Oily Streaks Research Articles

Microstructure and optical properties of nematic and cholesteric liquid crystals doped with organo-modified platelets

The nematic (5CB) and cholesteric (the mixture of 5CB with chiral M5) liquid crystals (LC) doped with organo-modified inorganic platelets Laponite® (LO) were studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), microscopic image analysis, and optical density measurements. Cetyl-trimethylammonium-bromide (CTAB) was used as an organo-modifier and “cold” and “hot” methods were applied for obtaining of LOc and LOh samples, respectively. The data of DSC and XRD for nematic 5CB+LO compositions evidenced the presence of strong interactions between 5CB molecules and LO platelets and the better exfoliation for LOc as compared with LOh platelets. Microscopic structure, optical density and selective reflection for cholesteric compositions evidenced the strong integration of LOc platelets inside LC medium. The observed optical behavior reflects combination of initial perturbation of cholesteric matrix by LOc particles and initiation of formation of topological defects at small concentration of LOc, C<0.1%, as well as the partial trapping of LOc by large scale oily streaks at higher concentrations, C≥0.1%.

Persistence of Smectic-A Oily Streaks into the Nematic Phase by UV Irradiation of Reactive Mesogens

Thin smectic liquid crystal films with competing boundary conditions (planar and homeotropic at opposing surfaces) form well-known striated structures known as “oily streaks”, which are a series of hemicylindrical caps that run perpendicular to the easy axis of the planar substrate. The streaks vanish on heating into the nematic phase, where the film becomes uniform and exhibits hybrid alignment. On adding sufficient reactive mesogen and polymerizing, the oily streak texture is maintained on heating through the entire nematic phase until reaching the bulk isotropic phase, above which the texture vanishes. Depending on the liquid crystal thickness, the oily streak structure may be retrieved after cooling, which demonstrates the strong impact of the polymer backbone on the liquid crystal texture. Polarizing optical, atomic force, and scanning electron microscopy data are presented.

Oriented Gold Nanorods and Gold Nanorod Chains within Smectic Liquid Crystal Topological Defects.

We show that the use of oriented linear arrays of smectic A defects, the so-called smectic oily streaks, enables the orientation of gold nanorods (GNRs) for a large range of GNR diameters, ranging from 7 to 48 nm, and for various ligands. For the small GNRs it enables oriented end-to-end small chains of GNRs when the density is increased from around 2 GNRs/μm2 to around 6 GNRs/μm2. We have characterized the orientation of single GNRs by spectrophotometry and two-photon luminescence (TPL). A strongly anisotropic absorption of the composites and an on-off switching of GNR luminescence, both controlled by incident light polarization, are observed, revealing an orientation of the GNRs mostly parallel to the oily streaks. A more favorable trapping of GNRs by smectic dislocations with respect to ribbon-like defects is thus demonstrated. The dislocations appear to be localized at a specific localization, namely, the summit of rotating grain boundaries. Combining plasmonic absorption measurements, TPL measurements, and simulation of the plasmonic absorption, we show that the end-to-end GNR chains are both dimers and trimers, all parallel to each other, with a small gap between the coupled GNRs, on the order of 1.5 nm, thus associated with a large red-shift of 110 nm of the longitudinal plasmonic mode. A motion of the GNRs along the dislocations appears as a necessary ingredient for the formation of end-to-end GNR chains, the gap value being driven by the balance between the attracting van der Waals interactions and the steric repulsion between the GNRs and leading to interdigitation of the neighboring ligands. We thus obtain electromagnetic coupling of nanorods controlled by light polarization.

Anomalous optical properties of photoactive cholesteric liquid crystal doped with single-walled carbon nanotubes

ABSTRACTOptical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At small concentrations of CNTs (C ≈ 0.01–0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density D was shown to be essentially non-linear and a minimum at certain concentration of CNTs, C ≈ 0.05–0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced trans–cis–trans isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.

Optical density and microstructure-related properties of photoactive nematic and cholesteric liquid crystal colloids with carbon nanotubes

The optical density, microstructure and electrical conductivity in nematic (ZhK440, 5CB and their mixtures) and cholesteric (the mixtures of ZhK440, 5CB with chiral M5) colloids with single-walled carbon nanotubes (CNTs) were studied. The azoxy nematic ZhK440 is photoactive, and the effects of UV-irradiation and restoring irradiation were also investigated for the LC composites containing ZhK440. In the nematic mixtures ZhK440+5CB anomalous behavior of optical density and electrical conductivity close to the compensation point of dielectric anisotropy was observed. It was speculated that CNTs can increase the tendency towards nematic ordering of LC molecules and destroy the intermolecular complex by preferential adsorption of 5CB molecules on the surface of CNTs. In cholesteric mixtures of ZhK440+ M5, the optical density vs. CNT concentration D(Cn) dependences were markedly non-monotonous, with a pronounced maximum and minimum. For these composites, UV-irradiation resulted in shifting the location of these extremums. The effects were explained accounting for interactions between cholesteric oily streaks and CNT networks, localisation of CNT inside the cholesteric defects, impact of CNTs on the structure of defects, and impact of cholesteric medium on the structure of CNT aggregates, their compactness and shape anisometry.

Study on the formation and rheological properties of sucrose stearate lamellar liquid crystals

ABSTRACTThe phase diagrams of S1570–Brij 97/oleic acid/water system at two different S1570/Brij 97 mass ratios of 1/1 (system S) and 7/3 (system P) were determined at 37°C, respectively. The microstructures and rheological properties of liquid crystals at constant surfactants/water mass ratio of 1/1 were studied with increase in oil content. Small-angle x-ray scattering measurements showed that system S exhibited a typical lamellar liquid crystals and the repeat distance (d) decreased as oil content increased. While for system P, two couples of scattering peaks were observed when oil content reached 28 wt%. This indicated that adding oil content may give rise to a multilamellar structures in the higher S1570/Brij 97 mass ratio system. The rheological tests indicated that a transition from elastic gel-like properties to viscoelastic liquid-like properties appeared in both system S and P when oil content reached 20 wt%, corresponding to the polarizing optical microscopy textures changing from oily streak to cruciate flower. This supplies a way to adjust structure, rheological properties, and even phase transition by altering the S1570/Brij 97 mass ratio and oleic acid content in system.

Synthesis and properties of new (−)-menthol-derived chiral liquid crystal compounds with alkyl or alkoxy terminal groups

ABSTRACTTwo series of chiral mesogenic compounds derived from (−)-menthol with varying length of alkyl or alkoxy terminal groups respectively were designed and synthesised. Their chemical structures were characterised by FT-IR and 1H-NMR spectra. The thermal properties and optical textures were investigated by differential scanning calorimetry and polarising optical microscopy. Bragg selective reflection spectra of the compounds with the alkoxy chain in the N* phase were measured by ultraviolet/visible spectrometer. The results showed that the alkyl series homologues melt directly to the isotropic phase on heating and display cubic blue phase and focal conic textures of chiral nematic phase on cooling, whereas the alkoxy series displayed oily streak textures with iridescent colours on heating, and platelet textures of blue phase and focal conic textures were observed on cooling cycles. The chain structure and length of the terminal groups have profound influence on the isotropic temperature and a large odd–even effect is observed for the compounds. The selective reflection colours of alkoxy series shifted to longer wavelength with the increasing of temperature.

Chiral oily streaks in a smectic-A liquid crystal.

The liquid crystal octylcyanobiphenyl (8CB) was doped with the chiral agent CB15 and spin-coated onto a substrate treated for planar alignment of the director, resulting in a film of thickness several hundred nm in the smectic-A phase. In both doped and undoped samples, the competing boundary conditions - planar alignment at the substrate and vertical alignment at the free surface - cause the liquid crystal to break into a series of flattened hemicylinders to satisfy the boundary conditions. When viewed under an optical microscope with crossed polarizers, this structure results in a series of dark and light stripes ("oily streaks") of period ∼1 μm. In the absence of chiral dopant the stripes run perpendicular to the substrate's easy axis. However, when doped with chiral CB15 at concentrations up to c = 4 wt%, the stripe orientation rotates by a temperature-dependent angle φ with respect to the c = 0 stripe orientation, where φ increases monotonically with c. φ is largest just below the nematic - smectic-A transition temperature TNA and decreases with decreasing temperature. As the temperature is lowered, φ relaxes to a steady-state orientation close to zero within ∼1 °C of TNA. We suggest that the rotation phenomenon is a manifestation of the surface electroclinic effect: The rotation is due to the weak smectic order parameter and resulting large director tilt susceptibility with respect to the smectic layer normal near TNA, in conjunction with an effective surface electric field due to polar interactions between the liquid crystal and substrate.

Self-organized arrays of dislocations in thin smectic liquid crystal films.

Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.

Non-symmetrical liquid crystal dimers armed with azobenzene and 1,2,3-triazole-cholesterol

New non-symmetrical dimers which contain respective side units of azobenzene and 1,2,3-triazole-choloesteral have been synthesised from the reaction between 6-azido-[4-(4-acetylphenylazo)phenoxy]alkane and cholesteryl-4-(prop-2-ynyloxy)benzoate. The results show that the derivatives of 6-bromo[4-(4-acetylphenylazo)phenoxy]alkanes which possess the odd alkyl spacers exhibit dimesomorphism of N and SmA phases whilst the members with even numbers are non-mesogenic. However, the 6-azido-[4-(4-acetylphenylazo)phenoxy]-alkanes show N and SmA phases in a wide temperature range. On heating and cooling the target compounds cholesteryl 4-[(1-(6-(4-(4ʹ-acetylphenylazo)phenoxy)alkyl)-1H-1,2,3-triazol-4-yl)methoxy]benzoate show chiral nematic (oily streak and fan-shaped texture) and focal conic texture characteristics of smectic A phases with high thermal stability. Present investigation also reveals the effect of the spacer length upon the phase transition temperature. Intercalated arrangement is favoured by the molecules in the smectic phase. The observation is well established by XRD measurement wherein the actual molecular length is smaller than the theoretical value.

Branched-arm macromolecule liquid crystals-containing fluorine and isosorbide-structure and properties

A new series of cholesteric fluorated branched-arm liquid crystals (CHFBALCs) compound (MFA1–MFA4), with isosorbide as chiral core, was synthesized. Four precursors of branched-arm FA1–FA4 were obtained firstly and then were esterized separately with isosorbide to get four chiral branched-arm liquid crystalline (MFA1–MFA4). The structures and properties were characterized by conventional spectrum methods, thermal analysis and texture analysis. The results showed that FA1–FA4 all were nematic phase with schlieren, threaded-like or droplet texture. When the isosorbide was successfully inducted, the phase of FA1–FA4 turned into cholesteric phase with oily streak texture, finger print texture or cholesteric droplets. The values of melting point and clearing point for FA1–FA4 and MFA1–MFA4 all changed irregularly, which may be due to the effect of different branched arm, warping of chiral agent and fluorine atom. MFA1–MFA4 had the same handedness as isosorbide core and did not show selective reflection. MFA3 and MFA4 were hopeful to realize the combination of the processability of polymers and the mobility of small molecules.

Enhancement of Photoluminescence Intensity of CdSe Nanorods Doped in Cholesteric Liquid Crystals

The enhancement of photoluminescence (PL) signals of CdSe nanorods caused by embedding in the cholesteric liquid crystals (CLCs) is demonstrated in this article. Three kinds of different CLCs have been used in this experiment, and the results have shown that the phenomenon of PL enhancement generally occurs in each sample. Moreover, the relations between the enhancement and the pitch of CLCs have been analyzed as well. It displays an inversely proportional property, that is, a greater enhancement of the PL signal is achieved in the samples with shorter pitches of CLCs. The highest PL amplification acquired in this study is 3.31 times. The enhancement phenomenon is attributed to the presence of oily streaks in CLCs, which possess advantages due to the excitation of CdSe nanorods. With the versatile properties that CLC have, this study suggests that the method could provide a potential way for PL signal manipulation in many optical fields.

Modeling the optical properties of self-organized arrays of liquid crystal defects.

Local full Mueller matrix measurements in the Fourier plane of a microscope lens were used to determine the internal anisotropic ordering in periodic linear arrays of smectic liquid crystal defects, known as 'oily streaks'. We propose a single microstructure-dependent model taking into account the anisotropic dielectric function of the liquid crystal that reproduces the smectic layers orientation and organization in the oily streaks. The calculated Mueller matrix elements are compared to the measured data to reveal the anchoring mechanism of the smectic oily streaks on the substrate and evidence the presence of new type of defect arrangement. Beyond the scientific inquiry, the understanding and control of the internal structure of such arrays offer technological opportunities for developing liquid-crystal based sensors and self-assembled nanostructures.

Thermotropic phase transitions of catanionic dodecylsulfates with multi-charged and multi-tailed quaternary ammonium centers

Five novel anhydrous catanionic dodecylsulfates containing multi-charged and multi-tailed quaternary ammonium centers were synthesized and examined with light microscopy, differential scanning calorimetry and X-ray powder diffraction. This study is an attempt to explain the relationship between chemical structure, molecular architecture, phase transition characteristics and thermodynamics and the nature of intermolecular interactions of the individual amphiphiles that ultimately lead to different mesomorphic product. All examined compounds are of typical layered structure at room temperature. The long spacing decreases linearly with the increase of either ionic head or n-dodecyl chain number. The thermal analysis of the examined multi-charged catanionics indicates thermotropic mesomorphism, whereas multi-tailed dodecylsulfates show only properties of soft crystals. Maltese crosses, oily streaks textures, stepped drops and fan-shaped textures affirmed the existence of various smectic mesophases at room and higher temperatures. Thermodynamically the most ordered compound is dimeric didodecylsulfate, and the most disordered is three-tailed dodecylsulfate. The addition of the new quaternary ammonium center or alkyl chain causes the increment of the lattice energy first, and it decreases by further changes in the structure. The temperatures of crystallization decrease by any mentioned addition.

Mesomorphic, micro-Raman and DFT studies of new calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates

The mesomorphic properties of newly synthesized homologous series of calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates, H2n+1CnOC6H4COOC6H4C(H)N C6H4COOCH3; n=6, 8, 10, 12, 14, 16 (MABBAB-n) containing ester and Schiff base groups as linker have been studied by temperature dependent micro-Raman study, differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All members of this series exhibit enantiotropic smectic A (SmA) mesophase with oily streak and focal conic textures. Analyses of Raman marker bands of phenyl rings, Schiff base and ester groups of MABBAB-10 confirm the phase transitions. The Raman study also gives an evidence of breaking of weak intermolecular hydrogen bonds associated with ester groups and formation of new hydrogen bonds through CN bond at Cr→SmA phase transition. The monomer and dimer were optimized and vibrational assignment of MABBAB-10 was also done with density functional theoretical (DFT) technique to understand the experimental results.

Structural rheology of focal conic domains: a stress-quench experiment.

We study the dynamics of focal conic domain (FCD) formation in a thermotropic smectic phase under shear stress. It is known that increasing the shear stress induces a non-equilibrium phase transition from a smectic phase with FCDs (SmAI) to another smectic phase (SmAII) in which the layers are oriented. By quenching the shear stress from the SmAII phase to the SmAI phase, we find three characteristic modes in the FCD formation process. The first mode is attributed to the edge dislocation dynamics induced by climb motions. The second mode results from FCD formation. The first and second modes show slowing down close to the smectic-nematic transition temperature, implying that the dynamics are dominated by dislocation unbinding. The third mode originates from the alignment of FCDs which form oily streaks. Such an alignment occurs when the shear stress balances the line tension of the oily streaks.

Synthesis and mesomorphic properties of chiral nematic liquid crystals based on cholesterol

A new series of mesogenic compounds having a cholesteryl moiety has been synthesised by condensing p-amino benzoate of cholesterol and methoxy substituted 4-n-alkoxy cinnamoyl chlorides and their liquid crystalline properties has been studied. All the members of the series are enantiotropic and exhibit chiral nematic (N*) mesophase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits odd–even effect and falling tendency for N*–isotropic transition temperatures. The compounds exhibit oily streak textures that on slight disturbance change to the plane textures and show iridescent colours. High anisotropy, linearity and intermolecular hydrogen bonding confer rich mesomorphic properties on the system. Intermolecular hydrogen bonding arising from amide linkage can lead to supramolecular motifs.

Structural peculiarities of micelles in lamellar mesophase of lyotropic liquid crystalline systems: Shape, sizes and anisometricity

In this work structural peculiarities of the disk-like (the plate-like) micelles in lyotropic liquid crystalline lamellar D mesophase have been investigated. Binary and ternary lyotropic systems, which are based on sodium (dodecyl) lauryl sulphate, were used. D mesophase in such systems was displayed by texture with oily streak formations on the pseudo-isotropic background. The method for the study of the electrical conductivity anisotropy in the orientational shear flow has been applied for estimation of structural peculiarities of micelles in D mesophase. The effective diameter, the parameters of anisometricity, the form factor and the volumes of the disk-like (plate-like) micelles have been determined. Comparative estimates of structural peculiarities of the disk-like (plate-like) micelles for binary and ternary lyotropic systems have been carried out.

Syntheses and Characterization of Chiral Branched-Arm Liquid Crystals-Basing on Isosorbide as Chiral Core

A new series of two-arm (MB1, MB2), four-arm (MB3), six-arm (MB4) chiral liquid crystalline compound containing isosorbide as chiral core were first synthesized. Four precursors of branched-arm B1~B4 were obtained at first and then were esterized separately with isosorbide and got four chiral branched-arm liquid crystals (MB1~MB4). The chemical structures and LC properties of the liquid crystalline molecule were measured by spectrum and thermal analysis techniques. XRD curves of MB1~MB4 samples only showed broad peaks at wide-angle, but no sharp peak was seen for all the samples. The results showed that MB1~MB4 appeared cholesteric phase with oily streak texture or lined texture and finger print texture. Isosorbide successfully induced cholesteric phase. The values of melting point and clearing point for MB1~MB4 decreased. The quantity of branched-arm and chiral core had effects on their liquid crystalline properties.

Shear-induced Onion Formation of Triblock Copolymer-embedded Surfactant Lamellar Phase

Shear-induced onion phase formation is an unsolved topic in soft matter. It has been widely reported that many different surfactant lamellar systems show the shear-induced onion phase formation, since Roux et al. found it. In order to understand the shear-induced onion phase formation mechanism, Nettesheim et al. traced the formation process by using small angle neutron scattering under shear (RheoSANS) and found an intermediate structure during the structural transition. Kato et al. presented that the coherently buckled lamellae is plausible as such intermediate structure by carefully observing a small angle X-ray scattering profiles under shear (Rheo-SAXS). Although new experimental results have been steadily accumulated, the comprehensive studies on the critical condition and membrane flexibility necessary for the onion phase formation are lacked. Our finding in our previous study is that the development of the oily streak network density, which is a typical structural defect found in the smectic phase, is essential to achieve the shearinduced onion phase formation. As the oily streak network density is increased, the focal conic domains (FCD) with a negative Gaussian curvature are crowded and fulfill the space. FCDs have unfavorable energy contribution originating from the layer compression because of the inhomogeneous layer spacing. Excess increase in the FCD density seems to destabilize the lamellar phase and trigger the realignment of the layer orientation from FCD with the negative Gaussian curvature to new state so that the inhomogeneous layer spacing is eliminated. This could be a driving force for the shear-induced onion formation. The bilayer membrane curvature has to be reorganized from negative to positive for the onion formation (Fig. 1). We have previously studied the shear-induced lamellar/ onion transformation behavior of the amphiphilic triblock copolymer-grafted lamellar phase systems. In such block Shear-induced Onion Formation of Triblock Copolymer-embedded Surfactant Lamellar Phase