Hybrid thermotropic liquid-crystalline block copolymers

A new approach to the preparation of two classes of block copolymers containing liquid-crystalline segments is reported. Copolymers 1 and 2 are constituted by polytetrahydrofuran and side-chain liquid-crystalline polymethacrylate blocks, whereas block copolymers 3 consist of polystyrene and main-chain liquid-crystalline polyester blocks. The synthetic procedures leading to copolymers 1-3 are extremely versatile and can be used to prepare a great variety of block copolymer architectures. In both copolymer classes the chemically different blocks are strongly segregated in the solid and melt phases and undergo individual phase transitions. The mesophase transition temperatures of the liquid-crystalline blocks are very similar to those of the corresponding homopolymers, and their enthalpies are directly proportional to the content of the liquid-crystalline block.     

Amphiphilic carbohydrate-based mesogens incorporating structural features of calamitic liquid crystals

Several examples of liquid-crystalline carbohydrate derivatives incorporating a carbocyclic ring in the side chain are reported and the influence of structural variations discussed. The mesophase observed for all compounds described is smectic A and the mesophase stability is strongly affected by the linking unit between the carbocyclic and carbohydrate moieties.     

Phase behaviour of amphiphilic side-chain polymers

The present study describes the phase behaviour of a low-molar-mass 1,3-diol with a linear alkyl chain in aqueous solution. From the combined results of differential scanning calorimetry and optical polarizing microscopy a phase diagram was constructed. The low-molar-mass diol forms a lamellar mesophase only in the presence of water. The influence of the linkage of the amphiphilic molecule to different polymer backbones on the stability of the mesophase was investigated. It is evident that amphiphilic side-chain polymers show a distinct stabilization of the mesophase. Moreover, comparison between the polyacrylate, polymethacrylate and polysiloxane indicates a strong influence of the main-chain flexibility on the stability of the liquid-crystalline phase. The type of the liquid-crystalline phase of the side-chain polyacrylate in the water-saturated state was identified by X-ray investigations as lamellar. The hydration behaviour of the polymers was investigated by ²H-N.M.R. and isopiestic measurements.     

Phase behaviour of amphiphilic side-chain polymers

The present study describes the phase behaviour of a low-molar-mass 1,3-diol with a linear alkyl chain in aqueous solution. From the combined results of differential scanning calorimetry and optical polarizing microscopy a phase diagram was constructed. The low-molar-mass diol forms a lamellar mesophase only in the presence of water. The influence of the linkage of the amphiphilic molecule to different polymer backbones on the stability of the mesophase was investigated. It is evident that amphiphilic side-chain polymers show a distinct stabilization of the mesophase. Moreover, comparison between the polyacrylate, polymethacrylate and polysiloxane indicates a strong influence of the main-chain flexibility on the stability of the liquid-crystalline phase. The type of the liquid-crystalline phase of the side-chain polyacrylate in the water-saturated state was identified by X-ray investigations as lamellar. The hydration behaviour of the polymers was investigated by ²H-N.M.R. and isopiestic measurements.     

New liquid-crystalline materials containing a five-membered heteroalicycle: Trans-2, 4-disubstituted 1, 3-oxathiolanes

The synthesis of new liquid-crystalline materials containing a five membered heteroalicyclic ring is described. The trans 2,4-disubstituted 1, 3-oxathiolane derivatives exhibit lower clearing temperatures and melting points than the analogous six membered 1, 3-oxathiane compounds. However the mesophase stability of the 1, 3-oxathiolane derivatives is superior to that of comparable cyclopentane derivatives.     

Multi-Color Photoluminescence Based on Mechanically and Thermally Induced Liquid-Crystalline Phase Transitions of a Hydrogen-Bonded Benzodithiophene Derivative

Controlling assembled structures of π-conjugated liquid-crystalline molecules is of great interest in the development of stimuli-responsive luminescent materials due to their molecular motility in the ordered states. Herein, we describe a mechanoresponsive hydrogen-bonded benzodithiophene liquid-crystalline molecule that exhibits a tricolor photoluminescence switching at ambient temperature. The compound shows a shear-induced phase transition from a rectangular columnar to a metastable optically anisotropic mesophase, which is accompanied by the luminescent color change from yellow to sky-blue. The metastable mesophase exhibits a time-responsive transformation to another metastable mesophase showing a blue-green emission through isothermal aging at room temperature. The luminescent color of aged sample reverts back to the initial yellow color by thermal annealing at 150 °C. These dynamic structural changes accompanied by the emission color changes are governed by distinct π-stacking modes and hydrogen-bonded patterns. The shear-induced luminescent color change from yellow to blue is found to occur above the shear strain of 390 % at which the shear stress is 2.4×10⁵ Pa as determined from dynamic viscoelastic measurements.     

Amphiphilic carbohydrate-based mesogens incorporating structural features of calamitic liquid crystals

Abstract

Several examples of liquid-crystalline carbohydrate derivatives incorporating a carbocyclic ring in the side chain are reported and the influence of structural variations discussed. The mesophase observed for all compounds described is smectic A and the mesophase stability is strongly affected by the linking unit between the carbocyclic and carbohydrate moieties.     

Thermotropic liquid-crystalline polymers—VI: Comb-like liquid-crystalline polymers of the smectic and nematic types with cyanobiphenyl groups in the side-chains

The syntheses and properties are described for a number of new comb-like liquid-crystalline polyacrylates and polymethacrylates with the same cyanobiphenyl groups in the side-chains but different lengths of the n-aliphatic substituent. The effect of the chemical structure of these polymers on the type of the liquid-crystalline state was examined by means of optical and electron scanning microscopy, X-ray and DSC methods. The polyacrylates with short n-aliphatic chains form the nematic mesophase, the polyacrylates with long n-aliphatic substituents and polymethacrylates form the smectic mesophase. The dependence of thermal properties of the nematic and smectic polymers upon their intrinsic viscosities is established.     

A propeller-like uranyl metallomesogen

Uranyl triflate forms with three imidazo[4,5-f]-1,10-phenanthroline ligands a propeller-like complex that exhibits a hexagonal columnar phase. The ligand is not liquid-crystalline, but a mesophase is induced upon complex formation with the uranyl salt. The thermal behavior has been investigated by polarizing optical microscopy and by high-temperature X-ray diffraction. A model of the stacking of the molecules in the mesophase is proposed.     

Stabilization of the mesomorphic phase in a self-assembled two-component system

We reported here the two-component self-assembling building blocks capable of forming lyotropic liquid crystal and liquid-crystalline physical gel. One of the components has a molecular characteristic of C(3)-symmetrical trisureas containing three azobenzene groups, which can form liquid-crystal phase in a temperature range of 133-215 degrees C. Another one has a trisamide core, which can self-aggregate to fibrous network through hydrogen bonds of amide moieties. The mixture of these two components performs lyotropic liquid crystal as well as liquid-crystalline physical gel in a temperature range larger than that of sole compound, suggesting that the cooperation of hydrogen bonds between these components stabilizes the mesophase of the assembly. The mechanism of formation of the mesophase was investigated by infrared spectra and small-angle X-ray scatterings.     

Micellar and liquid-crystalline properties of bicyclic fragment-containing cationic surfactant

NMR spectroscopy is employed to study the aggregation of a cationic surfactant, 4-aza-1-cetyl-1-azoniabicyclo[2.2.2]octane bromide, in aqueous solutions. Self-diffusion coefficients are determined for micelles and monomers and the hydrodynamic radius and aggregation number are calculated for micelles. Polarization microscopy data demonstrate that the examined compound is an amphotropic substance. It is found that the lyotropic liquid-crystalline system is characterized by a wider temperature range of mesophase existence as compared to the thermotropic system.     

Investigations of comblike polysiloxanes. I. The influence of spacer length on dielectric relaxation studies of aligned samples

Dielectric relaxation studies on aligned liquid-crystalline comblike polysiloxanes are presented. The polymers differ only in the length of the flexible spacer separating the mesogenic side groups from the polysiloxane backbone. The characteristic features of the observed relaxation process as a function of frequency and temperature, in both the liquid-crystalline and isotropic phases, are described, and the effect of the spacer length is discussed. Within the liquid-crystalline phase a narrow single loss process is observed, due to the relaxation of the mesogenic side groups around the polymer backbone, whereas in the isotropic phase a much broader loss curve is seen. The transition from an aligned polymer mesophase to the randomly aligned isotropic phase is also studied in detial, and comparison with earlier work on comblike polyacrylate and methacrylate liquid crystals is presented.     

Morphological studies of liquid-crystalline cellulose derivatives. I. Liquid-crystalline characteristics of hydroxypropyl cellulose in 2-hydroxyethyl methacrylate solutions and in polymer composites prepared by bulk polymerization

The formation of liquid-crystalline structure in hydroxypropyl cellulose (HPC) in the solvent 2-hydroxyethyl methacrylate (HEMA) is described. In addition, an attempt is made to preserve the ordered structure of HPC in a composite by polymerizing the solvent. Optical evidence, including refractive index, absorption spectra, polarized-light microscopy, and x-ray diffraction, indicates that HPC-HEMA solutions exhibit the cholesteric nature of the mesophase over limited concentration and temperature ranges. The polymer composite (HPC-PHEMA) prepared from the liquid-crystalline solution by polymerization of HEMA is endowed with anisotropic organization reflecting liquid-crystalline character. Detailed morphological observations of the composite by electron microscopy show many round particles composed of parallel-stacked, disklike lamellae, and aggregate bodies developed by coalescence of neighboring particles.     

Self-organization of supramolecular complex composed of rigid dendritic porphyrin and fullerene

A second-generation 1,3,5-phenylene-based dendritic porphyrin decorated with flexible alkyl chains exhibited a liquid crystallinity, and the inclusion of fullerene within the nanospace of the dendritic porphyrin strongly affected the mesophase structure in the thermotropic liquid-crystalline phase.     

Pentaerythritol Derived Tetrapode Exhibiting a Nematic-Like Mesophase at Ambient Temperatures

The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase ‘N_X’ to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the N_X phase to exhibit an electrooptic response consistent with a nematic-like phase.     

Self-assembly of folic Acid: a chiral-aligning medium for enantiodiscrimination of organic molecules in an aqueous environment

Weak orienting medium: Self-assembly of alkaline salt of folic acid yielded a weak liquid-crystalline phase in an aqueous environment. This medium has the ability to discriminate enantiomers. The mesophase exists over a broad range and has the physical parameter dependent tunability of degree of alignment.     

Imidazo [2, 1-b]-1,3,4-thiadiazoles. A new class of liquid-crystalline compounds

A series of new liquid-crystalline compounds called imidazo [2,1-b]-1,3,4-thiadiazoles has been synthesized. The new compounds contain alkyl, aryl, trans-alkylcyclohexyl groups in positions 2,6 and lateral Br- and CN-substituents in position 5 of the heterocyclic fragment. The new mesogens normally have a wide mesophase range and a high thermal stability (T_c1 ∼ 300° C). The mesophase type depends significantly on the nature and position of the substituents, in particular the cyclohexane ring. Some chemical, physico-chemical and spectral properties of the new compounds have been investigated. The results are compared with the corresponding characteristics of their aromatic and heterocyclic analogues.     

Synthesis and mesomorphic behaviour of a homologous series of 1,1'-bis[4(4'-alkyloxy)benzaldimine]ferrocene dicarboxylates

A new series of ferrocene derivatives, 1,1'-bis[4(4'-alkyloxy) benzaldimine]ferrocene dicarboxylates, (FeESADCn, where the number of carbon atoms in the alkyloxy chain, n, varies from 4-12) have been synthesized. The thermal behaviour was investigated by means of polarizing microscopy and differential scanning calorimetry. All members of the series exhibit liquid-crystalline behaviour. While the shorter chain members exhibit a nematic mesophase, the longer chain members exhibit a smectic A mesophase. The electronic spectrum of the n = 7 member is also presented.     

Molecular orientation of paramagnetic metal complex by melt-drawing thermotropic liquid crystalline polymers

New liquid-crystalline polyesters and paramagnetic vanadyl complex were prepared as host polymer and guest complex. Melt-drawn fibers were made from the blend of host polymer and guest complex at the mesophase temperature. ESR spectra of the melt-drawn fibers indicated that the base of pyramidal vanadyl complex is completely oriented parallel to the fiber axis.     

Structural variations of liquid crystalline polymers with phasmidic-type mesogens

The fixation of phasmidic-type mesogens through spacer groups to polysiloxane backbones provided a method of obtaining liquid crystalline polymers with phasmidic mesogens as side groups. The polymer 6b showed a bilayer structure in the mesophase range. The tendency to form liquid-crystalline phase depends on the number and the length of alkoxy substituents.