Dielectric spectroscopy of liquid crystalline dispersions

We describe dielectric spectroscopy measurements on dispersions of two thermotropic liquid crystals (5CB and 8CB) in a poly(dimethylsiloxane) matrix. 5CB exhibits nematic and isotropic phases, while 8CB exhibits smectic, nematic, and isotropic phases. The spectra of the dispersions exhibit a temperature-dependent dielectric relaxation in the interval from 100 to 1000 Hz, with relaxation times that depend strongly on whether the dispersed phase is isotropic, nematic, or smectic. The dielectric relaxation times also depend on the viscosity of the matrix fluid. These results suggest a coupling between the electric field and the mechanics of the interface that affects the spectrum of the dispersed phase and shifts the Maxwell-Wagner interfacial polarization peak.     

Progress in liquid crystalline dispersions: Cubosomes

Dispersed particles of bicontinuous cubic liquid crystalline phase, cubosomes, are self-assembled nanostructured particles that can be formed in aqueous lipid and surfactant systems. Contributions to cubosome research have come from the fields of biology, material science, medicine, and mathematics and much is known about their formation and properties. At the center of much of the discovery and innovation is the technique of cryo-transmission electron microscopy. Most of the research into cubosomes is motivated by potential applications in drug delivery and material synthesis although no commercialized product based on cubosomes is known. Recent advances in understanding and use of cubosomes are discussed in the context of some of the more promising application areas and the opportunities for microscopy techniques to make unique contributions to these areas.     

Smectic ordering in side-chain liquid crystalline polymers (LCPs) and in LCP-silica nanocomposites

The mesophase behaviour of a side-chain liquid crystalline polyacrylate (LCP) grown by drying a solution has been investigated. This LCP, characterised by a short spacer (four carbon atoms) and a long tail (10 carbon atoms), displays, at increasing temperatures, SmC and SmA_d phases. The effect of the mean molecular weight, i.e. the mean number of side chains per polyacrylate main chain (18 and 51) on the lamellar width, was studied. LCP-silica nanocomposites have been synthesised by a sol-gel process in the presence of LCP in the solution, followed by subcritical drying. The mesophase behaviour of these nanocomposites was compared to that of the corresponding bulk LCP. The experimental methods were polarised optical microscopy, differential scanning calorimetry and synchrotron X-ray scattering.     

Conformational transitions in macromolecules of complex chemical structure during liquid crystalline ordering

Conformational transitions in thermotropic main-chain polymers of complex chemical structure including homopolymers with possibility of conformational transformations in mesogens and random copolymers (CPLs) containing mesogens of the same type and spacers of different lengths were investigated. It was demonstrated that liquid crystalline (LC) state influences conformational transformations in fragments of chain restraining them in comparison with an isotropic melt. It was found that CPLs studied form LC order of smectic type. Peculiarities in spacers behaviour during transition to LC state ensure formation of such type of LC order. Conclusion about dependence of mechanism of smectic LC order formation on chemical structure of rigid components of CPLs is made.     

Liquid crystalline dispersions of complexes formed by chitosan with double-stranded nucleic acids

Double-stranded molecules of nucleic acids (NAs) were shown to interact with chitosans to form under certain conditions (chitosan molecular mass, content of amino groups, distance between amino groups, pH of solution, etc.) multiple types of liquid crystalline dispersions. The dispersions formed are different in their spatial structures, and hence in the sense and magnitude of the abnormal optical activity. The physicochemical properties of these dispersions were investigated. Time- and temperature-stabilization of dispersions that possess abnormal optical activity were achieved by chemical crosslinking of chitosan molecules in the liquid crystalline dispersions formed from NA–chitosan complexes. The accessibility of these ‘NA–liquid crystalline elastomers' with respect to enzyme and drug action was tested. The multiplicity of liquid crystalline forms of DNA–chitosan complexes was possibly explained by the influence of the character of the dipole distribution over the surface DNA molecules on the sense of the spatial twist of the cholesteric liquid crystalline dispersions resulting from these complexes.     

Liquid crystalline dispersions of complexes formed by chitosan with double-stranded nucleic acids

Double-stranded molecules of nucleic acids (NAs) were shown to interact with chitosans to form under certain conditions (chitosan molecular mass, content of amino groups, distance between amino groups, pH of solution, etc.) multiple types of liquid crystalline dispersions. The dispersions formed are different in their spatial structures, and hence in the sense and magnitude of the abnormal optical activity. The physicochemical properties of these dispersions were investigated. Time- and temperature-stabilization of dispersions that possess abnormal optical activity were achieved by chemical crosslinking of chitosan molecules in the liquid crystalline dispersions formed from NA-chitosan complexes. The accessibility of these 'NA-liquid crystalline elastomers' with respect to enzyme and drug action was tested. The multiplicity of liquid crystalline forms of DNA-chitosan complexes was possibly explained by the influence of the character of the dipole distribution over the surface DNA molecules on the sense of the spatial twist of the cholesteric liquid crystalline dispersions resulting from these complexes.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Molecular complexes in liquid crystalline structures as a stabilizer at liquid interfaces

Summary Phase equilibria in systems of water/p-xylene/amines have been investigated. The results show that the polar interactions giving rise to molecular complexes in liquids and solids do not have the same structural influence in the liquid crystalline phase.Regarding this the opinion that molecular complexes act as stabilizers for emulsions and foams is criticized. The basic stabilizing element is rather a liquid crystalline phase at the interface and consequently definite molecular ratios of surfactants do not have a decisive effect on the stability. The more general interaction of the stabilizer with both the aqueous and with the nonpolar phase is pointed out.     

NMR study of ordering in liquid crystalline solution of polybenzylglutamate of various molecular mass

¹H and ²H NMR spectra of fractionated poly-γ-benzyl-L-glutamate solutions are recorded at various temperatures. NMR signals of both these nuclei of solvent molecules (mixture of CD₂Cl₂ and CH₂Cl₂) appear to be doublets. Their splitting (30-130 Hz for protons and 200–600 Hz for deuterons) reduces when the polymer molecular mass grows. As the splitting is proportional to degree of orientation of solvent and solute molecules, it means that longer macromolecules are less oriented than shorter ones. One can explain such behavior in terms of persistent chain model, since the bended rods of less length can be better oriented along the director.     

Electrostatic stabilization of liquid crystalline ordering in binary melts of sodium and potassium alkanoates

Abstract

The mesophase electrostatic stabilization energy (E) has been examined by using the ionic continuous solid solution model for binary systems of sodium and potassium alkanoates with a common anion. It is found that E increases with a decrease in chain length of the alkanoate anion and that there is an inverse proportionality between E and the square of the mesophase bilayer spacing. The electrostatic stabilization factor is shown to be responsible for the formation of an ionic mesophase in binary acetate and propionate systems derived from two non-mesomorphic components.     

Comparison of calculated and observed CD spectra of liquid crystalline dispersions formed from double-stranded DNA and from DNA complexes with coloured compounds

The CD spectra of dispersions of DNA, in the form of cholesteric liquid crystalline droplets, in an aqueous continuum have been studied. Calculated curves have been fitted to experimental spectra. The amplitude and the sign of the intense absorption band of the purine and pyrimidine bases vary with the droplet size, the pitch and the twist sense of the cholesteric phase. The CD spectra of dispersions of the complex formed by DNA and a coloured intercalating antibiotic have been similarly studied. A general satisfactory level of fitting between observed and calculated CD spectra was found.     

Electrostatic stabilization of liquid crystalline ordering in binary melts of sodium and potassium alkanoates

The mesophase electrostatic stabilization energy (E) has been examined by using the ionic continuous solid solution model for binary systems of sodium and potassium alkanoates with a common anion. It is found that E increases with a decrease in chain length of the alkanoate anion and that there is an inverse proportionality between E and the square of the mesophase bilayer spacing. The electrostatic stabilization factor is shown to be responsible for the formation of an ionic mesophase in binary acetate and propionate systems derived from two non-mesomorphic components.     

Self-assembly of liquid crystalline triphenylene-oligo(ethylene oxide)-triphenylene molecules and their complexes with lithium triflate

Mesomorphic dimeric molecules consisting of discotic triphenylene rigid units and flexible ethylene oxide spacers have been prepared. The liquid crystalline behaviour is greatly dependent on the length of the ethylene oxide chains. The miscibility and phase behaviour have been examined for mixtures of the materials with lithium triflate.     

Development of a liquid chromatography-based screening methodology for proteolytic enzyme activity

A new methodology for the detection and isolation of serine proteases in complex mixtures has been developed. It combines the characterization of crude samples by electrospray tandem mass spectrometry (ESI-MS/MS) in a multi-substrate assay and the differentiated sensitive detection of the responsible enzymes by means of liquid chromatography hyphenated online to biochemical detection (BCD). First, active samples are identified in the multi-substrate assay monitoring the conversion of eight substrates in multiple reaction monitoring in parallel within 60 s. Hereby, the product patterns are investigated and the suitable peptide as substrate for BCD analysis is selected. Subsequently, the active proteases are identified online in the continuous-flow reactor serving as BCD after non-denaturing separation by size-exclusion chromatography and ion-exchange chromatography. For BCD, the selected para-nitroaniline (pNA) labeled peptide is added post-column and is cleaved by eluting proteases under release of the coloured pNA in a reaction coil (reaction time 5 min). The method was optimized and the figures of merit were characterized with trypsin and chymotrypsin serving as the model proteases. For trypsin, a limit of detection in LC–BCD of 0.1 U/mL corresponding to an injected amount of 0.4 ng protein (∼18 fmol) was observed. The BCD signal remained linear for an injected enzyme concentration of 0.3–10 U/mL (1.3–42 ng enzyme). The method was applied to the characterization of the crude venom of the pit viper Bothrops moojeni and the extracellular protease of the pathogenic amoeba Acanthamoeba castellanii. In the two samples, fractions with proteolytic activity potentially interfering with the blood coagulation cascade were identified. The described methodology represents a tool for serine protease screening in complex mixtures by a fast ESI-MS/MS identification of active samples followed by the separation and isolation of active sample constituents in LC–BCD.     

Dinuclear gold(I) isocyanide complexes with luminescent properties, and displaying thermotropic liquid crystalline behavior

Dinuclear gold(I) complexes [mu-(4,4'-CN-R-NC){Au(C6F4OC4H9)}2] [R = 1,4-phenylene, n = 8; R = 4,4'-biphenylene, 2,2'-dichloro-4,4'-biphenylene, 2,2'-dimethyl-4,4'-biphenylene, n = 4,6,8,10] have been prepared and their liquid crystal behavior and optical properties studied. Although the free ligands are not mesomorphic, all the gold(I) derivatives described, except the phenylisonitrilegold(I) derivative [mu-(1,4-CN-C6H4-NC){Au(C6F4OC8H17)}2], display liquid crystal behavior, giving rise to a nematic mesophase. The transition temperatures decrease in the order 4-4'-biphenylene > 2,2'-dichloro-4-4'-biphenylene > 2,2'dimethyl-4-4'-biphenylene. All compounds show photoluminescence in the solid state and in solution. The single-crystal X-ray diffraction structures of [mu-(4,4'-CN-R-NC){Au(C6F4OCnH2n+1)}2] (R = 4-4'-biphenylene and 2,2'-dichloro-4-4'-biphenylene) have been determined confirming the rodlike structure of the molecule, with a linear coordination around the gold atoms. There are Au...Au interactions in the 2,2'-dichlorobiphenyl derivative but not in the 4-4'-biphenyl compound.     

Preparation of amphotropic liquid crystalline poly(alkoxycarbonylphenylene)s

Poly(alkoxycarbonylphenylene)s with long alkoxy groups were prepared by dehalogenative polycondensation of alkyl dichlorobenzoates with nickel bromide/zinc/triphenylphosphine catalyst. The poly(alkoxycarbonyl‐p‐phenylene)s and poly(alkyloxycarbonyl‐m‐phenylene)s are liquid crystalline, although the latter are composed wholly of kinked repeating units. While poly(hexyloxycarbonylphenylene)s exhibit only thermotropic liquid crystalline behavior, poly(dodecyloxycarbonylphenylene)s and poly(hexadecyloxycarbonylphenylene)s show amphotropism.