture of the mesophases of the isomers is observed to be different and the mesophase of the meso soap is stable over a wider temperature range than that of its racemic counterpart. The behaviour is indicative of the effect of stereochemical configuration on the thermal properties of the soap.     

Thermotropic liquid-crystalline comb-like polymers with polyacrylamide main chain—I. Synthesis and mesomorphic structures of polymers with lipobiphenyl side chains

The synthesis of polymerizable lipobiphenyls, the study of the kinetics of their radical polymerization giving comb-like polymers, and the study of the thermotropic behaviour of the polymers are reported. Utilization of all the information provided by X-ray diffraction diagrams showed that polymers with polyacrylamide main chains and lipobiphenyl side chains exhibit successively, between room temperature and isotropization temperature, two mesophases: an ordered tilted double layer smectic phase SI2, and a disordered tilted double layer smectic phase SC2.     

Small‐Molecule Modulation of Soft‐Matter Frank–Kasper Phases: A Method for Adding Function to Form

Incorporation of small amounts of α‐tocopherol (vitamin E) in blends with the cellobiose–triazole‐linked atactic poly(4‐methyl‐1‐pentene) (CB‐aPMP) sugar–polyolefin conjugate can be used to exert external control over thermotropic phase behavior and provide access to non‐canonical soft matter Frank–Kasper A15 and σ phases. These results establish a paradigm that can be used for the further design and development of scalable quantities of soft matter FK phases of increased structural complexity and functional capability.     

Quasi-binary picture of thermotropic liquid crystals and its application to cubic mesophases

A quasi-binary (QB) picture of thermotropic liquid crystals is proposed on the basis of thermodynamic observations. The experimental conformational entropy of long alkyl chains attached to a (semi)rigid core of mesogenic molecules indicates that the chain is highly disordered in liquid crystalline states. These disordered chains serve as ‘intramolecular solvent’ or ‘self-solvent’ judging from a close resemblance between phase diagrams of neat (against chain length) and binary (against composition) systems. The application of the QB picture to the classic examples of thermotropic cubic mesophases (in ANBC series) shows that the essential structural motif is triply periodic minimal surface.     

Substitution effects on the liquid crystalline properties of D,L-xylitol amphiphiles

In this article we describe the self-assembling properties of alkyl substituted xylitols in relation to both thermotropic and lyotropic liquid crystalline mesophases. Three series of substituted xylitols were prepared where aliphatic chains of varying length were attached to a xylitol moiety via ether, thioether and ester linking groups. The thermotropic properties were investigated by thermal polarized light microscopy and differential scanning calorimetry, and evaluated as a function of chain length and linking group. The lyotropic phase behaviour was investigated via the addition of water to each material at room temperature. The efficiency for forming thermotropic phases was found to be reversed for the lyotropic phases in respect of the three series, i.e. as a function of the linking unit.     

Supramolecular main chain liquid crystalline polymers utilizing azopyridine derivatives

A series of main chain liquid crystalline polymers were formed through intermolecular hydrogen bonding between a functionalized bisazopyridine phenol and aromatic bisacids. The behaviour of these complexes was studied through differential scanning calorimetry and thermal polarizing optical microscopy. The presence of the hydrogen bonds was confirmed through infrared spectroscopy. These complexes formed thermotropic mesophases. The phases were determined to be nematic in nature from the schlieren textures of the optical micrographs. As the length of flexible spacer groups separating the mesogenic portions increased, the clearing temperatures of the mesophases decreased. As the length of the rigid component increased, the clearing temperature increased. A new bisacid species based on 2-hydroxy-6-naphthoic acid was used to increase clearing temperatures while remaining within an acceptable temperature window.     

Synthesis and liquic crystalline properties of copper tetra-4-(n-alkoxycarbonyl)phthalocyanines

Copper tetra-4-(n-alkoxycarbonyl)phthalocyanines were synthesized, and their thermotropic and lyotropic mesomorphism at different lengths of lateral substituents was studied. Their possibility of forming both thermotropic and lyophilic mesophases (amphotropy) was found. The length of lateral substituents affects the type of supramolecular packing in the mesophase rather than the temperature of the crystal→mesophase phase transition. A rarecase of the mesogeneity of disk-like compounds of the same chemical nature possessing only four substituents is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 129–136, January, 2000.     

Synthesis and liquic crystalline properties of copper tetra-4-(n-alkoxycarbonyl)phthalocyanines

Copper tetra-4-(n-alkoxycarbonyl)phthalocyanines were synthesized, and their thermotropic and lyotropic mesomorphism at different lengths of lateral substituents was studied. Their possibility of forming both thermotropic and lyophilic mesophases (amphotropy) was found. The length of lateral substituents affects the type of supramolecular packing in the mesophase rather than the temperature of the crystal→mesophase phase transition. A rarecase of the mesogeneity of disk-like compounds of the same chemical nature possessing only four substituents is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 129–136, January, 2000.     

Preparation and cholesteric mesophase properties of (Butyl-co-pentyl) propylcellulose

Butyl and pentyl ether derivatives of (2-hydroxypropyl) cellulose (HPC) and butyl/pentyl mixed ethers of HPC (BPPC) with different alkyl compositions were prepared in nonaqueous solution and their thermotropic cholesteric properties examined. The temperature dependence and the composition dependence of the optical pitch, nP, were then determined for all of the ether derivatives. The molecular conformation and chirality of BPPC appeared to be to be variably smooth with the side chain composition of the polymer. The response rate of cholesteric configurational change of the thermotropic mesophases arising from a temperature jump from 45  to 75 °C, was also determined. The transformation of the cholesteric mesophase formed by an equimolar ether derivative (BP-50) was faster than that of the cholesteric one formed by a single-alkyl (pentyl) ether derivative (BP-0). This seemed to be a general result reflecting a pseudo-copolymer effect of cellulose derivatives on the properties of their thermotropic cholesteric mesophases.     

Metal organic framework (MOF) liquid crystals. 1D, 2D and 3D ionic coordination polymer structures in the thermotropic mesophases of metal soaps,including alkaline earth,transition metal and lanthanide soaps

Taken together, the body of existing literature on metal soap crystal structures and mesophases supports the view that much is to be gained by treating the soaps as metal organic frameworks (MOF's) when relating their structure and liquid crystallinity.We argue that metal soaps mesophases often consist of disordered metal organic (carboxylate) frameworks (MOF's). Metal atoms are linked by bridging carboxylates, and the metal–oxygen networks form semi-flexible rods, chains and sheets of M–O polyhedra within their co-bonded, mesotructured, self-assemblies of lipidic chains. The packing of the molten hydrocarbon chains allows otherwise unconnected MOF networks to coexist as spatially isolated units in the same unit cell. For instance the lamellar phases are true 2D MOF's or layers of 1-D MOF's. The phase transitions can then be regarded as coupled disordering/re-ordering transitions involving rotational and conformational disordering of the hydrocarbon chains balanced with disordering of MOF symmetries, MOF topological transformations, depolymerizations and dimensionality reductions ultimately leading to anisotropic melts.By way of demonstration, thermotropic phase transitions of homologous series of lanthanide soaps are systematically studied using a variety of experimental methods, and the data are used in a topological model for testing the consistency with the MOF concept of metal soap crystal structures and thermotropic mesophases. Finally, an interpenetrating bicontinuous MOF comprised of SrO₆ polyhedral rods is presented as an atomically resolved model for the network topology of the cubic mesophase of strontium soaps.Metal soaps are therefore shown to afford a bridge between liquid crystals and metal organic framework (MOF) materials.